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Limit Switches on Compressor Bleed Valve
Looking for detailed explanation about limit switches on compressor bleed valve.

Good day,

Please i need detail explanation on working principle of limit switches 33cb on compressor bleed valve VA2-1 and VA2-2, and logic state of the limit switches when in open and close position.

Thanks

2 out of 3 members thought this post was helpful...

Don't you have a Device Summary, Control Spec. or a set of drawings? The limit switch is closed when the bleed valves are Open (de-energized).

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

You didn't say what Frame unit you are referring to, nor who the packager of the unit was.

IN GENERAL, the axial compressor bleed valve limit switches are installed as normally open switches--which are actuated by a lever on the bleed valve shaft to close the switch when the bleed valve is fully open.

In other words, the switch contacts are to be held closed when the compressor bleed valve is fully open. To protect the axial compressor during starting and acceleration the compressor bleed valves must be fully open prior to initiating a START--and the closed contact (actuated by the lever) provides the indication to the turbine control system that bleed valves are fully open.

Also the bleed valves must be fully open during shutdown to protect the axial compressor. If the fuel stop valve is still open during shutdown and the compressor bleed valves are not fully open the turbine control system will trip the turbine by immediately closing the fuel stop valve.

Hope this helps!

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

We had a problem with the bleed valve twice before during shut-down.

During turbine shut-down, the bleed valve should be opened when the turbine speed reaches 95% (14HS - Operating Speed). The problem was that the mechanism that pushes the limit switch didn't fully reach out. The bleed valve opened at 95% but the feedback signal didn't go to the Mark V, so the control 'thought' it was still closed and tripped the turbine without executing the shut-down sequence. We just needed to replace the adjustment screw with a longer one.

Put your e-mail and I will send you some photos.

Hi,

here is my email j_omizu@hotmail.com. I am interested in the pictures.

thanks

By 6bturbinetech on 26 March, 2018 - 7:49 am

My concern is, if the bleed valve actuated the switch in the past, why does the screw need to be changed?

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

A good wiring diagram guide for reference
http://www.daltco.com/sites/daltco.com/files/resource/schneider-wiring-diagram-book.pdf

Regards!

Bleed Valves in heavy duty gas turbine are used to protect the axial compressor during start-up and shutdown against stalling and surging conditions. These valves are typically open during start-up, acceleration, normal stop & shutdown.

L20 CB is the signal assigned to the Solenoid valve which control the air taken from axial compressor (stage).

the bleed valves are spring return valves, normally open. during the start up when the machine accelerate & just before complete sequence, the SOV will be energized allowing the air taken from the axial compressor stage, to actuate the bleed valves than L33CB1, 2 change status feedback.

during shut down the SOV de-energizes, cutting the air from the bleed vales actuators, than both bleed valve will open.

Then what will be the logic for l33cb, when the valve is in open state and when it is in closed state?

Thanks

Read your drawings Logic "1" & "0"

dannys,

Typically, the logic signals associated with 33CB-1 and 33CB-2 are l33cb1o and l33cb2o. The limit switches are usually wired to the normally open contacts, and the switches are actuated when the valves are closed--which means the contacts will be closed when the valves are closed. The inputs are usually NOT inverted, so l33cb1o and l33cb2o will both be logic "1"s when the compressor bleed valves are open.

Learning to "read" signal names is very helpful when trying to learn and understand logic and signal names. Most logic signals names are written so that they describe when the signal will be a logic "1". For example, l33cb1o:


l33cb1o
| |||||
| ||||o for open
| |||1 for the number 1 valve
| ||b for bleed valve
| |c for compressor
| 33 is the ANSI device number for a limit switch
l for logic signal
So, the signal name literally reads: "the logic signal for the #1 compressor bleed valve will be a logic "1: when the #1 compressor bleed valve is open." And, the same for the #2 compressor bleed valve limit switch signal--it will be a logic "1" when the #2 compressor bleed valve is open. The last character, being a lower-case 'o' means open.

Now, not ALL logic signals are written exactly that way--but the overwhelming majority of them are. And, if you obtain a copy of the ANSI device numbers and commit them to memory you will help yourself immensely when learning GE-design heavy duty gas turbine control signal naming.

Hope this helps! By the way, a couple of years back, there was a series of posts about how to "read" GE logic, beginning with the most important logic signal of all: L4, the Master Protective Logic signal. It was a pretty good introduction to signal reading and naming. The GE-design heavy duty gas turbine controls community here at control.com has been pretty active for more than 15 years, I believe, and there are LOTS of posts about LOTS of things that are all accessible by using the 'Search' feature at the far right of the Menu bar of every desktop version of every control.com webpage. It's recommended that you use the Search Help the first few times you use the 'Search' feature until you get the hang of it. But, it's fast and thorough. It's one of the best features of control.com--being able to see past posts, and because so many people post feedback about the information or help they receive to their posts it's even MORE helpful because you can see what worked, or what didn't. Feedback is what makes control.com very useful; you haven't provided much feedback in the past but we hope you do going forward!

Great answer from CSA as always.

"The limit switches are usually wired to the normally open contacts, and the switches are actuated when the valves are closed--which means the contacts will be closed when the valves are closed.The inputs are usually NOT inverted, so l33cb1o and l33cb2o will both be logic "1"s when the compressor bleed valves are open."


Assuming that l33cb1o and l33cb2o are not inverted, I believe you were trying to say that when the signal is 1 "valve is open" then the switch will be closed since the signal is not inverted.

I endeavour NEVER to assume anything. But I am human, after all.

Look, these two inputs are rarely inverted. The switches are actuated when the valves are closed, and so that means if the inputs to the Mark* turbine control system are wired to the normally open contacts of the switches the contacts of the switches will be closed when the valves are open. And, the signal names are usually l33cb1o and l33cb2o (or L33CB1O and L33CB2O), which were chosen to indicate when the signals would be a logic "1"--when the respective compressor bleed valves are Open.

It's no more complicated, or simple than that. Switches actuated when the valves are closed. Normally open contacts of said switches will be closed when the valves are open. Non-inverted inputs will be logic "1" when the valves are open (and said switches are actuated and contacts of said switches are closed when actuated).

It's not rocket science. Not even close.

Thanks

is it applicable to l33hr and l33cs? looking forward to hear from you.

dannys,

>is it applicable to l33hr and l33cs? looking forward to hear
>from you.

Are you providing the ENTIRE signal name? Because l33hr is usually l33hrf, and l33cs is usually 33cse.

In these two specific cases, the inputs are typically NOT inverted, and--again--the ENTIRE name describes when the logic signal will be a logic "1":

l33hrf is a logic "1" when the hydraulic ratchet has completed a forward stroke
l33cse is a logic "1" when the starting means clutch is ENGAGED

Can you see how the ENTIRE signal name is useful when trying to understand when the signal will be a logic "1" (and, conversely, when it will be a logic "0")?

In the case of l33hrf, when the hydraulic ratchet mechanism completes a forward stroke, l33hrf will go to a logic "1"--and it WILL REMAIN a logic "1" until the hydraulic ratchet retraction stroke is completed.

Thanks a lot.

So when i later check a running unit, i found out that the logic for l33cb1o and l33cb2o is "0".

dannys,

>So when i later check a running unit, i found out that the
>logic for l33cb1o and l33cb2o is "0".

The compressor bleed valves MUST be open during starting (prior to a START), and during shutdown. That's why there are limit switches to indicate when the valves are open. And, the limit switch is to be actuated when the compressor bleed valve is fully open, not partially.

The compressor bleed valves are extremely important to the protection of the axial compressor during starting and shutdown. They must be open, and they should be fully open.

Both of the limit switches are used as part of the Start Check permissive string--because it is so important for the compressor bleed valves to be fully open prior to initiating a START.

Once the unit gets to 14HS or FSNL or sometimes shortly after generator breaker closure, the compressor bleed valves are told to go closed (by energizing l20cb1x (that's the usual signal name)). And, as soon as the valves BEGIN to close the open limit switches will be de-actuated and the normally contacts will open and the logic signals will go to a logic "0".

During a shutdown, there is a short timer that looks for both compressor bleed valves to be fully open after l20cb1x is de-energized, and if both logic signals don't indicate both valves have gone open in the prescribed time the unit will be tripped.

Hope this helps!