Hi there,

we are running on GT Frame 9E. Recently, the CPD dipped by 0.4 bars (usually at 10.3 bars when on base load) when we went from minimum to maximum. We were struggling to get the load we wanted. The most we could generate was 100 MW. AA pressure was at 12.5 bars. When we inspected the GT, these were what we inspected:

1. IGV opening (required and actual position, servocurrent and LVDT)

2. Air filter house (DP at 1.4 inches H2O)

3. Compressor bleed valves (no alarm was annunciated) Local indication shown close for two bottom compressor bleed valves based on position of lever.

4. Any unusual compressor turbine leakage and i.e

When we inspected the turbine compartment for leakage, we noticed the increase in temperature. The heat was really intense although turbine compartment temperature from HMI showed only 51C. We used a rod and tied a cloth at the end of the rod and pointed it around in the turbine compartment for any unsuspecting leakage. Subsequently, we found FA 6-1 which is the porostone filter to solenoid vlv for compressor bleed valve 20 CB-1 air leaking from it.

That night, that GT was shut down and gasket was changed. This morning when we started up, GT CPD was back to normal at 10.3 bars. Load was up to 108 MW. Can the leakage of the porostone filter FA 6-1 really cause the CPD to dip? Previously we had leakage from FA 3-1 and FA 4-1. There was loud hissing noise which was obvious due to their location outside the turbine compartment beneath the grating. Appreciate your feedback.

 

If there isn't sufficient air to keep the compressor bleed valves fully closed, then THAT could be the reason for the dip in CPD. Fixing the leak could have restored sufficient pressure to keep the compressor bleed valves fully closed.

And remember, there should be CONTINUOUS blow-downs in the low-points of all air tubing/piping that uses compressor discharge air for valve actuation. Please have a look at the P&IDs to see them. They are not "leaks"; they are necessary to remove condensate from the lines. There SHOULD BE a continuous flow through the very small orifices of the continuous blow-downs, which is very commonly mistaken for a "leak".

 

Dear CSA,

Again, thank you for your reply. I hope every1 finds inspiration and motivation to share in you. Anyway, back to the topic, all the continuous blowdown drains at our sites are clogged up. There are no calls to repair them at the moment.

After replacing the gasket for FA6-1, the problem was solved. However, i couldn't help to be suspicious and wary bcoz the leakage did not trigger any of the limit switches of the 4 compressor bleed valves. There wasn't any sign that the compressor bleed valve is leaking else we wud have heard a sharp hissing noise. Perhaps the air supplied to the 20 cb was just sufficient to close the compressor bleed valves.

One initiative thought of during the incident was to supply external air to each of the compressor bleed valves since we have modified the air piping to the compressor bleed valves. That way, we can force the logic for 20 cb to allow the system to think its still energized to close the compressor bleed valves but on actual fact, close the manual valve before the 20 cb and see if the pressure rises back to 10.4bars.

We didn't do that after further discussion due to the heat. Turbine compartment was hotter than usual that time. We took a chance and replaced the gasket and it worked. We have also thought of sticking masking tape around the flanges, compressor casing and joints to detect the unsuspecting leakage. If there is leakage, we will be able to see it from the masking tape. Wonder if any1 has ever tried that? Would like to hear your comment regarding what i have stated above.

Thanks CSA.