I searched for the answer on Control.com but did not find the exact answer, hence I ask some questions here again.

We have five gas turbines, two Mark-2s and three Mark-4s. The two Mark-2 based turbines develop 119 PSI compressor discharge pressure (CDP; Also wish to ask, why some people refer to it as PCD? I came across this term while searching for answers on this site) in temperature control mode. Still turbine - 1 (T1)generates 2.5 to 3.0 MW extra power than turbine -2 (T2) in temperature control mode. The monthly fuel consumption of T2 is also higher than T1. Both T1 to T5 are fed the same quality gas. The Mechanical department engineers have changed the nozzles and combustion liners also. The Exhaust temperature of T2 is always 20-25 degrees higher than T1. The IGV is fully open. 84 Degrees angle in both cases, verified in field. Both turbines are twins commissioned in the same year. Can the firing temperatures vary so much in two turbines? What could be the reasons for this behaviour? Please illumine me.

Regards,
Grishma

 

First, thanks for using the search feature of control.com; it's a very powerful one which many sites such as this lack.

Second, to answer your first question: compressor discharge pressure was referred to as CDP in one version of Speedtronic turbine control, PCD (Pressure, Compressor Discharge) in another version of Speedtronic turbine control panel, and CPD (Compressor, Pressure - Discharge) in later versions of Speedtronic turbine control panels. It just reflects an attempt to standardize on signal naming conventions (for each version of Speedtronic--ain't this fun?).

One can buy two identical new automobiles or diesel trucks or motorcycles or scooters--with the same color paint--on the same day from the same dealership and those two vehicles, even driven over the same roads by the same driver will never get identically the same gas mileage or have the same mechanical failures or breakdowns. Even the tires will probably not wear or last identically! The same can be said for electric razors or air conditioners--they won't have the same identical operating characteristics.

Now, having said that, a 2.5-3.0 MW difference is slightly unusual--depending on the Frame size of the units. Are they Frame 5s or Frame 9s? You didn't say how the Mk IV-controlled units differ, but it's presumed they weren't all installed at the same time....

There are MANY factors that affect the performance of a heavy-duty gas turbine. Compressor cleanliness (do you perform "regular" Off-line Compressor Water Washes?); IGV angle (did you physically measure the IGVs or just look at the pointers?); the conditions of the turbine nozzles, buckets, and seals (the seals at the blade tips and the rotor-to-case seals); the exhaust duct backpressure (are these simple cycle units, or combined-cycle units with an HRSG (Heat Recover Steam Generator, or a "boiler"?); the inlet filter cleanliness; the condition of the IGV (Inlet Guide Vane bushings; the condition of the compressor blading (rotating and stationary); the condition of the transition pieces; and the condition of the seals between the transition pieces and the first-stage nozzle assembly and the condition of the seals between the transition pieces.

It's not just a matter of nozzles and liners. When was the last HGPI (Hot Gas Path Inspection) or Major Inspection for each unit? How long between nozzle/blade replacements for each unit? Are you certain that the exact same turbine nozzles and turbine buckets are in both machines? Quite often, during turbine nozzle or bucket replacements, upgraded parts are ordered--or refurbished parts are used, or third-party parts are used, or parts with different coatings and materials are used.

When was the last time you calibrated the compressor discharge pressure transducers of both units? What are you using to measure compressor discharge pressure--gauges or the feedback voltages from the compressor discharge pressure transducers? If you're looking at gauges, when was the last time they were calibrated? What is the accuracy of the gauges? How old are the gauges?

What are you using to measure MW outputs of the two units? The meters on the turbine- and/or generator control panels? When was the last time they were calibrated? Have you tried calculating power output using the generator watt-hour meters? (If your units were packaged and provided by GE, there should be a procedure in the Instruction Manuals for monitoring revolutions of the watt-hour meter disc over time and a formula for using the meter characteristics for calculating power output--much better than any meter or transducer--as long as it's been calibrated and serviced on a regular basis.... The formula and procedure were in a document about Performance Testing.)

And, the gas fuel flow measurement--how it is done? Are you using orifice plate dp-measurement? How accurate are the transmitters? If the flow measurement instruments and orifice plate were installed at the same time as the turbines, when was the last time the orifice plates were removed and examined for wear and bore diameter? Are the edges of the bore sharp, or worn? Are they flat, or warped?

The point of all this is, that typical instrumentation (meters and gauges, in particular) can be horribly misleading. Especially if they're not maintained and calibrate regularly. There can even be problems with metering PTs (Potential Transformers) and CTs (Current Transformers).

And this is just the measurement of power and operating conditions--not the mechanical condition of the equipment.

There are just too many unknowns here, and these machines are fairly "old"--not ancient or unserviceable--and so are there instrumentation. If you want a fair comparison, you need to know exactly what hardware is installed in each machine, the time since their last "major" inspection--and what hardware was replaced, if any, at that last inspection, AND, if the hardware was original equipment, or some higher-performance variety (upgraded coatings or nozzle cooling or bucket tips/shrouds, etc.), or.?.?.?

And, the instrumentation needs to be verified--including the IGV angles by a physical measurement of the angles.

Lastly, Mk II Speedtronic turbine control systems employ a lot of analog circuitry--which is calibrated with a lot of potentiometers. These circuits, and the components, drift over time--not like digital circuits. When was the last time the exhaust temperature feedback- and reference circuits were calibrated and tested/verified?

Don't know if this was much help--but it should point out what some of the difference may be caused by or related to.

markvguy

 

We have 3 Frame 6 GE Gas Turbines.
I had the same problem in the Unit 1.
TXset depends of PCD value.
Our trouble was in PCD Transducer Calibration.
Refer to Control Spec.

 

Dear Sir,

How could you know that Gas Turbine is over firing?

I raised this question because we have from our two units same Model # W501D5A same capacity and regularly conducted compressor blade washing using Detergent & water with IGV in full open and it come out that the other unit are much higher load for about 4 to 5 MW difference from other unit.

And I found out thru this formula (T3)/(T4) = [(P3)/(P4)]k-1/k or this formula an ISONTROPIC PROCESS of Gas Turbine, result is much temp. compare to designed temp.

I compare the Exhaust temp. set point, the other is much higher for around 4 degrees C to other.

Thank you and I need your help.

Regards,

Cesar B. Ledesma
Sr. Shift Engineer
Cogeneration Plant