The material in this article has been extracted from the book Plant Design and Operations.
After a piece of equipment or piping that operates under pressure has been opened and then reassembled (buttoned up) it must be pressure tested before being put back into service.
There are two types of pressure test. The first is a tightness or leak test, performed after the equipment has been opened but not modified in any way. Typically this type of test is conducted after the item was opened for cleaning, inspection or routine maintenance. Normally, the only change made to the system is that new gaskets are installed. The tightness test must be completed before process fluids are introduced into the vessel or before temperatures and pressures are increased. The test ensures that the equipment is leak-free but it does not test the integrity of the vessel or piping itself.
The tightness test pressure is generally 1.5 times design pressure or MAWP (Maximum Allowable Working Pressure) for Section VIII, Division 1 vessels, and 1.25 times MAWP for Division 2 vessels. Piping should be tested at 1.1 times its design pressure.
Tightness tests are not conducted at a pressure above the equipment or system design pressure or relief valve set pressures. (If the test pressure exceeds the set pressure of the section’s relief valves or rupture disks then they must be removed or blinded off.) A pneumatic strength test cannot be used to substitute for a hydrostatic test for power boilers fabricated in accordance with ASME BPV Code, Section I.
The second type of pressure test is a strength or hydro test. It is used when the equipment has been modified, say by having some welding done on it, or when structural repairs have been made. Strength test pressures are generally above the equipment or system normal design pressures or relief valve set pressures.
Piping is usually tested in the field with a pressure of 1.5 times the design pressure, corrected for temperature. When testing a piping system attached to a pressure vessel, and it is not considered practicable to isolate the piping from the vessel, the piping and the vessel can be tested together.
Large systems can be tested in sections. When vessels or process equipment are interconnected in such a way that blinding of each is not possible or practical, the combination can be considered as one unit that can be blinded and tested.
Regardless of the type of test the following issues should be considered before and during the testing process.
- Equipment should be visually inspected before testing to ensure the equipment item has been assembled according to procedures and vendor instructions. The inspection should make sure that: all welds are complete, all valves are closed with plugs installed, all the blinds have been installed, all supports are in place, and all low-pressure filling lines and other appurtenances that are not part of the test system have been disconnected or isolated.
- The test apparatus should be examined for tightness.
- Some internal components such as bellows and heat exchanger tube sheets are designed only for operating differential pressures. It is critical not to apply higher than design differential pressure on them when applying test pressure to only one side of the component.
- Equipment that has been in service under severe operating conditions should be considered as having a higher probability of failure during the test.
- For most materials, the metal temperature for the strength test generally should not be less than around 15°C and should not exceed 50°C throughout the duration of the test. If the system is subject to thermal expansion or contraction, precautions should be taken to avoid excessive pressure or vacuum being created during the test.
- As already noted, if the test pressure will exceed the set pressure of any pressure relief valves and/or rupture disks in the test system, they should be removed and their flange blinded (failure to do so was a major factor in the Piper Alpha catastrophe). Alternatively the disks of the relief valves can be held down with a test clamp — not by applying additional load to the valve spring by turning the compression screw.
- The venting of nitrogen or other inert gases should be done to open areas to avoid accumulation of the inert gases.
- An indicating gauge, visible to the person controlling the pressure applied to the test system, should be connected to the pressurized sections. Hydrostatic head on the gauge should be considered. Dial pressure gauges should have dials graduated over their entire range or about double the intended maximum test pressure, but in no case should the range be less than 1.5 times that pressure.
- Low-pressure filling lines and other appurtenances that should not be subjected to the test pressure must be disconnected or isolated.