LESER Type 821/810 Pilot Operated Safety Relief Valve

Applications – Functional areas
Across applications, there are four main functional requirements covered by the LESER Pilot Operated Safety Valve (POSV).

High Back Pressure Applications

• LESER POSVs can be operated in applications with a back pressure ratio (i.e. a ratio of back pressure/set pressure) of up to 70%. Spring loaded safety valves can be typically used up to 50% of back pressure.
• The absolute maximum back pressure is determined by the pressure class of the main valve outlet. Typically, LESER POSVs can be used for much higher back pressures than spring loaded safety valves.

Applications Requiring Set Pressure Independence of Back Pressure

The LESER POSVs open and operate independently of back pressure (within back pressure operating limits, see previous). The set pressure of the POSV is not affected by back pressure of any kind, i.e. superimposed, constant or variable.

Applications with High Inlet Pressure Losses (above 3%)

In these applications, POSVs with remote sensing should be utilized (refer to API 520 Part 2).

Applications with Increased Tightness Requirements

Since closing forces increase when approaching set pressure, LESER POSVs are particularly suitable for applications with high tightness requirements. Tightness is ensured up to 97% of set pressure because the closing forces increase approaching set pressure. Together with the defined blowdown, this allows operating the sytem close to the set pressure of the valve.

In a POSV, the system pressure acts on the main valve piston trying to push it open. It is, however, opposed by the same pressure because system pressure is also re-directed to the dome area above the piston.

Since the area of the piston exposed to pressure is larger in the dome than on the system side, this creates a greater net closing force on the main valve disc/nozzle. Approaching set pressure, closing forces increase