Hey guys,

I want to know the reason and solution of the problem that we are facing at our power plant (CCPP) with steam turbine admission control valve. We have installed the new servo valve a month ago. what we are facing is command for valve is increasing/decreasing as per load demand and process control, but the feedback or valve's actual response is changing after 30 sec delay which is not acceptable. We have checked control oil pressure as well and that is normal.

So guys please give me a solution.

Thanks

 

There might be some extraneous valve packing leaks impinging on the actuator, causing the feedback to be intermittent or slow. There also could be some interference in the feedback arms due to valve insulation getting in the way. We had similar issues when first starting up our Combined Cycle plant.

 

Hi,

This is yet another excellent example of how electro-hydraulic servo-valves get blamed for all sorts of turbine operating problems and issues. I don't know how the turbine OEM is, but it really doesn't seem to make a difference: lack of understanding about servos and what they do--and <b>DON'T do</b>--causes a lot of servos to be replaced, with no change in operation and no problem resolution.

There are a lot of possibilities for what might be causing the problem. Referring to past posts by Wajid29 it is very likely that plant operation/design is a culprit. A lot of steam turbines used in combined cycle plant applications are nothing more than followers--that's all they can really be since steam production (temperature, pressure and flow) is a function of gas turbine exhaust (unless the HRSGs have auxiliary duct firing, which usually isn't enabled unless the GTs are already at full load). So, asking the ST to change load if the GTs aren't changing load or are already at full load is simply not very possible--or has knock-on effects depending on how the plant controls are designed and programmed.

Most of the CCPPs I have worked at simply open the ST control valves fully once the GT load is above some point, and the ST load is a simply a function of steam production as a function of GT exhaust. AS GT load increases, and GT exhaust temperature and flow increase, steam production and temperature increase--and ST load increases. But the control valves are just held wide open and ST load is a function of GT load. The ST is just recovering heat energy from the GT exhaust that would otherwise be lost--so it's really just kind of an "after-thought." One doesn't operate a CCPP by changing the ST load setpoint; the ST load changes as GT load changes, unless there is some kind of bypass around the ST to the condenser.

And many GTs with low emissions combustors have some very limited operating ranges, which plant operations personnel try to work around by changing ST load setpoints--without understanding physics and plant design.

Sometimes, there is some kind of inlet pressure control or extraction pressure control that is also enabled, but ST load change commands--without load changes on the GT--usually don't result in much. The ST output goes up and down as GT load changes--and GT exhaust temperature and flow changes. The ST can't be expected to go to maximum output if the GTs are only at 80% of load, though sometimes plant personnel (mistakenly) believe it should. It's not like a typical thermal power plant with large fossil fuel-fired boilers supplying steam at a constant temperature and pressure as steam turbine load changes. That's not how CCPPs work, through some people seem to think they do or should.

There's a LOT Wajid29 would have to tell us about the configuration of his plant and how it's being operated before we could make any meaningful suggestions about why the ST control valves are slow to react. It doesn't even seem as if it's known if the current to the servo valve is changing if the reference (command) is changing (the two can be very different depending on other factors in the control system). So, while a different ST load setpoint might be entered into the control system (the DCS, or the ST governor?) it might take a long time for the other factors to allow the load setpoint change to actually effect an change in servo current which would cause the valve to move.

We don't even know--but I suspect--that off-line valve movement with the old, as well as the new, servo-valve is just fine; that is, valve position changes with command change, and there is little or no difference.

We don't know if the problem started some time after the plant was originally commissioned, or has existed since plant commissioning. It could be that the ST control valves/actuators are improperly sized for the application (yes; it does happen), in which case replacing a servo with the same one won't cause any appreciable change in behaviour.

It's not known if the problem only occurs at certain loads, or at any load. Or what the operating regions of the GTs are--and how the GTs are being operated when this "probelm" occurs.

We don't know that the GT load(s) is(are) relative to the ST load setpoint.

We don't know what mode the ST is being operated in (load control; pressure control; extraction control; etc.).

If the servo current "increases" but the valve position doesn't change, until the current reaches a certain level and then the valve "jumps" to a new position that's the result of mechanical problems with the valve/actuator--but not the servo valve (though the servo gets the blame for this a lot of the time).

There's just a lot we don't know, and it all can affect how the ST will react to load setpoint changes. We don't even know what is driving the ACV command: load setpoint, inlet pressure control, etc.

Someone really needs to understand how the plant was designed to run, versus how it's being operated because a lot of time, the operating practices don't match the design philosophy, and that can be because the operators are trying to operate the plant differently because operating conditions and requirement have changed and the plant can't be operated in the manner required, or because plant operations and management don't understand how the plant was designed, or both.

But, this matter isn't a simple one--and it's likely not related to a problem with the servo-valve. Unless the servo-valve isn't the correct one for the application, or the control system providing the signal to the servo-valve isn't properly configured/programmed.

If Wajid29 can provide the answers to a lot of the questions above, we might be able to provide some useful ideas or suggestions. But without more information, a lot more, there's not too much we can do.

 

After reading the post again, it almost seems like the ST is set on IPC (inlet pressure control) that would explain the varying feedback and valve movement.

Normally, this oper mode is not used in a CC plant, as the ST is indeed a "follower" as CSA states--the whole purpose of CC operation is to maximize the steam cycle--IPC does not do this, except in off-normal operations (ie startup).

 

Thank you guys for your answers.

We first suspected the servo, but the problem persisted even after installing new servo. Then we checked our control oil and filled our control oil tank with new oil. After that our issue was resolved and valve command and feedback were same and the response of valve was good as well.

 

Wajid29,

Thanks very much for the feedback! "Feedback is the most important contribution!"(c) here at control.com, as it let's others who read these posts in the days, weeks, months and years to come what worked, what didn't, and how problems were eventually solved. Feedback provided by people is what also sets this forum (control.com) apart from many others in the World Wide Web.