Pressure Relief Valves for Potable Hot Water Heaters PDF

Objective 4 Describe the construction and operation of temperature and pressure relief valves PRESSURE RELIEF VALVES for POTABLE HOT WATER HEATERS Safety relief valves may be installed on commercial and industrial potable water heaters and hot water storage tanks, designed according to ASME BPVC IV...

Objective 4 Describe the construction and operation of temperature and pressure relief valves PRESSURE RELIEF VALVES for POTABLE HOT WATER HEATERS Safety relief valves may be installed on commercial and industrial potable water heaters and hot water storage tanks, designed according to ASME BPVC IV, Part HLW. These heaters and tanks operate at pressures not exceeding 1100 kPa, and temperatures not exceeding 99°C. The heaters are supplied with cold potable water, and are only used to supply hot potable water. Though safety relief valves may be used, most potable water heaters and potable hot water storage tanks use temperature and pressure safety relief valves. These valves protect against the additional explosion dangers associated with super-saturated hot water. ASME BPVC Section IV, Part HLW-800 refers to safety relief valve requirements for industrial and commercial potable water heaters. HLW-800.1 states: Each water heater shall have at least one officially rated temperature and pressure safety relief valve or at least one officially rated safety relief valve… The pressure setting shall be less than or equal to the maximum allowable working pressure of the water heater. However, if any of the other components in the hot water supply system (such as valves, pumps, expansion or storage tanks, or piping) have a lesser working pressure rating than the water heater, the pressure setting for the relief valve(s) shall be based upon the component with the lowest maximum allowable working pressure rating. According to HLW-801, these pressure relief valves must be installed: a) Not lower than 100 mm from the top of the shell. b) Directly to one of the following: • A tapped or flanged opening in the top of the water heater • A fitting connected to the water heater by a short nipple • A y‐base • A valveless header connecting water outlets on the same heater c) With their spindles upright and vertical, with no horizontal connecting pipe, unless connected directly to the water heater vessel, with no more than 100 mm of interconnecting piping. In this case, the valve may be installed in the horizontal position with the outlet pointed down. As well: d) No piping or fitting used to mount the safety relief valve can be of a nominal pipe size smaller than that of the valve inlet. e) Safety relief valves shall not be connected to an internal pipe in the water heater or a cold water feed line connected to the water heater. ASME does not prohibit the installation of relief devices sensitive only to temperature. However, ASME considers these devices to be insufficient for vessel protection. Therefore, if they are installed on hot water heaters or tanks that actuate only on temperature, additional overpressure protection must be provided, in the form of either a safety relief valve or a temperature and pressure relief valve. TEMPERATURE and PRESSURE (T&P) RELIEF VALVES The combination temperature and pressure relief valve provides protection for both overpressurization, and dangerously high water temperatures. The pressure element opens to relieve excess pressure, usually caused by thermal expansion of water during times of low hot water consumption. The temperature element opens when the water temperature rises to about 99°C. This allows hot water to escape from the system before becoming a supersaturated liquid. Cold supply water then enters, and reduces the temperature. The pressure-sensitive element is similar to the construction of a safety relief valve. A nonadjustable compression spring holds a disc shut against a seat. Unlike the safety relief and safety valve, the T&P valve does not operate with a pop action when over-pressurization occurs. The temperature sensitive element consists of a wax-filled thermostatic sensor probe that extends into the water storage tank. When exposed to high temperature, the wax in the thermal element expands against the force of the compression spring, and lifts the valve disc off its seat. This causes the valve to discharge the overheated water. When the temperature returns to a safe level (below 99°C), the wax in the thermal element contracts, and allows the spring to reseat the valve. Figure 12 shows a combination temperature and pressure relief valve. It is recommended that T&P valves receive annual try lever tests. Figure 12 – Combination Temperature and Pressure Relief Valve TEMPERATURE RELIEF DEVICES Various types of temperature relief devices can be used. However, they must always have additional pressure relief. One type of temperature relief device employs a fusible element which melts at 99°C, and allows the hot water to escape. Unlike the T&P valve, such a fusible element does not reseat. The water continues to flow until the device is replaced. Another type of temperature relief device uses the expansion and contraction of a rod and tube arrangement. This opens the device at 99°C, and closes it at 71°C. Like the T&P valve, this device automatically recloses.