Hydrostatic vs Air Test: Pipeline Pressure of 16 Bar

[Ashokkk]

Can somebody explain the the difference between Hydrostatic test and Air test. Instead of hydrotest can we test the pipelines with air pressure. The working pressure of pipeline is 16 bar.

 

[ank_gl]

air is compressible, hydraulic fluid is not. compressed gases are also explosive, whereas liquid at higher pressure can leak, nothing as destructive as compressed gases.

i don't know about the tests though..

 

[Q_Goest]

Do you know what industry standard the piping needs to be pressure tested to? For example, in the US, process piping is tested to ASME B31.3. There's a fundamental difference in the philosophy of what pressure test is to be used depending on whether a structural test or a leak test is needed. Generally, the industry standard will tell you which needs to be done, at what pressure and using what kind of fluid media along with exceptions for example, in the case where a pipe may be 'contaminated' by water or other liquid.

 

[Ashokkk]

Thank you Goest, My doubt is about the pipe test on board a tanker ship, actually as per specification the pipe should be checked with 16 bars (hydro) but the ambient temperature is below zero because of that they worried about freezing of water, so the maker is suggesting the pressure test with air. And practically if the air pressure is above 0.5 bar the leak finding with soap water is not recommended. And I would like to know about the other differences between the pressure test with water and air.

 

[stewartcs]

Thank you Goest, My doubt is about the pipe test on board a tanker ship, actually as per specification the pipe should be checked with 16 bars (hydro) but the ambient temperature is below zero because of that they worried about freezing of water, so the maker is suggesting the pressure test with air. And practically if the air pressure is above 0.5 bar the leak finding with soap water is not recommended. And I would like to know about the other differences between the pressure test with water and air.

What type of fluid is normally carried in the pipe? If contamination is not a concern (e.g. this is not a potable water line) then you may be able to use a glycol/water mixture for your hydro test. Again this assumes contamination is not a concern.

Testing with air may prove more difficult since air is a smaller molecule than water. 16 bar is not too high of a pressure to test with but will definitely have more potential for damage if something ruptures (air is a lot more compressible than water). Other than that, pressure is pressure.

If the duration of your hydro test isn't that long, say like 15 minutes or so, then the water probably won't have time to freeze anyway. Of course that will depend on the ambient temperature and wall thickness and material of the pipe. Just something to consider.

Hope this helps.

CS

 

[Q_Goest]

Hi Ashok.
Piping systems are typically designed and built per an applicable code. Certainly the use of ASME B31.3 could be applicable to ships carrying oil, but you should really be following the code that the piping system was designed to. Since I’m familiar with B31.3 and not the European (or other country’s) equivalent, I’ll base this response on B31.3.

ASME B31.3 requires “leak testing” of the piping system. This is not a structural test, it is only a test to determine if there are points within the system that are leaking.* On the other hand, there are codes which may require a structural test such as is done by the boiler and pressure vessel code. In that case, the hydrostatic test is performed to verify that the vessel and attached piping is structurally sound, not just leak tight.

ASME B31.3, Para. 345.1 states:

Prior to initial operation, and after completion of the applicable examinations required by para. 341, each piping system shall be tested to ensure tightness. The test shall be a hydrostatic leak test in accordance with para. 345.4 except as provided herein.
…
(b) Where the owner considers a hydrostatic leak test impractical, either a pneumatic test in accordance with para. 345.5 or a combined hydrostatic-pneumatic test in accordance with para. 345.6 may be substituted, recognizing the hazard of energy stored in compressed gas.

So per the code, a leak test using air may be performed if the system owner considers the hydrostatic test to be impractical.

It’s important to understand that the pressure at which the test is performed is a function of the design pressure. Design pressure is a function of the allowable stress limits on the piping which is also a function of operating temperature.
- For a hydrostatic test, para. 345.4.2 requires a pressure of not less than 1.5 times the design pressure.
- For a pneumatic test, para. 345.5.4 requires a pressure of not less than 110% of design pressure.

Next step is for an engineer (preferably the piping system designer or stress analyst) to create pressure test procedures. These pressure test procedures look at the possibility of low temperature brittle failure, which may be a concern at the temperatures you’re referring to. The pressure test procedures are actually a set of procedures (typically) which include such things as method of pressurizing system, valve positions, removal of relief devices, isolation of portions of the piping system, etc…

Regarding the low temperature, para. 345.4.1 states: “The fluid shall be water unless there is a possibility of damage due to freezing or to adverse effects of water on the piping or the process (see para. F345.4.1). In that case, another suitable non-toxic liquid may be used.” So glycol/water is allowed.

If the test is to be done pneumatically, test pressure should be raised to 25 psi at which time a preliminary check shall be made, including examination of all joints. The use of a low temperature bubble fluid is highly advisable.

So to conclude:
1. If the specification you’ve been given is to perform a hydro test at 16 bar, then that should be 1.5 times the design pressure of 10.67 bar. Per B31.3, a pneumatic test should therefore be performed not at 16 bar, but at 1.1 times the design pressure or 11.7 bar. Run the pneumatic pressure only as high as 11.7 bar.
2. The possibility of brittle failure should be reviewed by the appropriate engineer. In the case of temperature below 0 C, the material used should be examined to verify it is not below the minimum useable temperature for that steel.
3. A cognizant engineer needs to produce a set of pressure test procedures. Those procedures need to indicate what sections of pipe are being tested, what positions valves should be placed in, what relief devices need to be removed (or installed), etc…
4. The pneumatic test needs to start at 25 psig and a preliminary examination for leaks performed prior to increasing pressure.
5. Most important, the cognizant engineer must also examine the piping design specification for all requirements pertaining to leak or pressure testing.

*Although B31.3 describes this as a “leak test”, when performed hydrostatically at 1.5 times design, it is in affect, a structural test.