PERMISSIVES - Looking for good examples

When explaining permissives (DC Function block) I always struggle to come up with good real world examples.  Anyone have any good examples of where an interlock wouldn't work and you had to use a permissive? 

Thanks,

 Travis

7 Replies

  • Hi Travis,

    In my experience there are situations where a motor must be started under "no load" conditions (to avoid large, sustained inrush currents on induction motors for example) so a permissive signal is needed confirming that the motor is under a no load condition.  An air compressor, for example, may require valves to be open to atmosphere until the motor is up to speed.  Once the motor has achieved operating speed the valves are closed so the air compressor can perform its function.

    Similarly, the press section of a paper machine (paper is fed between two long rotating cylinders that are pressed together) may require the rolls(long cylinders) to be "unloaded" when the motor is started but, under normal operating conditions, these rolls are forced together under a specific pressure.

    It may be a stretch but I sometimes use the example of an automatic transmission.  The car's brake must be applied to take the transmission out of "Park" but, once out of park, different gears can be selected without the brake.

    Hope that helps.

    Glenn

  • In reply to GPellegrin:

    Nice. Thanks Glenn.

    -Travis

  • Travis,

    I have used permissives on valves associated with totalizers. If the totalizer is complete don't allow it to open but it is not an interlock to close the valve.

    There are occasional uses but they are rare for sure.

    Regards,
    Matt
  • Hi Travis,

    Some examples where I use permissives rather than interlocks include:

    * Requiring a minimum level in a pump box before starting the pump.  I'll use a permissive rather than an interlock if I want to ensure adequate level in the pump box to start up and run the circuit but don't want to trip the pump on low level.  

    * Large conveyors and extreme cold weather don't do well together.  I've used a permissive for conveyor belt protection based on a curve defined by ambient temperature and time since the conveyor was last run.  At -40 the conveyor could only be started if it had been off less than an hour.  At -30(F) we could restart after 4 hours, and at -25 we could restart anytime.

    * A large motor may be self lubricating when running but needs a lift pump running prior to start.  The lift pump is set to stop after the motor successfully starts.

    * Motor protection in drives has lots of good permissive examples.  Simple drives may limit start attempts to X starts in a period of time.  More capable drives may have a thermal profile calculation and not allow a start attempt when thermal capacity is above 75% and each start may use up to 25%.

    * Requiring a pump to be started with a closed discharge valve.  I'd also commonly use that discharge valve as an interlock if it closes during pump operation (bypassed during pump start).

    Cheers,

    Kevin

  • In reply to fitzgerrell:

    Another place where I think of permissives rather than interlocks is when I can only have one active path.  I set up permissives that basically look at each other's module.  

    Take Matt's example about totalizers - If I have a totalizer on a line going to two tanks, I probably want to have a permissive on the valve at each tank so I can only send material to one tank at a time.  

    A double block and bleed valve arrangement is another example.  I can only open my bleed valve if my two block valves are closed.  And I can only open a block valve if the bleed valve is closed.

    No need for interlocks if permissives never allow the valves to be in an improper position in the first place.

    I hope this helps,

    Gareld

    Gareld Butler

  • In reply to Gareld Butler:

    Thanks Everyone!  

    Kevin's lift pump is my favorite so far.  

  • In reply to GPellegrin:

    Interesting.
    Thanks
    - Vitthal