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

Subsequent to repairs and heat treatment and prior to initial operation or commissioning of any plant it has to be ensured that there is no evidence of leakage by Pressure Testing.

Types of Pressure Testing

In the process Industry, two methods are widely used for pressure testing of piping or equipment. They are

  • hydro-static testing and
  • pneumatic testing

What is Hydro-Test or Hydrostatic Test?

Hydro test or Hydrostatic test is a type of pressure test performed on piping and pressure vessels to check system integrity under pressure condition. A hydrostatic test is performed by using water as the test medium.

What is the Pneumatic Test?

The pneumatic test is a type of pressure test for checking system integrity under the pressurized condition and applied to systems where the hydro-static test is difficult to apply. A pneumatic test uses air, nitrogen, or any non-flammable and non-toxic 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 the 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 the 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 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 a detrimental way after taken in service due to severe extended service conditions.  

Comparison of Hydrostatic and Pneumatic Testing

The following table lists down the major differences between hydrostatic and pneumatic testing.

Hydrostatic TestingPneumatic Testing
Test pressure is normally 50% higher than the design pressureTest pressure is normally 10% higher than the design pressure
Recommended for high-pressure applicationsRecommended only for low-pressure applications
Test media (Water) used is not compressible by pressure applicationTest media (Air) used is compressible by pressure application
Energy stored per unit volume of water under test pressure is very negligibleEnergy stored per unit volume of compressed air under test pressure is very high
Needs thorough cleaning after the test to eliminate moisture especially for service which are reactive to moisture/fluidsEasy to clean after testing
Pressure Relief devices are recommended to control a sudden increase in pressure during testingPressure relief devices are must during the test to ensure no over-pressurization
Chances of equipment/ Pipe / test apparatus failures are very lowChances 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 arrangementsWeight of equipment with the test medium as air is comparatively less
Needs verification and examination of joints and connections before testingNeeds very careful checking of weld joints thoroughly before testing
Test media can be reused or transferred to other places after testingTest media can not be reused or transferred to other places after testing
Skilled and semi-skilled personnel can carry out the testNeeds involvement of senior experienced staff to monitor the test.
Recommended where large volumes are to be tested at the same time (example pipelines )If pipelines are tested should be done with small segmental lengths at a time.
Damages made by failures are less compared to failures in pneumatic testingDamages 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 worksiteNeeds special attention and safety precautions
 Pressure changes finite amount by an infinitesimal change in volume Pressure change proportional to volume change
Table: Hydro Test vs Pneumatic Test

Difficulties with hydrostatic testing

  • Supply & disposal of water, disposal of fluid additives
  • A 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 that 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.

How to calculate Hydrotest 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 the test temperature
Sd=allowable stress value at design temperature.
The maximum allowable value of St/Sd is 6.5

How to Calculate Pneumatic Test Pressure?

The pneumatic test pressure shall be as per the following equation:


The basis for Pressure Test Method Selection

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

  1. If the fluid handled by the piping system is liquid then the pipe must be hydro tested.
  2. If the fluid handled is vapor or gas then the internal design pressure dictates the testing method. For pipe design pressure 10 bar and above the pipe is hydro tested. For pipe design pressure below 10 bar pipe is pneumatically tested.
  3. If fluid handled is stream then hydro testing is suggested.
  4. If the fluid handled is two-phase flow then hydro testing is suggested. However, for big size flare headers, pneumatic testing is performed.

Pressure Test Duration

Normally every EPC company has its own standard for the duration of tests. However, the recommended practice is that a QC inspector has 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. The duration of this activity varies with the span of the piping system. For larger piping system time taken for this activity is enough to clear the pressure test. In the case of a piping system having a smaller span, 1 hour time may be made as standard practice for hydrostatic testing. For the Pneumatic test, the test time is far lesser.

Few important points to consider while hydro-testing:

  • 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 the hydro test in case of big-bore, it should be decided first that line is to be hydro tested on rack or grade if the hydro test is done on the rack, the hydro test load should be considered in case of vapor lines while providing structure/rack loading information.

Hydrotest Fluid

As the name signifies, the test has to be performed using clean potable water free from suspended solids. However different codes specify different requirements for water quality. Hence, hydro tests need to be performed following those instructions.

Hydrotest system preparation

The piping network and the connected equipment must be prepared thoroughly before proceeding for pressure tests like hydro or pneumatic testing. Codes like ASME B31.3/ ASME B 31.1 and local company instructions normally provide guidelines for such preparation.

Piping systems that are normally open to the atmosphere, such as drains, vents, discharge piping from pressure-relieving devices, sewers, and stack downstream of the seal drum, need not be subjected to the piping test pressure.

For some countries, The provincial local inspector shall be notified at least 48 hours in advance of pressure testing of piping under the jurisdiction of the provincial safety code for witnessing at his/her option.

The following equipment shall be excluded from all piping pressure tests:

  • Vessels and tanks
  • Heat exchangers
  • Rotating machinery, such as pumps, compressors, and turbines
  • Equipment and Supplier furnished piping specifically recommended by the manufacturer not to be tested

Underground portions of piping systems may be tested and covered before testing aboveground portions.

Roughly, The following steps should be followed for preparing the piping system for leakage testing.

Comparison of P&ID/PEFS and Isometric Drawing

The first step is to compare the piping isometrics with the P&ID drawings to check if any discrepancy exists. All valve types, flow directions, branch tie-ins, and any material changes, etc are reviewed thoroughly. In-line components are verified and ensured that they are able to withstand the test pressure.

Thorough Inspection before testing

All sorts of ultrasonic and radiographic inspections or checks must be carried out before the actual pressure test is started. 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 be waived off if the pipeline is to be hydrostatically tested.

Joints shall be exposed

All joints, welds (including structural attachment welds to pressure-containing components), and bonds shall be left uninsulated and exposed for examination during leak testing as per Section 345.3.1 of ASME B31.3, except that joints previously tested in accordance with this Code may be insulated or covered.

Provision of Temporary Supports

If required Pipings designed for vapor or gas services shall be provided with additional temporary supports, to support the weight of test liquid as the test liquid is heavier than service fluid.

Spring Supports in Piping System

All Spring supports shall be kept in the locked condition during hydrostatic testing. Holding pins shall not be removed from spring supports until testing is completed and the system is drained. Care shall be taken to avoid overloading any parts of the supporting structures during hydrostatic testing.


  • All in-line Instruments shall be either removed or blocked prior to hydro testing to prevent damage during hydro testing (e.g. meters).
  • Instrument take-off piping and sampling system piping, up to the first block valve, shall be tested with the piping to which it is connected.
  • Instrument lead lines, between the first block valve and the instruments to which they are connected, shall be pressure tested to the test pressure of the associated piping. Testing may be performed separately or at the same time as the piping is tested, but the instruments shall be disconnected.
hydro-testing scheme
A sample figure showing hydro-testing scheme

Hydrotest for Piping with Expansion Joints

If there is an expansion joint in the piping system under the leakage test, the following criteria shall be applied.

  1. An expansion joint that depends on external main anchors to restrain pressure end load shall be tested in place in the piping system.
  2. A self-restrained expansion joint previously shop-tested by the manufacturer
  3. A piping system containing expansion joints shall be leak tested without temporary joint or anchor restraint at the lesser of
  4. 150 % of design pressure for a bellows-type expansion joint, or
  5. the system test pressure determined in accordance with para. 345

In no case shall a bellows-type expansion joint be subjected to a test pressure greater than the manufacturer’s test pressure.

  • When a system leak test at a pressure greater than the minimum test pressure specified in (c), or greater than 150% of the design pressure within the limitations of para. 345.2.1(a) is required, bellows-type expansion joints shall be removed from the piping system or temporary restraints shall be added to limit main anchor loads if necessary.

Limits of Tested Piping

Equipment that is not to be tested must be either disconnected from the piping or isolated by blinds or other means during the test. A valve may be used provided the valve is suitable for the test pressure.

Rotating Machinery

For rotary machinery, such as pumps, compressors, or steam turbines,  have lube and seal oil systems that could be impaired by the presence of water. These systems shall not be subjected to the piping test pressure.

Test Water

Clean water that will not corrode and/or damage the test system shall be used for hydrostatic tests. If municipal water is not available, supply water may be obtained from a native water supply. Water containing silt or suspended material shall not be used, and a suitable filter with 40 mesh should be provided in the filling line. A mixture of glycol-water shall be used where the ambient temperature may reach less than 0°C (32°F) during testing or prior to dry-out

Temporary Spades and Blanks

If Temporary spades and blanks are installed for testing purposes, those must be designed to withstand the test pressure without distortion. The presence of spades shall be clearly visible during testing. The recommended practice is to use standard blind flanges as per ASME B16.5 or B16.47 and spades acc. to ASME B16.48.

Check Valves

Check Valves shall have the piston or flap removed for testing,  where pressure can not be located on the upstream side of the valve. The locking device of the flap pivot pin shall be reinstated together with the flap and a new cover gasket shall be installed after completion of the test.

Completion of Hot Work

Wherever applicable All hot works, related to welding or the post-weld heat treatment have to be completed before the Hydrostatic test.

Installation of Barriers

It is necessary to install safety barriers around the piping system under test prior to starting pressurization related to testing. Public Address announcements and access restriction procedures such as permit to work shall be implemented. Under no circumstances should anyone other than an authorized person be allowed within the safety barriers.

Control Valves

Prior to the leakage test, Control Valves and soft-seal block Valves shall be removed from the piping and replaced with pipe spools. However, All welded Valves need to be tested along with pipings otherwise Radiographic Testing of golden joints (welded) will be required.

Physical Inspection

Prior to test following should be checked :

  • All gravity supports installed.
  • Proper pipe routing.
  • Vents or other high point connections shall be opened to eliminate air from and plugged during the test.
  • lines that are to receive a hydrostatic test.
  • Completed and torqued flanges with no missing bolts or gaskets.
  • Proper material type verified using color codes or markings, and heat numbers recorded if required by the codes.
  • Correct valve type and orientation.
  • Vents and drains installed to allow proper filling and draining
  • When a pressure test is required to be maintained for a period of time during which the testing medium in the system would be subject to thermal expansion, the provision shall be made for the relief of any pressure greater than the maximum test pressure.
  • All required piping stress relief, weld examinations, and welding documentation completed and acceptable. A relief valve to be added such that the test pressure is not exceeded beyond a safe amount.
  • When conducting tests at freezing temperatures, the test shall not take more than 4 hours, and special precautions, such as using glycol/water mixture, shall be observed to avoid freezing damage

Hydrotesting Documentation

The individual system documentation i.e. test pack shall be available prior to any testing and shall include information such as test limits, test pressure, test medium, duration, test blinds, blind flanges, vents, and drains.

The use of marked up P&IDs coupled with isolation registers should be utilised to identify the locations of blinds, Valves, vents and drains.

Records shall be made of each piping system test. In general, these records shall include the following information:

  • Date of test
  • Identification of piping system
  • Test medium
  • Test pressure
  • Minimum ambient temperature
  • Test medium temperature
  • Certification by examiner
  • Test duration
  • Pipe standards or specifications for the test
  • Pressure and temperature recording charts and logs
  • Location and cause of leaks/failures
  • Repair procedures used in the event that the pipe leaked of failed
  • Test results
  • Name of test operator
  • Minimum metal temperature (if applicable)
  • Test gauge calibration date

Forms, when completed, shall be retained in the Construction Contractor’s QC file as a permanent record.

Hydrostatic Testing Equipment

Testing equipment such as pumps, manifold, pressure, and temperature recorders, pressure gauges should be within calibration/certification (as per company procedures) and connected to the lowest convenient connection within the system to ensure the best results.

Hydrostatic Testing Problems

Even though proper industry-approved methods are followed for pressure testing, still due to some negligence hazards do happen. Click here to know about 5 such hazards.

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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.

36 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


  14. Please specify to standard re:
    Pt= 1.5 x Pd * (St/ Sd)

    The Normal practice is 1.5 x Pd,, is there a standard update on this? If yes, please mention the std and edition.

    Thank you.

  15. Thanks for sharing the information.
    As Aquilino De Belen says, I also like to know more about and where to get this info (St/Sd).
    With a max. of 6.5, Test Pressure is nearly 10 times Design Pressure.
    Thank you.

  16. Excellent article & very detailed explanation. Probably we can add 1). Pneumatic testing is not applicable for systems that handle hot liquids (as the expansion of piping during the operation will result in leaks and there is no way to identify such leaks during pneumatic test). 2). Water quality used for hydrostatic testing is governed by the maximum allowed Chloride content in case of Stainless steel piping.

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