We all know that different control valve style will have different relative shutoff capability
such as Reg-Sliding stem, bar stock, partial ball all have excellent shut off capability.
And choosing appropriate shut off pressure of control valve to assure leak tight shut off is important.
I would like to ask a very simple and mundane question. Please bear with me. In comparing with system design pressure and control valve shut off pressure, why do we always make control valve shut off pressure or 1.5 times control valve shutoff pressure smaller than system design pressure?

The criteria for actuator selection varies among companies.

Control valves are designed for throttling.  Not all control valves are designed for positive shutoff.  You can buy control valves with FCI 70-2 or IEC Class V or Class VI.  These are tight shutoff valves.  Typically line class valves comply with API 598 for seat leakage.

The actuator force helps to shutoff the flow in most control valves.  I have never seen the specification of 1.5 times anything.  Depending upon the client standard I either specify the design pressure (the more liberal requirement) or the maximum ASME flange class pressure (perhaps at 100 degree F) as the more conservative value.

The actuators are available in specific sizes.  We buy the actuator that meets the required force at the lowest air pressure.  The selection nearly always is a bit larger than required.  The available air pressure is usually greater than the minimum specified.

If the actuator is excessively large the valve positioner may work against lots of overshoot.  Also the speed could be too fast risking "water hammer" in liquid service.  Selecting an actuator larger than required is also money not well spent although usually not an issue.  The weight and space could be an issue.

Any control valve as indicated by JLSEAGULL is throttling purpose, and control valve shall be design following ASME 16.34, 31.3, ISA 75, IEC 60534, FCI 70-2, these are the main standard that every oil and gas company in the world follow to design control valves.

The 1.5 factor that you are talking about, maybe is related to trim selection, due there are three main trim types (plus modifications to these ones): Quick Opening, Equal Percentage and Lineal, the difference among then is because a) Lineal trim produce an similar flow performance since the plug start opening, b) EQ% shape have a smooth performance, so this one is the first choice, due loop tuning at DCS are more accurate and easy to set.
Depending on trim type, the influence on the CV is affected.

So, some oil companies are ruled for the following:

The sizing Cv for equal percentage characteristic valves (specified in data sheets) shall be based on 1.8 times the normal flow calculated Cv or 1.2 times the maximum flow calculated Cv, whichever is the greatest.

- The sizing Cv for linear characteristic valves shall be based on 1.5 times the normal flow calculated Cv or 1.1 times the maximum flow calculated Cv, whichever is greater.


But returning to Shutoff pressure, we usually select the actuator based on the minimum actuator capable to handle the high inlet pressure that the control would received.

These factor helps the instrument guy in the follow
1.- Avoid vendor to sell you a actuator that is oversized (cost, civil, pds, pipping issues)
2.- Control Valves vendor have usually actuator size standard, but if required, the can produce an different one, not recommended!
3.- The actuator function is to Close or Open the control valve, if the vendor confirm that the actuator is suitable to handle the higher inlet pressure,ok, you have to find a actuator that match that pressure value or even better is higher that than.

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