Hi, we have GE make Gas turbine frame 6FA having mark 5 control.

Hydraulic pump make: THE OIL GEAR
COMPANY,Milwaukee.
Hydraulic supply pressure switch low setting:
l63HQ1A &1B= 102 kg/cm2.
Accumulator(2 no's) pressure setting : 54 kg/cm2

When one try to change over Hydraulic pumps, at the instant of giving command "LEAD selection" to stand by pump it starts but SRV (Stop ratio valve-gas pressure control valve) having zero position.

From this we have doubt in accumulator pressure, it is found that one having 0 kg/cm2 & other at 50 kg/cm2.
I have doubt in mind
1) Is it due to accumulator problem?
2) How much minimum pressure required to have smooth operation of SRV,GCV & IGV.

Please guide in this matter.

 

One of the most common problems with hydraulic accumulators is the positioning of the valves. GE rarely (unfortunately) labels the accumulator valves, and almost no site I've ever been to labels them either. Are you certain all the valves on both accumulators are in the correct positions?

There are usually two valves per accumulator--one to "block" and one to "bleed." During normal operation the block valve (usually the larger of the two) is to be open; the bleed valve (usually the smaller of the two) is to be closed. The two accumulators should both be "valved in" as they are meant to be redundant.

The other common accumulator problem is incorrect connection and use of the hose and gauge used for charging and checking the charge which results in loss of charge. Incorrect connection and valve positioning when trying ro charge can also result in damage to the bladder.

Also, have you confirmed the operation/setpoint of the two pressure switches? They shouldn't be the cause of the problem but if they are giving false indications they could be causing a problem.

Lastly, there are air-bleed check valves and relief valves in the system. Are you sure they are working properly? The relief valves are quite often incorrectly used to set hydraulic system pressure and do fail when continuously "passing." They are meant to protect the system, not to set system pressure. The air-bleed check valves also have been known to fail.

 

Thank you for your reply and valuable guidelines.

1) I have confirmed that during normal operation the position of the both “block” & “bleed” valves are correct as you have mentioned in your reply, and also found that both pressure switch working correctly (no false indication).

2) And during charging of accumulator we will take care that no charge loss due to incorrect connection and checking.

3) As far as checking of air bleed valves and relief valves is concern I don’t know or don’t have any specific procedure to check the proper functioning of these valves.

So if possible, you guide me in this matter.

 

Umesh,

> When one try to change over Hydraulic pumps, at the instant of giving command
> "LEAD selection" to stand by pump it starts but SRV (Stop ratio valve-gas
> pressure control valve) having zero position.

When are you changing this selection? (during crank / FSNL /). SRV & GCV only will open after fire command.

Do you have 2 aux. pump for hydraulic supply?

What is output pressure from the hydraulic pump?

> From this we have doubt in accumulator pressure, it is found that one having 0
> kg/cm2 & other at 50 kg/cm2. I have doubt in mind
> 1) Is it due to accumulator problem?

Normally both accumulator will be in service, if you are facing problem only at changeover means it could be a motor issue or As CSA said PRV may be failed, we had faced similar issue.

> 2) How much minimum pressure required to have smooth operation of SRV,GCV & IGV.

80 +/-10 bar

I hope you have have gear driven pump for hydraulic supply. What is the output pressure after crank?

Take care
G.Rajesh

 

Hi, please follow the following steps and give the feed back:

1. Start the auxiliary lub oil and the auxiliary hydraulic oil pumps - record the delivery pressure for each. what are the vaues it normal?

2. if you get the normal operating pressure values, go for stroking the SRV/GCV- does it work properly?

If every thing is working properly with the auxiliary pumps. that means there are no problem with accumulators put the gear pump and its setup.

If not, there is a suspected oil leakage any where:
1. Pressure accumulator defected or low pressure inside--> charge to 54+/- 1.75 bar or replace the defected accumulator

2. check the flexible metal hoses on the hydraulic oil system it shall be replaced every 5 years. one of them if damaged shall be the cause of low pressure in the hydraulic oil system.

 

G.Rajesh,

Thank u for reply.

> When are you changing this selection? (during crank / FSNL /).
Change of selection after synchronizing turbine.

> Do you have 2 aux. pump for hydraulic supply?
We have 1 main and 1 stand by pump.

>I hope you have gear driven pump for hydraulic supply. What is the output pressure after crank?
We have Variable displacement pump for hydraulic supply.

Now I am getting something out of this; that is, we have another issue which might be related to this.

>>>
We have two hydraulic pump in a system, recently problem is that we keep both pumps(main & stand-by) running, because if we keep only one pump running the other get tripped on overload.

So from this I think that the point of “PRV fail" or "wrong procedure-to set/adjust pump discharge pressure by adjusting PRV instead of pressure compensator(stroke of the pump)" might be a reason as it was mention in earlier post by CSA.

 

G.Rajesh,

F-class turbines do not have Accessory Gears, so they do not have Acc. Gear-driven Main L.O. and Main Hydraulic pumps. They have redundant AC motor-driven pumps for both L.O. and Hydraulic systems. As such, they have Lead-Lag selection, and each system has two low pressure switches to start the pump designated as the Lag pump should the pump designated as the Lead pump have low pressure.

The new information about one of the AC motor-driven Hydraulic pumps tripping is very telling. There's either something wrong with the pump, or the pressure relief valve. If the whole story were known, it would probably turn out that someone has been "adjusting" or "calibrating" the hydraulic system pressures--and doing it incorrectly by using the pressure relief valves instead of the pump compensators.