10 replies to this topic

[Sridhar P]

why the pneumatic test pressure for vessels is low at 1.1 times the design pressure against the 1.3 times for hydrotest. why pneumatic test is not at 1.3 times design pressure as was with hydrotest pressure or the otherwise(both at 1.1 times). What makes the difference in different pressure testing with hydro and pneumatic test.
Pl. guide us.

P.Sridhar

[Rama]

Sridhar,

In contrast to water, air is a highly compressible fluid. Any failure of the part to be tested can have catastrophic consequences with possible loss of life. (imagine an improperly closed end cap taking off) Whereas in the case of a hydro test, a little movement of the part releases the pressure, as the fluid is not compressible. (The energy stored in a vessel filled with compressed air is much more than the energy in the water filled vessel)

That is why the pneumatic test is generally not done unless there are specific reasons to do the same. (Like shortage of water on a particular day delaying the schedule, or when water in the vessel is difficult to clean out - some processes may require bone dry vessel after the test)

However I feel the pressure for hydraulic test is generally 150% of the design pressure, if I remember right, in cases which I have come across. Where do they recommend 130% ? Is this a recent amendment?

For pneumatic test the pressure may be even lower than 1.1 times the design pressure if the working temperature is higher than the testing temperature.

[fallah]

Where do they recommend 130% ? Is this a recent amendment?

Yes ,it is recent criteria but for piping hydrotest still remains 150%.

[ankur2061]

Hello Sridhar,

Shop fabricated piping sections or short (as well as straight lengths) of piping are hydrotested by the piping contractor before installation on the pipe rack. However in many petrochemical plants after the piping contractor has installed the piping and you want to do a complete leak integrity check of a system which includes piping and equipment the most convenient way is to do a pneumatic test.

Pneumatic test pressures should be as low as possible. As Rama described of the dangers of the stored energy of air at higher pressures there are other reasons also. For example if youa re doing a soap suds test, the best results for checking leaks are obtained when the air pressure is as low as possible. Pneuamtic testing has some peculiar characteristics. For example if you are doing a pneumatic hold test in a place where the 24 hour ambient temperature shows an extreme variance you can easily be misled. Let us take an example:

The system was pressurized at, say, 10:00 P.M. in the night when ambient temperatures were say 15°C to 30 psig. It has been found that if you checked the pressure at 2.00 P.M. afternoon the next day the reading on the pressure gauge may be showing 30.5 or 31 psig if there is no tangible leak from the system. How did the pressure increase? It is not such a mystery. The ambient temperature at 2:00 P.M was probably 45 or 47°C which caused an expansion of the air in the system. This can cause total confusion. But the explanation is simple. The best way to avoid this kind of confusion is to take a pressure hold test reading at the same time 24 hours later i.e. at 10:00 P.M. when the ambient temperature is around the same as was when the test was started.

Pneumatic test are extremely effective when done properly and are routinely done before commissioning and start-up of the plant. They can combine large sections of the plant. Also, certain pipes and lines which cannot tolerate water (process requires zero moisture) are pneumatically tested.

So, good luck with your pneumatic testing.

Regards,
Ankur.

[Rama]

Yes ,it is recent criteria

Thanks, Fallah.

Ram.

[Sridhar P]

Thanks very much, Ram and Ankur.

Regards,
P.Sridhar

[Markel]

Hello All,

I am doing Nitrogen Leak Test which is based by my understanding that it is consider pneumatic test.
What book or API or ASME book acknowledge that it has to be tested at 110%?

Thanks

[ankur2061]

Markel,

Using nitrogen as a pressure test medium is, indeed, a pneumatic test. Where did you get the value of 110% for pneumatic test? What is 110%? Is it 110% of design pressure? You need to clarify the refernce for 110%.
I haven't come across pneumatic test with 110% in any API or ASME standard. If anybody knows about this, it would be very valuable information to me and other readers.

Regards,
Ankur.

[breizh]

Ankur and others,
is this document still valid ?
337.4.4

http://cstools.asme....PDF/R071413.pdf

Regards
Breizh

[ankur2061]

Breizh,

The latest ASME B31.3-2010 has the following to say with a differnt clause:

345.5 Pneumatic Leak Test

345.5.4 Test Pressure

The test pressure shall be not less than 1.1 times the design pressure and shall not exceed the lesser of
(a) 1.33 times the design pressure
( the pressure that would produce a nominal pressure stress or longitudinal stress in excess of 90% of the yield strength of any component at the test temperature

Hope this helps.

Regards,
Ankur.

[breizh]

thanks
Breizh