Reduction in RPM During Normal Operation of Steam Turbine Driving a Pump

Contact the field services office of the turbine manufacturer immediately!!!!!

I'd give you the website if you had bothered to provide any useful information.

Awaiting your reply, and the resolution. WPF.

From Wikipedia

Check your feedwater.

Reduce the flowrate through the pump.

Yes, at least for study purpose.

Reduce the flow by closing the discharge valve carefully and observe the discharge pressure. If the discharge pressure doesn't rise then suspect the pump performance.

Also observe, when the flow is reduced, the required power by the pump should be less. This should speed up the turbine and governor should react towards maintain the set speed. If this doesn't happen, suspect governor performance.

Caution: don't keep the pump running at reduced flow for long time since the pump might seize.

My dear 2 month before pump and turbine was going normal with same parameters.

When you say 'RPM come down', what changed between 3000 and 2780? Did the turbine run at 3000 uncoupled from the pump and then ran at 2780 with the pump coupled and in service? Or was the pump running at 3000 one day and 2780 the next day with no noticeable change in load? Also, you have given the inlet pressure, but what are the exhaust conditions as well?

1. I observed that the discharge pressure of pump came down from 43 bar to 37 bar

then I checked its RPM which were 2790. After this I did all checks which I described in my question.Exhaust pressure of turbine4 bar.

Dear every thing is happening in coupled position.Motor driven pump is running on full load. We start the turbine driven pump slowly increase the rpm up to 3100 then open the discharge valve of turbine driven pump and close the discharge valve of motor driven pump slowly. Turbine driven pump take load when 80% discharge valve of motor driven pump is closed. Now the turbine speed is 2950 rpm as we close the existing 20% discharge valve of motor driven pump turbine speed become low to 2790 rpm.

As far as my understanding goes, there can be two reasons.

First Reason: The regulation of governor is approx. 7%. In general mechanical governor used for industrial application have approximately 3 to 7% inherent regulation. This is avoid lugging of prime mover on sudden block loads. Please check specification/ Data Sheet of governor used. If it is an electronic governor plus hydraulic actuator then regulation shall be better than 1% and this shall not be the reason.

Second Reason: That power output at coupling of turbine at 3000 rpm is less than input required by pump at 3000rpm. Or at 7.3% speed drop (2780rpm) the power required by pump matches with that of output at shaft of Turbine. The Power Required by Centrifugal pump is proportional to cube of speed. (Flow proportional to speed and Head proportional to square of speed (velocity square/ 2 times density times g). Can be that turbine is short of 24% (=1.07^3)output which results in reduction in speed of the turbine.

Reason 2 can be checked by adjusting speed set point on Governor. Raising speed of Governor shall results in increase of steam flow (provided Steam Regulating Valves are already not 100% open and have some margin to open). If flow increases that design capacity of Boiler and Condenser will result in lower superheat of steam. Thus ultimately no practical improvement in speed can prove this point. Similarly increased flow more than condenser can handle (specially in summer) will result in drop of Vacuum - increase in temperature of LP stage blades.

Basically I am an Electrical Engineer and have tried to reason out from whatever little knowledge of machines I have, may not be correct and therefore please give more consideration to suggestions from other members.

Best regards,

Ramesh

Thank my dear as you said about governor there a TG 13 is controuling the speed but when we close the stand by pump's discharge valve and open the complete discharge valve of this pump the governor open the full steam inlet valve and speed come down to 2790 rpm. we have also increase the opening of governor valve from 12mm to 16 mm but result is same

Speed returning to 3000rpm by partially closing discharge valve means the designed input power required to produce full output of pump at 3000 rpm is more than the capacity of the pump (reason # 2 of my comments) and nothing to do with Speed Governor.

By partially closing discharge valve of pump, you are producing same results as turbine is doing itself by regulating its speed to match pump.

Better shall be to run turbine at 2790rpm by self regulation. This will have following advantages over partially closing discharge valve:

1. Increase life of seat of discharge valve from any pitting etc.

2. Better efficiency of operation as pressure drop across the discharge valve is eliminated thus saving on fuel.

3. Ensures maximum possible output from turbine + pump combination.

After few years of use, when the pump impeller has slightly worn out, you may find that system returns to 3000rpm.

Best regards,

Ramesh

3 bar pressure differential between steam supply and nozzle sounds high. Re-check everything between the 2 points where the pressures are read. Also, if turbine has hand valves, how many are open and how many are closed? Try opening another one. Let us know what you find.

There is no capacity control hand valve on this turbine. One thing we did that was removal of trip valve which is situated after control valve by doing this we gained 50 rpm increase but we cannot run the turbine with out over speed trip valve so we reversed this action.

Sounds like it's about time to call the turbine service man.

Before you do, you could re-check:

* What are the pump power requirements at flow and RPM? How does this compare to original turbine specification? How does this compare to the electric motor nameplate and spec?

* How do current steam supply pressure and temperature, and exhaust pressure compare to original specifications? You don't have a lot of available energy going from 12 to 4 bar, especially if you are losing 3 between the supply and the nozzle.

* Is there a restriction, faulty or broken block valve perhaps, in the inlet or outlet piping?

* Is there a minimum flow/recirculation line on the pump which would be causing extra flow and power requirement?

There may be internal leakage of inlet steam to the exhaust. I once saw an extraction turbine that had an internal drain plug come loose, allowing steam from the nozzle chest to bypass to the extraction port. Needless to say, the extraction temperature was way above design.

Noted that this is a back pressure turbine - exhaust pressure 4 bar.

There are so many variables. And we hope that problem expressed is with steam turbine driven pump in solo (not in parallel with motor driven pump)

Also some one commented that 2 months back this was fine (possibly he is representative of Turbine supplier and wants to only contribute that 2 months before my turbine was OK now what happened, he does not want to support anymore.)

Also we do not know whether there is any change between operating condition of pump between 2 months before and now.

The layout of system is not very clear. Is there any intermediate extraction stage(s). Has the flow of extraction at intermediate stages increased? Did you check the flow of steam at intermediate extraction??

Also do you have test reports of turbine at Dynamometer or it is just sold to your firm on calculated value.

You have two pumps, electric and turbine driven. Do you have their performance test reports? did you compare which one is more efficient of two?

Assuming the motor being an Induction motor, it can be run at 3000 rpm only if it is equipped with VFD, otherwise it will run on load at its Slip speed, which shall be approximately between 97% and 98% of synchronous speed (3000rpm).

Do you have dynamometer test report of motor? if available then this will also have the efficiency vs load curve of the motor. If available, suggest do following to eliminate problem with turbine:

1. Connect watt meters in power supply of motor.

2. Start and run the pump at 100% flow condition of Pump.

3. Record input power reading of watt meters (select meters of accuracy class 0.5 or better).

4. From Performance efficiency curve of motor calculate the out put power of the motor. This is the input required by pump at slip speed.

5. Measure speed of motor either using tachometer or stroboscopic light or read from performance curve of motor.

6. To calculate power required by pump at 3000 rpm multiply reading of point 4 above by cube of ratio of 3000: slip speed of motor. This shall be required designed rating of turbine under current operating conditions of the pump and will be approximately 10% more than value estimated in point 4 above.

If rating of turbine meets or exceeds value calculated in point 6 above - then turbine is fine and problem is elsewhere.

Did you try running the pump with exhaust of turbine opened vented to atmosphere for short time- this is to eliminate possibility of reduced consumption of extraction resulting in reduced flow - in turn lesser power output from turbine. If not so far - may try out this also.

Now a days very sophisticate Electronic speed governors are available (example Woodward 505E?? part number may not be correct, pl surf the web). wherein one can program the priority - say turbine speed or extraction pressure. If you are using similar controller and regulation of extraction pressure is set as priority then probably you can reprogram it to regulate speed. This will ensure that pump runs at constant speed under all conditions of load on extraction. (A vent will have to be operated by third control valve if consumption of extraction steam drops).

Hope I could provide useful inputs without causing any confusion.

Best regards,

Ramesh

Plz note the position of steam control valve and rpm at full load. Keep monitoring it and verify control valve travel when rpm fall down from the initially set rpm. I am sure you will catch the root cause of your problem.

yes I have marked on flul open position on valve stem and observed that when the pump was on full load(discharge valve full open) the steam control valve was on full open position. Even opening of steam control valve was increased by 4 mm.

Provided that initial and exhaust steam conditions are within permissible limits; then there is a need to check any changes in pumping media characteristics, flow and head to arrest the root cause.

What is the position of the governor valve? Is it 100% open and turbine speed is not increasing, then there may be some problem with the governor valve plug. We have faced similar problem and found that the governor valve equalizing valve had looseness and preventing full lift of the governor plug.

We have inspected plug and seat of control valve so many time during this problem and I am sure that plug n seat, both are in good condition.