Hi all,


We have a BHEL-supplied GE Frame VI Gas Turbine with a Speedtronic Mark VI control system. Recently, we faced an issue with low tank vacuum pressure. Normally, the tank vacuum pressure (96QV-1) stays around -300 mmWC, but suddenly it dropped to -50 mmWC.


On a doubt basis, we checked the calibration of the transmitter (Tx), which was found to be okay. We then flushed the tubing with instrument air and taking transmitter in line, the reading returned to around -300 mmWC.


We attempted to trace the tubing, but it was not possible since it runs down through the gauge cabinet into the diesel compartment. We also referred to the P&ID, but the exact tapping location is unclear.


Although the problem is currently resolved, we are still unsure about one point: if it is indicating tank vacuum, why are two tappings used? It seems to be measuring differential pressure (DP) across something, which is unclear to me (snapshot of the P&ID is attached for reference).


As it is measuring tank vacuum, ideally one side of the transmitter should be open to the atmosphere and the other side connected to the tank.


If anyone has an idea of what exactly 96QV-1 is measuring differential pressure across, please share. Also, if anyone has information about the probable tapping location, please share that as well.

 

you have to choose which lead to vent to atmosphere and which lead to connect to the tank. In complete design, so strictly speaking, you should contact the design engineer.

 

you have to choose which lead to vent to atmosphere and which lead to connect to the tank. In complete design, so strictly speaking, you should contact the design engineer.

As attached in above P&ID there are two tapping coming from tank and that is why it is creating doubt that Tx is measuring DP across what?
Ideally one tapping should be connected to tank and other should be open to atmosphere but that is not like that in above case.
Also tracing of tubbing tapping is difficult and not possible while machine is in running condition.

 

given that this is an existing design, you have to contact "BHEL-supplied GE Frame VI Gas Turbine with a Speedtronic Mark VI control system" or the supplier.

your instrument maintenance staff can easily fix it, but it is contrary to the original design drawing provided by your paid supplier.

 

@Tapan,

The P&ID is a really crappy drawing but I believe you are correct--one of the sensing legs should be measuring tank vacuum and the other should be measuring atmospheric (ambient) pressure to achieve a differential pressure (vacuum) measurement.

It would seem that you would need to wait until the machine is not running and is OFF COOLDOWN to try to trace the two sensing legs. It's entirely possible that one of the sensing legs is measuring the atmospheric (ambient) pressure inside the gauge cabinet or is connected to an open "tap" on the side of the gauge cabinet and the other sensing leg does, indeed, penetrate the diesel fuel tank and measures the vacuum inside the L.O. tank/reservoir below the diesel fuel tank. But, as you say, it's NOT easy to trace pipes/tubes into the L.O. tank/reservoir when the machine is running or when it's ON COOLDOWN.

And then make the appropriate changes to the P&ID to properly show the positions of the openings of the sensing legs....