Pressure Relief Valves for Hot Water Heaters

Safety Relief Valves in Potable Water Heaters and Hot Water Storage Tanks

• Combination temperature and pressure safety relief valves are typically used in potable water heaters and hot water storage tanks.
• These heaters and tanks operate at a maximum pressure of 150 psig (pounds per square inch gauge).
• According to ASME BPVC IV, Part HLW-800.1, each water heater should have at least one officially rated temperature and pressure safety relief valve.
• The maximum allowable working pressure of the water heater is 150 psig according to ASME BPVC Section IV, Part HLW-800.1.
• The heaters and tanks do not exceed temperatures of 99°C (210°F).

Purpose and Function of Safety Relief Valves

• The purpose of using temperature and pressure safety relief valves is to prevent overheating and overpressure in potable water heaters and hot water storage tanks.
• A combination temperature and pressure relief valve combines the functions of a temperature relief valve and a pressure relief valve.
• The temperature sensitive element of a combination temperature and pressure relief valve opens to relieve excess pressure when the temperature reaches a certain level.

Temperature and Pressure Relief Valves

• A fusible element temperature relief device melts at 99°C (210°F), allowing hot water to escape from the system.
• When the water temperature rises to about 99°C in a hot water system with a temperature and pressure relief valve, the valve opens to relieve excess pressure.
• The purpose of a safety relief valve on hot water heaters or tanks is to protect the vessel from excessive pressure and temperature.

Additional Requirements and Functions

• If relief devices sensitive only to temperature are installed on hot water heaters or tanks, an additional pressure-relieving device is required.
• In a combination temperature and pressure relief valve, a spring-loaded pressure element opens to relieve excess pressure.
• The function of the safety relief valve is to prevent damage to the vessel and ensure safe operation.
• To make relief devices sensitive only to temperature sufficient for vessel protection according to ASME, additional pressure-relieving devices are required.
• A device employing a fusible element which melts at 99°C, allowing hot water to escape from the system, is a temperature relief valve.
• A device that opens at 99°C and closes at 71°C, allowing hot water to escape from the system, is a temperature-sensitive relief valve.