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Pressure Tests

May 13, 2014

Pressure Test Gauge

The material in this post is extracted from Chapter 2 – Maintenance and Inspection – of 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 test, used 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. The test ensures that the equipment is leak-free but it does not test the integrity of the vessel or piping itself. Tightness tests are never conducted at a pressure above the equipment or system design pressure or the relief valve set pressure.

The test pressure is generally 1.5 times design pressure or MAWP (see below for an explanation of this term). Therefore, once a vessel has been installed, or anytime it is opened (say for inspection), it will be tested to that pressure before the process fluids are introduced into the vessel.

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 pressure.

Regardless of the type of test being carried out, it is important first to visually inspect the equipment and to carry out the actions discussed above to do with Mechanical Completion.

Maximum Allowable Working Pressure

Pressure vessels must always operate at a pressure lower than their Maximum Allowable Working Pressure (MAWP). The manner in which MAWP is determined can be illustrated using pressure vessel V-101 in the second standard example (Chapter 1).

  1. In the initial design the process engineer specifies that V-101 be designed for a maximum operating pressure of 6.5 barg. This is the design pressure or pressure rating of the vessel as measured at the top of the vessel.
  2. The process engineer’s requirements are transmitted to the vessel engineer who designs the vessel using standard sizes for wall thickness and flange size, thus generating the Maximum Allowable Working Pressure (MAWP) value. Generally MAWP will be higher than the design pressure because wall thicknesses are in discrete sizes and the designer will always choose a value greater than that called for. In the example, since it is unlikely that he can design for exactly 6.5 barg, the designer selects the next highest level, which, in this example, is 7.0 barg. Therefore the vessel’s MAWP is 7.0 barg.
  3. Once in service, the vessel can be operated at any pressure up to MAWP without violating any safety limits. However, if the operating pressure does ever go over MAWP for any reason the vessel should be checked and re-certified. Management or supervision can never operate at a pressure exceeding MAWP without going through the Management of Change process.
  4. Generally emergency systems such as interlocks and pressure relief valves will be set at a value just below MAWP.
  5. If the internal pressure in a vessel continues to rise then the walls will probably start to yield and around two times MAWP. The vessel or associated piping may be slightly distorted, but any leaks are most likely to occur at gaskets. At this pressure is likely that the vessel itself will not rupture.
  6. At two to four times MAWP there will probably be distortion of the vessel and it can be assumed that gaskets will blow out. The vessel’s burst pressure will typically be in the range of 3.5 to 4 times MAWP. Therefore, for this example, therefore, the burst pressure would be in the 27 to 34 barg range. (It is difficult to predict this value accurately because so few vessels actually fail, so there is not much field data.)

Increased temperatures quickly reduce the strength of the steel. For example, the MAWP for a certain vessel may be 10 barg at a temperature of 300C. At 550C the same item may fail at just 1 barg (metal temperature refers to the average metal temperature through its entire depth.). Therefore, each MAWP value must have an associated temperature.

Although low temperatures generally enhance metal strength (and so raise MAWP) very low temperatures may cause sudden and catastrophic embrittlement. This can be a serious problem in cryogenic services. For example, carbon steel equipment and pipe is liable to fail — even when there is no load on it — if its temperature falls below -20C. This can occur, for example, if a cryogenic liquid such as liquid air enters a carbon steel flare header.

Test Medium

The test medium can be either liquid (hydrostatic) or gas (pneumatic). Of these two, liquid is strongly preferred, particularly for strength or hydrostatic tests. (Gas may be preferred for routine tightness tests since it is much simpler to use and there are fewer disposal problems once the test is complete.) The potential energy of a compressed gas is much greater than that for a non-compressible liquid. Hence, if the item were to fail, it could disintegrate violently. Pneumatic testing should only be used when hydrostatic testing is impractical — for example, when testing a vacuum system that does not have the physical strength to support the piping when full of liquid.

Water is normally used as the test medium unless it could cause problems such as corrosion. The test water should be clean and should be of such quality as to minimize corrosion of the materials in the test system. If the equipment being tested is made of stainless steel the chloride content of the water should not exceed 30 ppm. If water is not used then the liquid selected must be a temperature below its boiling point. The use of combustible liquids should be avoided.

When liquid is used the effect of its weight on equipment (particularly the bottom heads of large, tall vessels), their supports, and foundations must be carefully evaluated. One chemical plant, for example, had large (72 inch) overhead lines coming from its vacuum columns. Because the normal operating pressure in those columns was very low the wall thicknesses were correspondingly low and the structural supports were minimal. Had liquid been introduced into these overhead lines they and the supporting structure would have collapsed — hence testing had to be pneumatic.

Tightness Tests

 As discussed above tightness test pressures should not exceed the equipment MAWP or system design pressure or the vessel’s relief valve set pressure.

Once the system is at test pressure, it is held at that pressure for 15-20 minutes (one hour maximum). If the pressure does not fall then the system is tight. If the pressure does fall then the leak must be found and repaired. Testing can be done using a soap solution if gas is being used as the testing medium. The solution is squirted on to the flanges that were had been parted and then put back together. Any gas leak will create bubbles in the solution.

Strength Tests

Non-destructive methods should be used to determine thickness loss should be performed before conducting a strength pressure test.

If the vessel or piping has been modified in any way then a strength test is required in order to ensure the vessel’s integrity. Strength tests are conducted above the vessel’s design pressure and above the set points of associated relief valves/rupture disks (which are either removed, blocked in or have their valve disk clamped). A strength test must be conducted in accordance with the applicable codes and/or standards for which the equipment is constructed. For pressure vessels the test pressure is 1.5 times the MAWP for ASME Section VIII, Division 1 vessels; 1.25 times MAWP for Section VIII, Division 2 vessels; and 1.5 times MAWP for Section I, Power Boilers. Systems designed for vacuum service should be strength tested at a pressure 1.5 times the difference between normal atmospheric pressure and the minimum design internal absolute pressure. In the test is pneumatic the test pressure may be lower than these values.

Vessels can be strength tested before being installed. However piping is usually tested in the field. 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 may be tested together.

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