NOM-020-STPS: what it requires, how to classify your equipment and how to prepare for an inspection

NOM-020-STPS-2011 is the Mexican Official Standard that sets the safety requirements for operating pressure vessels, cryogenic vessels and steam generators or boilers in workplaces. Mexico's labor ministry (STPS) published it in the Official Gazette (DOF) on December 27, 2011; it took effect six months later and replaced NOM-020-STPS-2002.

It has been amended twice: in 2015 the requirement for a verification unit to witness the tests of category III equipment was removed, and in 2017 the notices were allowed by electronic means and the standard clarified when their deadlines, and the validity of compliance opinions, start to run.

The standard defines a pressure vessel as an apparatus built to operate above atmospheric pressure or under vacuum (section 4.29). At a plant that includes, for example, compressed-air receivers, autoclaves, heat exchangers and process tanks, as well as cryogenic vessels (nitrogen, oxygen, argon or CO₂, among others) and steam generators or boilers.

It does not apply to the equipment excluded in section 2.2, including:

• Vessels used as fire extinguishers.
• Portable vessels holding compressed gases, such as cylinders.
• LPG vessels, which are regulated by Mexico's Energy Ministry.
• Containers that work at atmospheric pressure.
• Piping and distribution headers that are not used to accumulate fluids.
• Pressurized fittings up to 0.15 m nominal diameter installed on vessels (for mixing, filtering, metering or control, among others).
• Vessels interconnected in a process line that operates between 29.42 and 196.14 kPa and ends open to the atmosphere.
• Vessels in the braking systems of mobile equipment, tank cars for compressed gases, diving bells and hyperbaric chambers.

The category drives almost every obligation, so it is the first step (sections 5.1 and 7). For pressure vessels it depends on three data points: the fluid, the set pressure of the relief device (the pressure at which the safety valve opens, not the operating pressure) and the volume.

Under this standard a hazardous fluid is one that poses a health, flammability or reactivity hazard under NOM-018-STPS, or one that operates above 50 °C or below −10 °C (section 4.12). That is why a hot-water or condensate tank can land in a higher category than you might expect.

Cryogenic vessels are category II up to 1 m³ and category III above 1 m³. Steam generators or boilers are category II when their set pressure is up to 490.33 kPa and their heat capacity up to 1,674.72 MJ/h (400,000 kcal/h); above either value they are category III. Neither type has a category I.

Once the category is set, the obligations in chapter 5 break down as follows:

All equipment must also meet the safety conditions in chapter 12: a marked serial number or tag, a pressure gauge and control instruments in good condition, and a relief device. Categories II and III add, among others, physical protection where vehicles could hit them, signage of the fluid, grounding if they handle flammables, and relief discharge routed to a place where it can't hurt anyone.

Category II and III equipment must undergo pressure tests (hydrostatic, pneumatic or hydro-pneumatic) or non-destructive examinations (NDE) on this schedule (section 13.1):

• New equipment with a manufacturer's certificate or code stamp: the first test before ten years from the certificate or stamp; after that, at least every five years.
• Used equipment, or new equipment without certificate or stamp: the first test before putting it into service; after that, at least every five years.

Each periodic test is done within the 60 calendar days before the five-year period ends. Qualified personnel appointed by the employer may set a shorter interval, but never longer than five years (13.2).

Pressure tests are performed and signed off by an engineer with knowledge of the subject, and NDE by certified personnel (13.2). NDE are combined: at least one volumetric examination (radiography, ultrasound, neutron radiography or acoustic emission) and one surface or flux-leakage examination, such as liquid penetrant, magnetic particle or eddy current (13.5). Pneumatic tests are allowed only on air receivers that meet certain pressure and volume conditions (13.3), and boilers also need functional tests of their safety shutdowns and alarms (13.6).

Since the 2015 amendment, the standard no longer requires a verification unit to witness the tests of category III equipment.

For each category III item, the employer must notify the STPS, in writing or electronically, that the equipment complies with the standard (section 5.13 and chapter 16). The process:

1. 1.An accredited and approved verification unit (type A, B or C), or the workplace's own (type B), assesses the equipment and issues the compliance opinion (dictamen); a type C unit can issue it together with a service report (19.2).
2. 2.The employer files the notice before putting the equipment into service and within 60 days of the opinion's date, attaching the opinion (16.1 and 16.4).
3. 3.The STPS checks that the notice matches the opinion and issues the control number for each item within the next five business days (16.5).
4. 4.The notice is repeated after ten years for new equipment (five for used equipment) and then every five years, counted from the last satisfactorily completed pressure test or NDE, within the 60 calendar days before each period ends (16.1).
5. 5.If the equipment is altered or relocated, a new notice is filed before it runs again (16.2).

A compliance opinion is valid for ten years for new equipment and five for used equipment, counted from the last satisfactorily completed test (19.7). For category I and II equipment, hiring a verification unit is optional.

The file is the core evidence in an inspection, and its contents grow with the category (chapter 9):

• Category I: name and serial number or tag, data sheet (fluid; design, operating, set and maximum allowable working pressures; volume and temperatures), a short description of its operation, inspection and maintenance records, and location.
• Category II: in addition, year of manufacture, construction code, manufacturer's certificate if any, hydrostatic test pressure, records of the last pressure test or NDE, of modifications and of welded repairs, a drawing of the equipment and a location sketch.
• Category III: in addition, the STPS control number, a photo or rubbing of the nameplate, a description of operating hazards, chronological summaries of inspections, tests, alterations and repairs, a drawing with sections and dimensions, design calculations signed by an engineer with their professional license number and the verification unit's compliance opinion.

Each item maps to a specific requirement of the standard:

• Is every item on the list, including small receivers mounted on machines or compressors? (8.1)
• Did you classify by the safety valve's set pressure rather than the operating pressure? (Table 1)
• Is the set pressure at or below the maximum allowable working pressure and above the operating pressure? (14.2)
• Does the gauge range from 1.5 to 4 times the operating pressure, or put it in the middle third of the dial? (12.2.1)
• Does each item have its serial number or tag marked? (12.1)
• Are the pressure tests or NDE of category II and III equipment less than five years old? (13.1)
• Do category III items have a valid compliance opinion, a filed notice and a control number? (16)
• Are there operating, inspection and maintenance records with the responsible person's name and signature? (18)

Mexico's Federal Labor Law sets fines of 250 to 5,000 times the UMA (Mexico's daily reference unit) for employers who don't follow safety and hygiene standards in their facilities (article 994, section V). Beyond the fine, equipment without current tests or reliable relief devices is exactly the risk the standard exists to prevent.

At CSE Industrial we organize the inventory, classify each item, build the equipment files and leave the evidence ready for the inspection or the verification unit, at plants in Tijuana, Mexicali, Ensenada and Tecate. See our NOM-020-STPS service or request an assessment.