Measuring static pressure in GT exhaust system


Thread Starter

Mike H

I have a need for accurate measurement of static pressure loss in the exhaust system of an LM6000 GT. Based on the information I have gathered thus far, this is not as simple a task as I expected. A simple manometer measurement has yielded results anywhere from minus 0.5" to +2" WG.

Is there equipment or companies that have capability to do in duct measurement? Given the exhaust temp at 950F, anything in duct would need to be fairly substantial.

What is available to accomplish this type of testing?
GE usually provides what are called "pancake probes" which must be placed in an area that is not subject to changing or swirling exhaust gas flow. There must be some kind of penetration through the ductwork using something like a 1/2-inch or 3/4-inch piece of pipe through which a piece of tubing with a disk of approximately 4 inches diameter welded to it is inserted (from the inside of the ductwork out). There is usually a very tiny (sorry; I've never measured the diameter) hole drilled in the face of the disk. The outer edge of the disk are usually ground almost to almost a knife edge, though I don't think that's very critical; they just need to be smooth and not be produced with a cutting torch. The disk is also usually a piece of stainless steel (capable of withstanding the heat you describe).

So, the disk is centered and welded over one end of the piece of tubing (usually a piece of 1/2-inch stainless steel tubing) with the tiny hole in the center of the disk. Looking down through the tubing (which is usually 16-20 or so inches long), the tiny hole can be seen through. The disk is located about 4 to 6 inches from the inside wall of the duct work. The tubing is usually held stationary with a tubing fitting which is screwed to the pipe penetration on the outside of the ductwork; it's usually a 1/2-inch FPT by 1/2-inch compression fitting, and another tubing fitting is attached to the open end of the tubing and the pressure measuring instrument is attached to that tubing fitting. (I would also recommend mounting the pressure sensing instrument *above* the pancake probe, so that the tubing doesn't fill with condensate.)

The best installations I have seen have used two pancake probes, opposite each other in the duct work, and connected together with a piece of tubing, to kind of "dampen" any pressure pulsations experienced by one or the other.

A picture's worth a thousand words, but I never took a picture of one!