Low pressure closure test 3

[arash807]

What is the main purpose of low pressure closure test in API598.according to mentioned standard there are also high pressure closure test which is done by higher pressure.
so when we use it?
and what is the advantage of that?

 

[EMorel]

Looking at the 7th edition of API 598 that I have available, Section 3.3.3 and 3.3.4 (which point to Table 1-A and 1-B respectively)explain what types of valves need the High and Low pressure closing tests.

Another important thing to note is that the low pressure closure and low pressure back seat tests can only be performed with air or gas, whereas the high pressure test can be performed with liquids.

Even if the test is listed as optional in the Tables 1-A or 1-B, the valve is still required to be ABLE to pass the test as described.

The reason there is a high pressure and low pressure sealing test is because some valve styles and seating styles will seal better at higher pressures (harder seating materials typically) and not so great at low pressures, and vice-verse. This specification ensures that the design is capable of sealing to the required extent at a wide range of pressures.

 

[bcd]

In some valve designs, a low pressure test is harder to pass and will be a better check of quality. For example, a floating ball valve. The higher the test pressure, the more force pushing the ball into the downstream seat, and the tighter the valve will seal. A low pressure gas test is a more discerning check of the construction quality, ball roundness, and surface finishes. Same is true for many high-performance type butterfly valves.

Also consider that in these types of valves with plastic seats, testing at the maximum pressure applies higher forces of the plastic sealing parts and can cause them to deform more than they would when they are installed and running at lower pressures. More deformation can shorten the operational life of the plastic sealing elements.

Keep in mind that the factory tests are intended to be checks of the production quality and should be selected to be the most discerningand minimize the impact to the future operational life. In such plastic seated valves, the ability to seal at maximum pressure is a design validation that should be done indepedent of the factory tests to prove the design is appropriate.

 

[Gorman3]

I have alot of experience testing API 600 gate valves and in that case even with hard seats the low pressure air test is more difficult to pass. Small bubbles will more easily escape through scratches or poorly seated valves revealing flaws