Since my area of expertise is in steam boilers, I will not discuss the hydrostatic test (HT) in unfired pressure vessel or power piping.
Hydrostatic test is a form of NDT (nondestructive test) and is the most preferred method to detect leakage and cracks in boiler under water pressure. Other NDT’s such as magnetic particle test (MPT) or penetrant test (PT) are normally done to confirm the extent of damage after the leak point has been discovered. Only water pressure is allowed for HT. Steam or air is not allowed. “Hydro” itself means water. Water for all practical purposes is incompressible, and the pressure developed by the hydrostatic pump can be relieved instantaneously by releasing very small quantity of water, which would happen on leak occurring. Air is compressible and could be dangerous under pressure. The same goes with steam. Under steaming conditions, any failure of the structure would result in the boiler water flashing into steam, or steam leaks, subjected personnel to a danger of burn.
In Malaysia, the local jurisdiction, the Factories & Machinery Act 1970, stipulates that a boiler shall be hydrostatically tested every 7 years in which the authorized inspector will issue a Certificate of Hydrostatic Test in 6th Schedule, stating the date, test pressure, holding time, result, and authorized safe working pressure. The Certificate is valid for 7 years until the next HT. However, this is only a time frame, and your local jurisdictions may require different interval.
As a preparation for the HT, clean boiler waterside and fireside, nozzle flanges, manhole surfaces, and welding joints. Observe all safety precautions (procedures can be obtained from the eBook). The safety valve must be removed. However, if for some reason the safety valves cannot be removed, a test gag may be used following the safety valves’ manufacturer guidelines. Remove electrical connections such as high pressure limit switches and sensors. Note that when a HT is being applied to a boiler, a calibrated test pressure gage must be connected to the hydrostatic pump. This precision test gage shall have the accuracy of 0.5%. Countercheck it with the calibrated pressure gage mounted on the boiler. Blind the nozzles by installing blind flanges or blind plates to fittings connection such as safety valves, main stop valves, feed nozzles, mombrey connections, and gage glasses. Rubber or normal gaskets will do just fine.
Fill up with treated water (at ambient temperature of 26oC) for the entire volume of the boiler and vent air from the system. Use water hose and fill up through one of the top nozzles until water overflows from the nozzle. Because the boiler may remain standby after the test, it is important that the water in the boiler is treated to prevent corrosion pitting. Consult your local chemical vendor for the treating method. The metal temperature for HT shall not be less than 60oF (16oC) and not more than 120oF (50oC). This is as per requirement in ASME Code AT-352. I have heard an engineer used boiler makeup water from the feed tank at 90oC for the HT just because he wanted to use the feed pump. Don’t do this! If the temperature of the water is above the recommended temperature (exceeds 50oC), the boiler metal is subjected to brittle fracture. This brittle fracture is a phenomenon where rapid crack propagation occurs when temperature decreases under extreme stress condition. Moreover, there is a direct relationship between pressure and temperature. As the temperature drops, the pressure also drops, so you will notice a pressure drop from the pressure gage despite there is no leakage.
Make sure all air is removed before connecting the pressure hose from the hydrostatic-test pump to the boiler venting line. If the water overflows from the vent, shut off the valve tightly. Do not allow air pockets from forming inside the boiler. Do not use feed pump during HT although it is the fastest way to build up pressure. You have to standby an operator at the pump and he must be alert enough to stop the feed pump immediately once the test pressure is attained. Any negligence can be catastrophic, so why take chances? Hydrostatic-test pump or hand pump is preferred as the pressure and throughput can be easily controlled whereas the pumping rate of feed pump is high and could subject the boiler to overpressure if there is no precaution. Operate the hydrostatic pump and increase the pressure at the rate of 1 bar per minute and look at the gage carefully. Raise the pressure up to 1.5 times the maximum allowable working pressure (MWAP). MAWP is another term for design pressure. If the MAWP of the boiler is 2,000 kPa (20 bars), the required test pressure is 2,000 X 1.5 = 3,000 kPa (30 bars). Stop the pump operation at the test pressure and close the ball cock of the pump.
Wipe dry or spray with compressed air at all flanges and manholes to get rid of water residue. Hold the water under static pressure for 30 minutes. Visually examine all areas for leaks and cracks, giving particular attention to welds. I recommend you to not spend longer because besides exerting extra stress to the pressure parts, there shall be no leakage if there is no pressure drop within 30 minutes. In addition, 30 minutes is long enough to satisfy the code requirement. Some engineers maintained HT for only 15 to 20 minutes, which is OK IMO. If there is a leak, mark the location, snap photos showing water shooting out from the crack (important for documentation), and if no pressure drop, the HT is conducted successfully. Release the water via the bottom blowdown connection. As a standard procedure, after completion of any repair involving welding at pressure parts of the boiler, carry out a HT at 1.5 X MAWP again.
After the HT is successful, fill up the HT report, stating testing conditions, accessories, and procedures. Include pressure increase and holding time data in the report. This record must be kept for future reference.