What is industry acceptable practice for measuring Control valves stroking speed? Is it upon Positioner/DCS signal failure or Air Failure?

Dear Sirs:

I have one general basic question on the industry acceptable practice for control valves stroking speed requirement. As we know in control valve industry critical service valves such as Anti-surge, Turbine bypass valves specifically require fast stroking speed upon their signal failure / fail safe return condition. Based on my limited knowledge, control valve stroking speed tests shall be performed only under below conditions.

  1. Only upon Positoner / DCS signal failure (Typically <= 2 ~ 3 Sec for Anti-surge valves)
  2. Only upon Solenoid valve trip condition (Typically <= 1 ~ 2 Sec for Anti-surge valves)

How about “only air” supply failure case? If I am not wrong, the measured stroking speed to achieve fail safe state under only air fail case would take little longer time than the direct signal failures as it may involve Air-set spring sense according to Upstream and downstream trip pressure difference.

Does customer really demand same stroking speed requirements upon “only air” failure for anti-surge or turbine by pass applications? If yes, how frequent this air failure case might happen in a plant?

Please bear with my limited knowledge on this topic, kindly clarify my queries. 

Thanks in advance!

Ashokan Giridharan

  • Ashokan,
    It is good question. Antisurge control is critical application, stroking speed is dictated by process conditions and compressor controllers crunching algorithm, decides exact response time required based on various inputs available to controller. It is true that meeting stroking speed is key for critical antisurge applications but along with that accurate control as well to improve performance and availability of loop.

    Air Fail or DCS Signal Fail valve will move valve to desired FAIL SAFE state attaining normal speed based on accessories’ orifice size and air volume in pneumatic schematics of control valves. Generally, redundancy is built in critical controls like Antisurge to avoid such failures (like DCS signal or air) but accidents happens and such condition may arise. If that happens than it is NOT surge condition.

    To achieve requirements of Antisurge valves critical conditions, Digital Valve Controller, Boosters and other accessories are employed in pneumatic schematics of control valve. Sensing of "DCS Electrical Signal" or Air "Pneumatic signal" is done through in built sensors of Digital Valve Controller for pressure, input signal, travel etc. Response time from microprocessor based device is pretty higher compare to conventional spring sensed pressure etc.

    Stroking speed has more meaning when process demand requires valve to move to a desired position quickly and accurately to avoid surge.

    Surge is characterized by fast flow reversals through a compressor and is caused by a large-scale breakdown of flow patterns within the compressor. Such situations should be strictly not allowed.

    Hence the need of digital valve controller for optimized Antisurge Valve, designed with both open- and closed-loop performance in mind. The open-loop response is tied directly to stroking speed while the closed-loop response is directly related to the actual control function of the antisurge controller. Better control yields improved system gains, which equates to faster action and tighter control. This tighter compressor control allows the compressor to operate more efficiently while increasing compressor throughput. For new units, this means that a smaller compressor could be used when using an Optimized Antisurge Valve compared to one that uses a valve selected by traditional means.

    Hope this helps.

  • In reply to Riyaz Ali:

    Dear Riyaz Ali

    Thanks a lot for your detailed explanation!

    Per your explanation my understanding in short:
    Air supply failure is not related to compressor surge condition.Hence applying the same quick stroke requirement under this case is not necessary and its not plant operators expectation.Having said that Control valve action for surge condition MUST be addressed thru their proper DCS/Solenoid signals, hence the required quick stroke MUST be achieved without any lagging/dead time.Adding to that selecting best Fisher pneumatic accessories ,tubing size and best higher DVC tiers (E.g PD,ODV) can help to minimise the dead band,hystyresis & linearity errors to its lowest values and help the valve to operate with its best dynamic performance for this critical service.

    Thanks a lot


    you got it right.