Are you tired of pressure testing your process piping after the pressure boundary has been breached?

Are you tired of spending thousands of dollars on high-cost nitrogen to pressure test your process piping?  Are you tired of all the procedural documents you must have in place to perform this testing?  And all of the training you must do for the personnel performing this testing?  Well, I have a special deal for you…

This article provides alternatives to pressure testing after repairs or alterations based on ASME PCC-2, 2008

A pressure test in itself is a useful tool with respect to newly constructed equipment. Application of a pressure test, to equipment that has been in service for some time, is a matter that REQUIRES CAREFUL CONSIDERATION OF A VARIETY OF FACTORS INVOLVED. There are instances where the application of a pressure test is not desirable, such as, the application of a pressure test may create damage.

This practice applies to equipment for which

1. Non-Destructive Examination (NDE) provides better assurance of integrity in future operation for elevated temperature or cyclic operation where crack initiation and propagation is a concern. Large flaws may not result in failure during a pressure test but may propagate in cyclic or creep service.
2. a pressure test is not practical and NDE can be shown to provide appropriate integrity assurance. A pressure test of equipment that has been repaired is primarily a leak test, or, in some cases, a test for gross fabrication flaws that could compromise structural integrity. The structural integrity of the design is usually not an issue for repairs. Even for most alterations, the integrity of the design can be verified by engineering analysis.
3. a pressure test is practical, but NDE can be shown to provide equivalent integrity assurance. In this case, the overall cost may be a major consideration.

It is essential that the appropriate NDE be performed based on the damage mechanisms anticipated during repairs or alterations.

Pressure testing consists of three primary methods.

1. Hydrostatic Test. The fluid used is typically water; however, another suitable liquid can be substituted if there is a risk of damage due from any adverse effects of having water in the system.
2. Pneumatic Test. This is performed in some situations where the presence of any water or weight of the water in the system is an issue. The pneumatic test is potentially hazardous due from the stored energy of the compressed gas.
3. Hydro-Pneumatic Test. This is a combination of the two other test methods.

NDE has been defined as comprising those examination methods (see Mandatory Appendix 502-I) used to examine an object, material, or system without impairing future usefulness.

It is used to investigate the material and component integrity. Determining the structural integrity of a pressure-retaining device or component can be accomplished by a process involving a quantitative engineering evaluation coupled with NDE to obtain current wall thicknesses and provide detection and sizing of any in-service flaws or cracks (National Board Bulletin, Volume 61).

Pressure Test

Reasons for Pressure Testing in New Construction

1. Checks for leakage of mechanical and welded joints
2. May avoid an in-service failure and associated safety issues
3. Screens out gross design, material, fabrication deficiencies
4. Reduces the stress-multiplying capability of sharp notches, metallurgical defects, discontinuities (flaw tip blunting)
5. Provides mechanical stress relief

NDE Methods

The following is a limited list of the more common alternative NDE methods. 

Alternative NDE Methods: Volumetric

1. Radiography
2. Ultrasonic shear wave
3. Automated ultrasonics, such as time-of-flightdiffraction (TOFD) and phased array ultrasonics

Alternative NDE Methods: Surface

1. Magnetic particle
2. Liquid penetrant
3. Eddy current
4. Magnetic flux leakage

Performing pressure tests of an in-service device or component with inadequate fracture toughness could result in brittle fracture. Once a device or component has been subjected to a one-time (at new construction) hydrostatic test, any additional hydrostatic tests over the life of the component will serve minimal useful purpose regarding structural integrity or benefits of redistribution of stresses. In addition, performing a hydrostatic test above the normal working pressure of an in-service component can result in significant exposure to brittle fracture – especially if the material of construction:

1. has been subjected to some degree of embrittlement under normal service conditions
2. possesses poor fracture toughness as a result of steel melting practices
3. contains an undetected critical flaw from in-service exposure, or
4. will be pressure tested below the DTB transition temperature of any component

Examples Where Pressure Testing May Be Inadvisable

Foundation or Support Structure – Where the foundation or supporting structure has not been designed to carry the weight of liquid-filled pressure equipment. 

Undesirable Reactions or Consequences – A safety concern where the application of test fluids could lead to an undesirable reaction with the residue of fluids contained in the pressure equipment.

Design Reasons – Where the design of the pressure equipment is based on other factors, such as bending, where stresses due to pressure may not be governing or where a vessel for which the thickness of the pressure boundary components is governed by external pressure (buckling) considerations

Painting/Coating/Lining Issues – Where painting/coating/lining could mask leaks that would otherwise have been detected during a pressure test. This includes damage to refractory or other insulating internal materials as well as damage to internal linings.

Repairs and Alterations for Which Pressure Testing Is Not Normally Required (ANSI/NB-23)

The following types of repairs and/or alterations may be exempt from pressure testing or where pressure tests may be optional depending upon the needs of the owner– user:

1. welding or brazing that does not penetrate the pressure boundary at any point
2. seal welds
3. cladding application/repairs
4. hard surfacing
5. welding to flange seating surfaces, when less than 50% of the axial thickness is replaced by welding
6. tube-to-tubesheet welds, provided less than 10% of the total number of tubes are replaced at any time after a full operational cycle
7. tube plugging or sleeving of heat exchangers, steam generators, or boiler tubes
8. hot tap fittings 

EXAMINATION — NONDESTRUCTIVE EXAMINATION (NDE)

The specific type and amount of surface and/or volumetric NDE that should be specified in lieu of pressure testing is at the owner–user’s discretion, the extent of which should be based on the risk of leak or failure of the equipment. The NDE specified needs to match the likelihood of the type of defects that could occur with the particular materials and welding methods in use.