Pneumatic Pressure Test Hazards Requirements

When to perform a pneumatic test instead of hydrostatic pressure test?

1. (a) The equipment, piping, and/or supports, including foundations, cannot adequately support the liquid weight.
2. (b) The equipment or piping cannot be dried and traces of the test liquid may result in contamination of the system or its contents after returning to service.
3. (c) The equipment or piping contains internal linings that could be damaged by the test medium.

What are the pneumatic test hazards?

It is necessary to evaluate the risk analysis of brittle fracture. This fracture may result from several reasons as follows:

• The metal tempera test temperature is not MDMT+30 F for thickness greater than 2 inch and MDMT+10 F for thickness less than 2 inch.

• The in service damage mechanisms such as temper embrittlement. In this case the pneumatic test is not recommended below MPT (minimum pressurizing temperature). The MPT may be calculated by API 934F.
• The Pre-Service Deficiencies: According to ASME FFS-1, Clause 2B.3 the are five type of Pre-service Deficiencies which may be discovered at initial inspections as follows:

1- Material Production Flaws – Flaws which occur during production including lamination and laps in wrought products, voids, segregation, shrinks, cracks, and bursts in cast products.

2- Welding Related Flaws – Flaws which occur as a result of the welding process including lack of penetration, lack of fusion, delayed hydrogen cracking, porosity, slag, undercut, weld cracking, and hot shortness.

3- Fabrication Related Flaws – Imperfections associated with fabrication including out-of-roundness, forming cracks, grinding cracks and marks, dents, gouges, dent-gouge combinations, and lamellar tearing.

4-Heat Treatment Related Flaws or Embrittlement – Flaws associated with heat treatment including reheat cracking, quench cracking, sensitization, 475°C (885°F) embrittlement, and sigma phase embrittlement. Similar flaws are also associated with in-service elevated temperature exposure.

5-Wrong Material of Construction – Due to either faulty material selections, poor choice of a specification break or due to the inadvertent substitution of a different alloy or heat treatment condition due to a lack of positive material identification (PMI), the installed component does not have the expected resistance or needed properties for the service or loading.

The Recommendations for Pneumatic pressure tests:

1- Stored Energy and Safe Distance Consideration :. The stored energy should be calculated by ASME PCC2 article 501 and The maximum calculated stored energy of any vessel or piping system being pneumatically pressure tested should not be greater than 271MJ (200,000,000 ft-lb). When calculating the stored energy for a vessel, the total volume shall be considered. When calculating the stored energy of a piping system, a maximum volume based on a length of 8 pipe diameters may be considered for any single failure analyzed. If the calculated stored energy is greater than 271 MJ (200,000,000 ft-lb), then one of the following shall occur:

(1) The system shall be divided into smaller volumes such that each subsystem has a stored energy not greater than 271 000 000 J (200,000,000 ft-lb).

(2) A minimum distance shall be calculated per Mandatory Appendix 501-III, eq. (III-1), and this distance shall be adhered to.

2- NDE Requirements : As mentioned some flaws may be discovered during inspection and result to catastrophic explosion during pneumatic pressure test by stored energy at equipment, so it is recommended that some susceptible material such as tee joints or non pressure parts to pressure part will be MT / PT before test as ASME Sec VIII - Div1 UW-50 is mentioned.

In next articles we will discus about How to evaluate FFS for brittle fracture mechanisms.

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