Liquid fuel change over.

A

Thread Starter

a-w-p-d

Hi, I am fairly new to my plant ( CCGT 9FA GT ) and have been tasked with finding out the procedure for changing over fuels on load (from GAS to Distillate). All I can find in the manuals is the principles on how it works i.e.: How the bypass works, How the divider works and so on, But it does not say how to initiate a change over or what to look for when it is taking place.

I have been told that we would need to reduce load to do this but not how much. Is this true and why?

Any help would be most appreciated.

Thanks in advance.
 
We'll presume that the liquid fuel system is "primed and ready for operation." The operator clicks on LIQ FUEL or DIST or whatever the liquid fuel button on the HMI says and the control system does the rest.

There's plenty to look for when operating on Liquid Fuel. GE has a new document on liquid fuel operation for heavy-duty gas turbines, GEK-111717. Contact GE for a copy; it has some excellent information for understanding how liquid fuel systems operate and what to look for. Remember: liquid fuel systems have many components, and many of them are *not* controlled or monitored directly by the Speedtronic turbine control system, and they must all work as designed/intended for the turbine to run properly on liquid fuel.

One of the keys to operating a turbine on liquid fuel when it's primarily operated on gas fuel, is to operate it periodically on liquid fuel to keep all those components in working order.
 
Thank you CSA for your reply.

I shall look into obtaining a copy of the said document on liquid fuel operation for heavy-duty gas turbines, GEK-111717.

However, would you have any insight into how one would initiate a distillate change over from Gas whilst on line?
Would I have to reduce Load and if so too what?
Can it be done whilst in remote or do would I select to auto?
Can it be done with the Steam Turbine on line or would we take the ST unit off?
 
So, you must have a single-shaft machine, with the gas- and steam turbines and generator all directly connected? This is also called a single-shaft STAG unit.

Presuming all the permissives are met for liquid fuel operation, while the unit is running all the operator need do (under normal circumstances) is click on LIQ or DIST or whatever the button text reads and then confirm the action and the fuel transfer is initiated. The liquid fuel forwarding pumps should be started and pressure allowed to build up, the Liq. Fuel Stop Valve will open, the High-pressure Liquid Fuel Pump will be started, and the Liq. Fuel Bypass Valve will begin to close to force fuel to the liquid fuel nozzles. The gas fuel will be automatically reduced as the liq fuel flow-rate is increased. If everything works properly, the power output will remain fairly constant (but, if the unit hasn't been run on liq fuel for a while, it *never* works properly).

It is recommended not to perform a liq fuel transfer while running at Base Load. If there is a bobble in fuel supply pressure or atomizing air pressure can cause a sudden increase in exhaust temperature which can cause an exhaust overtemperature alarm and/or trip. So, it is recommended to reduce load approximately 10-15% before initiating a fuel transfer (whether going from liq to gas or gas to liq).

The steam turbine is commonly referred to as a "follower", meaning that it's just "along for the ride" producing whatever power it can from the steam which is being produced from the exhaust heat of the gas turbine. If you have single-shaft STAG unit, you can't just trip the steam turbine while the gas turbine is running--the steam turbine *needs* steam flow through it, believe it or not, for cooling. If there is no steam flow through the turbine and it is operated at rated speed the blades will get very hot. So, if the unit is a single-shaft STAG unit you must leave the steam turbine running while the gas turbine is running.

Usually, a liq fuel transfer can be initiated whilst either in auto or remote modes.

But, if it's been a while since the unit was last operated on liquid fuel, you should definitely think twice about just initiating a transfer to liq fuel while the unit is running under load because it will most likely trip (sorry; but it's true). I would recommend taking the unit to very light load or even FSNL operation just before a shutdown, and then initiating a liq fuel transfer. That way if the unit trips, you won't be scrambling to try to get it back on line.

If it's been a long time since the unit was last run on liquid fuel, I would also recommend shutting the unit down and exercising all the liquid-fuel related devices. All the solenoid-operated valves; remove and examine the check valves. Start and run the Liq. Fuel Forwarding pumps and make sure there is no air in the piping up to the Liq. Fuel Stop Valve. Open the Liq. Fuel Filters and make sure they are okay; bacteria can grow in liq fuel filters if left unchecked for months or years. Also, open the liquid fuel stop valve with the Liq. Fuel Forwarding Pump running and open the liq fuel filter air bleeds to make sure they have no air trapped in the tops of them after checking/changing the filters.

Go through the system thoroughly; have a field engineer assist if you're unsure. *Then* try a liq fuel transfer. Or even, a liquid fuel start, and a gas transfer while running then a transfer to liq fuel.

And, follow the operational recommendations for liq fuel system reliability in GEK-111717.
 
B

Bob Johnston

As CSA says, there are a lot of things which can go wrong. To answer your question, it depends whether a Gas to Liquid transfer has been performed successfully recently or not. Let's say that it has not, so the answer to your questions are:

1) Load reduction - depending on how critical it would be to your network if you lose the machine, I would reduce the load to as low a value as possible, at least the first time.

2) Better in Local and have as many two-way linked observers as you can get. There are many things to watch.

3) Pretty much the same as answer 1.

Once you have the changeover working reliably, and you want to make a change-over. I would always try to reduce below Base Load, it doesn't need to be too much. You will always get a little combustion instability during transfer and therefore a slight chance of tripping.

I should have asked at the beginning, do you have DLN on your Gas System or do you have Wet Nox reduction on your Liquid Fuel System. If the answer is Yes to either or both, repost the reply as you will need to use a completely different procedure.

Good Luck and report back.
 
Hi, thank you both for the information,
I would just like to say sorry for the delay in my response. I did post this response a little while back but for some reason it did not make it through and I have been a bit busy with an outage.

and so to answer you questions;

We are a CCGT with independent Gas and Steam Turbines not single shaft ( I think this would be “STIG” ) in which the Gas turbine runs with the DLN4 Gas System.
If you mean water injection with the Liquid fuel to reduce the Nox then Yes we do you have Wet Nox reduction.

So does this mean that I am going to have “BIG” problems with the Liquid fuel change over?
 
The magnitude of issues with liquid fuel transfers will be directly proportional to the amount of time since the unit was last operated on liquid fuel. If that was last month, or two or three months ago, you'll probably only have some air in the piping which will cause some instability in output, maybe even a trip on loss of fuel depending on how much air is in the piping.

Some Frame 9Es have recently experience problems with "coking" of the liquid fuel nozzles caused by leaking liquid fuel check valves. This results in high exhaust temperature spreads which can also trip the unit.

Again, as was said before, if it's been some time since the unit was last operated on liquid fuel, it's recommended that you post-pone the operation/transfer until after you've inspected the liquid fuel system, particularly the liquid fuel nozzles and the liquid fuel and atomizing air check valves. If that's not possible, *at a minimum* you should try to purge all air from the liquid fuel piping up to the liquid fuel stop valve, including any filters (all filters should have air bleed vent valves and transfer fill valves which can be used to purge the air through/with).

You should become very familiar with the Liquid Fuel-, Atomizing Air, and Cooling & Sealing Air piping and instrumentation diagrams (P&IDs to most people, Schematic Piping Diagrams to GE). There are LOTS of potential areas for problems.

You can always get a field engineer to help with the first couple of transfers until everyone is familiar with the systems and potential problems.

Again, the magnitude of your problems is mostly going to be related to how much time has passed since the last time the unit was run on liquid fuel, as well as your (you and others on the site) familiarity with the systems and components. As Bob Johnston said, there are *many* components which all have to work together in order for the Liquid Fuel system to work correctly, many of which are not directly controlled or even monitored by the Speedtronic turbine control system. Which makes troubleshooting difficult even with a good understanding of how the systems and components are to work.
 
After relaying all of the points and help that I have been give from Control.com, it has been decided that we are to give the whole system a cleaning and flush before we a tempt and kind of fuel change over. As for a field engineer to help with the first couple of transfers, that remains to be seen.

But just reading your responses has given me a greater insight and better understanding of our liquid fuel system and for that again I thank you.
 
This seems like a prudent course of action.

After the flush is complete, it is important to ensure that as much air has been removed from the piping between the Liquid Fuel Forwarding pumps/storage tank(s) and the Liquid Fuel Stop Valve, usually VS1-1, as possible. Use the Liquid Fuel Forwarding pumps to do this.

Presuming the discharge pressure of the Liquid Fuel Forwarding pumps is less than the Liquid Fuel Check Valve cracking pressure, you should then open the Liquid Fuel Stop Valve and fill the Liquid Fuel Filters, using the filter transfer/fill valves and the air bleeds at the tops of the filter canisters. (It goes without saying, do this with the unit not running, and immediately after the cleaning and flushing.)

Removing the air from the piping between the pumps and the Liquid Fuel Stop Valve can be easier said than done. Most piping does not have any high-point vents installed and is not pitched up to the vent to assist with removing air. If the piping has any high-points between the pumps/tank(s) and the stop valve, and there are no high-point vents, you will need to establish flow in order to displace the air. So, some kind of recirculation will need to be arranged. If all the piping is underground between the pumps/tank(s) and the stop valve, you are probably in good shape just flushing through the stop valve and the air bleed vents on the tops of the liquid fuel filters.

There are many F-class turbine owner/operators who are considering purchasing or installing some kind of permanent liquid fuel recirculation system to keep the piping between the tanks and the stop valve purged of air and "primed" and ready for emergency operation, such as when gas fuel pressure and/or supply is lost. I believe I saw this described in the GE document quoted above.

Study the piping diagrams for the Liquid Fuel System, the Atomizing Air System, and all related systems (Water Injection, if used; all purges (Liq. Fuel, Water Injection, etc.)). Let us know if you have any questions.

Write back and let us know how things go after your flushing/cleaning.
 
Hi,

Thank you again for your continued support. I shall be in touch and let you know how it all went as soon as we have a go.

"Permanent liquid fuel recirculation system"
I have mentioned this system to others, which has brought great interest; however I have looked high and low for the said document but cannot find GEK-111717 anywhere. Could you point me in the right direction?

Again many thanks.
 
Top