SRV position at valve full close position was found negative 4.2% and LVDT voltage was 0.625V. we did calibration after adjusting LVDT to 0.7 v and auto calibration showed 0.2 %. After one day operation, when the unit was stopped the valve position again shows negative 4.2%. Actual valve position is also below the zero position mark at the valve side. How can I bring back the position to zero?

 

Iqbal,

When you are posting with a problem like this, it's very helpful if you would provide some more information about the equipment at your site. We don't know what the turbine is, what kind of gas valve assembly is in use; nothing. Except what you tell us, which wasn't much. It's great that you've told what you've done, but you haven't provided other necessary information. (And telling us you have a Hitachi 20MW turbine (which you've mentioned previously) doesn't tell us if it's a GE-design Frame 5 or anything about the auxiliaries of the unit.)

One has to remember: <b>We want to know the position of the VALVE PLUG</b>, and on some of the gas valve assemblies used with GE-design heavy duty gas turbines <b>the LVDTs are *NOT* attached to the valve plug.</b>

Please read http://control.com/thread/1294809227#1295331639. While the description is for the GCV of a typical GE-design heavy duty gas turbine Frame 5 or Frame 6B combined gas valve assembly (usually provided with units with conventional combustors), it also applies to the SRV of that same combined gas valve assembly. If the unit is a Frame 7E/EA or a Frame 9E with a combined gas valve assembly, I don't recall exactly which valve it also applies to but it applies to one of them as well (either the SRV or the GCV, and I just can't remember which it is).

If you have a combined gas valve assembly (where the SRV and GCV are in a single casting/assembly), this is normal and to be expected. The LVDTs are attached to a valve stem that is <b>*NOT</b> attached to either the actuator rod or the valve plug. <b>The true zero stroke position is when the top of the valve stem is fully up against the bottom of the valve plug.</b> There are situations where, during shutdown or while operating on liquid fuel, that the valve position feedback could be negative--<b>if the valve stem is not in the position where the top of the valve stem is in contact with the bottom of the valve plug.</b> This happens quite often when the valve is shut by the force of the internal closing spring during a normal fired shutdown or during an emergency trip.

Sometimes the seal on the valve stem will keep the valve stem from falling down when the valve is closed by the internal closing spring; and sometimes it doesn't.

But, because the LVDTs are attached to the valve stem <b>and *NOT* the valve plug</b> and because the valve stem is free to move up and down when the valve is closed and there is no hydraulic pressure in the actuator, then the valve position can be zero or slightly negative, just as you have observed.

Take a piece of lumber (a 2" x 4" or something similar) and while the unit is shut down and there is no hydraulic pressure on the actuator, gently pry down on the top of the bar the LVDTs are attached to and which is clamped to the valve stem. It shouldn't require much force; don't bend the bar! (NOTE: It's best to put the end of the 2" x 4" nearest the valve stem when prying, either from the top or from the bottom, so as not to inadvertently bend the bar. Sometimes this will require another block of wood to use as a fulcrum; you are using the 2" x 4" as a lever and you need a fulcrum. Just remember it shouldn't take too much force!)

Then gently pry up on the underside of the same bar (again--don't use too much force which could bend the bar!) and it should rise ever so slightly. You should then be able to see a small gap between the actuator rod and the valve stem, and when the valve stem is in contact with the bottom of the valve plug, that is the true zero stroke position (0% open).

Again, depending on how tight the seal is around the valve stem, if you release the pressure on the underside of the LVDT bar it may fall, slightly or even all the way back down to contact the actuator rod, or it might just stay in place. To be sure the top of the valve stem is in contact with the bottom of the valve plug, insert some feeler gauges in the gap between the valve stem and actuator rod before removing the piece of lumber (enough to completely fill the gap between the bottom of the valve stem and the top of the actuator rod), and that is the 0% opening (zero stroke position).

Just be aware, that when you remove the feeler gauges or when they are not installed, the valve stem might move down and in that case the LVDTs will indicate a slight negative position, which is "correct" because the LVDTs are attached to the valve stem and NOT to the valve plug, and we're only trying to measure the position of the valve plug.

You should be able to find the mechanical drawings of the gas valve assembly and see a very good representation of what's happening inside the valve. They are usually in the service and maintenance manuals provided with the package (turbine and driven device). It should confirm what's in the referenced thread and described above. As always, when one encounters a problem like this one should always investigate the entire "loop" which, in this case, includes the gas valve assembly and the the LVDT arrangement.

Again, it's very difficult to provide concise answers when we don't have very much information to work with. The type of control system isn't important in this thread; it's the type of device the control system is monitoring. This is exactly how the control system gets blamed for something that's not it's fault! One must understand the devices which the control system is using to control and protect the turbine.

Please write back to tell us what you were able to determine.

 

CSA,

Thanks for the quick response. Out GT is Hitachi H25 (which is a compact version of Frame6) SRV/GCV is in one assembly similar to GE.

Control system is Mark V

I will try the procedure in the next shutdown. Meanwhile, as you mentioned, this negative 4.2% is normal and cannot be bring back to zero permanently. Am i right?

Thanks CSA..

 

Iqbal,

You are welcome.

And, you are 100.0000% correct.