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What is the difference between pressure containing and pressure retaining parts? and how does NACE MR0175 apply?

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ANSI / NACE MR0175/ISO 15156 defines a pressure-containing part as follows:

part whose failure to function as intended results in a release of retained fluid to the atmosphere

EXAMPLES Valve bodies, bonnets and stems.

NACE MR0175 does not provide an exhaustive list of “pressure containing parts”. Some users require a definite definition between pressure-containing parts and pressure-retaining parts. Regardless of a design code definition of pressure containing or pressure retaining parts,  NACE MR0175, in part 1 Clause 5, states that the equipment user shall determine whether or not the service conditions are such that NACE MR0175 applies and to ensure that any material specified for use is satisfactory in the service environment. This means that the equipment user must identify which parts require compliance with NACE MR0175 (including pressure containing and pressure retaining parts).

At Oil & Gas Corrosion, we get this question all the time “What is the difference between pressure-containing and pressure-retaining parts?”. The answer to this will depend on the applicable regulatory framework and design codes, nevertheless we have put together a few examples of how these parts are defined according to relevant references.

  • Pressure Containing Member: A component which is exposed to and contains pressure
  • Pressure Retaining Member: A component which holds one or more pressure containing members together but is not exposed to the pressure.
Various documents from API such API 16A, API 16AR, API Std 674 These API documents follow a very similar approach:

  • A pressure-containing part is exposed to the fluid exerting the pressure and failure to function as intended results in release of fluids. These parts act as a barrier between the fluid and the environment.
  • A pressure-retaining part is not exposed to the fluid exerting the pressure, it is stressed due to the effects of a differential pressure, failure to function as intended results in release of fluids.
  • pressure-containing: component whose failure to function as intended results in a release of retained fluid to the atmosphere.
  • pressure-retaining is not included in this document.
Pressure Equipment Directive (PED) 2014/68/EU (formerly97/23/EC)
  • PED 2014/68/EU does not address pressure containing or retaining parts, the term used is “pressure-bearing parts”. The main pressure-bearing parts are the parts, which constitute the envelope under pressure, and the parts which are essential for the integrity of the equipment.

These are just some examples of how pressure-containing and pressure retaining parts are defined in different codes. When pressure equipment is exposed to sour service conditions, the equipment user must define which parts need to comply with the relevant materials requirements such as NACE MR0175 or NACE MR0103. There is no general rule that covers all possible scenarios and as such, a materials engineer must review which pressure-containing and pressure-retaining parts have to comply with NACE MR0175.

At Oil & Gas Corrosion we help manufacturers and users to carry out a material selection review for their design and to identify the parts that need to meet the requirements of NACE MR0175 / ISO 15156.

We can help you review and document that your design complies with NACE MR0175 / ISO 15156 and the relevant regulatory requirements such as ASME VIII, PED 2014/68/EU, etc.


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In 2003 an inquiry was sent to the NACE MR0175 Maintenance Panel:

Question: Definition of pressure-containing parts: “Those parts whose failure to function as intended would result in a release of retained fluid to the atmosphere. Examples are valve bodies, bonnets, and stems.” Are stems always defined as pressure-containing parts, regardless of features that by design keep the stem intact? Example #1: Internal entry stems for ball valves that have a shoulder that rests against the body around the stem bore. Example #2: Shafts for butterfly valves that have a retaining ring holding the shaft inside the valve.


The NACE MR0175/ISO 15156 maintenance panel cannot interpret design issues.

This question is in relation to NACE MR0175/ISO 15156-2 Clause 3.14

Reference: ISO 15156 Maintenance Panel Inquiry #2003-12 Q2.

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