As mentioned Valve which are intended for use for shutdown in case of emergency or normal operation of maintenance are called Isolation/Shutdown valve.
They are also sometimes referred as Actuated On/Off valves as the whole intend behind it use is to shutdown or open the process
Valve Seat Leakage
Since these valves are TSO( Tight shut off) valves so for testing seat leakage recommended standard used for the same is API598, If not defined then should be generically Class VI
Valve body is generally of same size as of the line. The end connection flange rating shall be ASME/ANSI B16.5 for lines upto 24 " and ASME/ANSI B16.47 for sizes above 24 ".
Recommended Standard for Face to Face dimension shall be ISO 5752/ANSI B16.10/ API 609
Things to check for while working on Isolation/Shutoff Valves:-
1) The most important care which needs to be taken care for selecting isolation or shutdown valve is timing for shutdown
General thumb rule is shutdown time should be half of PST ( Process safety time)
Process Safety Time is time available between the ignition or start of event and Hazard to occur.
Generally Isolation Valve shut down is measured as seconds/per inch of valve
2) Generally Valves identified with SIL rating are advised to have Partial stroke testing as identified in SIL report
Partial Stroke Testing is the process which valves are partially stroked at predetermined interval to test and certify that these valves will be available in case emergency need arises and wont fail to operate.
Partial stroke testing is generally done from DCS/Automation System and limits switches or positioner feed back are recorded to test functioning of the valve,
3) Fireproofing is done for actuator with the jacketed material which helps to prevent non functioning of the valves in case of fire. The recommended standard for Fire proofing is API607
4) Special Fire resistant cable s are selected for solenoid valves on the actuator of the Isolation valves so the valve is able to operate in case of cable fire.
5) Handwheel Operation overide switch is provided so that in case of emergency the valve could be operated with Handwheel
6) Generally the valve provided at inlet of the pump suction are isolation valves which have remote operation capabilities and can also be operated from field from some distance from valves. (around 20-25 meters)
7) These valves are same size as of line size
a) Ball valve for 6" or smaller.
b) Butterfly valve for 6" or greter
Butterfly valve is triple offset valve as it has inherent Tight shut off capabilities (TSO)
and hard seating material
8) Ball valves are used for bidirectional flows and
Generally for Fail last operation Double acting valves are selected and for Fail close or Fail open Single acting valve are selected.