Pressure Tests of Piping systems-Hydrotest Vs Pneumatic Test

A pressure test is always required for a new pressure system before use or an existing pressure system after repair or alteration  to ensure the following:
  • Safety
  • Reliability of operation
  • and leak tightness of pressure systems
  • Also The last physical quality test in the fabrication process

Nomally two methods for pressure testing: hydrostatic and pneumatic, are widely used in process industry.

A hydrostatic test is performed by using water as the test medium, whereas a pneumatic test uses air, nitrogen, or any non-flammable and nontoxic gas. Pressure tests (both hydrostatic and pneumatic) must always be performed under controlled conditions, following an approved test plan, and documented in a test record. A single approved test plan could be used for several similar tests, but for each test a separate test record is required.
When to perform a Pressure Test:
As per the governing code, pressure test has to be performed on piping system after all hot works have been completed on a certain piping system. Here the term hot work means  everything related to welding or the post weld heat treatment (PWHT). PWHT also has a potential of degrading the mechanical properties of piping, in case not addressed properly. And this is the reason why code calls for NDT after PWHT operation. In case after carrying out pressure testing, some modification has to be made requiring hot work, it calls for a retest as per code. Here code specifies that the minor repairs / modifications may be waived off provided adequate measures have been taken to ensure sound construction. Now taking this decision as to which repairs or modifications may be waived off should be taken very carefully. Normally, the maximum extent of repairs not requiring retesting shall be the tack welding of any piping support or pad. Anything else shall be done following a retest.
Pressure testing and conducting 100% radiography or ultrasonic inspection shall not be interchanged. In case carrying out of hydrostatic or pneumatic test stands impractical then 100% radiography or ultrasonic testing may be performed but in addition to this it is advisable to check that the whole piping and its components have been supplied against acceptable ASTM standards and required test certificates are available. Conducting 100% radiography of all the weld joints assure that your weld joints are defect free but can never provide you with the assurance of mechanical integrity of a system. This is also to be noted that radiography / ultrasonic inspection shall also not to be waived off if the pipeline is to be hydrostatically tested. This may pose an additional safety risk during the test. Moreover, some defects in the weld zones may prove to be detrimental way after taken in service due to severe extended service conditions.
Comparison of Hydrostatic and Pneumatic Testing:
Hydrostatic Testing
Pneumatic Testing
Test pressure is normally 30% higher than the design pressure
Test pressure is normally 10% higher than the design pressure
Recommended for high pressure applications
Recommended only for low pressure applications
Test media (Water) used is not compressible by pressure application
Test media (Air) used is compressible by pressure application
Energy stored per unit volume of water under test pressure is very negligible
Energy stored per unit volume of compressed air under test pressure is very high
Needs thorough cleaning after test to
eliminate moisture especially for service which are reactive to moisture / fluids
Easy to clean after testing
Pressure Relief devices are recommended to control sudden increase in pressure during testing
Pressure relief devices are must during test to ensure no over pressurisation
Chances of equipment/ Pipe / test apparatus
failures are very low
Chances of equipment/ Pipe / test apparatus
failures are high
Weight of equipment along with test medium as
water is high hence special attention should be given to floor and supporting arrangements
Weight of equipment with test medium
as air is comparatively less
Needs verification and examination of joints and connections before testing
Needs very careful checking of weld joints thoroughly before testing
Test media can be reused or transferred to other place after testing
Test media can not be reused or transferred to other place after testing
Skilled and semi skilled personnel can carry out the test
Needs involvement of senior experienced staff to monitor the test.
Recommended where large volumes are to be tested at same time (example pipe lines )
If pipe lines are tested should be done with small segmental lengths at a time.
Damages made by failures are less compared to failures in pneumatic testing
Damages made by failures in testing are very huge and extensive
Hydrotest is a regular day to day practice and safe procedure and it can be followed in any work site
Needs special attention and safety precautions

 Pressure changes finite amount by infinitesimal change in volume

 Pressure change proportional to volume change

Difficulties with hydrostatic testing:
  • Supply & disposal of water, disposal of fluid additives
  • Water leak can cause equipment damage
  • Freeze susceptibility
  • Structural support limitations
  • Contamination
  • Operational impacts – process contaminant
  • Affect dry-out of internal refractory linings


Difficulties with Pneumatic testing:

Pneumatic tests are potentially more dangerous than hydrostatic tests because of the higher level of potential energy stored during compressing the gas. Care must be exercised to minimize the chance of brittle failure during testing by initially assuring the system is suitable for pneumatic testing. Pneumatic tests could be performed only when at least one of the following conditions exists:

  • When the systems are designed in such a way thet it cannot be filled with water.
  • When the systems are such that it is to be used in services where traces of the testing medium cannot be tolerated.

Using a pneumatic test instead of hydrostatic requires approval from proper authority or body.

Test Pressure:

ASME Code B 31.3 provides the basis for test pressure. The minimum hydrostatic test pressure for metallic piping shall be as per the following equation:



Pt=minimum test gauge pressure
Pd=internal design gauge pressure
St=allowable stress value at test temperature
Sd=allowable stress value at design temperature.
The maximum allowable value of St/Sd is 6.5The pneumatic test pressure shall be as per the following equation:   Pt=1.1*PdNormal Basis for Test Method Selection:

Normally the following basis is followed in process piping industry for selection of hydro or pneumatic tests.

  1. If the fluid handled by piping system is liquid then the pipe must be hydrotested.
  2. If the fluid handled is vapour or gas then the internal design pressure dictates the testing method. For pipe design pressure 10 bar and above the pipe is hydrotested. For pipe design pressure below 10 bar pipe is pneumatically tested.
  3. If fluid handled is stream then hydrotesting is suggested.
  4. If fluid handles is two-phase flow then hydrotesting is suggested. However for big size flare headers pneumatic testing is performed.
Test Duration:
Normally every EPC company have their own standard for duration of tests. However the recommended practice is that a QC inspector have to walk through the whole piping system and check for leaks. Every single length of piping, welds, bolted connections shall be visually examined for any leakage. Duration of this activity varies with the span of piping system. For larger piping system time taken for this activity is enough to clear the pressure test. In case of piping system having smaller span, 1 hour time may be made as standard practice for hydrostatic testing. For Pneumatic test the test time is far lesser.Few important points to consider while hydrotesting:

  • The tower overhead lines which are normally hydro tested shall be decided by stress engineer group whether to hydro test after erection or at grade, based on the capacity of standard clip available as per engineering specification for pipe supports.
  • During hydrotest in case of big bore it should be decided first that line is to be hydrotested on rack or grade if hydrotest is done on rack , hydrotest load should be considered in case of vapour lines while providing structure / rack loading information.
Please provide your input in the Comments section

Anup Kumar Dey

I am a Mechanical Engineer turned into a Piping Engineer. Currently, I work in a reputed MNC as a Senior Piping Stress Engineer. I am very much passionate about blogging and always tried to do unique things. This website is my first venture into the world of blogging with the aim of connecting with other piping engineers around the world.

27 thoughts on “Pressure Tests of Piping systems-Hydrotest Vs Pneumatic Test

  1. Hi,
    Thanks for providing valuable information on hydrotest and pneumatic test. One of the questions I had on my current project is that we are buying Stainless Steel inline instruments on Carbon steel lines. Also it is noticed that the hydrotest pressure on such lines is calculated based on corresponding operating pressure for Stainless steel material at that temperature.
    My opinion is that by doing so we are derating the hydrotest pressure for carbon steel and are not testing it to the required pressure. Is it ok to do so.

    Please provide your feedback.


  2. Very good article but comparison table shows hydrostatic test = 30% higher than design pressure when it should read 50%
    i.e. Pt=1.5*Pd*(St/Sd)


    1. Piping designed as per B31.3 is never hydro-tested at 1.3times of test pressures,1.3 is for pressure vessels designed on ASME Sec VIII Div 1(UG-99(b) is 1.3 x MAWP at Design Temp. x lowest ratio of S at test temp to S at design temp). However, B31.3 states Pt=1.5*Pd*(St/Sd) for piping.

    1. Normally Flare lines are not hydrotested.
      Check the project document.
      If pneumatically tested then separate load case is not required.
      If hydrotested then same load case (WW+HP)—HYD need to be prepared what you make for other systems.

  3. Nozzle is getting fail in hydro Case. Almost 15 times the allowable. What could be the reason behind it? Burdon effect is activated.
    Hydro Pressure is OK. Thickness of Pipe is OK. Units are OK.
    Any other reason ?

  4. Stored energy in a pipe during Pneumatic testing :

    The process inside the system is assumed as adiabatic compression.
    As per the ideal Gas equation-
    PV= mRT
    P= Pressure
    V= Volume
    m= Mass of Gas
    R= Gas constant
    T=Temperature of the Gas
    Work done in an adiabatic compression = mCv DT
    = mCv (T2-T1) Constant Volume process
    =R/( -1) m (T2-T1)
    =R/( -1) mT2( 1-T1/(T2 )) mT2= P2V2/R – Ideal gas equation
    =R/( -1) P2V2/R {1-( V2/V1) (-1)}
    =R/( -1) P2V2/R {1-( P1/P2) (-1/)} as per adiabatic equation PV  = C
    = P2V2/(-1) {1-( P1/P2) (-1/)}
    P2= Test pressure(abs)….psi
    V2=Volume of the pipe (Total Volume under test pressure)….m3
    = Ratio of specific heat for the test fluid (gamma)
    P1=Abs Atmospheric pressure….psi

    This formula is similar to the one given by the ASME standard ASME-PCC-2-2008 regarding the energy calculation in its Article No. -5.1, Mandatory Appendix II.

  5. All water piping shall be tested to hydro-static test pressure of at least one and a half times the maximum
    operating pressure but not less than 10 kg/sq cm for a period of not less than 24 hours. Why 24 hours is required?

  6. We have situation where a Discharge Pipe for an LPG storage bullet (pressure vessel) is encased (sleeved) inside a larger diameter pipe. The weld joint connecting the discharge line to the bullet is a golden weld (GW). Our project Spec does not allow GW. However this GW is located inside a larger diameter pipe sleeve which is welded to the bullet at one end and to the discharge pipe at the other end. The sleeve pipe can be tested (pneumatically) to the same required test pressure for the Discharge Pipe. My question is as follow: If we pneumatically test the sleeve pipe to the same test pressure of the Discharge Pipe, which in turn will be subjecting the GW joint on the Discharge Pipe to external test pressure, can this be considered a substitute to subjecting the Discharge Pipe GW to internal pneumatic pressure testing? Your reply will be much appreciated.

  7. Can u give some information about pressure relief device that at what pressure and at what pipe dia we should use pressure relief device

  8. Good comparison , While critical line analysis , How Caesar ll Hydro load case look like in case of pnumetic test requirement , please put more light., We generally put same hydro case (Ww+HP)=HYD to see the impact. HP is considered 1.5 times of Pd but WW weight should not come.

  9. @ Vinay Sharma. The hydrotest of the main line which is in Carbon Steel materials should be the basis for your testing. Any components connected to it shall be subjected to the same during operations. However, at cases where there piping component is not designed to be subjected to such pressure, can be dismounted and shall be tested on its own.

  10. I wish to know. If I test a Valve @ 10Kg/SqCm Hydro Pressure at what pressure should I test the same with Pneumatic Pressure. Is there any formula for applying it and any limitation of Size of the valve.

  11. Please explain me one doubt. This article says, hydrotest is the last physical test. No hot work should be done after that. In case of over head lines, what we need to ensure when hydrotest is done on ground level and then pipe is erected by hot work?

  12. Dear your blog is very interesting and good, that i can’t tell how knowledgable it is. It’s such a rewarding work you had done.

  13. Hi thanks for the great explanation
    We are looking to test Rubber Bulk hoses Dia 6″ at WP 10bar
    I need to confirm what sort of pressure test should be done i.e. either
    1. Hydrostatic (filling the hose with water and then pile up the pressure to WP10bar)
    2. Directly fill the hose with air compressor up to WP 10bar?(not filling with water just directly attaching compressor outlet to hose with other end of hose blocked with a dead plate)
    Would really appreciate your technical input . Thank you very much


Leave a Reply

Your email address will not be published. Required fields are marked *


Recent Content