Hi all;

We have two 7FA's on site and currently have a problem with one them tripping on high exhaust pressure. I have pulled a trend from the transmitter and it looks like the pressure is spiking every few minutes. Normally the pressure runs around 9 inH20 at base load but like I said spikes every 10 or 15 minutes. Because the transmitter shows the spike and the switches also show the spike I believe it is a real pressure. This has always been an issue but seems to be worse right now.

GE's response was to check the slope of the tubing and to change it to larger tubing. I have changed the slope of the tubing and moved from 1/4" to 3/8" but am still seeing the issue. I have also been told by a TA to try heat tracing the tubing as moisture could be freezing up and flashing off into steam. I have not heat traced the tubing yet as the spikes are evident when the temperature was well above freezing. The last two trips were below freezing though. Both of the units pressure spike but one seems to have higher spikes than the other.

What else could be causing this pressure spike? Or what else should I check. I am having a hard time believing it is the tubing, or that heat tracing would help.

 

hi

you confirm the transmitter? or remove to lab check the current of transmitter.

 

> you confirm the transmitter? or remove to lab check the current of transmitter.

Transmitter has been confirmed. I have not lab checked the transmitter for current spikes as I believe the pressure in the line is indeed spiking as there are also three pressure switches in line with the transmitter. The switches are tripping the turbine and the pressure spike can be seen on the transmitter.

 

Dear MPAC,

I have experienced this same type of issue on 2 frame 7EA machines that I maintain. We are located in California where the temperature is typically above freezing. Our machines run near 14inH2O at baseload (switches are set to trip @20inH2O), they are combined cycle, you did not say what config your machines are.

We ended up re-tubing the lines from the exhaust collector to the switch/transmitter cabinet to give as much slope as possible. We also put drain lines with manual valves that we can open to drain down the tubing.

We suspect that moisture is accumulating in the lines at times, and then migrating causing false high indications. We have a transmitter in the HRSG that does not indicate a rise in pressure like what the one for the turbine is seeing. Our setup uses a single transmitter and 2 switches, logic must see the transmitter indicate high and one switch pickup to trip.

The holes for the probes in the exhaust collector are very small orifices. Again we suspect moisture from the exhaust is condensing in the tubing and can't get out. As it collects and exhaust pressure rises/falls it forms a small column in the tubing and indicates a false high pressure.

That is what we have found, hopefully this might help you. Let us know how you fare.

 

I am currently experiencing this same issue on a 7FA GE turbine in a combined cycle configuration. The exhaust spikes are definitely real and not a calibration issue as they have been seen on all three switches, the transmitter, and the local gauge. The tubing is 1/2" most of the way and the slope is pretty good (really good above the turbine and not quite as good above the exhaust duct). My next step is to try to re-slope the section above the exhaust duct or try to get heat trace installed.

Odd thing is that the spikes only seem to occur at extremely high loads. We've been lowering load slightly to reduce the chance of tripping and the spikes go away.

Thanks in advance for any other information.

 

We did find our problem. We were told to try heat tracing the tubing and I found it hard to believe this would solve the problem, but it did. Looking at our trends the spikes start when the temp got below around 55F. So due to the fact the heat trace worked, I assume the difference in temp between the outside air and air in the tubing was causing condensation. This condensation would run down the tube until it flashed off and caused a pressure spike. It was obvious it worked for us because as soon as it was heat traced and insulated the spikes stopped. The heat trace and insulation has to go right to the turbine.

I also takes to others in the area with similar equipment and they also heat traced there.

 

> We did find our problem. We were told to try heat tracing the tubing and I
> found it hard to believe this would solve the problem, but it did. Looking
> at our trends the spikes start when the temp got below around 55F.

---- snip ----

I've heard of other units in this area has their tubing heat traced. I, myself, would like to go that route as well.

Thank you for the knowledgeable insight.

 

We have are approximate 12:1 feet gradient and still observed some random alarms...Our tubing (rather it is piping) was having a ID of slightly above 1/2 inch (1/2 "NB) and we made a large volume of a cylinder shaped joint of diameter 3" by height 1-1/2 feet with a manual drain valve onto which the impulse piping and the switches were mounted. After which we have not faced it till now..

 

We are getting the same problem on both unit we check everything the slope of the tubing, the two pressure switch 63-ET and the 96-EP.

You guys can tell us what more we can do to get this problem fix.

 

Have you tried putting a drain leg in the tubing runs close to the transmitter and leading to a drain pot? Any condensation should run to the drain pot and can be emptied periodically. Alternatively, can the transmitter be located above the tubing so no moisture settles on it?