One of our Frame-7 turbine controlled by Mark VI and driving gas compressor - trips on high high vibration at around 68% every time it is started.

We have starting motor connected through torque converter and SSS clutch to turbine shaft.

Also there is a VFD helper motor that is directly coupled to the other end of the gas compressor and helps the startup from 28% of TNH.

The sequence of events as below:

Turbine reaches around 68% speed.
CPD drops from 30 psig to 18 psig in about 0.2 seconds.
Simultaneously vibration increases upto trip levels in bearing No.2 and alarm levels in bearing No.3
Turbine trips on high high vibration

Any thoughts would be much appreciated.

 

Compressor surge/stall

Though i have not personally experienced such exigencies. from what you have described it looks like a case of compressor stall/surge. That is the only phenomenon which will explain such a huge pressure drop in 0.2 sec and the associated high vibration. It is not advisable to start the machine without asking GE why such a stall should occur in a compressor. one of the reasons why this should happen is because of a sudden loading during the startup , is the gas compressor coupled to the frame 7 in loaded or unloaded condition at and during startup. Another area to look into is the position of the bleed off valves. during normal startup the bleed off valves must be open to prevent such stall/surge conditions. how many bleed off take offs are there and have you ensured that they remain open during the startup?

without looking into the plant operating procedures and PID's this is as much as i can help you.

Please report back if you found this useful and also if you have solved this problem , how was it done.

ps - On a curious note , i would like to know what kind of plant uses such a compressor. most Probably petrochemical/process industry (middle east ??) , can you share with us in brief the nature of the plant ?

 

When did this problem start? After a maintenance outage? After an off-line water wash? After a trip from load?

Have you verified that the Compressor Bleed Valves are fully open at start-up? Not just from the limit switch indications, but by visually checking the bleed valves to ensure they are open?

Sometimes there are manual isolation valves upstream of the air-actuated Compressor Bleed Valves. Are you certain they are fully open?

It's very odd that CPD is decreasing so quickly. What is the exhaust temperature doing when this drop in pressure is occurring? Does the flame indication go out when CPD drops?

What happens to the turbine speed when this sudden drop in pressure and increase in vibration occurs?

How quickly after the

What other Process Alarms are annunciated during this event? Specifically, what alarms are annunciated for 20 seconds prior to the high vibration trip?

What Diagnostic Alarms are annunciated prior to and after this event?

Is anyone near the unit when this occurs? If so, are there any sounds, like a "bang" coming from the unit when this happens?

Are you sure this isn't a problem being caused by the compressor being driven by the turbine? Like a surge during starting? Because this kind of seems like a compressor stall event of either the axial compressor of the gas turbine or the driven compressor.

It's very difficult to understand how CPD can be decreasing so quickly unless there is a surge of the axial compressor or something is causing a sudden change in speed (decrease).

Please answer all the questions when you respond, not just the ones felt relevant.

 

Thanks for all the support. Finally turbine was started to FSNL when the fuel gas wobbe index was increased.