3-Way Purge Valve Coking in the Purge Air Port

Greetings,

We had some 3-way purge valves passing liquid to the can while on gas operation. We have opened one of them and there was small coking in the purge air line port. So there are two problems liquid passing to can + liquid present in the purge port. I don't know why, but one of the GE told me that this is the common place for coking (he means usually coking happens in the purge air port). I can't understand why this happens, because this tells us that there was diesel in the purge port.

Thank you in advance and here is the picture of the 3 way valve.

https://postimg.org/image/pwr6w4e1z/
 
Dual fuel systems--<b>particularly</b> on GE-design F-class machines--are an engineering work-in-progress. Studying the P&IDS and reviewing the physical layout of the systems (multiple systems) involved--AND the sheer number of devices and components should just re-enforce the concept of an engineering work-in-progress.

The system designs change with just about every turbine (or group of turbines built for a multi-unit site), and will likely continue to evolve and "improve" (because the system designers believe every design or component change is an "improvement ") for decades.

If cost were no impediment, then certainly the systems would be more reliable and robust. But, every buyer/site wants to spend the least possible while getting the most value--and that value for money is determined by the supplier, until the buyers get educated and have experience to specify and/or review systems and system designs and layouts to evaluate them for suitability.

Once the units are past their initial warranty the owners/operators are free to make any design or component system changes they deem necessary and/or prudent to improve reliability. While few sites actually undertake such changes, some have proven to be very successful and have resulted in greatly improved reliability.

One of the things I have encountered which has caused quite a few problems with reliability is the poor attention paid to assembly/reassembly of systems, particularly after maintenance outages. MANY fitters and laborers DO NOT use proper practices when assembling or reassembling tubing, and I'm specifically referring to the use of backing wrenches and tightening procedures. The sheer volume (number) of tubing fittings in these systems is staggering, and many times loose fittings found during troubleshooting are not reported--but do contribute greatly to issues. Site personnel have to develop--and enforce--the use of proper tubing reassembly practices and procedures and ensure that workers are trained and follow the procedures and practices. Sites which do this seem to have fewer issues than before the procedures and practices were put in place.

When liquid fuel is found in places it shouldn't be or isn't anticipated, one has to remember that for liquid fuel to get there it had to have a path to flow to get there. And loose tubing fittings allow for air and eventually liquid fuel to flow to places where it shouldn't be. That's why fitting tightness is so important--because even if devices/components (check valves; solenoid-operated valves; etc.) are working properly if the tubing fittings on those devices aren't tight then air and eventually liquid fuel can flow to where it shouldn't be.

We fully empathize with your situation, but the system designs are not stable and the physical layout of every site is usually a little different--enough so to make for big problems in small things. Another thing which is commonly encountered when troubleshooting problems like this is that the Notes which are written in P&IDs and Piping Arrangement drawings about distances and piping/tubing pitches (angles) are never fully implemented during construction. Many times piping/tubing has several low points and no high-point vents that were specified in the Notes which can mitigate some problems.

The take-away here is: It is (They are) your turbine(s), and while you do have a reasonable expectation that the supplier should provide a reliable system there are few guarantees in life or business. There are MANY so-called PCs on drawings (including P&IDs)--Purchaser's Connections--which means the constructor has to connect between these PCs, following the information in the Notes as well as sound engineering and construction practices--in order to construct a reliable system. If proper inspections of these systems and connections weren't performed during commission and turnover/acceptance then Operations can experience reliability issues, some of which are easily solved by correcting piping, some by proper reassembly techniques, and some by a thorough understanding and review of systems and components and making modifications to achieve desired reliability.

From your posts on these systems, it's not clear you are working to try to resolve these issues--either on site or with the assistance of the supplier. There are so many variables and site-specific peculiarities that even if we study the P&IDs we could never understand exactly how the system is configured on YOUR site in order to be able to provide useful suggestions. And, as was first said--these systems, from the OEM, are engineering works-in-progress. Simply consider how many devices and components are sourced from different vendors and manufacturers to assemble into these systems and one has to understand or at least be sympathetic to the complexity of the system--as well as the situation being addressed by the systems.

Sorry, but site personnel simply must be part of the solution--including assembly-reassembly practices and procedures, thorough reviews of system layout and construction to ensure all Notes were followed as well as sound engineering and construction practices for those parts of the systems that were built on site to connect the various components, even including designing and/or specifying and sourcing components to improve reliability.
 
Thank you CSA

I don't see why loose fitting would allow liquid to the purge line. liquid goes to the purge line through the 3-way valve (possibly through the seals).

Actually OEM is assisting us. OEM emphasizes that at each outage all the 3-way valves have to undergo 4 tests in the workshop. but actually we don't do it. this is one point

the second point our system has a reticulation system and nitrogen purge system (unfortunately the nitrogen system is not ready yet) which, if we isolate the recirculation system[for any reason] and didn't purge the liquid remaining in the lines with nitrogen, coking will form in the 3 way valve. this is the second point. but they didn't give me a clear answer of why there is coking in the purge line and why there is liquid passing to the cans. they are saying do the test, but I wanted to understand what causes these problems. this is why I have put this thread.

Thank you CSA.
 
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