Hello,

We have 9F DLN1 dual fuel units - 20+ years old. Mostly running on gas, but we have to exercise them on liquid fuel periodically. Many times liquid fuel pipes running from splitter found clogged with sludge and also a number of check valves always found stuck.

Is there any experience with this problem on this type of machines? Flushing procedure, producing under pressure while on standby etc.?

Thanks everyone for dropping a word

 

Hello,

There have been lots of technological advances in systems and components over the last twenty-plus years. I don't recall the publication number but the OEM has published, and revised, a document that describes their recommendations for F-class dual-fuel machines. They sell a number of upgrades to both hardware and software that can improve the reliability--but it all comes at a cost.

I'm sure there are after-market suppliers who can engineer similar, or even in some cases, better systems to help improve reliability.

The major cause of the problem you are describing is leaking liquid fuel check valves which allow hot combustion gases to flow backwards into the liquid fuel lines, which results in the carbonization of the liquid fuel in the lines. When this occurs (backwards-leaking liquid fuel check valves) it's very common for the liquid fuel stop valve to leak backwards, also, which allows the "flow" through the system that's necessary for the hot combustion gases to get into the liquid fuel lines.

There are several different types of liquid fuel check valves; some water-cooled; some air-operated. Basically, if you can stop the liquid fuel check valves from leaking backwards you can eliminate most of the problems.

If your liquid fuel system absolutely has to be reliable, you may want to look into some kind of liquid fuel recirculation system, which periodically recirculates liquid fuel all the way up to the liquid fuel check valves. These will greatly increase the amount of tubing and piping in the turbine compartment, but just about any of the solutions will do the same. Combine one or two of the solutions and you're looking at significant tubing/piping additions and modifications in the turbine compartment. Sorry; but reliability comes at a cost--including system complexity.

Again, contact your OEM/packager for a copy of GE's dual-fuel recommendations publication and have a read of it. I would also suggest doing some Internet research on various liquid fuel system check valves--there are some really good alternatives out there to those recommended/sold by the OEM. Look at jasc-controls.com; they have some very good recommendations and products and I've found them to be very informative and helpful at various gas turbine users' group trade shows.

Hope this helps!

 

I have read and assimilated all the liquid fuel "fixes" for the 7Fa's. Ours were 7FA+e dual fuel peaking machines. The liquid fuel check valves had a habit of leaking. I resisted all mods to this, as the leaking actually blew liquid fuel back out of the combustors, and valves keeping them clean for the next fuel swap from gas to liquid. Our Peakers had a 97.7 % reliability swap to liquid fuel from gas-a- major achievement, in spite of GE'S recommendations. The only detriment to the swap was the variable speed drive for the water injection-specifically--the water control valve-it was controlled by the water supply pressure, rather than an air signal.

 

sggoat,

> I resisted all mods to this, as the leaking actually blew liquid fuel back out of the combustors,
> and valves keeping them clean for the next fuel swap from gas to liquid.

I'm confused. There should be a liquid fuel purge system that purges the "nozzles" downstream of the liquid fuel check valves using atomizing air. Most of the F-class machines I've worked on have small liquid fuel flow dividers ("distributors") on the end covers which are usually also purged to prevent carbonization of the fuel in the distributors. So, why would leaking check valves be a good thing? Hot combustion gases flowing backwards through the liquid fuel piping is what causes the "slushing" the original poster is complaining about.

I suspect, sggoat, that you exercise the liquid fuel system more frequently than the original poster. I also suspect the liquid fuel check valves in use at your site are "improved" models, newer than those in use at the original poster's site/--so leakage isn't as much of a problem as at the original poster's site.

 

We swapped to liquid fuel once per week--rain or shine.

When the check valves have small leaks,(air operated) the stagnant liquid fuel is blown back into the system(actually back-pressurizing the fuel system)--when there's no fuel there, there's no carbonization(is that a word?) Since I left(retirement), the utility has succumbed to GE's admonition and installed water cooled check valves----the reliability has dropped to 57% on liquid. Some due to the originally poorly designed water injection control valve, but lots due to the water leaks onto the casing from the leaking coolant to the check valves.

I actually think GE came close to getting it right the first time--just no full stop 'check' valves. let the compressor air blow the liquid clear when running on gas--maybe into a collection tank--keep the flow open til the unit gets to FSNL, then close it off. If they had similar 80 psi(or what ever pressure) checks like the 7Bs, it would be easy enough to swap over to liquid when the time came--(liquid pressure would overcome the check setting and provide fuel.

My .02