NOM-020-STPS-2011, Pressure vessels, cryogenic vessels and steam generators or boilers - Operation - Safety conditions

That on 30 November 2010, in compliance with article 46, section I, of the Federal Act on Metrology and Standardisation, the Ministry of Labour and Social Welfare submitted to the National Consultative Committee for Standardisation on Occupational Safety and Health the Preliminary Draft Amendment to this Official Mexican Standard, and that the aforementioned Committee considered it correct and agreed that it be published as a Draft in the Official Journal of the Federation; That to comply with articles 69-E and 69-H of the Federal Act on Administrative Procedure, the corresponding Preliminary Draft was submitted for the consideration of the Federal Commission for Regulatory Improvement, which issued a favourable opinion on it; That on 27 December 2010, in compliance with the Agreement establishing the organisation and Operating Rules of the National Consultative Committee for Standardisation on Occupational Safety and Health, and with article 47, section I, of the Federal Act on Metrology and Standardisation, the Draft amendment to Official Mexican Standard NOM-020-STPS-2002, Pressure vessels and boilers - Operation - Safety conditions, to become PROY-NOM-020-STPS-2010, Pressure vessels, cryogenic vessels and steam generators or boilers - Operation - Safety conditions, was published in the Official Journal of the Federation so that, within the 60 calendar days following that publication, interested parties could submit their comments to the Committee; That having received comments from 21 petitioners, the Committee proceeded to study them and resolved upon them in due course, this department publishing the respective responses in the Official Journal of the Federation of 30 November 2011, in compliance with article 47, section III, of the Federal Act on Metrology and Standardisation; That as a result of incorporating the comments submitted on the Draft amendment to Official Mexican Standard NOM-020-STPS-2002, Pressure vessels and boilers - Operation - Safety conditions, to become PROY-NOM-020-STPS-2010, Pressure vessels, cryogenic vessels and steam generators or boilers - Operation - Safety conditions, as well as of the final review of the draft itself, various amendments were made to provide clarity, consistency and legal certainty as regards the provisions applying in workplaces, and That in view of the foregoing considerations, and since the National Consultative Committee for Standardisation on Occupational Safety and Health granted its approval, the following is issued: OFFICIAL MEXICAN STANDARD NOM-020-STPS-2011, PRESSURE VESSELS, CRYOGENIC VESSELS AND STEAM GENERATORS OR BOILERS - OPERATION – SAFETY CONDITIONS

Reference Guide I Equivalences of pressure, volume and thermal capacity values Reference Guide II Classification of Pressure Vessels, Cryogenic Vessels and Steam Generators or Boilers under the Decimal Metric System Reference Guide III Non-destructive examinations

To establish the safety requirements for the operation of pressure vessels, cryogenic vessels and steam generators or boilers in workplaces, so as to prevent risks to workers and damage to installations.

2.1 This Official Mexican Standard is in force throughout the national territory and applies to all workplaces where pressure vessels, cryogenic vessels and steam generators or boilers operate.

2.2 This Official Mexican Standard does not apply to the following equipment: a) Diving bells; b) Hyperbaric bells or chambers; c) Vessels used as fire extinguishers; d) Containers working at atmospheric pressure; e) Vessels working interconnected in the same process line, where the operating pressure of the set of equipment and of each piece of equipment is between 29.42 kPa and 196.14 kPa gauge pressure and, at the end of the process line, they are open to the atmosphere; f) Pipework, distribution headers not used as fluid accumulators and their components (expansion joints and connections); g) Portable vessels containing compressed gases; h) Pressurised fittings used as components or mechanisms serving for mixing, separation, spraying, distribution, measurement, filtration or control of fluids that do not exceed 0.15 m nominal diameter, installed on the pressure vessels; i) Vessels installed on mobile equipment associated with their braking systems; j) Vessels containing liquefied petroleum gas, governed by legal provisions whose enforcement falls to the Ministry of Energy, and k) Tank cars transporting compressed gases, whose regulation falls to the Ministry of Communications and Transport.

For the correct interpretation of this Standard, the following standards in force, or those superseding them, shall be consulted:

3.1 NOM-002-STPS-2010, Safety conditions - Fire prevention and protection in workplaces.

3.2 NOM-018-STPS-2000, System for the identification and communication of hazards and risks from hazardous chemical substances in workplaces.

3.3 NOM-022-STPS-2008, Static electricity in workplaces - Safety conditions.

3.4 NOM-026-STPS-2008, Safety and hygiene colours and signs, and identification of hazards from fluids conveyed in pipework.

3.5 NOM-008-SCFI-2002, General System of Units of Measurement.

3.6 NOM-093-SCFI-1994, Pressure relief valves (safety, safety-relief and relief), spring- and pilot-operated; made of steel and bronze.

3.7 NMX-B-482-1991, Training, qualification and certification of non-destructive testing personnel.

For the purposes of this Standard, the following definitions are established:

4.1 Alterations: Modifications made to a piece of equipment that change its capacity to withstand the original design temperature or pressure, even where they do not involve physical changes to the equipment. The replacement of fittings or instruments by others of the same characteristics is not considered an alteration.

4.2 Labour authority; employment authority: The competent administrative units of the Ministry of Labour and Social Welfare that carry out inspection functions in occupational safety and health matters, and the corresponding units of the federal states and the Federal District acting in support of them.

4.3 Workplace: Those places such as buildings, premises, installations and areas in which activities of production, marketing, transport and storage or provision of services are carried out, or in which persons who are subject to an employment relationship work.

4.4 Manufacturing certificate: The document issued and endorsed by the original manufacturer of the equipment, setting out the design, materials, data and information for its use, tests and reviews, consistent with the code or standard used for its construction.

4.5 Code or standard: The set of technical rules on which the design, construction, installation, testing, repair or alteration of the equipment is based.

4.6 Operating conditions: The operating variables of the equipment, including the pressure and temperature limits accepted and recognised as safe according to the design and manufacturing characteristics, and which neither activate the safety devices nor exceed the safety ranges of their control instruments.

4.7 Pressure relief devices; safety device: The fittings or any other element set to relieve an overpressure, such as safety valves, pressure relief valves, rupture discs, among others.

4.8 Equipment: The closed vessel or vessels subject to internal or external pressure, cryogenic vessels and steam generators or boilers.

4.9 Auxiliary equipment: The fitting or component required for the operation of steam generators or boilers, such as containers for the supply of water, air or fuel; water treatment, pumping and condensate equipment; distribution headers; fan(s) and extractor(s), among others.

4.10 New equipment: Equipment that has not been put into operation since its manufacture; that has a manufacturing certificate or the stamp of compliance with the code or standard; that retains the integrity conditions with which it was designed and built, and that has not undergone alterations.

4.11 Non-destructive examinations (NDE): The types of test carried out on the material of a piece of equipment and/or on its joints, which do not permanently alter its physical, chemical, mechanical or dimensional properties.

4.12 Hazardous fluids: Those chemical substances that, by their intrinsic characteristics and their temperature in the process, meet the following: a) They present health, flammability and/or reactivity risks, being oxidising substances or substances that react on contact with water, as established in NOM-018-STPS-2000, or those superseding it, or b) Their operating temperature is above 50 ºC, or c) Their temperature is below -10 °C.

4.13 Operation: The safe running of the equipment and of its safety elements, underpinned by the actions carried out in its review and maintenance.

4.14 Liquefied petroleum gas; LP gas: The fuel in whose chemical composition the hydrocarbons butane and propane predominate.

4.15 Steam generator or boiler: The apparatus used to generate water vapour or to heat a fluid in the liquid state, by applying the heat produced by the combustion of materials, chemical reactions or solar or electrical energy.

4.16 Safety instructions or procedures: The description, in logical and sequential order, of the activities and safe conditions that workers shall follow in the operation, review, maintenance, pressure testing and non-destructive examination of the equipment.

4.17 Control instruments: The elements installed on the equipment to manage, regulate and/or monitor its operating variables, including, among others, water columns, level indicators and pressure controls.

4.18 Employer: The natural or legal person in whose workplace a piece of equipment operates and who is responsible for its operation.

4.19 Qualified personnel: The person with knowledge and experience to carry out repairs to equipment requiring welding, or alterations.

4.20 Trained personnel: The person with knowledge to carry out activities of operation, maintenance or repair of equipment.

4.21 Certified personnel: The person with the technical capacity and experience to carry out, supervise, interpret and evaluate the results of non-destructive examinations, in accordance with NMX-B- 482-1991, or those superseding it, or an equivalent.

4.22 Set pressure: The pressure value at which the opening of a pressure relief device is set.

4.23 Design pressure: The pressure value established in the manufacture of the equipment, based on the most severe pressure and temperature conditions expected during its operation, and according to which the strictest specifications for wall thickness and for its components are determined.

4.24 Operating pressure: The gauge pressure at which a piece of equipment works under normal conditions.

4.25 Test pressure: The pressure value to which a piece of equipment is subjected, on the basis of the construction code or standard, to verify that its constituent parts are safe in operation.

4.26 Maximum allowable working pressure: The lowest of the pressure values calculated for any of the essential parts of the equipment according to its design, or recalculated using the current thicknesses, without permanent deformation occurring.

4.27 Conformity assessment procedure: The methodology for assessing conformity with this Official Mexican Standard.

4.28 Cryogenic vessel: The pressure vessel containing a cryogenic fluid -oxygen, nitrogen, argon, helium and hydrogen, among others-, or condensed or liquefied gases -carbon dioxide and nitrous oxide, among others-, of double wall, with an inner tank and an outer one, as applicable. Between the two tanks there is an annular space under vacuum and a material with insulating properties to prevent heat transfer by conduction, convection and radiation from the outside environment to the gases in the liquid state inside it.

4.29 Pressure vessel: The apparatus built to operate at a pressure above atmospheric or subjected to vacuum. The pressure may be exerted on the inner surface, the outer surface and/or the components of the equipment. That pressure may come from external sources or through the application of heat, from a direct or indirect source or any combination of these.

4.30 Record: The objective evidence of operation, review and maintenance activities having been carried out, as well as of the history of the pressure tests or non-destructive examinations performed on the equipment.

4.31 Repair: The work carried out, in accordance with a code or standard, that restores the equipment to an appropriate state so that it operates in safe conditions.

4.32 Services report: The document issued by a type "C" verification unit, accredited and approved under the Federal Act on Metrology and Standardisation, in which it certifies that, at the employer's request, the services of preparation, execution and validation of the requirements established in this Standard were provided.

4.33 Review: The activities carried out by knowledgeable personnel to determine whether the equipment operates in safe working conditions and whether the operating procedures and maintenance programmes are complied with.

4.34 Imminent risk: The condition of operation outside the normal parameters of a piece of equipment and/or the absence of control instruments, which endangers its physical integrity, that of the workers and/or the installations of the workplace.

4.35 Ministry: The Ministry of Labour and Social Welfare.

4.36 Heating surface: The heat transfer area of a heat exchanger, or of a steam generator or boiler, that is in simultaneous contact, on one side with the source of thermal energy and, on the other, with the working fluid or cold part.

4.37 Design temperature: The temperature expected in the equipment under the maximum extraordinary operating conditions, which may be equal to or higher than the operating temperature.

4.38 Operating temperature: The maximum temperature of the metal in the equipment reached under normal operating conditions, which shall never exceed the design temperature value.

4.39 Extreme temperature: The outer temperature of a piece of equipment, above 50 ºC or below -10 ºC, which, should it come into contact with the skin, causes burns to the worker.

4.40 Relevant transients: Those momentary conditions occurring outside the normal parameters of safe operation of a piece of equipment.

4.41 Type "A" verification unit: The natural or legal person, accredited and approved under the Federal Act on Metrology and Standardisation, to verify the degree of compliance with this Standard by issuing a conformity assessment report.

4.42 Type "B" verification unit: The natural or legal person, accredited and approved under the Federal Act on Metrology and Standardisation, constituted by the workplace itself and forming an integral part of it, with independence of judgement, integrity in relation to its activities and without conflict of interest, responsible for verifying the degree of compliance with this Standard by issuing a conformity assessment report.

4.43 Type "C" verification unit: The natural or legal person, accredited and approved under the Federal Act on Metrology and Standardisation, having personnel to carry out independently: a) The services of preparation, execution and validation of the requirements established in this Standard, by issuing a services report, and b) The verification of the degree of compliance with this Standard by issuing a conformity assessment report.

5.1 Classify the equipment installed in the workplace into Categories I, II or III, in accordance with Chapter 7 of this Standard.

5.2 Have an up-to-date list of the equipment installed in the workplace, in accordance with Chapter 8 of this Standard.

5.3 Have a file for each piece of equipment installed in the workplace, in accordance with Chapter 9 of this Standard.

5.4 Draw up and apply specific review and maintenance programmes for the equipment classified in Categories II and III, based on Chapter 10 of this Standard.

5.5 Draw up and apply programmes for the review and calibration of the control instruments and pressure relief devices of the equipment, as applicable.

5.6 Have and apply procedures for the operation, review and maintenance of the equipment, in Spanish, in accordance with Chapter 11 of this Standard. The procedures may be drawn up per piece of equipment or per set of interconnected equipment or equipment of common application.

5.7 Carry out the maintenance and repair of equipment not requiring welding, with personnel trained in the subject.

5.8 Carry out repairs to equipment requiring welding, or alterations, in accordance with the procedures developed for that purpose and with qualified personnel.

5.9 Comply with the safety conditions of the equipment, as applicable, in accordance with Chapter 12 of this Standard.

5.10 Determine and carry out pressure tests or non-destructive examinations on the equipment classified in Categories II and III, in accordance with Chapter 13 of this Standard.

5.11 Demonstrate that the pressure relief devices of the equipment are in working condition, based on Chapter 14 of this Standard.

5.12 Have an emergency response plan for the equipment classified in Categories II and III, in accordance with Chapter 15 of this Standard.

5.13 Give notice to the Ministry that the equipment operating in the workplace, classified in Category III, complies with this Standard, in accordance with the arrangements provided for in Chapter 16 of the same.

5.14 Inform the workers and the safety and hygiene committee about the hazards and risks inherent in the equipment and in the fluids it contains.

5.15 Train the personnel who carry out activities of operation, maintenance, repair and pressure testing or non-destructive examination on equipment classified in Categories II and III, in the subjects applicable to them, in accordance with Chapter 17 of this Standard.

5.16 Have the operating records of the equipment installed in the workplace, classified in Categories II and III, in accordance with Chapter 18 of this Standard.

5.17 Have the records of the results of the review, maintenance and pressure tests or non-destructive examinations carried out on the equipment classified in Categories II and III, based on Chapter 18 of this Standard.

5.18 Present to the labour authority the documents, records and information that this Standard requires the employer to draw up or hold, when it so requests.

6.1 Review the state of the equipment before operating it.

6.2 Operate, review and maintain the equipment, as applicable, in accordance with the safety instructions and/or procedures.

6.3 Inform the employer and the safety and hygiene committee about anomalies and unsafe operating conditions of the equipment, even where these have been remedied.

6.4 Inform the employer and the safety and hygiene committee about the conditions of imminent risk they detect in the operation of the equipment.

6.5 Take part in the training and instruction provided by the employer.

7.1 The equipment installed in the workplace shall be classified as follows:

7.1.1 Pressure vessels shall be classified into the following categories (See Table 1): Table 1 Types of Categories for Pressure Vessels*

                               Hazardous                Lower than or equal to 686.47 kPa       Lower than or equal to 1 m3
      III        Water, air and/or non-hazardous fluid Greater than 490.33 kPa and lower than or               Greater than 1 m3

                 Water, air and/or non-hazardous fluid        Greater than 784.53 kPa             Any volume
                               Hazardous                Lower than or equal to 686.47 kPa             Greater than 1 m3
                               Hazardous                   Greater than 686.47 kPa             Any volume
 *          See Reference Guides I and II to identify the equivalent values of pressure and volume in

7.1.2 Cryogenic vessels shall be classified into the following categories (See Table 2): Table 2 Types of Categories for Cryogenic Vessels Category Volume II Lower than or equal to 1 m3 III Greater than 1 m3 a) Category II: Cryogenic vessels with a volume lower than or equal to 1 m3. b) Category III: Cryogenic vessels with a volume greater than 1 m3.

7.1.3 Steam generators or boilers shall be classified into the following categories (See Table 3): Table 3 Types of Categories for Steam Generators or Boilers*

               Category                              Pressure**                      Thermal capacity
                    II                        Lower than or equal to 490.33 kPa        Lower than or equal to 1 674.72 MJ/hr
                    III                       Lower than or equal to 490.33 kPa           Greater than 1 674.72 MJ/hr
                                                 Greater than 490.33 kPa                  Any capacity
   *      See Reference Guides I and II to identify the equivalent values of pressure and capacity

8.1 The list of the equipment installed in the workplace shall contain the following: a) The generic name of the equipment; b) The serial or unique identification number, the equipment code or TAG number; c) The classification corresponding to each piece of equipment, in accordance with Chapter 7 of this Standard; d) The fluid(s) handled; e) The set pressure, where applicable; f) The volumetric capacity, in the case of pressure vessels and cryogenic vessels; g) The thermal capacity, in the case of steam generators or boilers; h) The area where the equipment is situated; i) The number of the report, or of the report with services report, issued by a verification unit, in the case of equipment classified in Category III, and j) The control number assigned by the Ministry, referred to in paragraph 16.5 of this Standard, in the case of equipment classified in Category III.

9.1 The file for each piece of equipment classified in Category I installed in the workplace shall contain the following: a) The generic name of the equipment; b) The serial or unique identification number, the equipment code or TAG number; c) The technical data sheet of the equipment, covering at least: 1) The fluid(s) handled and their type of risk, where applicable; 2) The design pressure(s); 3) The operating pressure(s); 4) The set pressure(s), where applicable; 5) The maximum allowable working pressure(s); 6) The volumetric capacity; 7) The design temperature(s), and 8) The operating temperature(s); d) The brief description of its operation; e) The record of the results of the reviews and maintenance carried out, and f) The location of the equipment.

9.2 The file for each piece of equipment classified in Category II installed in the workplace shall contain, as applicable, the following: a) The generic name of the equipment; b) The serial or unique identification number, the equipment code or TAG number; c) The year of manufacture; d) The applicable construction code or standard; e) The manufacturing certificate, where one exists; f) The technical data sheet of the equipment, covering at least: 1) The fluid(s) handled and their type of risk, where applicable; 2) The design pressure(s); 3) The operating pressure(s); 4) The set pressure(s), where applicable; 5) The maximum allowable working pressure(s); 6) The hydrostatic test pressure(s); 7) The volumetric capacity, in the case of pressure vessels and cryogenic vessels; 8) The thermal capacity, in the case of steam generators or boilers; 9) The design temperature(s), and 10) The operating temperature(s); g) The brief description of its operation; h) The record of the results of the reviews and maintenance carried out; i) The record of the last pressure test or non-destructive examinations performed on the equipment; j) The record of the modifications and alterations carried out; k) The record of the repairs that involved welding; l) The drawing, simple plan or document (project book, manual or catalogue) of the equipment, and m) The location sketch of the fixed equipment within the workplace and, in the case of mobile equipment, the location logbook.

9.3 The file for each piece of equipment classified in Category III installed in the workplace shall contain, as applicable, the following: a) The generic name of the equipment; b) The serial or unique identification number, the equipment code or TAG number; c) The control number assigned by the Ministry; d) The year of manufacture; e) The applicable construction code or standard; f) The manufacturing certificate, where one exists; g) The photograph or rubbing of the equipment nameplate, affixed or stamped by the manufacturer; h) The technical data sheet of the equipment, covering at least: 1) The fluid(s) handled and their type of risk, where applicable; 2) The design pressure(s); 3) The operating pressure(s); 4) The set pressure(s), where applicable; 5) The maximum allowable working pressure(s); 6) The hydrostatic test pressure(s); 7) The volumetric capacity, in the case of pressure vessels and cryogenic vessels; 8) The thermal capacity, in the case of steam generators or boilers; 9) The design temperature(s); 10) The operating temperature(s); 11) The type of pressure relief devices, and 12) The number of pressure relief devices; i) The brief description of its operation; j) The description of the risks relating to its operation; k) The safety elements for the control of the main variables of its operation; l) The chronological summary of the reviews and maintenance carried out, in accordance with the programme drawn up for that purpose, duly recorded and documented, endorsed in writing and signed by the person responsible for the maintenance or operation of the equipment in the workplace; m) The chronological summary of the pressure tests or non-destructive examinations performed on the equipment; n) The chronological summary of the modifications and alterations carried out, duly recorded and documented, endorsed in writing and signed by the person responsible for the maintenance or operation of the equipment in the workplace; o) The chronological summary of the repairs that involved welding, endorsed in writing and signed by the person responsible for the maintenance, operation or inspection of the workplace; p) The drawing, plan or document (project book, manual or catalogue) of the equipment, covering: 1) The cross and longitudinal sections of the equipment; 2) The dimensions of the equipment, such as diameter, lengths and manufacturing thicknesses; 3) The relevant details, such as the location of nozzles, fittings and types of heads, among others; 4) The location of the pressure relief devices, whether on the equipment itself, on pipework or on other equipment with which it is interconnected, and 5) The basic arrangement of the support or foundation system; q) The up-to-date design calculation report, backed by the signature, professional licence number and name of an engineer with knowledge in the subject, containing the following: 1) The maximum internal pressure withstood by the equipment in its critical parts, such as shells, heads, furnace, tubesheets and tubes, among others, as applicable; 2) The minimum thicknesses required in its parts; 3) The relief area of the safety devices for the operating conditions. Where such a device is not present, the way in which the equipment is protected against overpressure shall be justified; 4) The heating surface, in the case of steam generators or boilers; 5) The volumetric capacity, in the case of pressure vessels and cryogenic vessels, and 6) The generating capacity, in the case of steam generators or boilers; r) The location sketch of the fixed equipment within the workplace and, in the case of mobile equipment, the location logbook, and s) The conformity assessment report, or the conformity assessment report with services report, issued by a verification unit.

10.1 The specific review and maintenance programmes for the equipment classified in Categories II and III shall provide for, at least, the following: a) The dates of execution; b) The period of execution; c) The type and general description of the activities to be carried out, and d) The name of the person(s) responsible for the scheduling and execution of the activities.

11.1 For operation.

11.1.1 For the equipment classified in Category I, the corresponding instructions or procedures shall be held.

11.1.2 For the equipment classified in Category II, the operating manual shall be held, covering at least the following: a) The safe start-up and shutdown of the equipment; b) The safety measures to be adopted during its operation; c) The response to emergency situations, and d) The recording of the operating variables of the equipment.

11.1.3 For the equipment classified in Category III, the operating manual shall be held, covering at least the following: a) The safe start-up and shutdown of the equipment; b) The use of the measuring instruments; c) The interpretation of the values of the safe operating limits and of the relevant transients; d) The safety measures to be adopted during its operation; e) The personal protective equipment specific to the activities to be carried out; f) The response to emergency situations, and g) The recording of the operating variables of the equipment.

11.2 For review.

11.2.1 For the equipment classified in Category I, the corresponding instructions or procedures shall be held.

11.2.2 For the equipment classified in Categories II and III, the review manual shall be held, containing at least the following: a) The checklist for the operation and maintenance of the equipment; b) The confirmation of compliance with the general and specific safety conditions, as applicable, in accordance with Chapter 12 of this Standard; c) The verification that the tests on the pressure relief devices have been performed, as well as pressure tests or non-destructive examinations and functional tests on the equipment, every five years or after a repair or alteration has been carried out, and d) The criteria for determining whether or not the equipment may remain in operation.

11.3 For maintenance.

11.3.1 For the equipment classified in Category I, the corresponding instructions or procedures shall be held.

11.3.2 For the equipment classified in Category II, the maintenance manual shall be held, covering at least: a) The scope of the maintenance; b) The safety measures to be adopted during its execution; c) The personal or collective protective equipment to be used for each type of work activity; d) The apparatus, instruments and tools to be used, and e) The work permits required, where applicable.

11.3.3 For the equipment classified in Category III, the maintenance manual shall be held, covering at least: a) The scope of the maintenance; b) The description of the main activities, in order of execution; c) The safety measures to be adopted during its execution; d) The personal or collective protective equipment to be used for each type of work activity; e) The apparatus, instruments and tools to be used; f) The work permits required, and g) The emergency response instructions.

12.1 General conditions.

12.1.1 For the equipment classified in Category I, the following shall be complied with: a) Have the serial or unique identification number, code or TAG number marked or painted on it; b) Have the pressure gauge and, where applicable, the control instruments; c) Keep its control instruments in safe operating condition; d) Have the pressure relief device, and e) Have sufficient space for its operation, review and, where applicable, the performance of maintenance operations, in accordance with the manufacturing manual or the installer's recommendations.

12.1.2 For the equipment classified in Categories II and III, the following shall be complied with: a) Have the serial or unique identification number, code or TAG number marked or painted on it; b) Have physical protection, such as containment barriers or perimeter fencing, among others, in the case of equipment located in areas or zones where it may be exposed to being struck by vehicles; c) Keep its support or foundation system in a condition such that its operation is not affected; d) Have the space required for the operation of the equipment and, where applicable, the performance of maintenance operations, pressure tests and/or non-destructive examinations. The minimum dimensions shall be equivalent to those of the element requiring the most space (tubes, heads, baffles, burners or other components), and to the operations envisaged in maintenance, pressure tests and/or non-destructive examinations; e) Have physical protection or insulation elements, to prevent risks to workers from contact with extreme temperatures; f) Be marked to identify the fluids it contains, in accordance with Official Mexican Standards NOM-018-STPS-2000 and NOM-026-STPS-2008, or those superseding them; g) Be connected to an earth electrode, in the case of equipment containing or handling flammable liquids and/or gases, in accordance with NOM-022-STPS-2008, or those superseding it; h) Keep its control instruments in safe operating condition; i) Have elements that direct the relief of its fluids through pressure relief devices, consistent with the state of the fluids -gases, vapours or liquids-, to places where they do not harm workers or the workplace, in accordance with Appendix B, subparagraph B6, of NOM-093-SCFI-1994, or those superseding it; j) Have fire-extinguishing means, on equipment containing or handling flammable liquids or gases, or combustible substances, in accordance with NOM-002-STPS- 2010, or those superseding it; k) Be subject to the review and maintenance programmes; l) Have the safety data sheets of the fluids contained in the equipment, based on NOM-018-STPS-2000, or those superseding it, and m) Maintain the original design conditions of the heating systems, such as the burner and/or fuel, in the case of heat exchangers, and steam generators or boilers.

12.2 Specific conditions.

12.2.1 For the pressure vessels classified in Categories II and III, the following shall be complied with: a) Have pressure relief devices or elements that control the operating pressure so that it is lower than or equal to the maximum working pressure; b) Have their safety devices set in accordance with paragraph 14.1 of this Standard; c) Have pressure measuring instruments, with the measuring range between 1.5 and 4 times the operating pressure, or in the second third of the dial scale; d) Fit shut-off valves between the equipment and the pressure relief devices only in the cases provided for in Appendix B, subparagraphs B3 and B3.1, of NOM-093-SCFI-1994, or those superseding it; e) Keep at least one of the pressure relief devices in service where there is a three-way connection, and f) Set the first pressure relief device (rupture disc) to a value lower than that of the second or last pressure relief device, where these are installed in series.

12.2.2 For the cryogenic vessels classified in Categories II and III, the following shall be complied with: a) Be installed outside the premises on a concrete base and with perimeter fencing; b) Avoid the storage of materials and objects unrelated to the area where they are installed; c) Allow access to the area where they are situated only to authorised personnel; d) Have signage in the area where they are situated in accordance with NOM-026-STPS-2008, or those superseding it; e) Have at least two pressure relief valves, connected to the inner vessel in the gas-phase part. One of these valves may be replaced by a rupture disc; f) Keep the first safety valve set at the design pressure and the second valve or the rupture disc 10% above the design pressure. Both devices shall be capable of relieving the overpressure; g) Have the gasifiers outside the tank, anchored to the foundation; h) Have connection pipework sufficiently flexible to avoid the effects of expansion and contraction caused by temperature changes; i) Apply the emergency procedure corresponding to the fluid contained, where applicable; j) Avoid contact with oils, greases or other flammable materials, and k) Mark on the tank the information (name and telephone number) of the owner of the cryogenic vessel so that they may be contacted in an emergency.

12.2.3 For the steam generators or boilers classified in Categories II and III, the following shall be complied with: a) Have the pressure relief devices and control instruments that record the safe operating limits; b) Have their safety devices set in accordance with the calibration programme, and subject them to the review and maintenance programmes; c) Have pressure measuring instruments, with the measuring range between 1.5 and 4 times the operating pressure, or in the second third of the dial scale; d) Have pressure relief devices installed on the body and not on remote connections; e) Prohibit the fitting of shut-off valves between the equipment and the pressure relief devices; f) Have the safety elements to prevent them operating in critical conditions of combustion, pressure and/or water level; g) Keep the control instruments in conditions that ensure safe operation; h) Review and test their functioning periodically; i) Verify that the start-up and combustion control system is in good condition to carry out the gas purge, prior to start-up, normal shutdown or in the event of a failure; j) Verify that, in the start-up and combustion control system, in the event of a combustion failure, the fuel supply is automatically shut off, the combustion failure alarm is triggered, automatic re-ignition is prevented and continuous flame monitoring is maintained; k) Confirm, as applicable, the proper functioning of the water level safety elements, so that: 1) The minimum water level specified by the manufacturer is covered; 2) Visual level references are provided, positioned so that the lowest visible part of the reference is at the minimum water level; 3) The mechanical protection systems are suitable for the level indicators, and 4) Where the protection systems are external to the body of the boiler or steam generator, they are provided with blowdowns with a connection for safe drainage; l) Verify on the pressure relief devices, as applicable, the following: 1) That they are accessible and free from obstacles preventing the operator's actions; 2) That the set pressure is never higher than the maximum allowable working pressure; 3) That the relief area calculated for the discharge is equal to or lower than the sum of the relief areas of the pressure relief devices installed; 4) That their connections are independent of any other steam connection; 5) That they are positioned as close as possible to the steam generator or boiler and that in no case are there shut-off valves between the two; 6) That the discharge pipe of the pressure relief devices does not discharge into traffic, operating or working-scaffold zones; 7) That the discharge pipe has an area equal to or greater than that of the pressure relief device; 8) That they are equipped with drainage devices that prevent the accumulation of sediment in the upper part of the pressure relief device; 9) That where an elbow is fitted for the discharge of the pressure relief device it is at a distance of not more than 60 cm from it, and the pipe is fixed independently of the device, and 10) That where silencers are used on the valve, their outlet area is wide enough to prevent back pressure from hindering operation or reducing discharge capacity; m) Verify, as regards the blowdown systems, the following: 1) That the fittings and control/safety elements remain clean, so as to prevent accumulations of residue or the formation of sediment that hinder their operation, and 2) That the blowdown discharge is directed to blowdown pits and/or systems allowing the reduction and damping of the discharge pressure and the cooling of the fluids, and n) Prevent the formation of scale, oxidation or progressive corrosion through the formation of critical thermal zones that weaken materials or joints in the body of the equipment.

13.1 For new equipment classified in Categories II and III that has a manufacturing certificate or the stamp of compliance with the construction code or standard, the first pressure test or the first non-destructive examinations shall be carried out before ten years have elapsed from the issue of that certificate or from obtaining the stamp, and the subsequent tests or examinations at least every five years, within the sixty calendar days prior to the end of each five-year period. In the case of new equipment classified in Categories II and III that does not have the manufacturing certificate or the stamp of compliance with the construction code or standard, or of used equipment of the same categories, with or without the certificate or stamp mentioned above, the first pressure test or the first non-destructive examinations shall be carried out before it is put into operation and, thereafter, at least every five years, within the sixty calendar days prior to the end of each five-year period.

13.2 The hydrostatic, pneumatic and hydrostatic-pneumatic tests, non-destructive examinations and alternative methods approved by the Ministry, carried out on the equipment classified in Categories II and III, shall meet the following requirements: a) Be carried out with the frequency determined by the qualified personnel in the subject appointed by the employer, which shall in no case be more than five years; b) Be selected on the basis of: 1) The results of the reviews of the equipment; 2) The characteristics of the fluids they handle, and 3) The feasibility of their application; c) Be performed in accordance with the requirements and/or guidelines established in nationally or internationally accepted codes or standards; d) Be carried out with the safety measures required before, during and after their performance, as applicable; e) Be developed step by step on the basis of the procedures designed for their execution; f) Be carried out by certified personnel, in the case of non-destructive testing, and by an engineer with knowledge in the subject, in the case of pressure tests; g) Have the acceptance/rejection criteria applied to the results of the pressure tests and/or non-destructive testing; h) Serve as a basis for determining, after their performance, whether or not the equipment assessed may remain in operation; i) Have their results endorsed by certified personnel, by name and signature, in the case of non-destructive examinations, and by an engineer with knowledge in the subject, in the case of pressure tests; j) Be carried out in the presence of a type "A", "B" or "C" verification unit, in the case of equipment classified in Category III, and k) Have their results recorded.

13.3 Pneumatic pressure tests shall be applied only at pressures below 1 961.33 kPa, where the pressure vessels meet the following characteristics: a) That the setting of their safety device is equal to or lower than 980.67 kPa; b) That the volumetric capacity is lower than 10 m3; c) That the maximum internal pressure is greater than 1 961.33 kPa, taking as a reference the current thicknesses of the equipment, and d) That the fluid is air only.

13.4 Hydrostatic-pneumatic pressure tests shall be applied only to cryogenic vessels, under the following conditions: a) Be disconnected from the line supplying the fluid to the process; b) Contain the same cryogenic fluid with which it operates; c) Be at least at 60% of its capacity in the liquid state, and d) Have flow control diagrams of the vessel.

13.5 For the application of non-destructive examinations, at least a combination of one volumetric examination and one surface or flow-loss examination shall be selected and carried out in the same period, as a result of a visual review, from among the following: a) Volumetric: 1) Industrial radiography, or 2) Industrial ultrasound, or 3) Neutron radiography, or 4) Acoustic emission, and b) Surface: 1) Penetrant testing, or 2) Magnetic particles, or 3) Electromagnetism (Eddy currents), or c) Flow loss: 1) Halogen detector, or 2) Mass spectrometer, or 3) Bubble chamber.

13.6 In the case of steam generators or boilers, functional tests shall also be applied, as applicable, for: a) The safety shutdown for: 1) High working-fluid temperature; 2) High steam pressure; 3) High and/or low fuel temperature; 4) High and/or low fuel pressure; 5) High stack temperature, and 6) Off-specification gas emissions, and b) The safety shutdown and alarm for: 1) High and/or low water level, and 2) Flame or combustion failure.

13.7 Where alternative methods are applied that replace the pressure tests or the non-destructive examinations provided for by this Standard, the employer shall hold the authorisation that, where applicable, is granted by the Directorate-General for Occupational Safety and Health, in accordance with the Federal Act on Metrology and Standardisation and its Regulations, and the Federal Regulations on Occupational Safety, Hygiene and the Working Environment.

13.7.1 The application for authorisation of alternative methods shall contain, at least, the following: a) The technical justification for requesting the performance of alternative methods on the equipment; b) The methodology for their development, containing at least: 1) The step-by-step procedures for carrying out the tests; 2) The description of the utensils, materials, fittings and characteristics of the apparatus and instruments -with calibration certificates in force- that will be used in developing the procedure; 3) The drawing of the equipment, with a graphic indication of the zones or points to be inspected where necessary, and 4) The personnel appointed by the employer to develop the tests and to interpret and evaluate the results, with the justification of the experience or training received for that work; c) The criteria for accepting or rejecting the results obtained, which will serve as a basis for determining whether the alternative method performed is satisfactory, and d) The safety measures needed to develop the procedures, where applicable.

14.1 To demonstrate that the pressure relief devices of the equipment are in operating condition, it shall be necessary to: a) Carry out the functional test with instruments having traceability, in accordance with the Federal Act on Metrology and Standardisation, as applicable, on: 1) The equipment itself, or 2) A test bench, where the operating characteristics of the equipment or the fluids contained in it may generate a risk, or b) Hold a manufacturer's quality record or calibration certificate issued under the Federal Act on Metrology and Standardisation.

14.2 Verify, for the functioning of the main pressure relief device, the following criteria: a) The set pressure shall be lower than or equal to the maximum allowable working pressure, and b) The set pressure shall be higher than the operating pressure of the equipment.

14.3 It shall be technically demonstrated that the pressure relief device(s) protect(s) the equipment interconnected with others in a process, where the set pressure value of that or those devices is below the operating pressure value of any of them.

14.4 Equipment classified in Categories II and III that lacks pressure relief devices shall have a technical justification in its design calculation report.

The emergency response plan for equipment classified in Categories II and III shall provide for, at least, the following: a) The identification and location of the areas, premises or buildings where the pressure vessels, cryogenic vessels and steam generators or boilers are situated; b) The identification of the evacuation routes, emergency exits and staircases, lower-risk zones and assembly points, among others; c) The alerting mechanism should an emergency occur; d) The instructions for evacuating workers, contractors, employers and visitors, among others, taking into account persons with disabilities; e) The mechanism for requesting assistance from specialised emergency response services, taking into account the directory of those specialised services in the locality; f) The instructions for the return to normal operating activities after the emergency, and g) The means of communicating the emergency response plan for the equipment.

16.1 The notice that the equipment classified in Category III complies with this Standard shall be given by the employer to the Ministry before the date on which it is put into operation. In the case of new equipment, the employer shall give the notice ten years after having given the first one, and thereafter every five years, within the sixty calendar days prior to the end of each period. In the case of used equipment, the employer shall give the notice five years after having given the first one, and thereafter every five years, within the sixty calendar days prior to the end of each period.

16.2 Where an alteration is carried out or the equipment classified in Category III is relocated, the employer shall give notice to the Ministry that it continues to comply with this Standard, before putting it back into operation with the new operating conditions or the modifications carried out.

16.3 The notices referred to in paragraphs 16.1 and 16.2 shall contain the following: a) Workplace data: 1) The name, denomination or business name; 2) The full address, and 3) The name and signature of the legal representative; b) Equipment data: 1) The generic name of the equipment; 2) The serial or unique identification number, the equipment code and/or TAG number; 3) The control number assigned by the Ministry, where applicable; 4) The fluid(s) handled; 5) The operating pressure(s); 6) The set pressure(s), where applicable; 7) The volumetric capacity, in the case of pressure vessels and cryogenic vessels; 8) The thermal capacity, in the case of steam generators or boilers; 9) The operating temperature(s); 10) The type of pressure relief devices; 11) The number of pressure relief devices, where applicable, and 12) The area where the equipment is situated; c) Manufacturing certificate data, where applicable: 1) The name of the manufacturer; 2) The manufacturing certificate number; 3) The date of issue of the certificate, and 4) The applicable construction code or standard; d) Report data: 1) The name, denomination or business name of the verification unit; 2) The accreditation number granted to the verification unit by the accreditation body; 3) The date on which the accreditation was granted; 4) The registration number granted to the report by the Ministry, and 5) The name of the person responsible for issuing the report, and e) Services report with the summary of the topics or chapters of this Standard addressed, where applicable.

16.4 The notices referred to in the preceding paragraph shall be accompanied by the conformity assessment report issued by a type "A", "B" or "C" verification unit, or by the conformity assessment report with services report issued by a type "C" verification unit. The employer shall have sixty days, counted from the date of issue of the verification report or of the report with services report corresponding to the equipment classified in Category III, to give notice to the Ministry that it complies with this Standard.

16.5 The Ministry shall assign a control number for each piece of equipment, as a result of the notices referred to in paragraphs 16.1 and 16.2, under any of the arrangements provided for in paragraph 16.4, as follows: a) The Ministry, through the federal labour delegations, shall proceed to: 1) Acknowledge receipt of the notice that the equipment complies with this Standard, as well as of its accompanying documentation; 2) Examine whether the information in the notice matches that of the report, or report with services report, issued by the verification unit; 3) Recognise the report or report with services report, and 4) Assign a control number to each piece of equipment, and b) The Ministry shall issue an official letter with the control number within the five working days following receipt of the notice.

17.1 Workers who carry out activities of operation, maintenance, repair and pressure testing or non-destructive examination on equipment shall receive theoretical and practical instruction, as applicable, so as to: a) Define and interpret the following concepts: 1) Design and operating pressure and temperature; 2) Maximum allowable working pressure; 3) Set pressure; 4) Volumetric capacity, in the case of pressure vessels and cryogenic vessels; 5) Thermal capacity, in the case of steam generators or boilers; 6) Drawings or plans of the equipment, main sections of the equipment, relevant details, basic dimensions and basic arrangement of the support system; 7) Signage system for the equipment and pipework; 8) Measuring instruments; 9) Pressure relief devices; 10) Values of the safe operating limits, and 11) Relevant transients; b) Identify the toxicity, flammability and reactivity characteristics of the fluid or fluids handled in the equipment; c) Recognise and address the risks generated by the pressure and temperature of the fluids in the equipment; d) Keep within the established value the operating limits of the equipment and of any pressure relief device or safety element, as well as of those variables that may affect them; e) Apply the procedures for operation, review, maintenance, repair, alteration and pressure testing or non-destructive examination of the equipment, as applicable; f) Apply the procedures for reviewing the pressure relief devices, safety elements and control instruments, as applicable, including emergency shutdown operations, and g) Control changes in the operating conditions of the equipment and/or of the fluids they handle.

17.2 The training of the personnel who carry out activities of operation, maintenance, repair and pressure testing or non-destructive examination on equipment held on loan shall be provided by the employer who owns that type of equipment, who shall deliver a copy of the respective documentation to the workplace where it is installed.

18.1 The records on the operation of the equipment classified in Categories II and III shall contain, as applicable, the following information: a) The generic name of the equipment; b) The control number assigned by the Ministry, where applicable; c) The operating pressures; d) The operating temperatures; e) Any observations that may arise, where applicable; f) The date and time of the records on the operation, and g) The name and signature of the person responsible.

18.2 The records on the results of the review of the equipment shall comprise, as applicable, the following information: a) The generic name of the equipment; b) The control number assigned by the Ministry, where applicable; c) The elements reviewed; d) The result of the review; e) The date and time of the records on the results of the review, and f) The name and signature of the person responsible for the review.

18.3 The records on the results of the maintenance of the equipment shall comprise, as applicable, the following information: a) The generic name of the equipment; b) The control number assigned by the Ministry, where applicable; c) The elements subjected to maintenance and the actions carried out; d) The date and time of the records on the results of the maintenance, and e) The name and signature of the person responsible for the maintenance.

18.4 The records on the results of the pressure tests and/or non-destructive examinations on the equipment shall comprise, as applicable, the following information: a) The generic name of the equipment; b) The control number assigned by the Ministry, where applicable; c) The type of pressure test or non-destructive examinations carried out; d) The equipment used and its characteristics; e) The results of the pressure test or of the non-destructive examinations carried out; f) The date and time of the records on the results of the pressure tests or of the non-destructive examinations carried out, and g) The name and signature of the person responsible for endorsing the results of the pressure tests or non-destructive examinations.

19.1 In the case of the equipment classified in Categories I and II, the employer shall have the option of engaging: a) The services of a type "A", "B" or "C" verification unit, to assess the compliance of the equipment with the Standard and issue the conformity assessment reports, or b) The services of a type "C" verification unit, to provide the services of preparation, execution and validation of the requirements established in this Standard and the verification of its degree of compliance, and issue the conformity assessment report with services report.

19.2 In the case of the equipment classified in Category III, the employer shall have the option of: a) Constituting its own Type "B" verification unit, to assess the compliance of its equipment with the Standard and issue the conformity assessment reports, with independence of judgement, integrity in relation to its activities and without conflict of interest, or b) Engaging the services of a type "A", "B" or "C" verification unit, to assess the compliance of the equipment with the Standard and issue the conformity assessment reports, or c) Engaging the services of a type "C" verification unit, to provide the services of preparation, execution and validation of the requirements established in this Standard and the verification of its degree of compliance, and issue the conformity assessment report with services report.

19.3 The services of preparation, execution and validation of the requirements established in this Standard provided by type "C" verification units shall comprise, among others: a) The preparation of programmes, procedures and design calculation reports, among others; b) The endorsement of the design calculation reports with their name, professional licence number and signature; c) The delivery of training to workers, and/or d) The application of pressure tests and non-destructive examinations, as well as the interpretation of their results.

19.4 The employer may consult the current directory of the verification units accredited and approved by the Ministry of Labour and Social Welfare at the following web address: www.stps.gob.mx

19.5 Verification units assessing conformity with this Standard shall apply the compliance criteria established in the conformity assessment procedure of Chapter 20 of this Standard.

19.6 Verification units shall issue a report or a report with services report, which shall contain: a) Workplace data: 1) The name, denomination or business name; 2) The full address, and 3) The name and signature of the legal representative; b) Verification unit data: 1) The name, denomination or business name; 2) The accreditation number granted by the accreditation body; 3) The dates on which the accreditation was granted and on which approval was granted by the Ministry; 4) The name and signature of the person responsible for issuing the report; 5) The place and date of issue of the report, and 6) The period of validity of the report; c) Equipment data: 1) The generic name of the equipment; 2) The serial or unique identification number, the equipment code or TAG number; 3) The control number assigned by the Ministry, where applicable; 4) The year of manufacture; 5) The fluid(s) handled; 6) The design pressure(s); 7) The operating pressure(s); 8) The set pressure(s), where applicable; 9) The volumetric capacity in the case of pressure vessels and cryogenic vessels; 10) The thermal capacity, in the case of steam generators or boilers; 11) The design temperature(s); 12) The operating temperature(s); 13) The type of pressure relief devices; 14) The number of pressure relief devices, and 15) The area where the equipment is situated, and d) Data on the pressure tests, non-destructive examinations or alternative methods performed on the equipment: 1) The type of pressure tests, non-destructive examinations or alternative methods, and 2) The result of the pressure tests, non-destructive examinations or alternative methods, and e) Number of the report or of the report with services report.

19.7 The period of validity of the reports, or of the reports with services report, issued by verification units shall be ten years in the case of new equipment and five years in the case of used equipment.

20.1 This conformity assessment procedure applies both to the verification actions carried out by verification units and to the enforcement actions falling to the labour authority. Table 4 Random selection sampling table Number of workers to be Number of workers interviewed 1-15 1 16-50 2 51-105 3 1 for every 35 workers up to a More than 105 maximum of 15

20.4 The documentation referred to in this Standard shall be kept for at least ten years in the case of new equipment, and for five years in the case of used equipment.

20.5 The documentary evidence or the records held in the workplace may be presented in printed form or on magnetic media.

Enforcement of compliance with this Standard is the responsibility of the Ministry of Labour and Social Welfare.

22.1 ANSI/ASNT CP-189-1995. American National Standard. ASNT Standard for Qualification and Certification of Nondestructive Testing Personnel U.S.A.

22.2 Recommended Practice No SNT-TC-1A 1996 U.S.A.

This Standard does not concord with any international standard, as no reference existed at the time of its drafting.

FIRST.- This Official Mexican Standard shall enter into force six months after its publication in the Official Journal of the Federation. SECOND.- During the period indicated in the preceding article, employers shall comply with NOM-020-STPS-2002, Pressure vessels and boilers - Operation - Safety conditions, or shall make the adaptations needed to observe the provisions of this Official Mexican Standard NOM-020-STPS-2011, Pressure vessels, cryogenic vessels and steam generators or boilers - Operation - Safety conditions and, in the latter case, the labour authorities shall provide, at the request of interested employers, advice and guidance for implementing compliance, without their becoming liable to penalties for non-compliance with the Standard in force. THIRD.- In the course of the period referred to in the preceding article, employers may obtain the operating authorisations, or the extension of the period of validity of the operating authorisation, for the equipment requiring them, in accordance with the procedures established in paragraphs 8.1 to 8.6 of NOM-020-STPS-2002, Pressure vessels and boilers - Operation – Safety conditions, or may submit to the Ministry the notice referred to in paragraph 5.13 of this Official Mexican Standard NOM-020-STPS-2011, Pressure vessels, cryogenic vessels and steam generators or boilers - Operation - Safety conditions, where the equipment falls within those classified in Category III of this Standard. FOURTH.- Employers holding equipment with an operating authorisation as at the date this Standard enters into force, issued under paragraph 8.2 of NOM-020-STPS-2002, Pressure vessels and boilers - Operation - Safety conditions, may submit to the Ministry the notice referred to in paragraphs 5.13 and 16.1, final subparagraph, of this Official Mexican Standard NOM-020-STPS-2011, Pressure vessels, cryogenic vessels and steam generators or boilers - Operation - Safety conditions, within the sixty days prior to the expiry of the period covered by that authorisation, where the equipment falls within those classified in Category III. FIFTH.- From the date on which this Official Mexican Standard enters into force, NOM-020-STPS-2002, Pressure vessels and boilers - Operation - Safety conditions, published in the Official Journal of the Federation of 28 August 2002, shall cease to have effect, with the exception of the provisions of the following subparagraph. As regards the authorisations for the operation of pressure vessels and steam generators or boilers, provided for by articles 29, section II, and 31 of the Federal Regulations on Occupational Safety, Hygiene and the Working Environment, paragraphs 8.2, 8.4 and 8.5 of NOM-020-STPS-2002, Pressure vessels and boilers - Operation – Safety conditions, shall apply, until such time as the Ministry issues the operating guidelines for granting those authorisations, in accordance with article 19, section XVIII, of the Internal Regulations of the Ministry of Labour and Social Welfare, where the equipment falls within those classified in Category III. Given in Mexico City, Federal District, on the sixteenth day of the month of December of the year two thousand and eleven.- The Minister of Labour and Social Welfare, Rosalinda Vélez Juárez.- Signature.

Equivalences of pressure, volume and thermal capacity values The content of this guide is a supplement for a better understanding of this Standard and is not mandatory. This guide relates the pressure, volume and thermal capacity values of pressure vessels, cryogenic vessels and steam generators or boilers, expressed under the General System of Units of Measurement, to those corresponding under the Decimal Metric System. Table I.1 Pressure values Units of Pressure General System of Units of Measurement Decimal Metric System 2

29.42 kilopascal (kPa) 0.3 kilograms per square centimetre (kg/cm ) 2 196.13 kilopascal (kPa) 2 kilograms per square centimetre (kg/cm ) 2 490.33 kilopascal (kPa) 5 kilograms per square centimetre (kg/cm ) 2 686.47 kilopascal (kPa) 7 kilograms per square centimetre (kg/cm ) 2 784.53 kilopascal (kPa) 8 kilograms per square centimetre (kg/cm ) 2 980.67 kilopascal (kPa) 10 kilograms per square centimetre (kg/cm ) 2 1 961.33 kilopascal (kPa) 20 kilograms per square centimetre (kg/ cm ) Table I.2 Volume values Units of Volume General System of Units of Measurement Decimal Metric System 3

0.5 cubic metres (m ) 500 litres (L or l) 3 1 cubic metres (m ) 1 000 litres(L or l) Table I.3 Thermal capacity values Units of Thermal capacity General System of Units of Measurement Decimal Metric System 1 674.72 Mega joules per hour (MJ/hr) 400 000 kilocalories per hour (kcal/hr) Category Fluid Pressure* Volume I Water, air and/or non-hazardous fluid Lower than or equal to 5 kg/cm Lower than or equal to 500 litres Water, air and/or non-hazardous fluid Lower than or equal to 5 kg/cm Greater than 500 litres Hazardous Lower than or equal to 7 kg/cm Lower than or equal to 1 000 litres III Water, air and/or non-hazardous fluid Greater than 5 kg/cm and lower than or equal to Greater than 1 000 litres Water, air and/or non-hazardous fluid Greater than 8 kg/cm Any volume Hazardous Lower than or equal to 7 kg/cm Greater than 1 000 litres Category Pressure** Thermal capacity II Lower than or equal to 5 kg/cm Lower than or equal to 400 000 kcal/hr III Lower than or equal to 5 kg/cm Greater than 400 000 kcal/hr

                     Water, air and/or non-hazardous fluid    Greater than 5 kg/cm and lower than or equal to 8      Lower than or equal to 1 000 litres
        II                                                                     2

                                 Hazardous                         Greater than 7 kg/cm                    Any volume
   *          Set pressure on its pressure relief device(s).
   a)         Category I:

Classification of Pressure Vessels, Cryogenic Vessels and Steam Generators or Boilers under the Decimal Metric System The content of this guide is a supplement for a better understanding of this Standard. This guide classifies pressure vessels, cryogenic vessels and steam generators or boilers on the basis of the pressure, volume and thermal capacity values expressed under the Decimal Metric System. II.1 The equipment installed in the workplace shall be classified as follows: II.1.1 Pressure vessels shall be classified into the following categories (See Table II.1): Table II.1 Types of Categories for Pressure Vessels 2 2 2 kg/cm 2 2 2 8 kg/cm 2 2 2 Pressure vessels containing water, air and/or any non-hazardous fluid, with a set pressure lower than or equal to 5 kg/cm2 and a volume lower than or equal to 500 litres. b) Category II: Pressure vessels that: 1) Contain water, air and/or any non-hazardous fluid, with a set pressure lower than or equal to 5 kg/cm2 and a volume greater than 500 litres, or 2) Contain water, air and/or any non-hazardous fluid, with a set pressure greater than 5 kg/cm2 but lower than or equal to 8 kg/cm2 and a volume lower than or equal to 1 000 litres, or 3) Handle hazardous fluids, with a set pressure lower than or equal to 7 kg/cm2 and a volume lower than or equal to 1 000 litres. c) Category III: Pressure vessels that: 1) Contain water, air and/or any non-hazardous fluid, with a set pressure greater than 5 kg/cm2 but lower than or equal to 8 kg/cm2, and a volume greater than 1 000 litres, or 2) Contain water, air and/or any non-hazardous fluid, with a set pressure greater than 8 kg/cm2 and any volume, or 3) Handle hazardous fluids with a set pressure lower than or equal to 7 kg/cm2 and a volume greater than 1 000 litres, or 4) Handle hazardous fluids with a set pressure greater than 7 kg/cm2 and any volume. II.1.2 Cryogenic vessels shall be classified into the following categories (See Table II.2): Table II. 2 Types of Categories for Cryogenic Vessels Category Volume II Lower than or equal to 1000 litres III Greater than 1000 litres a) Category II: Cryogenic vessels with a volume lower than or equal to 1 000 litres. b) Category III: Cryogenic vessels with a volume greater than 1000 litres. II.1.3 Steam generators or boilers shall be classified into the following categories (See Table II.3): Table II.3 Types of Categories for Steam Generators or Boilers* 2 2 2 Greater than 5 kg/cm Any capacity * Set pressure on the first safety valve. a) Category II: Steam generators or boilers with a set pressure lower than or equal to 5 kg/cm2 and a thermal capacity lower than or equal to 400 000 kcal/hr. b) Category III: Steam generators or boilers that: 1) Have a set pressure lower than or equal to 5 kg/cm2 and a thermal capacity greater than 400 000 kcal/hr, or 2) Have a set pressure greater than 5 kg/cm2 and any thermal capacity.

Non-destructive examinations The content of this guide is a supplement for a better understanding of the Standard and is not mandatory. III.1 Non-destructive examinations are the indirect physical methods whose purpose is to verify the integrity of equipment, materials, components or welds as regards the presence of internal or surface discontinuities. III.2 Non-destructive examinations make it possible to analyse indirectly the state of the materials making up a piece of equipment so that it may continue to withstand the conditions of its frequent use. In this way it is also possible to know the safety with which that equipment may continue operating. Although non-destructive examinations are techniques of great help, they require that their scope, limitations, advantages and disadvantages be taken into account before their application is selected. III.3 The application of non-destructive examinations does not permanently alter the physical, chemical, mechanical or dimensional properties of the equipment, and their results depend on the accuracy of the method and on the quality control system with which they are carried out. III.4 Non-destructive examinations, also known as non-destructive testing (NDT), may include the following, among others: a) Ultrasound (thickness measurement and flaw detection), UT; b) Radiography, RT; c) Neutron radiography, NRT; d) Acoustic emission, EAT; e) Penetrant testing, PT; f) Electromagnetism (Eddy currents), ET; g) Magnetic particles, MT; h) Leak tightness, LT, and i) Visual inspection, VT. The letters after the name of the examination correspond to the initials of their names in English. III.5 The procedures for applying non-destructive examinations require precise written instructions, set out in an orderly and logical form, describing in detail the means and the sequences in which they are to be applied to a piece of equipment, to one of its parts or to a component. To give certainty to the application of non-destructive examinations, as well as to their results, these shall be endorsed by qualified personnel. III.6 The reports on the results of the tests carried out in compliance with the applicable specific codes or standards, by trained personnel, give the user certainty as to the correct application of the non-destructive testing and the integrity of their equipment. Diagram 1 is presented below as an example of the way in which the operation of pressure vessels, cryogenic vessels and steam generators or boilers is analysed. ________________