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We are operating 2-GE frame 6B dual fuel machines with DLN1. We are tripping flame (after 52G opens) because CPD drops below 50psi before 14hs clears. Found that we have a leak. Line running from CDP casing to the 3 transmitters has developed a leak. Impossible at this time to get to it so we are running an exterior line. Question: what is the purpose of the .03 orifice blow down on that line?

Low point blowdown to keep the instrument tubing clear of condensation.

309Eguy is correct; the purpose of a low-point blowdown, sometimes also called a continuous blowdown, is to ensure any moisture or condensate is removed from the tubing. It's a very small hole, usually drilled into a small piece of 1/4-inch pipe with a tapered on the outside, and it's meant to be continually blowing when the turbine is running. Otherwise, it's not a "continuous" blowdown, and it won't work.

Sometimes, these orifices get worn out and pass more air than they should and require replacement. Sometimes, people believe they are a leak and people remove them to "stop the leak". That would be a bad idea, as condensate will collect in the tubing and can negatively impact the measurement.

The "extraction" of the compressor discharge casing/combustion wrapper should have a manual hand valve (usually a ball valve) to shut off the line during water washing.

Somtimes there is an orifice placed in the tubing line, usually at a high point in the tubing, near the CPD transmitters. The purpose of this orifice is to limit pressure pulsations from impacting the pressure measurements. At one time, these orifices were also a kind of "safety" mechanism, in that if there was rupture downstream of the orifice they would stop all flow. Of course, if the unit had DLN combustors, it would then trip (can't run DLN without CPD measurement). Many times these orifices were not properly installed after a maintenance outage, and sometimes they became plugged by rust or dirt and wouldn't pass enough pressure to work properly.

Look at your Piping Schematics (P&IDs) for the Cooling and Sealing Air System to find the sensing line from the compressor discharge casing/combustion wrapper, through the manual isolation valve, with a low-point/continuous blowdown, and sometimes an in-line orifice as described above.

Please write back to let us know what you find.

Problem we found was that the swagelok ball valve (water wash isolation) was not spec'd for the high temperature. The seat had melted, limiting the CPD transmitters. We always had good pressure compared to the other unit. Tonight we are running so will see if this is the issue.

Thanks for the feedback! Hopefully this resolves the issue.