Hi,

What is the safety aspect for Gas Leak test? I saw old machines which run without any gas leak test. Is there a regulation to conduct Gas Leak test as mandatory?

Is there any instance where there is a catastrophic failure & hampered safety of plant personnel due to GLT.

I would like to know how the below constants were calculated.

K86GLTA---Gas Leak Test A Fail Calculation Pressure
K86GLTB---Gas Leak Test B Fail Calculation Pressure
K86GLT1---Gas Leak Test A Time constant
K86GLT3---Gas Leak Test B Time Constant

Regards,
Controlquest

 

Controlquest,

The Gas Leak Test is a test which has been implemented to protect against stop valve leakage which could lead to an explosion, damage to equipment and possible loss of life.

You are correct in that no such test was performed for decades.

I believe there is an API standard which at the least recommends some kind of test of the stop valve prior to starting. Some parts of the world are mandating this test, but this particular OEM (Original Equipment Manufacturer) has decided to implement it across the fleet as a safety precaution--to protect against lawsuits and monetary damages.

I'm not personally aware of any instance of an explosion attributed to a leaking gas stop valve, but it certainly is something to be concerned about with the increase in gas-fired plants and turbines. Part of the problem has been that there has been no OEM recommendation for any kind of gas valve leak test other than periodic maintenance of the gas valve assembly. So, most sites are blissfully unaware of a leaking gas valve--unless someone notices a problem when making rounds (which very few sites do these days).

The design of the typical gas fuel valve assembly/compartment on GE-design heavy duty gas turbines is such that if the stop valve leaks there is a vent between the SRV (Stop-Ratio Valve) and the Gas Control Valve(s) that vents pressure between the two valves to atmosphere through a goose-neck vent above the compartment where the gas valves are located. This could lead to an issue if there were a spark or open flame nearby and the magnitude of the leak was severe enough.If the gas fuel is odorized, someone making rounds could detect the smell of gas and alert the maintenance department to a problem. If the gas is not odorized, well, then detecting a leak is difficult without some kind of sensor or some kind of test like the gas leak test.

It should be noted that on combined gas valve assemblies (ones where the SRV and GCV are located in the same casting) the valve packing leak-offs are also directed to the same goose-neck vent so the source of gas coming out of the goose-neck vent must be verified to prove it is coming from a leaking stop valve or a packing leak-off.

The OEM has a Safety Review Board which periodically reviews technical regulations and standards (like API) for new or amended requirements, as well as investigating any "close calls" which were reported to the OEM. They also look at failures which resulted in damage or human injury/death. They then make recommendations about new standards for safety to be implemented on their fleet. This includes new units and any OEM control systems being provided as upgrades or retrofits to existing control systems.

And, of course, there is always the Legal Department review of these matters along with their proclamations to protect the OEM against lawsuits and monetary damages--but, worse, bad publicity--which might arise from prudent implementation of safety measures, whether they are required by local technical regulations or standards.

For sites which have been operating for years or decades without the gas leak test which purchase new OEM control systems provided with the gas leak test because the supplier deems as the OEM they are "required" to provide this new safety feature, it can quickly become a problem if there are leaks which haven't been known about for years which suddenly become a problem when the new control system is being commissioned. This causes frustration and some very heated, at times angry, exchanges during commissioning, or sometimes shortly thereafter. Part of this is that the purchaser was not made aware of the new test and its purpose, but part of it is also that the equipment may have been operating for some time with an unknown problem--and discovering it during starting after installing a new control system is usually not an advantageous time for anyone to have to stop and repair a leaking valve when spares may not be immediately available.

As for how the Control Constants you have listed are calculated, there are formulas depending on which version is in use. And, the formulas use industry-standard limits for acceptable leakage when calculating the alarm- and trip levels. And, the worse thing about this test is the lack of documentation from the OEM about its purpose and how the limits are decided upon.

If the cause of a failure of the Gas Leak Test can be attributed to an actual leaking stop valve and not incorrectly calculated Control Constants the issue becomes will the site make the necessary repairs to rectify the situation. Usually, the "discussion" (really, it's degenerated to an argument at this point) centers around what is an acceptable limit, or, how much deviation from the Control Constant values can be "safely" tolerated until such time as repairs can be affected. And, again, without OEM documentation the answer is not possible to define.

But, even if it were, there are still sites which would decide to bypass the protection and continue to run the unit.

So, it's really down to two things:

1) Is the stop valve leaking?

2) How much leaking is acceptable?

In the first case, that's not too difficult to determine. If the valve is found not to be leaking, then the accuracy of the Control Constants comes into question. Without OEM documentation about how the Control Constants are calculated it's difficult to determine if they were calculated "correctly" and how to change them to be "correct."

If the stop valves is found to be leaking, then it's the Owner's/Operator's decision about how to proceed. If there was OEM documentation about how the Control Constants were calculated then an intelligent decision could be made about how to proceed. Without documentation, the argument, ..., er, ..., uh, ... discussion is at a stalemate because there is no reference about how the limits were derived at and what is an acceptable deviation (and for how long--until repairs can be made).

In the second case, as above, if the valve is found to be leaking then what's an acceptable leakage limit? And, usually, what is an acceptable deviation--and for how long can the deviation be "safely" tolerated--until repairs can be made? Again, without OEM documentation the decision is difficult.

But, in the end, it's the site Owner's/Operator's decision. It's just like vibration levels--sometimes an Owner/Operator decides to keep running for some time because the OEM vibration trip level is deemed not applicable for the situation. It's really about how much risk the site Owner/Operator is willing to accept. Without OEM documentation the decision is much more difficult--but, even when OEM documentation was available there will still be sites where the decision is made to keep running the unit with a leaking stop valve because they deem the OEM recommendations don't apply in this situation.

If the unit is new and under warranty from the OEM/supplier, then the OEM/supplier should be tasked with making the appropriate determinations: Is the stop valve actually leaking? Should it be repaired/replaced? Or, are the Control Constants unnecessarily "tight"? And, without documentation even the OEM is hard-pressed to justify their position if they decide the leakage is safely acceptable at the present time.

I wish I had better information--but these are the facts of the situation with the Gas Leak Test. And, again, it's always up to site Owners/Operators to overrule any document or recommendation in order to meet their deliverables. OEM documentation would be extremely helpful in making their determination, but, in the end--especially if the stop valve is leaking (and it usually is in these cases)--the ultimate decision is up to site Ownership/Operations.

I also wish you would provide more details about the situation at your site. Is this a new unit installation? Is this a control system upgrade? Is the gas stop valve actually leaking? (A lot of times, the control system gets blamed for causing trips when no determination has been made about whether or not the condition is real or just "another" control system problem.... Again, this is an all-to-common site Owner/Operator perception whenever a "new" (previously unexperienced) trip occurs.) Others learn from these posts, so the more information you can provide the more people can understand and be informed.

If you want some help with calculating these Control Constants, you would need to provide some more information--but, without being able to see the version of the Gas Leak Test running in the turbine control system at your site it would be difficult to provide a concise answer. The information you would need to provide would be the Control Constant values FPKGNG, FPKGNO, TNK14HM1, as well as the actual firing speed of the unit experiencing the problem, as well as the maximum P2 pressure during the Gas Leak Test. It would be best if you could provide the P2 pressures at 1 second intervals during the Gas Leak Test to be most helpful.

Hope this helps!

 

The constants Controlquest refers to are part of the "bottle test" that is done during each startup and shutdown. K86GLTA is part of the SRV test, where the vent valve is closed and the P2 pressure monitored for excessive increase. K86GLTB is part of the vent valve and control valve test, where the SRV is briefly opened, allowing P2 to see line pressure. Then the SRV is closed, and the P2 pressure checked for excessive decay, indicating vent valve or control valve leakage. Additional tests are performed when the vent valve is opened to check that P2 pressure decay is within limits. The calculation methods would be similar to what you mentioned, based on acceptable leakage rates and the volumes involved, and verified by testing. The obscurity of the engineering behind these constants, outside of the OEM's engineering staff, is also similar.

 

Hello CSA,

That is really great answer. I will get back to you by pulling those constants from my site.

Regards,
ControlQuest

 

ControlQuest,

Please provide the current values of the four gas leak test Constants, as well.

And please tell about the situation the site is in. Is this a new unit (turbine) installation or a controls upgrade/retrofit?

And, what is the actual firing speed of the unit?

And most importantly, has it need been determined if the SRV or GCV is actually leaking?

Thanks in advance for helping others in their desire to learn and understand!

 

Hello CSA,

Good Morning.

Below are the requested constants.

TNK14HM1- 14.0%
K86GLTA- 0.17
K86GLTB- 0.93
FPKGNO-- -38.5
FPKGNG-- 4.60
K86GLT1-- 30.0Secs
K86GLT2-- 6.0Secs
K86GLT3-- 10.0Secs

It is retrofit project...TA adjusted some constants and test got passed now machine was running succesfully so I am curious to know how K86GLTA & B were arrived & on what basis they tune it.

Regards,
Controlquest