In Steam Turbine Applications, where we have Main & Stand By Lube Oil Pumps, do we stop the Lube Oil Pumps on Discrepancy (between command & Feedback).

If YES, is it necessary to do that way?

Consider that there was a discrepancy (false discrepancy) on Pump A and STOP command is sent to Pump A and Pump B starts. Due to some reason, if Pump B also fails to start, then we will have a Low Lube Oil Pressure on the header and this would TRIP the Turbine.

 

premkumar,

What is a 'discrepancy?' You mention something about command and feedback? What is the feedback--motor starter status or pump discharge pressure? (Motor starter status is an auxiliary switch on the L.O. Pump motor starter that provides an indication to the control system of whether or not the motor starter is energized or not, the <b><i>implication</b></i> is that if the motor starter is energized that the motor is running--which is not really a positive indication that a pump motor is running. Only pump discharge pressure is positive indication that a pump is running--just because a motor starter is energize and supplying current to a pump motor does NOT mean the pump is actually running and producing sufficient pressure and flow (the coupling between the pump and motor may have come loose or failed) and developing sufficient pressure (there may be a problem with the pump impeller, or the suction is restricted (choked), or there's a problem with a check valve in the pump discharge, etc.).

If the turbine is running and L.O. Pump A (the "main") is running that would imply that L.O. Pump A is providing sufficient pressure (and flow) to keep the turbine running. So, if the motor starter status of L.O. Pump A is indicating the pump is NOT running, the first thing to be done is to send someone to the L.O. skid to see what the L.O. Pump A discharge pressure is, then to the motor control center to check on the motor starter status of L.O. Pump A.

It wouldn't be prudent to start L.O. Pump B (the "standby") just because the motor starter status of L.O. Pump A indicates the pump is not running--because if the turbine is running and L.O. Pump B is not running that implies L.O. Pump A <b>IS</b> running and providing sufficient pressure (and flow).

Without visual verification that L.O. Pump A is running or not (and, again, if the turbine is running then it would certainly seem L.O. Pump A is running if L.O. Pump B is NOT running) it's extremely risky to issue a command to start L.O. Pump B if that action also stops L.O. Pump A. Because if L.O. Pump B does fail to start, then, yes, it's very likely the turbine will trip on low L.O. pressure.

Having said all of the above, it would be VERY remiss of the turbine packager NOT to monitor L.O. Pump discharge pressure for the "lead/lag" operation. In other words, in the case where A was running and an operator commanded B to run, A should not shut down until the discharge pressure from B was sufficient to maintain unit operation.

The HMI display for the L.O. Pumps should also have some indication of L.O. system pressure(s)--if not actual pump pressures (from one or more transmitters), then at least indication from one or more pressure switches that the pressure is either good or low or low-low. Simply operating pumps based on motor starter status is not a good idea--just for the reason you cited. Simply taking action based on display indications of motor starter status is not good practice; someone needs to go out to check and verify what pump is running and what the discharge/header pressures are.

 

premkumar,

Further, if the turbine was running and the discharge pressure from L.O. Pump A (the "main") were to drop below some level, that should automatically trigger L.O. Pump B (the "standby") to start. An operator shouldn't have to take any action to start L.O. Pump B--it should be automatic.

But, if the turbine is running and it's just the motor starter status indication that the main pump isn't running and the standby pump is also NOT running, then that's a pretty good bet there's something wrong the motor starter status indication for the main pump. And the proper response by the operator would be to visually verify the discharge pressure from the main pump before taking any other action. If the main pump is running and producing sufficient pressure and flow, the operator should issue a work order for the I&C- or electrical department to fix the main pump motor starter status indication.

 

Thank you Very much CSA for such detailed explanation. It really helped me understand to depth.

Yes, the feedback that I was mentioning was about the Electrical feedback from MCC and not the feedback from the Discharge Pressure.

Once, again thank you very much.

 

premkumar,

Thanks for the feedback (no pun intended!)! Glad to have been of help.