Hello,

Can anybody explain to me the Command state (CMDSTATE) blocks (especially the L43FG rung) used in Mark 6e control system in Gas turbine control? I am fairly new to reading the logics in mark 6e and the mask constant part is really confusing me as well as the preset and permissive variables for L43FG.
Our unit is trying to run 1 GT (ms 5001) on mix fuel for the first time so any help is much appreciated.

 

Have you tried the trick of right-clicking on the block and choosing 'Item Help' or 'Block Help'?

 

Dear wtf (interesting name),

I did alrey explore the help button wherein I came across the mask constants about which I asked in the question. I want more clarification on the rung block as the help section did not prove useful at all. There's some mention about the MSB and LSB of the mask constant being used which did not make any sense to me. So any explanation regarding the block will be of much help to me.

 

Post a clear photo of the block in question (or a screen capture of the block). Provide the values of the Control Constant for the mask preset and permissive values (the MSB and LSB).

The gist of command state blocks is they are used to choose a logic output based on operator selections and presets and permissives (also logic values). In the case you are asking about it is the fuel selection (Gas; Liquid; Mix; etc. depending on the capabilities of the machine).

For mixed fuel operation, GE-design heavy duty gas turbines usually have to be at rated speed (not during STARTing or acceleration to rated speed or shutdown from rated speed), so you can't choose mixed fuel operation prior to STARTing or during acceleration or shutdown). These are part of the permissives and presets inputs to the block. Let's say the unit was at 25% of rated load (so at rated speed and producing megawiggles) burning natural gas and the operator was told to select mixed fuel operation and operate the unit at 30% liquid fuel and 70% gas fuel. The operator would initiate a transfer to liquid fuel and once the transfer had begun (reducing the amount of gas fuel and increasing the amount of liquid fuel) the operator would click on the "mix" button to stop the transfer at the desired amount. And that's important to understand: Operating on mixed fuel is really pausing a fuel transfer at some mid-point of the transfer (transfers, once in progress, usually take about 30 seconds to ramp one fuel up as the other is ramped down). [NOTE: achieving the exact 30/70 split is difficult to achieve; there were some machines that had the ability to set a desired amount of one fuel (usually liquid fuel percentage) after the transfer had been paused, but they were rate.]

ANOTHER really important thing to know about mixed fuel operation is that there are upper and lower "limits" that are usually stated in the Operations & Maintenance Manuals for mixed fuel operation. For example, it would not be good for the machine to operate at 5% or 10% of liquid fuel for extended periods of time because the liquid fuel flow-rate is so low that proper atomization of the liquid fuel would be very difficult so combustion would be poor (efficiency would also be very poor). (And you haven't said what fuels the machine at your site is capable of burning...!) These limits ARE NOT usually programmed into the machine--they have to be understood and communicated to operators and their supervisors--so make sure to read the Operations & Maintenance Manuals about mixed fuel operation carefully and closely. The general rule--for natural gas and liquid fuel operation (distillate liquid fuels that is)--is that neither fuel should be less than 25% of total fuel flow, so liquid fuel should not be less than 25% of total fuel flow and gas fuel fuel flow should not be less than about 25% of total fuel flow. Mixed fuel operation, in my personal experience, is not a common mode of operation--in fact, it's very UNCOMMON (unless there are multiple liquid fuels (distillate (diesel) and naphtha or crude oil, for example) or multiple gas fuels (for example natural gas and butane, for example). It would be helpful to know what fuels the machine at your site is capable of burning....

The MSB and LSB values are used like "switches" to allow permissives and presets--or to block them, as the case may be. The Command State block is really a fancy bunch of relay ladder logic that a very smart programmer implemented that make automated machine software configuration easier. It wasn't for technicians or operators to be able to easily understand how the machine is configured or operated; it was strictly for automated configuration and as such if the factory engineer didn't understand the automation or the block the values of MSB and LSB and the logic values supplied for them could be (and very often was) incorrect.

So, tell us what problems you are experiencing in trying to operate the machine on mixed fuel, what you are trying to do, and supply the requestion information above. It might also be helpful to supply a clear photo of the Main Display of the HMI prominently showing the Fuel Selection section of the display (specifically the fuels that can be burned and how mixed fuel operation would be selected (a "mix" button, for example). I will look for some possibly better description of the Command State block operation, which has been used since Mark* V production and was documented in GEH-6195, the SPEEDTRONIC Mark* V Application Manual, if I recall correctly.

 

Hello Again! apologies for the late response. Our machines are designed to run on NG, HSD and naphtha. The command state block is somewhat clear to me at this point. The presets and permissive are basically required to be true and then the corresponding logic-out bit is high and passed on.
Also, we are yet to run the unit on mix fuel so will be posting any problems we find. we will run the unit on NG+HSD shortly and see if that holds up without any spread issues, as we had high TTXSP1 when running on liquid fuel alone (flow dividers might be the suspect here, any more insight is welcome, although that maybe another topic for another thread!)
Thanks for the information regarding the command state block, however any more clarifications is gladly accepted. I am working as GT & STG panel operation engineer and any help in clearing the logic rungs is whole heartedly appreciated. Can you please mail me the documentation GEH-6195 as I am unable to find them in our GE libraries?

 

Hello

I permit myself to add some notes on this thread.

L43FM1 IS Signal name Mix fuel selected on the CMDSTATE Block

You got to check behaviour of this signal during Mix fuel operation

As @WTF? mentionned , The block is built by contact command preset and Mask binary contact and signal values

Mask bits signal is described in GEH-6195D like set of conditions stored in two two bytes configuration ( preset , permissives) that the command variable must pass before it is passed to the state output

Any time!
@ControlsGuy25

 

And without be able to check the Mark6e application code I don't know how the blcok is been configured for mixed fuel operation ...i mean I got one app code Mark6e with Preset for L43FML is setted to False
How about your app code?

 

" I permit myself to add some notes on this thread."
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"Any time!"

WTF???

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As for flow divider [singular] being the cause of high exhaust temperature spreads while running on liquid fuel, the purpose of the liquid fuel flow divider is to divide the liquid fuel flow-rate coming out of the high-pressure liquid fuel pump (controlled by the liquid fuel bypass valve) into 10 equal flow-rates so exactly the same amount of fuel gets delivered to each combustor fuel nozzle. This serves to help to ensure the same amount of fuel is being burned in each combustor.

There are a couple of types of liquid fuel flow dividers, but the most common type used on smaller GE-design heavy duty gas turbines is a horizontal liquid fuel flow divider which has ten "wheels" connected together as a common shaft, and it's HIGHLY unusual (but not impossible) for the wheels to become separated and not turn at the same speed.

The usual cause of high exhaust temperature spreads is something blocking the fuel flow through the liquid fuel portion of the fuel nozzle (usually metal debris on new machines). Liquid fuel check valves also commonly fail, and this can lead to poor atomization or low fuel flow. Dual-fuel machines will have liquid fuel purge systems and often the liquid fuel purge check valves fail and allow some or all of the liquid fuel flow to one or more combustors to go to the gas turbine drains tank. Some types of liquid fuel nozzles are also known to "come apart" internally which causes poor atomization and high exhaust temperature spreads.

But it's not usually the liquid fuel flow divider that causes high exhaust temperature spreads. Could be, but not very likely.

 

Revisiting this thread after a long time, sorry for the delay. GT mix fuel run was successful. Thanks to everyone who helped me in answering my questions. L43FG, L43FU, L43FM block is clear to me now. The high spread issue was solved was replacing the nozzles, check valves and fuel flow divider assembly. As for the lower percentage of liquid fuel limit during mix fuel operation, i think one more reason to not set a lower limit less than 10% would be high recirculation of the liquid fuel (HSD or naphtha) in the warren pump bypass valve which will lead to increased temperatures in the warren pump discharge line as it is a high pressure pump and liquid fuel has low flash point values as well. AS per my opinion i think its imperative to end these threads with proper status updates as it will help anyone with similar queries. Thank you!