Safety valve is an important kind of relieving attachment for the boiler, pressure vessel and other pressure equipments. The action reliability and performance of it is directly related to the safety of the equipment and the person and closely related to energy saving and environmental protection. However, some customers and design departments always choose the wrong type when purchasing. So we will analyze the selection of safety valve in this document.

Definition

Generally speaking, safety valve includes Relief Valve. In accordance with the administrative rules, it’s directly installed on the steam boiler or the first class pressure equipments with the necessary condition that it must be the approved valve by the technical supervision department which is called safety valve, and the others usually is called Relief Valve. Safety valve is very similar with Relief Valve on structure and performance. Both of them automatically discharge the inner medium when the pressure in the equipment is above the opening pressure to ensure the safety of the manufacturing plant. People often confuse them in using because of this essential similarity. In addition, some manufactures stipulate on the specification of the manufacturing plant that either kind can be used. So, the difference between them is often ignored, and consequently a lot of problems appear. To distinguish them definitely, we can understand them in accordance with the definition in the first section of the “ASME specification of boiler and pressure equipment”:

 Safety valve is a kind of automatic pressure discharging equipment which is driven by the static pressure of the medium before the valve. It features outstanding full activation. It’s used in the gas or vapor application as illustrated diagram 1.

Relief valve is a kind of automatic pressure discharging equipment which is driven by the static pressure of the medium before the valve which is also called overflow valve. It will be activated in proportion with the increase of the pressure which is over the breakaway force. It’s mainly used in the fluid application as illustrated diagram 2.

Safety relief valve is a kind of automatic pressure discharging equipment which is driven by the pressure of the medium which is also called safety overflow valve. It can be used as either safety valve or relief valve depending on the different situation. Taking the example of Japan, usually, there is no clear definition for safety valve and relief valve. Generally, the safety device used on large energy storage pressure vessel like boiler is called safety valve, and that used on pipe lines and other equipments is called relief valve. However, in accordance with the provision of “technical standard of thermal power generation” of MITI of Japan, safety valves are appointed to use on the major assurance parts of the equipment, which includes the boiler, superheater, reheater etc.. And it’s required to install relief valve or safety valve at the position under the reducing valve where need to be connected to the boiler and turbine. So, we can see that safety valve is more reliable than relief valve. In addition, the valve that ensures the discharge capacity is called safety valve, otherwise, it’s called relief valve according to the confirmation and provision for safe discharge capacity of the regulation of high pressure gas administration regulation of Ministry of Labour of Japan and regulation of Ministry of Transport and the ships association of all grades. Both the full opening and the low lift valves are called safety valve.