We have a 60MW GE steam turbine. We are facing sudden pressure dip in control oil pressure from 180 bar to 140 bar for a very short time and standby pump starts on Auto CutIn.
We have separate Control Oil System independent of Lubrication Oil of Turbine.
We have checked the PRVs / PSVs / NRVs and Nitrogen bladder pressures that are OK. The pressure drop issue emerges multiple times for two three days and then vanishes without any corrective action. Again emerges after couple of weeks.
Is it always the same pump which experiences the drop in pressure?
Have you checked the pump? Does the pump have a pressure compensator? Could it be failing?
Have you checked the pump to motor coupling?
When this problem occurs is there a large change in multiple valve positions?
The type of hydraulic accumulators typically used by GE have a very special type of charging hose assembly, that if not attached correctly can actually cause a loss of accumulator charge and a false reading of accumulator pressure. Are you certain the charging hose assembly is being used correctly?
Please write back with more details and to let us know how you fare in resolving the problem.
Don't forget to check the pump motor itself, and the motor starter. It's not likely that it's the turbine control system or the DCS/BOP control system controlling the pump motor, unless there are Diagnostic Alarms indicating a problem with the auxiliary control outputs to the motor starter, or the inputs to the lead/lag circuit.
>Is it always the same pump which experiences the drop in pressure?
Response: No it is not always the same pump, we experience the same drop in pressure for both Main & Stand By pumps. Even the pressure drop observed when both pumps were in operation
>Have you checked the pump? Does the pump have a pressure
>compensator? Could it be failing?
Response: We have checked pump shutoff pressure by closing isolation valve, and pump fluid was recycled by its PRV, and we found ok. Yes pump have compensator but we are waiting for available opportunity to check pump compensator.
>Have you checked the pump to motor coupling?
Response: Yes we have inspected coupling and its flexible spider and found ok.
>When this problem occurs is there a large change in multiple
Response: This problem usually occur when machine is running at its normal load (52MW), and there is no major change in multiple valves. Also no variation observed in steam inlet pressure & flow.
>The type of hydraulic accumulators typically used by GE have
>a very special type of charging hose assembly, that if not
>attached correctly can actually cause a loss of accumulator
>charge and a false reading of accumulator pressure. Are you
>certain the charging hose assembly is being used correctly?
Response: We understand that you are asking about kit for nitrogen filling in the bladder of accumulator. We have manifold and gauges supplied by GE that is installed at nozzle of bladder for nitrogen filling and pressure verification. So far we have checked bladder pressure both with Bottle fully drained from oil and with oil filled conditions. In fully drained oil condition the bladder pressure is ok (119 Bar).
If it's not always the same pump, then I would be looking for something that is common to both pumps. And I would presume that would include the control system that is turning the pumps on and off. It could be that the discrete outputs for the two pumps (be they solid-state relays or electromechanical relays) are experiencing some momentary problems which are causing the standby (lag) pump to start.
OR, there is some problem with the input channel to the control system that is sensing the hydraulic system pressure and is detecting a momentary drop in pressure that isn't really there that's causing the lag pump to start.
If the input is a pressure transmitter, a single pressure transmitter, then that could be the problem--an intermittent problem with the pressure transmitter. If it's a pressure switch, it's not really likely that the switch is failing intermittently--but it is a possibility as stranger things have happened. It could be a loose wire/termination in the circuit between the sensor (transmitter or switch) and the control system.
It's not likely that it's air in the system, because most GE hydraulic manifolds have some kind of air-bleed check valve in the pump outlet to remove the air. However, if the tank level is low, then that could be a problem (air in the line that can't be all removed by the air-bleed check valve).
Without being able to see the P&ID for the hydraulic system, and without knowing more about the control system that is starting-stopping the pumps, that's about all I can think of.
Please write back to let us know how you fare in resolving this issue! A lot of people read these threads, not just now, but in the future also, and feedback is VERY helpful and important.
>If it's not always the same pump, then I would be looking
>for something that is common to both pumps. And I would
>presume that would include the control system that is
>turning the pumps on and off. It could be that the discrete
>outputs for the two pumps (be they solid-state relays or
>electromechanical relays) are experiencing some momentary
>problems which are causing the standby (lag) pump to start.
Response: Solid state programmable relays are installed. Please also note that the relays are Latch type and can only be manually reset.
>OR, there is some problem with the input channel to the
>control system that is sensing the hydraulic system pressure
>and is detecting a momentary drop in pressure that isn't
>really there that's causing the lag pump to start.
Response: The pressure drop is actual as we have witnessed on local pressure gauge as well.
>If the input is a pressure transmitter, a single pressure
>transmitter, then that could be the problem--an intermittent
>problem with the pressure transmitter. If it's a pressure
>switch, it's not really likely that the switch is failing
>intermittently--but it is a possibility as stranger things
>have happened. It could be a loose wire/termination in the
>circuit between the sensor (transmitter or switch) and the
Response: We have checked and verified the transmitter and its circuit. In addition to this whenever there is pressure drop it resulted in severe hunting in Governor Vale opening. We believe it is more clear a sign of physical pressure drop than mere malfunction of a transmitter. Same drop in pressure has also been verified from local pressure gauges. Also governor hunting observation diminishes whenever the accumulators were isolated although drop in HP oil pressure still observed.
>It's not likely that it's air in the system, because most GE
>hydraulic manifolds have some kind of air-bleed check valve
>in the pump outlet to remove the air. However, if the tank
>level is low, then that could be a problem (air in the line
>that can't be all removed by the air-bleed check valve).
Response: Yes the air bleeder is available. Moreover tank level has been checked with Dip Stick to double check and its level is full as per defined level. Over and above we have cleaned air breather as well just to eliminate any chance of fumes accumulation in the oil tank.
We had a similar problem years ago with a GE boiler feed pump turbine where the standby oil pump auto started on low pressure. We hooked up a high speed recorder and were able to trace this infrequent event to be coincided with the speed probe missing just one pulse. The missing pulse caused the calculated speed to be momentarily substantially less and this caused the valves to quickly open and close causing a pressure dip. We changed out the speed probe and the problem went away and has never come back.
You can often troubleshoot a difficult problem such as this by hooking up a high speed recorder with inputs to monitor amps, voltage, speed, pressure, position, etc. There is quite a selection of high speed pressure transmitters (1 mSec response), LVDT's etc that can be purchased fairly cheap. High speed recorders can be rented if needed and once management sees the value in such equipment they will be more willing to purchase the needed equipment.
There is no doubt the pressure drop is real; it's the cause that is in doubt.
I'm not familiar with "latched solid-state relays;" I will have to do some investigating on those. Can you provide a manufacturer's name and model number?
In my personal opinion based on my personal experience, hydraulic accumulators are used to prevent or reduce the effects of a sudden increase in flow or decrease in pressure. However, it could be that the pressure drop is so severe that the accumulator can't compensate for the drop, and that results in governor hunting until the standby pump starts and can make up for the drop in pressure/increase in flow.
I don't understand how isolating the accumulator can reduce the effects of the pressure drop/governor hunting--unless the accumulator is not properly charged. Again, charging and checking the charge on the types of hydraulic accumulators most often used on GE-design hydraulic systems requires patience and an understanding of the valves/valving and charging hose assembly. Unfortunately, we can't know how the charge has been checked; we have to rely on what we are told. But, again--it's hard for me to understand how isolating the accumulator could improve the situation, unless there is something amiss with the valve positions and/or the accumulator is not functioning properly.
Troubleshooting sometimes is a long, methodical process of elimination. And, that process requires a patient and planned and thorough checkout of the system and its components. I cannot tell you how many times I and my colleagues have been told, "We checked that; it's not the problem." And, so we go on about checking everything else, and guess what: We end up back at the component that was supposedly checked out and find it to be the root cause of the problem(s) being experienced. And it does NO GOOD to ask how the component was checked and what the results were--because nothing was written down during the initial investigation, and assumptions were made about how the component worked and how it could be tested and what the test results should be. And, the "test results" were never questioned or discussed.
I did see one hydraulic filter assembly that had a bypass valve that would operate to relieve pressure if the filter(s) became excessively dirty ("choked") to prevent the filter(s) from rupturing and releasing the dirt back into the hydraulic system. This assembly would somehow also slightly relieve the filter differential pressure (probably by allowing some dirt to be washed away through the relief valve), and when the differential dropped low enough the relief valve would close. The differential pressure gauge was found to be not tubed properly, so it was not registering the actual differential pressure, and the high differential pressure switch was in parallel with the gauge, so it, too, was not sensing the actual differential pressure. This was causing intermittent control valve instability which was improperly being blamed on--what else, but the Mark* turbine control system. It took a couple of days of gathering data and reading manuals and looking at tubing runs and all the time the Customer's personnel kept saying, "The problem is that Mark* control system!" When, in fact, was never the Mark* control system, and the site had never questioned the fact that the unit had been in service for several years and yet no one had ever changed the hydraulic filter elements, and the hydraulic filter differential pressure had never been seen to be above zero.
I've also been to site where the filter differential pressure had been logged to be in excess of the high differential pressure alarm setpoint, and the alarm was annunciated on the Mark* HMI, and then, all of a sudden the differential pressure went to zero, and the site believed that the system was fine. That is, until the hydraulic control valves started having servo problems, and upon investigation it was found that the hydraulic filters had ruptured, which had caused the differential pressure to go to zero--and released a LOT of dirt and debris into the hydraulic system, which was causing problems with the servo-valves and valve instability.
So, the lesson is: When a root cause can't be found by quick investigation and it's necessary to start going through the system component by component that it is also necessary to thoroughly research and understand each component and how it works so that it can be properly tested and eliminated as the cause of the problem, or it can be identified as the cause of the problem and replaced, re-adjusted, or repaired. And only by a thorough process of elimination can these problems be identified and resolved. And the process requires a written plan for each component, and a thorough review of the results of the test by a group of people to determine if the test was sufficient to prove or disprove the component to be the cause of the problem(s) being experienced.
A LOT of us are VERY CURIOUS to know what you ultimately find out. It would also be helpful to know what control system is being used to control the hydraulic pumps and monitor hydraulic pressure.