Hard Seat and Soft Seat Safety Relief Valves
When to use Hard Seat Safety Valves VS. Soft Seat Safety Valves?
Hard Seat Safety Valve
A hard seat safety relief valve is a metal-to-metal seated valve. These typically use a metal disc or ball as the internal sealing surface against the orifice or internal passageway in a safety relief valve.
The ASME Boiler and Pressure Vessel Code does not require a specific seat tightness requirement. A certain level of leakage is allowed per manufacturers’ published literature. Kingston defines seat tightness standards as follows: Factory Standard Seat Tightness Performance: Hard Seat Valves – no audible leakage at 20% below nameplate set.
A hard seat safety relief valve is excellent in corrosive or extremely high temperature applications. It is normal for spring-operated safety valves to exhibit leakage. The simmer/warn appears as the operating pressure approaches the nameplate set pressure. This is typically in the 80%-90% range of nameplate set pressure.
Soft Seat Safety Valve
A soft seat safety relief valve typically uses an elastomer material, such as Silicone, FKM (Viton™), EPDM (Teflon™), Nitrile (Buna-N) or PTFE. The sealing material in the internals of the valve will depend on the compatibility of media flowing through the valve.
The Kingston standard for soft seat valves is: no audible leakage at 10% below nameplate set pressure. At very low set pressures (20 psi and below), the ratio of the downward spring force as compared to the upward pressure force is very small. In these cases, it may be impossible to achieve seat tightness.
Use soft seat valves for superior seat tightness in applications which fall within the soft seat material temperature limitations. Although soft seat valves will typically provide a higher degree of seat tightness than metal seats, Factory Standard does not ensure bubble-tight seats, regardless of seat material.