HI,

I WORK JUST IN NATURAL GAS AND CHECK THE MINIMUM TEMPERATURE OF NATURAL GAS INLET IS AROUND 23 DEG-C, AND THE DEW POINT OF HYDROCARBON IS -35 DEG-C, AND THE WATER CONTENT IN NATURAL GAS IS 8 MG/SM3, AND THE PRESSURE AFTER SRV VALVE IS 22.5 BAR.

THE TEMPERATURE NATURAL GAS INLET IS suitable.

I work just with natural gas and primary mode.

WHY WE see a lot of coke around the atomizing nozzle?

At any start the unit the pressure of atomizing is low alarm. after the increasing the load the pressure of atomizing air go higher than low alarm.

thanks a lot for reply

 

ABDI,

Hmmmm,..... Well,.....

You seem to be saying that the turbine (without telling us what kind of machine it is and what kind of control system it has and if it has DLN combustors and what kind of DLN combustors it has--if any) is dual fuel-capable (natural gas and distillate, I would presume--since you didn't say) and <b>IT</b> runs primarily on natural gas (or rarely on liquid fuel).

You say that there is coking/carbonization around the atomizing air tip of the dual fuel nozzles.

You say the gas fuel temperature is well above the gas fuel dewpoint temperature, and so therefore you believe there should be no gas fuel liquids condensing in the combustors.

First, the flame "ball" is NOT located right at the nozzle tip(s). It's usually at least a couple of inches away from the nozzle tips.

Second, many gas fuel liquids--when they do condense--do NOT result in coking/carbonization. Usually they result in load swings ("a quart of natural gas liquids is equivalent to 10 MW") and, in DLN combustors they can result in primary zone re-ignitions (in DLN-I combustors) and flashbacks (in DLN-2.x combustors).

Third, most coking/carbonization is the result of entrained hydrocarbons in the gas stream ("gas" being either natural gas or atomizing air). Yes; natural gas is quite frequently "contaminated" with compressor lubricating oil, hydraulic oil, diesel or even gasoline--and other hydrocarbon-based liquids. When there is no "knock-out" drum in the gas fuel supply piping upstream of the gas turbine fuel nozzles these entrained contaminants can cause coking/carbonization.

Fourth, the most common cause of coking/carbonization around atomizing air tips is lubricating oil entrained in the atomizing air--from bad seals in the main atomizing air compressor.

It's usually very easy to see evidence of entrained contaminants in the gas fuel as it will collect in low points of the gas fuel supply piping and manifolds, and it generally coats the inside of the piping and manifolds as well as the gas fuel nozzle tips and orifices where it will coke/carbonize when it comes in contact with the hot metal of the gas fuel nozzle tips.

The same is true of the leaking atomizing air seals, allowing lube oil to get into the piping--it will coat the inside of the atomizing air piping and pigtails and will coke when it comes in contact with the hot metal of the atomizing air nozzle tips.

So, have a look at the inside of the piping for evidence of contaminants. The main atomizing air compressor suction is supplied by axial compressor discharge air, so there shouldn't be any lube oil entrained in that. Though, if the ambient air has hydrocarbon vapours the evidence of that is a quickly fouling axial compressor and if it's bad enough, then the atomizing air compressor and associated piping will also be wetted by it.

But, leaking main atomizing air compressor seals are the most common cause of problems with coking/carbonization of the atomizing air tips. And, this usually happens because the atomizing air pre-cooler temperature regulating valve isn't working or isn't adjusted properly (EXTREMELY common).

As far as the low atomizing air pressure alarm on start-up, well, usually in a properly configured Speedtronic this condition is "blocked" until the turbine reaches 14HS (approx. 95% speed) because the axial compressor discharge pressure, and hence the main atomizing air compressor discharge pressure, is low because speed is low. How is the Speedtronic configured at your site for this condition?

However, you did NOT say precisely when this alarm occurs (after firing, during acceleration; for a brief period of time after 14HS; etc.). So, it could just be an improperly adjusted or non-working pressure switch. Have you tested ("calibrated") the low atomizing air pressure switch?

Remember that there is a bypass around the atomizing air compressor (you have looked at the Atomizing Air P&ID, correct?) that is open when the unit is operating on gas fuel and closed when the unit is operating on liquid fuel--because the atomizing air is only used for cooling and purging the atomizing air lines when running on gas fuel so the pressure doesn't need to be so high and by reducing the flow through the atomizing air compressor the turbine doesn't lose as much energy (horsepower) to the atomizing air compressor when operating on gas fuel.

There's a lot you haven't told us.

And there is a lot to check and reply with the findings.

Hope this helps!

 

hi,

you said check the sealing oil and air for main atomizing air compressor, but I don't know how to check it. please help me how to check it.

I have another question. DO you know what is the maximum temperature inlet natural gas temperature to gas turbine (frame 9e mark v, DLN TYPE)?

thank you

 

ABDI,

There is a low-point drain in the atomizing air piping--both the inlet piping to the suction side of the Main Atomizing Air Compressor, and on the discharge (outlet) side of the compressor. When the unit is shut down, remove the pipe plug from the drain weld-o-let and see if any oil drains out of the piping.

There is also a low-point drain where the Atomizing Air piping enters the Atomizing Air manifold around the axial compressor. When the unit is shut down open that drain by removing the pipe plug from the weld-o-let and see if any oil drains out.

Lastly, as was said before: Oil will coat the inside of piping (natural gas and atomizing air). That goes for the pigtails/hoses between the manifolds around the compressor casing as well as the pigtails/hoses between the manifolds and the nozzles. If you find evidence of oil mist on the inside of the pigtails/hoses in the form of an oily residue, you have your smoking gun.

The Atomizing Air P&ID clearly shows the presence of low-point drains--not the exact locations of them, the Piping Arrangement Drawings in the service manuals show them. But the easiest way to find the low-point drains is to just follow the piping and look on the bottom side of the piping in the low-points--and there will almost always be drains there. Especially if the unit has off-line water washing capability as there needs to be a method for draining the water out of the piping prior to starting. There will even be manual valves on those particular low-point drains (they are usually plugged with pipe plugs from the factory, but those pipe plugs need to be removed at least when draining off-line water wash water from the piping).

Hope this helps. Also, if your site is having to add lube oil to maintain the oil level in the tank it's a pretty safe bet that if the L.O. Mist Eliminator is working properly there might be leak in the seals of the Main Atomizing Air Compressor.

Don't forget to check the Atomizing Air Pre-cooler Shell Drain. If the cooler is leaking, or if the Atomizing Air Temperature Regulating Valve is not adjusted properly or is not working properly there will be fluid (cooling water; condensate) in the shell that should not be there. There is supposed to be a continuous blow-down orifice on the cooler shell but most sites think it's a leak, or a nuisance whistling noise, and either plug it or it gets plugged with rust and dirt (they--continuous blow-down orifices--do need periodic maintenance). Again, the presence of the shell drain and the continuous blow-down orifice should be clearly shown on the Atomizing Air P&ID. Any GE-design heavy duty gas turbine operator or maintenance technician (mechanical or electrical/instrumentation/controls) should have their own copy of ALL the P&IDs, complete with hand-written notes and the good ones (operators and technicians) should be able to go out and locate all the elements of the P&IDs on the turbine and auxiliaries.

The maximum gas fuel inlet temperature should be clearly stated in Sect. 05.02 of the Control Specification provided with every Speedtronic turbine control system. The Control Specification has a section entitled 'Expected Fuel Characteristics' or something to that effect that lists a lot of very pertinent information about the fuel(s) for that particular machine based on the analysis of the fuel(s) expected to be burned in the unit. Flow-rates at various loads; heating content; maximum- and minimum pressures and -temperatures. (Well, the minimum temperature is always specified as to be a minimum of 50 deg F of superheat so as to ensure no condensation of gas fuel liquids as the fuel passes through valves and nozzle orifices.)

 

Hi

I read the ger4189 about the range of minimum and maximum temperature required for fuel gas inlet.

but I DON'T UNDERSTAND ABOUT the maximum temperature, because the maximum temperature is obtained around 500 deg-c. IT'S impossible.

Please guide

 

ABDI,

I haven't read GER-4189, so I can't comment on the specifics of the document.

But I <b>CAN</b> tell you that some units in combined cycle application use various methods of heating natural gas before it enters the combustors. This is done to improve the gas turbine heat rate (it takes less energy to combust hot gas fuel than cold gas fuel). It's a small improvement, but for large units and over a long period of time it can be a significant savings. And, if there is a source of "waste" heat available to heat the gas fuel, a source that might otherwise need some kind of cooling, then it's an even bigger overall improvement for the plant process.

So, it's not impossible to achieve a very high temperature, though 500 deg C seems awfully high; I have seen 400+ deg F heated natural gas fuel.