Hydro Test or Hydrostatic Test | Purpose, Procedure & Precautions

Know the basics of the hydro test or hydrostatic test for systems with fluids under pressure, and find out what safety precautions need to be taken.

What is a hydro test or hydrostatic test?

Hydro test or hydrostatic test is an essential procedure used to verify the structural integrity of systems containing fluids under pressure, i.e., pressure vessels, boilers, gas cylinders, and pipes. It has saved countless lives by helping people ensure that pressure-containing components remain safe and reliable throughout their operational life. Hydrostatic testing involves filling the vessel or piping system with water or any other suitable liquid and then pressurizing it at levels considerably higher than those it will experience in its normal operating environment. The pressure is then tracked over time to look for leaks or other problems that could cause the system to fail. This lets any flaws or weak spots in the system's components be found and fixed before they become a safety risk. Hydro testing is used in many industries, such as oil and gas, chemical processing, and power generation, to ensure that pressurized systems are safe to operate.

Why is a hydro test performed?

Hydro testing aims to ensure that any equipment and pipework systems intended for use with liquids or gases are safe and reliable. It provides confidence that the equipment and pipework system can handle the pressure and volume of a substance it will be holding. The two basic purposes of hydro testing are:

Strength Testing

A strength test aims to lower the chance that a faulty part will cause a catastrophic failure in service. It is performed to prove the quality of the materials used and the fabrication of the equipment before it enters or re-enters service. The test pressure is normally above the design pressure of the system, typically 1.25 to 1.5 times the design pressure. The strength test may be carried out on individual system components prior to assembly, typically in a fabrication workshop. Alternatively, the strength test may be carried out on an assembly of components, either in a workshop or installation location.

Leak Testing

A leak test is performed to show that the assembled system is leak-free during initial installation, modification, changeover, maintenance, or other operations that may compromise the containment boundary. A leak test is used to confirm the integrity of component connections, joints, and gaskets that may not have undergone a strength test. Leak tests are normally carried out at 110% of the normal operating pressure. It is important to note that before performing a leak test, all fabricated system components must first undergo a strength test. Alternatively, suppose a strength test is performed on an assembled system at the installation location, and no component connections or joints are disturbed after the test. In that case, the test can then be considered a combined strength and leak test.

Broadly speaking, hydro tests may be carried out for one or more of the following reasons:

• To confirm the adequacy of a design.
• To confirm that the construction materials of equipment and pipework will withstand the design stresses.
• To verify the quality of the fabrication.
• To verify the mechanical integrity of an assembled system.
• To identify areas where additional reinforcement might be necessary for safety reasons.
• To establish fitness for continued service.
• To comply with applicable regulatory requirements.
• To check for leaks.
• To provide acceptance/rejection criteria for allowable leaks.

When is a hydro test required?

Hydro testing is usually required before equipment and pipework systems are used for the first time or after being changed, modified, or left idle for a long time. Also, hydro testing is done after every maintenance or operational activity that may compromise the integrity of the pressurized system. The frequency of hydro testing will depend on the type & contents of equipment, its age, and any changes made to it over time. Specific codes, i.e., ASME B 3.1.1, ASME VIII, BS 806, can be referred to define the frequency of hydro testing for equipment and pipelines. For firefighting equipment, NFPA codes specify requirements for hydro testing. For example, as per NFPA 10, the frequency for hydro testing of carbon dioxide, pressurized water, and wet chemical fire extinguishers is every five years, and for dry chemical fire extinguishers is every 12 years.

Hydro Testing Procedures

For larger pieces of equipment, the equipment or pipeline is filled with water to its maximum capacity, the air is evacuated, and the pressure is increased to within 1.5 times the design pressure limit of the equipment or pipeline. Once the desired pressure is reached, it is maintained for a certain period of time to allow for a visual inspection of the system for leaks. Tracer or fluorescent dyes can be added to the liquid to aid in visual inspection by highlighting the exact location of cracks and leaks.

Small pressure vessels and cylinders are often tested using one of the following hydrostatic testing methods:

Water Jacket Hydro Test Method

This technique requires filling the small vessel or cylinder with water and placing it inside a water-filled, sealed enclosure called the water jacket. The vessel or cylinder placed inside the jacket is subjected to pressure above its MAWP (Maximum allowable working pressure) for a predetermined period of time. As a result, the vessel or cylinder expands, and water is forced out into a glass tube, where the total expansion of the vessel or cylinder may be measured. Once the total expansion has been measured, the pressure is released, and the vessel returns to its original size. In case the vessel or cylinder does not return to its normal size, the second size value is called "permanent expansion." The difference between the total expansion and the permanent expansion shows if the vessel or cylinder is ready for service or not. The higher the permanent expansion, the more likely it is that the vessel or cylinder will be taken out of service.

Direct Expansion Hydro Test Method

Direct expansion hydrostatic testing is a method for determining a vessel's strength by gauging its unpressurized volume, much like the water jacket test. To accomplish this, water is pumped into the vessel to increase pressure, and the amount of water lost during depressurization is measured. The same measurements (percentage of permanent expansion and total expansion) can be obtained using this method, although it is not as precise as using a water jacket hydrostatic test.

There are some other methods for conducting hydrostatic testing, like Proof Pressure & Pressure Recession methods; however they have limited practical applications and are not often permitted by regulatory bodies.

Hydro test safety precautions

Before the hydro test is conducted, it is important to make sure that all safety procedures are followed to ensure that no accident or injury occurs during the test. This includes ensuring that all safety equipment, such as protective eyewear, gloves, and a first aid kit, are readily available. Additionally, personnel should be aware of any potential hazards in the area where the hydro test is being conducted, including possible flammable materials or hazardous chemicals. During the test itself, personnel should remain at least 10 feet away from the area being tested and should never be within range of any high-pressure lines or equipment. Workers should be well-trained in the proper operation and maintenance of hydro testing equipment and should follow all manufacturer instructions regarding the use of the equipment.

To be specific, the following is the comprehensive list of safety precautions to ensure a safe and successful hydro test:

• All hydro tests must be carried out in accordance with an authorized test method, prepared/approved by a responsible engineer knowledgeable in testing pressure equipment and piping.
• Hydro testing should always be done by a qualified professional with experience in this field in order to guarantee accurate results and avoid any unnecessary risks. 
• A thorough risk assessment of the activity should be carried out, and appropriate control measures should be implemented according to the identified hazards and risks.
• The risk assessment must take into account the risk of blanks, blind flanges, or plugs being ejected as missiles during the course of the test.
• The risk assessment must take into account how the system will be filled, drained, and vented.
• Where vessels or pipework are designed to a code, such as ASME VIII, ASME B31.3, or BS 806, they must be pressure tested in accordance with the code.
• The tests should be carried out at around ambient temperature. Where the design service temperature is higher than the ambient, the ambient test pressure may need to be adjusted. This must be determined using the relevant code.
• The possibility of brittle fracture must be considered when conducting hydrostatic tests at temperatures approaching the ductile-brittle transition temperature of the metals under test.
• The pressure gauges used to indicate the test pressure must have a range and accuracy appropriate to the test pressure and must be within their valid calibration period to assure appropriate accuracy. At least two pressure gauges must be installed, one at the bottom and the other at the top of the system.
• The chosen test fluid must be compatible with the materials of construction of all the system components, or any incompatible components must be removed and their connections plugged or blanked off.
• The system must be checked prior to the pressure test to verify that all the components subject to the test are capable of withstanding the test pressure. For example, sensitive instruments or safety devices may need to be removed and the connections plugged or blanked off.
• Any closed valves which are to be used as a system limit must be checked to ensure that they are capable of withstanding the test pressure without damage to the seats and closure component (e.g., gate). If not, the valves must either be removed and blanked off or slip-plates inserted.
• Whenever components have been removed, connections plugged or blanked, or slip plates fitted prior to strength testing, refitting the components after the test necessarily compromises the containment boundary. A further leak test must therefore be carried out to verify the integrity of the complete system. This leak test may be carried out by the controlled introduction of the service fluid.
• Any valves within the system to be tested must be in the open position so that the valve stem seal is subjected to the test pressure.
• Any non-return valves within the system to be tested must be disabled in the open position or removed from the system to allow free flow of fluid through the system during filling, testing, and venting.
• Where safety valves are used, they must be of adequate size and marked with the set pressure and must be installed in or close to the test supply line to prevent the test pressure from being exceeded. It must not be possible to isolate these valves from the testing medium pressure.
• Where flexible hoses are used as fluid-filling connections or to connect the hydrotesting equipment to the system, all hoses used must be suitably pressure-rated. Proprietary crimped hose fittings must be used. Jubilee clips should never be used.
• The system to be tested must be visually inspected before starting the test to check that there are no missing components or open connections, all fasteners and connections are in place and tight, all supports and anchorages are in place, there are no obvious defects or damage to the system, and the correct gaskets have been installed.
• When filling the system with test fluid, care must be taken to vent any air pockets from the system.
• Once the system is air-free, all personnel must leave the immediate area, and the pressure then gradually increases until the full test pressure is reached. If leaks are noted at any time, the pressure test must be stopped until appropriate repairs are made.
• During the test, the system pressure must be raised in a series of steps to prevent accidentally exceeding the specified test pressure. Fluid pressure may be applied using a hand-operated pump. If a motorized or hydraulic pump is used, measures must be taken to ensure that the system is not accidentally over-pressurized, either by the use of pressure limiting or safety valves or by close personal supervision of the pump operation. 
• The system under test must not be subject to shock loading (e.g., hammer testing) during the course of the test.
• If leakage at a connection or joint occurs during the course of the test, the system must be depressurized before any attempt is made to rectify the leak. Tightening connections under pressure should never be performed.
• Once the test pressure has been satisfactorily held for the required period, the pressure must then be reduced to the maximum operating pressure before the system is inspected for leaks.
• Close personal examination of the system must not be carried out until a reasonable period of time (at least ten minutes) has elapsed during the test. Colorant or fluorescent dye may be added to the test fluid to facilitate the detection of leaks.
• In the event of a leak or if the system fails the hydrotest for any reason, the system must be depressurized to resolve the problem.
• If a hydrostatic testing procedure is being performed on a gas system, it must be examined to ascertain that it can bear the load of test fluid. Installing temporary reinforcements while carrying out the hydrostatic test may be necessary.
• Once the test is complete, any vents must be opened to release the pressure, and the equipment or piping must be drained at a rate slow enough to prevent the formation of a vacuum.