Centrifugal Pump - Auto Operation - Sequence

[NickParker]

I would like to ask, how the centrifugal pump operation is done through PLC in Auto - sequence mode in general.

Starting of Pump in Auto Sequence:
1) Whenever the pump has to be operated, the discharge valve needs to be in "shut" or "close" position.
2) After the Motor/pump has been started/accelerated to full speed, the discharge valve should start opening slowly to "open" position.
3) What happens if the discharge valve got struck in the middle position?
4) There is a timer which starts after the motor/pump has been given a start command. If the discharge valve got struck, the timer would get run out and stops the motor regardless of the discharge valve position.

Stopping of Pump in Auto Sequence:
1) First the discharge valve should be shut to give "Close" signal.
2) Again What happens if the discharge valve got struck in the middle position?
3) There is a timer which starts after the motor/pump has been given a stop command. If the discharge valve got struck, the timer would get run out and stops the motor regardless of discharge valve position.
I'm concerned because if the discharge valve got struck in the middle, the motor would be stopped/ started frequently which would affect the motor's life. Also how the discharge valve "Open torque switch" & "Close torque switch" be put into use in the ladder programming.
Thank you for your insights!

 

[crshears]

Your description would seem to apply to larger scale pumps, as starting against a closed discharge valve certainly adds complications to the mix...

I'd want to use a quite robust valve actuator that would hold up to such repeated operation, with the specs of this embedded right into the request for quotes.

Depending on the type of valve used, the actuator would also have to develop sufficient torque to repeatedly and reliably break the valve from its seat when fully closed [I have a gate valve in view], as the dynamic friction loading would be considerable. I'd go even further and suggest that the starting sequence could be: [1] Demand for pump operation received. [2] Discharge valve operates from fully shut to cracked open. [3] Pump starts. [4] Discharge valve opens wide.

Shut down would occur in reverse sequence.

This would harmonize with the way I remember condensate extraction pumps being operated back in my Auxiliary Plant Operator days, viz., there were three such pumps, and when the unit was in operation all three pump discharge valves were wide open so that on failure of one pump the standby one could immediately be placed in service [two were required in operation when the unit was at full load]. When the unit came off line and was shut down, however, one of the discharge valves would be closed down to the cracked position, such that when a boiler top-up became necessary, the throttled pump would be started first to gently prime and pressurize the line from the pumps up the seventy feet or so to the deaerator inlet valve, at which point one of the other two pumps would be started and the one with the cracked open discharge valve shut back down.

I'd want to specify that as well as the types of control sequence you describe, you would want to specify watchdog features that monitor the movements of the valve and the operation of the motor, such that failure of design actions to complete within the allotted time would annunciate in the control centre as " PUMP/VALVE MISOPERATION".

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[LittleInch]

Well it depends what type of valve.

If it's a ball valve or gate valve, 50% open would give you a relatively low pressure drop.

However you would normally end up with a pump start sequence fail alarm which usually trips the pump if you don't hit the limit switches on the open valve.

But it's really very rare. I think you're over thinking this if you believe it happens "frequently"

Actuators are normally sized for full differential pressure when opening and the worst is at the first part of opening / last part of closure. So half way is rare.

It usually either doesn't open at all or gets to 90% then stalls.

Starting pumps against closed valve is common both to reduce initial power surge and stop the pump going off the end of the curve before the back pressure has climbed to give the pump something to pump against.

Some pumps sit on the Non return valve, but they often leak and result in wasted energy and flow going around in circles. Isolation valves are often a lot better.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[itsmoked]

You may have left out two important items.

1) You must limit the number of starts per period of big motors and prevent restarts under the required rest period.

2) With regards to #1 above you must handle power failures. The system must not ignorantly restart the motor(s) after system bootup as it my violate #1 above.

Keith Cress
kcress -

http://www.flaminsystems.com