GE 9FA gas turbine tripping on "Loss of blocking" alarm. What is meant by loss of blocking or LOB

 

@yogeshji,

When does this happen?

Is it happening often?

Which version of Mark* turbine control system is used to control and protect the machine?

Do you have any recorded data from the trip(s) you can share? (Most EVERY Mark* has the ability to view a Trip History display, which has some important information (not EVERYTHING--but the most important data) and a list of alarms prior to the trip. Files can be attached to posts here on Control.com. If you have a Mark* VIe turbine control system, it's a little more difficult to share data from the Trip History display, but not impossible.

And--MOST IMPORTANTLY--what alarms (Process AND Diagnostic) were present BEFORE and at the time of the trip? (Even if you think the alarms aren't relevant, often they can help understand the problem.)

Do you have someone at the site that is familiar with the Mark* system and can use the available tools on the HMI to troubleshoot the problem? Have they tried reviewing the sequencing/application code to try to understand the situation?

I don't know how you relate a "purge" valve to "blocking"--unless you are talking about the natural gas system purge valves (two of them per system). But, we need more information, and the list of alarms would be MOST helpful. Without any more information we can't be of too much help (unless someone is familiar with GE Belfort sequencing and cryptic alarm text messages).

 

@yogeshji,

The typical GE-design heavy duty gas turbine fuel gas purge system for DLN combustors uses a combination of two (2) valves which are usually mounted very close to each leaving a small space between them. These valves are OPEN when the particular gas fuel manifold is being purged (so when gas fuel IS NOT flowing through a particular gas fuel piping/manifold, and CLOSED when gas fuel IS flowing through this particular gas fuel manifold system. (There are multiple gas fuel piping systems/manifolds used on a DLN combustor.) This small space between the purge valves uses a pressure switch to detect if either valve is leaking when gas fuel is flowing in that particular gas fuel piping/manifold. (When there is NO gas fuel flowing in a particular gas fuel piping/manifold the two valves will be open and compressor discharge air will flow through both valves and into the gas fuel piping/manifold to prevent backflow of hot combustion gases through the fuel nozzles and into the axial compressor discharge.)

When gas fuel IS flowing through a particular gas fuel piping/manifold and the two (2) valves are closed the purpose of the pressure switch is to check to make sure that neither purge valve is leaking into the space between the two valves. Because if either valve is leaking then there's a high possibility of a serious problem when gas is flowing into the space between the two valves, or if both valves are leaking then there's a likelihood that gas fuel will flow into the axial compressor discharge and become ignited outside of the combustor and result in serious damage to the machine.

This problem can often occur if the actuator of either or both purge valves are not adjusted correctly. The hot axial compressor discharge air can make the valves not seat fully under certain conditions if the actuators aren't adjusted properly and result in a leak (or leaks). Often, all that's required is to perform a check of the actuator adjustment to make sure the valves are seating fully and won't leak. Sometimes, if the valves are old and haven't been refurbished, the valve seats and/or plugs may need replacing (some valves had replaceable seats and plugs, other valves did not; we don't know which type your machine has.)

Finally, there should be a P&ID (Piping & Instrumentation Diagram) for the Gas Fuel Purge system, and with that drawing you can see the two gas fuel purge valves for each gas fuel piping/manifold which requires them. Those valves usually have a specific device number--an example of a device number is VA13-1, for example (that may or may not be the device number used on the gas fuel purge valves on your machine). When writing to a World Wide Web forum like this it's extremely helpful is you can provide the device number(s) of the devices you are having problems with or that you suspect are having problems. I had to guess in my first response--and actually, I'm guessing in this response.

IF the "purge valve(s)" you are referring to are some other kind of valve in another system, please disregard all of the above.

 

Thank you for the information, i was referring to VA13-1 only.