GE Frame 6 Cooling water pressure hunting

Hi, I've recently found a problem in one of my Frame 6 GE GTG. A bit of an overview of the cooling water system, there is no selector switch to determine which cooling water pump is the lead, so by default, 88WC-1 is the main pump2, 88WC-2 will activate when pressure drops to 40PSIG. Current pump discharge when only 1 pump is operating is around 60 psig, however it is hunting and drops below 40 PSIG at an erratic interval, which always cause 88WC-2 pump to activate. Note: deactivation of 88WC-2 is manual by operator. I've tried to operate 88WC-2 as the main pump while open breaker 88WC-1, but it still showed the same symptoms of pressure drop.

Cooling water system is a closed cycle and components cooled by the system are turbine support legs, Atomizing Air Precooler and LO HEX. Water after cooling the AA Precooler, LO HEX and support legs first enters an expansion tank (venting is closed) and then pumped by 88WC to a Finned tube Heat exchanger which will then be used to cool the AA Precooler, LO HEX and support legs to complete the closed cycle.

I was wondering what would cause the hunting/ erratic pressure to drop from 60 to 40 psig? I've observed that around the turbine base at the precooler, there is a metal knocking sound which happened when the pressure drops and is wondering what the cause of the sound could be and whether there is a problem in the precooler.
Also, should the vent on the expansion tank be open during normal operations or venting should only be done during filling of cooling water system to vent out trapped air in the system and then closed?

Apologize for any unclear wording as english is not my main language.
Really appreciate any feedback or info for other part that I should look at regarding the problem.

Cheers
 
If you hear knocking sounds in the area of the AA Pre-cooler that's usually indicative of cooling water boiling in the pre-cooler because the temperature regulating valve is not working properly and/or there is insufficient cooling water flow in the AA Pre-cooler. It's very common for older temperature regulating valves to be adjusted incorrectly and/or the capillary tube often gets broken and goes un-noticed for long periods of time.

The reason for the AA Pre-cooler is to prevent damage to the Main AA Compressor from air above 300 deg F entering the compressor (which usually spins at a VERY HIGH speed and has tight internal clearances which can be damaged by excessive inlet air temperatures). Also, if the Main AA Compressor inlet air temperature is too cold then entrained moisture in the air entering the AA Pre-cooler can condense and if they get moved along into the Main AA Compressor they can also cause catastrophic Main AA Compressor damage. The AA leaving the AA Pre-cooler should not be less than about 225 deg F (which is just above boiling) to prevent condensation and Main AA Compressor damage. There should be a continuous blow-down vent from the shell of the AA Pre-Cooler (which most people FALSELY think SHOULD NOT be in service or present) to indicate if moisture is forming in the shell (it will be visible in the discharge from the continuous blow-down).

In my experience, the vent on the expansion tank should be open (it's not very large, and provides for expansion and contraction (rising and falling levels)) in the expansion tank. I'm not experienced with sealed cooling water systems, which would be like the radiator in an automobile, and should have a "lid" that would allow pressure not to get too high in the system (just like the cap on an automobile radiator). BUT, GE Belfort does things differently than most other divisions of GE, and if the unit originated in Belfort it's anyone's guess how the system is configured--WITHOUT SEEING the P&ID.

Are there check valves in the discharge of each of the Cooling Water Pumps to prevent backflow in the non-running pump? Could they be not working? (Usually, they are gravity swing check valves, or have light springs which can break.)

If both pumps are exhibiting the same behavior that suggests the coupling between the motors and the pumps are not slipping, but perhaps they both might be....?

Check the Cooling Water P&ID; it should say if the expansion tank vent should be open to atmosphere or closed. Also, check the Cooling Water system section of the Operation & Maintenance Manual provided with the unit to see if it says if the system should be continuously vented or pressurized.
 
Hi,

Can you share P&IDs of this unit..?

If not...
I Wil check with P&IDs that I got see if I can support you on this issue.. Even there an be some discrepancies from one unit to another one...

Regards
@ControlsGuy25
Sure, attached is the P&ID for the cooling water system

What about expansion tank level... Did you monitored it.. Does it stay regular or fluctuate also?
Sorry, the expansion tank gauge is blurry so I can't get a visual indicator, will try to change the level gauge and I'll share the update. I tried to check the pump inlet pressure gauge to monitor if its also fluctuating, but the maintenance team installed a 0-200 PSIG pressure gauge. I've informed the maintenance team and requested to change it, they plan to change it on the weekend, so I'll probably be able to give an update next monday.

If you hear knocking sounds in the area of the AA Pre-cooler that's usually indicative of cooling water boiling in the pre-cooler because the temperature regulating valve is not working properly and/or there is insufficient cooling water flow in the AA Pre-cooler. It's very common for older temperature regulating valves to be adjusted incorrectly and/or the capillary tube often gets broken and goes un-noticed for long periods of time.

The reason for the AA Pre-cooler is to prevent damage to the Main AA Compressor from air above 300 deg F entering the compressor (which usually spins at a VERY HIGH speed and has tight internal clearances which can be damaged by excessive inlet air temperatures). Also, if the Main AA Compressor inlet air temperature is too cold then entrained moisture in the air entering the AA Pre-cooler can condense and if they get moved along into the Main AA Compressor they can also cause catastrophic Main AA Compressor damage. The AA leaving the AA Pre-cooler should not be less than about 225 deg F (which is just above boiling) to prevent condensation and Main AA Compressor damage. There should be a continuous blow-down vent from the shell of the AA Pre-Cooler (which most people FALSELY think SHOULD NOT be in service or present) to indicate if moisture is forming in the shell (it will be visible in the discharge from the continuous blow-down).

In my experience, the vent on the expansion tank should be open (it's not very large, and provides for expansion and contraction (rising and falling levels)) in the expansion tank. I'm not experienced with sealed cooling water systems, which would be like the radiator in an automobile, and should have a "lid" that would allow pressure not to get too high in the system (just like the cap on an automobile radiator). BUT, GE Belfort does things differently than most other divisions of GE, and if the unit originated in Belfort it's anyone's guess how the system is configured--WITHOUT SEEING the P&ID.

Are there check valves in the discharge of each of the Cooling Water Pumps to prevent backflow in the non-running pump? Could they be not working? (Usually, they are gravity swing check valves, or have light springs which can break.)

If both pumps are exhibiting the same behavior that suggests the coupling between the motors and the pumps are not slipping, but perhaps they both might be....?

Check the Cooling Water P&ID; it should say if the expansion tank vent should be open to atmosphere or closed. Also, check the Cooling Water system section of the Operation & Maintenance Manual provided with the unit to see if it says if the system should be continuously vented or pressurized.
Thanks for the input, I'll check the AA comp incoming temperature and will give an update.

Confirmed that there are check valves on each 88WC. I've checked the check valves (no pun intended), and found that the check valve for 88WC-1 is passing (confirmed as 88WC shaft is rotating CCW when it is off), while the check valve for 88WC-2 is functioning properly, but considering that the main pump is always 88WC-1 (no selector switch), I think that the passing check valve of 88WC-1 is still acceptable for operating. But I've planned to change the check valve at the nearest shut down/time available.

Unfortunately the P&ID (attached) does not indicate whether the expansion tank vent should be open or closed, will check on the O&M Manual.
 

Attachments

There should kind of abrasive filters/joints check if they are in proper state ...

Also Expansion tank Valve shown here as PSV VR11 is to avoid suppressure in that Tank..

I got Frame 6B P&IDs and they are more accurate and showing rpecisely every equipments/devices and their functionnalities.

The drafts that you shared are not P&IDs but only general diagram I mean they are just to give an overview ( And not real overview) of the Cooling circuit system..
 
Do this unit got following valves /devices.

VA32-2
VPR68-3
PGA87

If so did you check that they are operating well according to O&M manuals /appcode on Mark * or whatever controls system associated to the unit..
 
  1. Drain the water from the turbine support leg if your GTG has a cooling water circuit to the turbine support leg
  2. clean Stainer of the water circuit
  3. vent water from the cooling water makeup tank
  4. change over the motors and observe the pressure hunting
  5. try to vent from AA cooler, turbine support leg, and makeup tank
 
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