question about alternative test

[pipeng90]

Hi everybody,
Q1. What is combination test (in piping)? And when can we use...?
Q2. There is a large diameter process pipe (24") with 10 bar design pressure(& 15 bar hydrotest pressure). Can we test it with N2 and 15 bar pressure, instead of water?
Thanks.

 

[bimr]

Hydrostatic testing is the preferred leak-testing method and perhaps the most often used. The most important reason for this is the relative safety of hydrostatic testing compared to pneumatic testing. Water is a much safer fluid test medium than air (or nitrogen) because water is nearly incompressible. Therefore, the amount of work required to compress water to a given pressure in a piping system is substantially less than the work required to compress air, or any other gas, to the same pressure. The work of compression is stored in the fluid as a potential energy, which could be released suddenly in the event of a failure during a pressure test.

Q1. A combination test is a two part test, the first part involves pneumatic testing with low pressure air (or nitrogen) for major leaks. The second part involves the standard hydrostatic test.

A low air (or nitrogen) pressure, most often 25 psig, is first used to see if there are major leaks. This low pressure reduces the danger of personal injury but still enables major leaks to be quickly located. Repairs, if needed, can then be done before the hydrostatic test. This method can be very effective in saving time, particularly if it takes a long time to fill a system with water only to find leaks on the first try. If leaks are found in a hydrostatic test, it will take longer to remove the water and dry the piping sufficiently to make repairs.

Q2. Testing to 15 Bar pressure with nitrogen is not recommended as a test with that high of a gas pressure is unsafe.

 

[rconner]

In addition to the response of bimr mentioning safety angles etc, and while I guess the significance of the following might be debated with whatever piping system and quality of installation and other inspection you have, a 10 bar test with anything will not test the structural capabilities of the system as strenuously as will a 15 bar hydrostatic test.

 

[LittleInch]

1) Not too sure but might refer to a short duration strength test followed by or preceeeded by a longer term lower pressure leak test, often in air / gas at < 7 bar to check leaking flanges etc

2) Yes you can, but there are a numbe rof issues to check first. This paper highlights some

http://www.engineering.com/Designer...as-an-Alternative-to-Hydrostatic-Testing.aspx

and if you search pneumatic testing on this site you find lots of others. A good risk assessment is key and protection of people and plant is required.

Normally requires special permission and you need to justify it. Cryogenic pipe is often tested this way to avoid water contamination.

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