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Low Hydraulic Pressure Lead to Closing GCV
Low Hydraulic Pressure closes GCV but IGV remains stable
1 out of 1 members thought this post was helpful...

Greetings to all

The unit I speak about is 7FA turbine.

Recently we had a trip on loss of flame because the hydraulic pressure went very low (650 Psi). the standby-pump started and got the pressure back to normal after 5 seconds. I believe the accumulator was not charged properly because there was no reaction to the decrease of the hydraulic pressure.

When the hydraulic pressure was decreasing, the WATT was also decreasing with it linearly. but the IGV remained in its position which caused Loss of Flame Trip.

My question is that, Based on my understanding of GCV and IGV control. Once the feedback equals the reference, the oil will be locked inside the actuator. So a loss of hydraulic pressure should not affect the control unless you want to actuate the valve. Maybe closing of GCV valve happened because it is constantly moving 0.5% up and down, so it is permitting a small amount of oil to the actuator. IGV was not affected because the degree usually is stable not moving up and down. this is my interpretation of the situation happened.

Anyone can comment and clarify?

ONE IMPORTANT NOTE: I said in the thread that the decrease in WATT is due to GCV closing. I am not sure about that, maybe the SRV was the one closing. I will plot P2 and GCV positions in the near time and see.

The main purpose of this thread is:

1- What is the minimum hydraulic pressure that the turbine can survive without any problem. I heard it is around 700 Psig. Is that true.

2- What happens to the GCV and IGV if the hydraulic pressure gets very very low and why ?? (for example huge leak in the hydraulic line). Is the scenario while on base load operation will be different from part load operation ? because if the hydraulic pressure gets really low, sometimes the turbine will trip on loss of flame and sometimes will trip in high exhaust temperature.

3- what happens to the GCV and IGV if the hydraulic pressure is fluctuating and why?

Thank you very much in advanced

1 out of 1 members thought this post was helpful...

Aptx4869,

> 1- What is the minimum hydraulic pressure that the turbine can
> survive without any problem. I heard it is around 700 Psig.
> Is that true.

I don't know if there's any "minimum survival" pressure published anywhere in any documentation. BUT, if the hydraulic supply pressure can't be maintained above approximately 1200 psig with two AC motor-drive Auxiliary Hydraulic Pumps, there's a problem (a real problem). Every site I have ever worked at or assisted will find and resolve any hydraulic pressure problems well before trying to start/run at less than approximately 1200 psig. Since this is an F-class machine, is Aux. Hyd. Pump pressure used for bearing lift oil supply, and is there a solenoid/valve arrangement on the Aux. Hyd. Pump to control flow/pressure to the lift oil system that may be malfunctioning?

>2- What happens to the GCV and IGV if the hydraulic pressure
>gets very very low and why ?? (for example huge leak in the
>hydraulic line). Is the scenario while on base load
>operation will be different from part load operation ?
>because if the hydraulic pressure gets really low, sometimes
>the turbine will trip on loss of flame and sometimes will
>trip in high exhaust temperature.

What will usually happen is that because the hydraulic supply pressure is low the servo current output will increase (in the negative direction) to try to maintain position versus reference. The GCV has a large closing spring to overcome with hydraulic pressure, and the IGVs are usually trying to open--because of the flow of air across them and so the turbine control system is trying to keep them from opening more than the reference. (There's no spring on the IGVs trying to close, or open, them. It's a double-acting piston trying to keep actual position equal to position reference.) If hydraulic pressure dropped quickly, and the accumulator wasn't working (and it can only respond to short hydraulic flow-rate increases--it has a limited volume, after all) then the IGVs and the GCV could be greatly affected. And, if the lag Aux. Hyd. Pump came on when the pressure was low while the unit was at or near Base Load, particularly, it's possible to envision a scenario where the servo current on the GCV was high trying to hold position and the sudden pressure increase caused a spike in gas fuel flow-rate which caused an exhaust overtemperature trip. And, if the unit survived the start of the lag Aux. Hyd. Pump without tripping on high-high exhaust temperature then it could very well trip on loss of flame if the hydraulic supply pressure didn't build back up to normal and/or kept dropping.

When the unit is operating at Base Load, the actual exhaust temperature is only 40 deg F (typically) less than the high-high exhaust temperature trip setpoint.

And, if the hydraulic supply pressure doesn't get restored fairly quickly (within a couple of seconds) of the start of the lag Aux. Hyd. Pump, then it's probably a certainty there's something seriously amiss with the hydraulic system (possibly a problem with the valves of the accumulator???).

>3- what happens to the GCV and IGV if the hydraulic pressure
>is fluctuating and why?

The turbine control system is going to be quickly changing servo current trying to keep the actual positions equal to the position references.

Lastly, tripping F-class turbines, repeatedly, IS NOT good for them. They are not the most robust machines around, and the OEM does a LOT of work on trip reduction to protect their F-class fleet, in particular. Trying to determine what the minimum hydraulic pressure a machine can start and run on without tripping is asking for trouble--now and in the future. Every trip is an incrementor in FFH (Factored Fired Hours) and maintenance planning. There is a limit to what a machine (and especially an F-class machine) can withstand.

CSA,

I really appreciate your constant presence for helping people. I hope you can bear with me a little and elaborate as much as possible

The piston hydraulic pumps supply both hydraulic and lift oil. but this has nothing to do with our problem. The hydraulic pumps will stop supplying the lift oil lines once the turbine speed exceeds 50% of rated speed.

What triggered my questions are the two trips happened in the previous months.

One is Loss of flame trip which I described in the first post of this thread.

The other trip was high exhaust temperature trip while the unit was operating on part load. the hydraulic oil line supplying the IGV valve bursted and the hydraulic pressure dropped sharply. what puzzles me is that before the sharp drop the IGV reference was around 70 degrees.Immediately after the sharp drop, the IGV reference was 29 degrees !! and the feedback was 21.9 degrees which is the mechanical stop. How the IGV closed during the sharp hydraulic drop ? I don't know. And why the IGV closed lower than the reference. As I mentioned the reference was 29 and feedback was 21.9 degrees.

>What will usually happen is that because the hydraulic
>supply pressure is low the servo current output will
>increase (in the negative direction) to try to maintain
>position versus reference. The GCV has a large closing
>spring to overcome with hydraulic pressure

I believe that you are trying to say the following:
The hydraulic pressure dropped --> As a result, the GCV starts closing and the error between the reference and the feedback starts increasing --> Since the error increased the turbine control system will increase the servo current trying to get the error back to zero.

> the IGVs are usually trying to open--because of the flow of air across
>them and so the turbine control system is trying to keep
>them from opening more than the reference. (There's no
>spring on the IGVs trying to close, or open, them. It's a
>double-acting piston trying to keep actual position equal to
>position reference.)

Effect of low of hydraulic pressure on GCV is clear now. Please elaborate on the IGV. Why the IGV changed the reference in the trip I mentioned above?

What causes the IGV to suddenly change the reference like in this case? Why it has gone to mechanical stop position?

>The other trip was high exhaust temperature trip while the
>unit was operating on part load. the hydraulic oil line
>supplying the IGV valve bursted and the hydraulic pressure
>dropped sharply. what puzzles me is that before the sharp
>drop the IGV reference was around 70 degrees.Immediately
>after the sharp drop, the IGV reference was 29 degrees !!
>and the feedback was 21.9 degrees which is the mechanical
>stop. How the IGV closed during the sharp hydraulic drop ? I
>don't know. And why the IGV closed lower than the reference.
>As I mentioned the reference was 29 and feedback was 21.9
>degrees.

Aptx489,

We sympathize with your frustration--but without 1) actionable data, and, 2) the ability to review the application code running in the turbine control system at your site, it's virtually impossible to say much more than has been said.

Can you briefly state how one can review the application code. I once tried to access the block diagram of the IGV in my site but couldn't. Some blocks are not accessible meaning that you can not see what is inside them.

What happens when you right-click on the block?

Aptx4869,

On the HMI(s) supplied with the unit, there are LOTS of GEH-nnnnn.pdf files, which are the instruction manuals and user guides for the various products GE Salem (Virginia, USA) publishes.

GEH-6700.pdf appears to the the ToolboxST User Guide for Mark VIe Control. GEH-6700U.pdf would refer to Revision U of GEH-6700.

There are LOTS of manuals, for various products, present on almost every GE HMI shipped with turbine control systems--many for products not on that particular HMI.

There is also a pretty good 'Help' feature of ToolboxST available at the Menu bar at the top of the application. There are usually at least three ways to search for various topics.

Algorithmic-type "blocks", such as TTXSPLVn, usually have what are called 'Item Help' or 'Block Help' descriptions. (The 'Vn' at the end of the block name stands for the Version number, where 'n' is a whole number.)

To easily access the Item Help or Block Help description for a block, simply right-click anywhere in the block's rectangle and either a small pop-up or dialog window will appear. If it's a pop-up window, there should be a selection 'Item Help', and if you (left-)click on it if there is a description supplied by GE with the block it will pop up in a new window. The window contents can be printed for a hard, paper copy of the description, on which you can use colored high-lighters and make notes to help you understand what is happening.

If there is no description supplied with the block by GE, then a dialog window will open with some message to the effect "No help is available for this block." Most turbine-specific blocks (such as the Allowable Exhaust Temperature Spread algorithm (TTXSPLVn) have descriptions.

GE is now using more and more functions called 'macros.' These are functions which can be used many times in a single unit's application code or across many different turbines and are assembled by a programmer from available basic functions in ToolboxST, such as MULTiply, or SUBtract, or SQuareRooT, etc.

Most macros DO NOT have descriptions in ToolboxST--but do not despair. If the block is a macro, you will see some single-line name in the tall, narrow Tree View pane at the left of the ToolboxST window. I believe, if you click on a macro block in the main pane on the right side of ToolboxST window, the macro block will be highlighted in the Tree View pane at the left. To the left of the highlighted macro block name will be a "+"--indicating it can be expanded. Click on the plus sign, and Voila'! The block will expand into the various basic functions, all connected as the programmer intended, for your viewing and analyzing pleasure.

Now, before you complain that the block help files or the macro block basic functions are too difficult to understand realize that it's all just basically following a signal into the block from the left side of the block (inputs are almost ALWAYS shown entering from the left side of a block) through the block (by following a simple line through the block as it is ADDed or MULTiplied or COMPared or whatever action is happening to some output(s) on the right side of the block.

Most blocks and macros are or can be graphically described as some kind of relay ladder diagram--which is just about the simplest way to describe logic there is. (If you, or anyone reading this, learned some other method of logic writing/reading--you will think RLD (Relay Ladder Diagrams) are incredibly difficult. They're not. It's just not what you know or have already learned.) So, just sit down and work through the block--we're here to help--and learn it, or be doomed to what most people who don't take the time or have the motivation to learn GE logic say: "It's a CLOSED architecture; no one can understand it." Which is simply, patently and undeniably false.

This is where many people fail when trying to learn and understand the "Mark VI" or the "Mark V" or the Mark VIe" or whatever Mark they have to work with. They go to "Mark [whatever]" school and think they are going to learn how the turbine operates. FALSE. They don't teach people how turbines operate at Mark [whatever] school. They teach people how to click around inside a window and how to put flashing buttons on CIMPLICITY HMI display--but they NEVER teach how to read relay ladder diagrams, or how to read block descriptions. It's just ass-u-m-e-d that everyone and anyone can read relay ladder diagrams and interpret GE graphical descriptions.

If you want to know how YOUR turbine operates--learn to read relay ladder diagrams as GE uses them to describe blocks which are used to operate the turbine(s) at your site. Do this, AND learn to read the P&IDs (which GE doesn't teach, either)--and you will have skills that will serve you very well in your career. GE P&IDs are about the simplest P&IDs ever created, though that is changing (and not for the better). The way your turbine operates is described in the logic (application code, as GE calls it in Mark VI and Mark VIe turbine control systems)--NOT in any sentence or paragraph in any manual or on any World Wide Web site. It's what's programmed in the machine at YOUR site that determines how the turbine at YOUR site operates. That, and the P&IDs (which depict MANY of the devices that either provide inputs to the turbine control system, or receive outputs from the turbine control system, or affect how the turbine control operates. And, that can only be determined by learning how to read the logic in the turbine control system and the P&IDs for the turbine(s) at YOUR site.

Sure there are some "generic" blocks and functions, but sometimes they get modified--either at the Customer's request, or because the commissioning person perceives it needs to be changed. Or, some unique function is required for some unique reason, in which case sometimes a generic function is modified, or some entirely new function is created/written/programmed. BUT, that's all still in the turbine control system of YOUR turbine.

Hope this helps! (I'm not just writing this to you, Aptx4869; many people read these threads and try to learn how their turbine is operating.) If you have questions, or need clarification, we're here to help.

And, the manuals provided by GE with their equipment are always a good place to start. GEH-6700 isn't going to teach you how your turbine operates, but it's going to help you understand how to use ToolboxST to understand and troubleshoot your turbine as you learn how it operates.