Steam Turbine Speed Fluctuation Problem

Governor problem?? In my opinion the OST test should be done under normal operating conditions that is coupled to whatever is being driven. Retest again under these conditions.

In our company this is the procedure of OST (we should make sure before starting OST that the coupling is not installed).

By the way, the turbine is controlled by Trisen system.

Please let the forum know how the phone call with Trisen went so that others can learn from this issue.

Sounds like you could have one of the sensors failing...I would look for any troubleshooting guides you may have and/or contact Tri-sen for guidance...

http://tri-sen.com/contact-us/

Only if the driven equipment can withstand the over speed safely.

Backpressure steam turbines are sensitive to both the inlet AND exhaust pressures.

Are you sure that there was no variation in the exhaust side backpressure ?

Differential pressure between outlet and inlet, have it monitored. Be sure it doesnot fluctuate, otherwise you lack mass rate at that speed.

Is your turbine simply changing speed once, or is it going into a speed oscillation from 4100 to 3800 in a cyclic fashion? How is the vibration sensor acting during this? With no coupling at the turbine output shaft, is it possible you have an unbalanced thrust load, and that is producing a damaging chatter against the thrust bearing?

Are you certain that the temperature of your steam at the inlet is steady? I find that somewhat suspect. Why is the pressure of the steam being reduced during spin-up to over speed? Your steam source must be ridiculously weak, if it cannot keep up with over speed at zero mechanical load.

Go back to the planning board, and start over, and make the over speed conditions match how the turbine is to be operated in real life. Consult first with the turbine manufacturer, and also consult with the controls people you mentioned in second post, and make sure their controls/instruments are functional and in good order.

Sounds to me like you are about to wipe the thrust bearing.

I tend to agree with poster1

I had the same problem when testing the Woodward Governor, when putting it from speed control to position control.The speed would drop drastically, without load.I was convinced of that by putting my finger on the stem of the steam valve and could not detect any motion, but the speed did drop drastically.Minor changes not even felt by the human hand can change the speed of turbine without any load

Agreed. It does appear that is the case, since no load condition really takes little steam at all.

Dear Rotating Engineer,

You Have to inform whether the exhaust steam was vented out to Atmosphere during OST, or back pressure was maintained between 1.0 to 1.5 KG/cm^2 gauge or actual design value of back pressure.

In a back-pressure Turbine, the Ehaust Steam should not be vented out unless the back pressure exceeds the prescribed limit. The reason being, the Sp.Vol of steam will increase for 0.0 Kg./cm^2 (g) when compared to a back-pressure of 1.0 to 1.5 Kg./cm^2 (g) or corressponding design back pressure.

If you refer steam tables you will find the specific vol. as at 1.0 to 1.5 Kg. Pressure (gauge) Sp.Vol., is 0.881 m^3 /Kg. to 0.668 m^3 /Kg., and at Atmospheric Pressure (0.0 Kg/cm^2 g) the Sp.Vol., is 1.673 m^3 /Kg. and degree of super heat is ignored. If Deg. Of Super heat is considered, the sp. Vol., will be more which you know.

Therefore the exit velocity of the exhaust steam will be more than double and this will result in speed shooting up and governor will regulate the steam flow and the inertial force of rotor will come down and speed will drop and then the governor will admit more steam. Further, the governor may be too sensitive.

Hence pl. throttle the exhaust valve and bring the back pressure to the design value and check the Over Speed Tripping. If need be increase the sluggishness of the governor.

Pl. post information to this forum, what was the reason and how it is set right.

DHAYANNADHAN.S

If one is sending the unit into over-speed the one would expect the speed to change, as this is the intent.

A reduction in inlet pressure would accompany a higher speed and a higher flow rate of steam causing it, and merely indicates that a bottleneck in the steam supply is approaching.