Frame 9E Low CPD


Thread Starter


We had a compressor failure in 2013 and had to carry out an unplanned major inspection to replace compressor. Initial startup with new compressor gave a CPD or 10.4 bar. However Initial startup of same unit with an old compressor in 2010 gave a CPD of 10.7 bar after major inspection when the compressor was steam washed.

We suspect bleed valves not sitting properly. What ways can be verify
the integrity of the bleed valves online?

Are there any other causes of this CPD performance degradation?

The "easiest" way to check to see if axial compressor bleed valves are passing (leaking) is to use an infrared heat sensor and check the temperature of the piping. When the bleed valve is closed and there is no flow through it, the piping before and after the valves should be at or slightly less than ambient temperature in the turbine compartment. If the valve is passing, the temperature of the piping should be higher than the turbine compartment temperature, both before and after the valve (because the temperature of the air passing through the valve to the exhaust will be higher than ambient).

It's important to properly use and "aim" the infrared heat sensor; improper use and aiming can result in false readings (and needlessly shutting down and disassembling the unit only to find the valves are working properly).

The best way is to remove the valves and using compressed air to operate them, and observe the closure, or even to test them with high-pressure air or -water on the upstream side and a gauge to monitor the pressure.

You didn't say if the turbine power output was also lower than before. There have been cases where the CPD transmitter was incorrectly "calibrated" (packages of GE-design heavy duty gas turbines typically don't use "Smart" transmitters, so they must be "pumped up" to verify or calibrate them, and lots of site don't have the proper equipment).

IGV LVDT calibration can also have an impact on CPD.

Finally, increased clearances in the axial compressor and high inlet filter differential pressures, as well as high exhaust duck back pressures can also impact unit performance and even CPD.

Many times these problems are the result of several factors, so be sure to investigate all of them.

Hope this helps!