Selection of Safety Valves

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ESD/ESV valves are one of the defense in protection layers against process hazards. These valves have a function which requires much more reliable performance than standard remotely operated on-off valves. IEC 61508 is a globally recognized new safety standard specific to process industry. Very strong guidelines are given there to process operators to test and prove the availability and functional safety of the safety loop including the final elements and ESD valves.Part 1 of the IEC 61508 demands certain documentation requirements for the development of safety critical devices. The selection of a specific type of safety valve is governed by several factors:

-    Cost
-    Valve construction
-    Approvals
-    Operating characteristics
-    Type of disposal system
-    Application type (Emergency closing or venting)
-    Pressure:  Shut-off pressure, operating pressure
-    Temperature: Soft parts, body material, coatings, seat construction bearings, high temp. extension, low temp./cryogenic
-    Fluid: Corrosive, solids, toxic, explosive (O2), Viscosity, errosion, corrosion, solidifying, coke build-up
-    Material: CF8M, WCB, LCC, C5, note temperature, chemical resistancy, NACE
-    Valve type:  Ball valve reduced bore or full bore, butterfly valve, fail open or close, tightness, operating speed, floating ortrunnion
-    Actuator type: Spring return or double acting, operating speed, handwheel, jammer, seel cylinder, fre box
-    Fire safety: API 607, BS6755
-    Accessories: SOV, LS, Booster, QEV, PPS, VT
-    Area classification:  EExi, EExd, EExn, FM, CSA, Cenelec
-    Power supply:  Supply pressure, SV voltage

In order to establish the set pressure correctly, the following terms require careful consideration:
-    Normal working pressure (NWP): The operating pressure of the system under full-load conditions.
-    Maximum allowable working pressure (MAWP):  Sometimes called the safe working pressure
-    (SWP) or design pressure of the system: This is the maximum pressure existing at normal operating conditions (relative to the maximum operating temperature) of the system.
-    Maximum allowable accumulation pressure (MAAP): The maximum pressure the system is allowed to reach in accordance with the specification of the design standards of the system. The MAAP is often expressed as a percentage of the MAWP.
-    Set Pressure (PS):  The pressure at which the safety valve starts to lift.
-    Relieving pressure (PR): This is the pressure at which the full capacity of the safety valve is achieved. It is the sum of the set pressure (Ps) and the overpressure (Po).
-    Overpressure (PO): The overpressure is the percentage of the set pressure at which the safety valve is designed to operate.


There are two fundamental constraints, which must be taken into account when establishing a safety valve set pressure:
1. The set pressure must be low enough to ensure that the relieving pressure never exceeds the maximum allowable accumulation pressure (MAAP) of the system.
2. The set pressure must be high enough to ensure that there is sufficient margin above the normal working pressure (NWP) to allow the safety valve to close. However, the set pressure must never be greater than the maximum allowable working pressure (MAWP).In order to meet the first constraint, it is necessary to consider the relative magnitudes of the percentage overpressure and the percentage MAAP (expressed as a percentage of the MAWP).

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