Since my area of expertise is steam boilers, I will not talk about hydrostatic testing (HT) on pressure vessels without fire or electrical pipes.
Hydrostatic testing is a form of NDT (non-destructive testing) and is the most preferred method of detecting leaks and cracks in the boiler under water pressure. Other NDTs, such as the Magnetic Particle Test (MPT) or Penetration Test (PT), are typically performed to confirm the extent of damage after the leak point has been discovered. Water pressure is only allowed for HT. No steam or air allowed. “Hydro” itself means water. Water for all practical purposes is incompressible, and the pressure developed by the hydrostatic pump can be instantly relieved by releasing a very small amount of water, which would happen if a leak occurred. Air is compressible and can be dangerous under pressure. The same goes for steam. In steamy conditions, any structural failure would cause the water in the boiler to turn to steam or steam leaks, exposing personnel to a burn hazard.
In Malaysia, the local jurisdiction, the Factory and Machinery Act of 1970, stipulates that a boiler must be hydrostatically tested every 7 years in which the authorized inspector will issue a Certificate of Hydrostatic Test in the sixth Program, indicating the date, pressure test, waiting 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 a different interval.
In preparation for HT, clean the fire and water side of the boiler, nozzle flanges, inspection well surfaces, and weld joints. Observe all safety precautions (procedures can be obtained in the e-book). The safety valve must be removed. However, if the safety valves cannot be removed for some reason, a test jaw can be used following the safety valve manufacturer’s guidelines. Remove electrical connections such as high pressure limit switches and sensors. Note that when HT is applied to a boiler, a calibrated test gauge must be connected to the hydrostatic pump. This precision test gauge should be 0.5% accurate. Check with the calibrated pressure gauge mounted on the boiler. Plug nozzles by installing blind flanges or blanking plates at the connection of accessories, such as safety valves, main shut-off valves, feed nozzles, manifolds, and gauge glass. Rubber or normal gaskets will work fine.
Fill with treated water (at room temperature of 26oC) for the entire volume of the boiler and ventilate the air in the system. Use a water hose and fill through one of the upper nozzles until the water overflows from the nozzle. Because the boiler can remain on hold after the test, it is important that the boiler water is treated to prevent corrosion pitting. Consult your local chemical supplier for the treatment method. The metal temperature for HT should not be lower than 60 ° F (16 ° C) and not higher than 120 ° F (50 ° C). This is per the ASME AT-352 code requirement. I heard that an engineer used 90 ° C feed tank boiler make-up water for the HT just because he wanted to use the feed pump. Do not do this! If the water temperature is above the recommended temperature (above 50oC), the metal in the boiler undergoes a brittle fracture. This brittle fracture is a phenomenon in which a rapid propagation of cracks occurs when the temperature decreases under conditions of extreme stress. Furthermore, there is a direct relationship between pressure and temperature. As the temperature drops, the pressure drops as well, so you will notice a pressure drop from the gauge even though there are no leaks.
Be sure to remove all air before connecting the hydrostatic test pump pressure hose to the boiler vent line. If water overflows from the vent, close the valve tightly. Do not allow air pockets to form inside the boiler. Do not use the feed pump during HT, although it is the fastest way to build pressure. You must wait for an operator at the pump and must be alert enough to stop the feed pump immediately once the test pressure is reached. Any negligence can be catastrophic, so why take the risk? A hydrostatic test pump or hand pump is preferred as pressure and performance can be easily controlled, while feed pump pumping speed is high and could put the boiler into overpressure if precautions are not taken . Operate the hydrostatic pump and increase the pressure at the rate of 1 bar per minute and watch the pressure gauge carefully. Increase the pressure to 1.5 times the Maximum Allowable Working Pressure (MWAP). MAWP is another term for design pressure. If the MAWP of the boiler is 2000 kPa (20 bar), the required test pressure is 2000 X 1.5 = 3000 kPa (30 bar). Stop the pump operation at the test pressure and close the ball valve on the pump.
Blow dry or blast all flanges and inspection wells with compressed air to remove residual water. Keep the water under static pressure for 30 minutes. Visually examine all areas for leaks and cracks, paying particular attention to welds. I recommend that you do not waste any more time because in addition to putting extra stress on the pressure parts, there will be no leakage if there is no pressure drop within 30 minutes. Also, 30 minutes is enough to satisfy the code requirement. Some engineers held HT for only 15-20 minutes, which is fine in my opinion. If there is a leak, mark the location, take photos showing the water coming out of the crack (important for documentation), and if there is no pressure drop, the HT is carried out successfully. Release the water through the lower drain connection. As a standard procedure, after completing any repair involving welding on boiler pressure parts, redo an HT at 1.5 X MAWP.
Once the HT is successful, complete the HT report, outlining test conditions, accessories, and procedures. Include pressure build-up and retention time data in the report. This record should be kept for future reference.