Specifications for the overpressure hydrostatic test pressure of boilers

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Provisions for the overpressure hydrostatic test pressure 1. Preparatory work before the hydrostatic test (1) Remove scale and soot. In particular, the scale must be removed thoroughly. When removing scale, be careful not to drop tools, gloves, cotton yarn, etc., into the boiler, to avoid blocking the furnace tubes or drain holes. (2) Remove the safety valve, block its outlet with a plug of sufficient thickness, and reinstall all other accessories. A vent valve should be installed on the furnace top; the main steam valve or safety valve can also be used to release air. (3) Remove some of the coverings on the boiler itself, such as the asbestos insulation on vertical boilers, the heat-absorbing materials on the top of Lancashire boiler shells, and the heat-absorbing materials beneath the upper and lower drum assemblies in double-drum boilers. (4) Install the pump. In areas without tap water, an electric pump or steam pump can be used to fill the pot with water. But it cannot be used during voltage boosting. Due to their large water intake volume and high pressure, these pumps are difficult to control, and overpressure can occur easily if not handled carefully. Therefore, only a hand pump can be used for pressurization, and a pressure gauge must be installed at the outlet of the hand pump to indicate the pressure at the pump’s outlet. (5) The water temperature should generally be maintained between 20~70°C. When the water temperature is too low, sweating tends to occur ； If the water temperature is too high, the water that leaks out will evaporate. The temperature of the air around must be no lower than 5°C; if it drops below 5°C, anti-freezing measures must be taken. After completing the above preparatory work, the over-pressure test can be carried out. During testing, the water pressure should be increased gradually. Once the pressure reaches the operating level, increase it no further and check all parts of the boiler for any leaks or abnormalities ； If there are no defects, the pressure can be increased further to the test pressure. The test pressure shall be based on the reading indicated by the boiler’s pressure gauge. After reaching the test pressure, the welded boiler must be held at that pressure for 5 minutes ； For riveted boilers, the pressure must be maintained for 20 minutes. During the aforementioned period, the test pressure must not drop; if it does, the cause must be identified. After the test pressure is maintained for the specified time, it is reduced to the operating pressure, followed by a thorough inspection. 2. Points to note during the hydrostatic test: (1) It is best to conduct the overpressure hydrostatic test on the boiler during the day. (2) The pressure increase or decrease must be gradual, with the pointer of the pressure gauge moving smoothly and evenly. (3) When conducting boiler inspections, the inspection staff should prepare a safety inspection checklist to avoid missing any items. (4) During inspection, no one shall enter the furnace for inspection until the test pressure has dropped to the operating pressure ； When inspecting welds or rivets, a testing hammer can be used to gently tap on both sides of them; it is not allowed to strike the welds and rivets forcefully. (5) Repair under pressure is not permitted. 3. After undergoing a superpressure hydrostatic test, the boiler must meet the following criteria to be considered qualified: (1) There should be no water droplets or mist on the metal walls of the pressure-bearing components or on their welds. (2) No leakage occurs after the pressure is raised to the operating pressure at the rivet joints and flared areas. (3) After the overpressure test, visual inspection revealed no residual deformation. It should be noted that pressure testing must be carried out using water pressure, and never air pressure. Since the test pressure during a boiler’s pressure resistance test is higher than the normal maximum operating pressure, the boiler may explode during such a test. If an explosion occurs, a container filled with water will have much less explosive power than one filled with gas. Under the same conditions of volume and pressure, the explosive energy of a gas is hundreds to tens of thousands of times greater than that of water (depending on the pressure; the lower the pressure, the greater this multiplier for explosive energy). Therefore, when using water pressure for a pressure resistance test, even if the boiler bursts under the test pressure, casualties generally do not occur. And if air pressure is used in place of water pressure, it will cause severe casualties.