We have 50 MW CCPP (30 MW GT, 125 SFF fired HRSG & 20 MW STG. At present we operate Gas Turbine and HRSG in Co-Generation mode i.e. STG in shut down condition. Our GT is GE frame-6 with Mark-VIe control. HRSG steam is generated as per Gas Turbine load. We need to maintain GT load fixed as per guidelines of state electricity board. Steam generated from HRSG is feed to process plant through centralized grid. Steam generation from HRSG is @ 5~10 tons more than process requirements. In order to maintain the steam grid pressure we have to vent the steam in atmosphere (thus incurring huge losses). or if sometime if steam pressure drops due to demand of process plant, we have to increase to GT load. As a result we have to vary GT load continuously between two MW (GT Load).

Is there any control method/loop in which we can maintain the required steam pressure by generating steam as per our requirement by operating GT between two load limits on auto by sensing the steam pressure of HRSG so that we can overcome losses due to steam venting and impact of load changes on life of GT?

 

DRP, most Speedtronic turbine control systems have a method for manually controlling IGV position--within limits, of course. You say you are operating the gas turbine in "CoGeneration mode" which is also called Combined Cycle mode, or IGV Exhaust Temperature Control Mode.

If the gas turbine has Manual IGV Control, you might be able to control the IGVs (manually, mind you) to limit exhaust temperature (by not closing the IGVs as far as IGV Exhaust Temperature Control would) to reduce steam production.

Yours is a problem where the plant owners/managers have not properly communicated the way the plant operates to the electric grid regulators. When the grid regulators understand how the plants operate they are usually a little more amenable to allowing slight changes in the operating agreement to accommodate equipment limitations.

Getting back to the Manual IGV control, it's a little difficult to explain without a graph, but we'll try. Let's say the turbine is operating at some load (less than Base Load) in Simple Cycle mode (IGV Exhaust Temperature Control is OFF), and the IGVs are at, let's say, 74 DGA. If Manual IGV Control was selected on, it might be possible to CLOSE the IGVs a little further--until the exhaust temperature, TTXM, reached the exhaust temperature control reference, TTRX. But, the IGVs cannot be closed any further, NOR can Manual IGV Control be used to open the IGVs more than 74 DGA.

So, there are limits to what can be done with Manual IGV Control--the IGVs cannot be opened beyond the current IGV reference (when in Simple Cycle/IGV Exhaust Temperature Control OFF), and can only be closed until the exhaust temperature, TTXM, reaches the exhaust temperature control reference, TTRX.

It might work for your application; it might not. But, it seems, if the ability is available via the operator interface on your turbine, it's worth a try. You will NOT trip the turbine with Manual IGV Control. The Speedtronic recognizes the limits described above, and unless you force logic (which shouldn't be done anyway without fully understanding all of the ramifications of doing so--which may extend well beyond the original intent of the force) the Speedtronic will not let you go beyond the limits and cause damage and/or trip the turbine.

If you (and your management) decide to attempt Manual IGV Control, please be patient when doing so. Closely monitor CSGV (actual IGV position, from the LVDT feedback), CSRGV (IGV Reference position), TTXM (Actual Exhaust Temperature), TTRX (exhaust temperature control reference--the maximum allowable exhaust temperature under ANY operating condition), and load, DW or DWATT. Just be patient with changes and give the turbine a few minutes to settle out after each change of IGV position to get some feedback. I also strongly recommend you plan to disable Pre-Selected Load Control for the period of the testing, at least the first part of the testing, and just let the gas turbine output be what it will be while gathering data. I think you will find the output (DW or DWATT) will not change very much, and if you change load during the test you will be changing the exhaust temperature and so might not be gathering good data.

As a second part of the test, you might want to enable Pre-Selected Load Control with Manual IGV Control enabled and see if you can maintain load with a reduced exhaust temperature.

Again, using Manual IGV Control it is NOT possible to close the IGVs below the point at which the exhaust temperature exceeds the exhaust temperature reference, and it's not possible to open the IGVs more than would otherwise be the normal IGV position when Manual IGV Control is OFF. So, while you may find you just can't quite get to the IGV position you want (which is probably open a little more than currently for the load you are operating at), you may just get pretty close.

And, to complicate matters even further, again--if IGV Exhaust Temperature Control is enabled (ON) (sometimes called Combined Cycle Mode), I suggest for the purposes of the testing you select it off prior to beginning the test. IGV Exhaust Temperature Control will keep the IGVs closed to maximize exhaust temperature (and steam production) when enabled (ON), so you just might find if you turn it OFF that you will much closer to the desired steam production without even enabling Manual IGV Control.

Not so easy. Please read, and re-read, and even have someone else read, and re-read, the information above, check to see if your operator interface has Manual IGV Control, plan your testing, and ask any questions you might come up with before testing. We'll do our best to help, but you may be asked to provide some detailed information about your turbine and configuration parameters (it would be nice to know if you have a Mark V or Mark VI or Mark VIe, for example).

By the way, I'm only offering a method for using existing control schemes to see if you can achieve what you're looking to achieve. I'm sure you could add some instrumentation to the Speedtronic (depending on the version), and hire someone knowledgeable in Speedtronic programming to make some control modifications to the sequencing/application code running in the Speedtronic to achieve what you want, but you may end up with a very complicated and unwieldy control scheme--or you may get lucky and end up with a simple and elegant control scheme. Without being able to see how your plant operates and understand all of the parameters and requirements, it's very difficult to predict which.

Please write back to let us know how you proceed.

 

DRP,

Sorry I missed the Mark VIe part of your post.

Was the Mark VIe an upgrade/retrofit of an older control system, or was it provided new with a new gas turbine?

 

Opening the IGV's may not reduce the steam production. It will reduce the steam temperature, but since opening the IGV's increases air flow through the gas turbine, and hence exhaust flow through the HRSG, the tons/hr of steam will not be significantly reduced.

If you could bypass the excess steam to a condenser you could at least save the water, but I am guessing that if the steam turbine is out of service, then the condenser is also.

Since there was no mention of an exhaust bypass, I assume you cannot divert some of the exhaust gas to a bypass stack.

You could ask the HRSG supplier for recommendations, but I think your best chance is to negotiate with grid authority to permit you to reduce load.

 

Dear CSA,

Thank you very much for your reply. Our management may not agree to operate our GT the way in which you described by change in IGV control On/Off command.

Also in this case we will have to control/change the IGV position manually with reference to steam generation. This is what we do at present i.e instead of IGV position we change GT load between two limits.But still you reply will be useful for further discussion/operating strategy for us.

Thank you once again.
If at all we do something I will let reply back.

 

> Sorry I missed the Mark VIe part of your post.

> Was the Mark VIe an upgrade/retrofit of an older control system, or was it
> provided new with a new gas turbine?

Dear Sir,
Our system was upgraded to Mark VIe from mark IV.
One alternative which came to my mind is partial closing of divertor damper. It's also loss of energy but we can save cost of feed water used for generating steam. Will this action have any adverse effect on GT life or performance.

 

DRP,

You would need to know if the diverter damper actuator and hinge mechanism are capable of withstanding the pressures which would be applied when the damper is partially open. The pressures can be VERY great. Some diverters are not designed to be operated with the dampers in a mid-stroke position (modulated) at all.

I know of a couple of plants which had diverter dampers, and which were equipped with switches to indicate when the dampers were either fully "open" or fully "closed" and would initiate a trip if either switch changed state during operation and would also prevent a start if either switch was not actuated prior to a start. Both of these plants tried modulating the diverter damper position and both suffered major damage as a result. They defeated the switches and when the pressure on the damper overcame the attachment mechanism (hinges in both cases) the diverter dampers broke loose and did serious damage in the exhaust, one damaging most of the superheater tubes.

So, you need to understand how your dampers can be operated--and if they could be modified to be modulated to prevent possible damage.

 

> One alternative which came to my mind is partial closing of divertor damper.
> It's also loss of energy but we can save cost of feed water used for generating
> steam. Will this action have any adverse effect on GT life or performance.

It will not likely have an adverse effect on GT life or performance. However, you need to make sure the diverter damper is designed for sustained operation in a partially open condition - many are essentially designed for full open or full closed operation, with intermediate position only during startup or shutdown of the GT and HRSG.

You also may need to check with the HRSG supplier. The exhaust gas from the gas turbine is rotating, and a partially open damper can really disturb the gas flow pattern. I don't recall whether you said you have supplemental firing of the HRSG, but if you do, you should turn it off while operating with the damper at an intermediate position.