Dear Sir,

We are having 2 frame5 gas Turbines each 20MW mark V control system. Recently one more new 20MW mark VI control Gas Turbine we added due to our refinery expansion. Both GTs are running on naphtha fuel & HSD is kept as standby fuel. HSD line is always in charged condition, where as naphtha we are taking from separate line. One three-way Valve is located at the junction of Both naphtha & HSD fuel line to supply fuel to the main fuel line to GTs.

Scheme is as below:

Naphtha line--->50m filter--naphtha pump--accumulator(50lt)--Control Valve--filter SKid--3WAY VALVE---THEN SHUT OFF VALVE to GTs

HSD line--->50m filter--HSD pump--
accumulator(50lt)--Control Valve--filter SKid--3WAY VALVE--THEN SHUT OFF VALVE to GTs

PROBLEM in EXISTING SCHEME :

When ever we are giving fuel change over command (naphtha to HSD or HSD to Naphtha),For fully C/O it's taking 13Sec to 15 Sec. In the mean time Fuel Pressure is getting reduced to < 1.5kG/cm2 & GTs are getting Tripped on low liquid fuel Pressure. We tuned our existing Naphtha & HSD Control Valves, also provided recirculation Line.

BUT the above said problem still Persists, even in the newly commissioned Mark VI GT. So now we are afraid of Fuel change Over during Emergency condition also. From the date of commissioning till now > 20 times GTs has got tripped on Fuel change Over.

REMEDY :

In order to avoid the problem sir,

1) Can we put one accumulator after the 3-WAY Valve & before the SHUTOFF VALVE? Normally for that 10 Sec Fuel requirement will be 25lt. Please note that already one accumulator(50 lit) is connected at the discharge end of fuel Pumps, which is needed only during Fuel Pump change Over.

2) Can we put one accumulator of 25-35 lt capacity? Or can we shift that existing accumulator to this new place?

3) If we will put one accumulator what will be the impact on the respective Control valve during that 10 Sec change Over time?

4) Sir is there any software available to simulate the control Valve & accumulator or any precess device, from which we can analysis?

5) Or any other idea about preventing the above said frequent TRIPs of GTs?

I m waiting for your reply

Very Much Thank You,
Samarendra Das
smrndrdas [at] gmail.com

 

It's clear there's a problem. Have you spoken to the supplier of this fuel change-over scheme?

I've been trying to understand what you've been describing, and it's just not clear. Perhaps that's why others haven't been responding,
either. I can't understand if this three-way valve is upstream of the line feeding all three turbines, or if all three turbines each
have a three-way valve upstream of the high-pressure liquid fuel pump.

I'm going to take a stab at this though.

You say the units are running and when a fuel change-over command is initiated the fuel pressure is reduced and the GTs (all of them) are
tripping.

Did this same phenomenon occur before the new GT was added?

Has this scheme ever worked? When did it start having problems?

Do the operators initiate the fuel change-over command, or does some other control system initiate the change-over command and on what set of circumstances, or does a low naphtha supply pressure initiate the fuel change-over command?

You say the HSD line is "charged". Does that mean that a HSD forwarding pump is continually running to maintain pressure on the HSD supply line, or that the HSD supply line is pressurized and then the pump is shut off? Does the pump cycle to keep the pressure from falling below a certain point? What does "charged" mean?

I was going to make a guess and say that it was taking too long for the transfer valve to switch from naphtha to HSD, but that's just a guess at this point. As I write, I realize I just don't understand what's happening or the configuration of the system.

Are you sure the HSD supply pressure isn't falling when the transfer is occurring?

Are you sure the naphtha supply pressure isn't falling when the transfer is occurring?

I saw a site once that shut off the liq fuel forwarding pumps 30 seconds after a transfer to gas fuel was initiated and more often that not the unit tripped when transferring to gas fuel from liquid fuel. That's because the Speedtronic hadn't even begun to start decreasing the liquid fuel flow-rate as it increased the gas fuel flow-rate. The liq fuel forwarding pump should not have been shut down until the transfer to natural gas was complete.

I don't have any experience with units running naphtha, but I wonder if the transfer valve might not also be sized properly to account for the change in BTU contents of the two fuels.

You say the pressure is dropping, but is that because the fuel flow-rate of HSD is not increasing quickly enough as the naphtha fuel flow-rate is decreasing? Pressure can drop if flow-rate doesn't increase fast enough.

Are you certain the accumulator you already have is working properly? If this system worked properly before the new turbine was installed and put on the system, are you sure the accumulator is sized correctly?

There aren't a lot of people with experience with naphtha-fired units. And all of them don't read this forum. Repeatedly posting the same question, or similar, isn't going to get a response if people don't understand the question or they don't have any experience or anything constructive to offer. We do the best we can. No one here gets paid for the time they spend responding to queries, nor for the experience that allows them to respond knowledgeably. Neither are you paying for any responses you might receive.

We try to help everyone, but if there's no one who can respond with something positive then that's that, as they say.

 

I have some experience on Naphtha fired units with more or less the same kind of arrangement (HSD as standby fuel & Naphtha as primary fuel) that you are talking of. The 3-way valve timing mentioned by you is pretty okay. I think there are a few simple things that come to mind are

a) change the upstream fuel filters prior to intiating a planned change-over (may be even upstream filters of forwarding pumps close to your tankages)

b) Manually control the fuel oil pressure before the 3-way valve on the higher side (around 6 - 6.25 kg/cm2)for the fuel that is supposed to take over (HSD or Naphtha) so that the pressure does not drop immediately to the value stated by you after initating a change-over

c) Lastly, you have not mentioned of the load on the generator during the changeover. If possible, try initiating a change-over at the lowest possible value. D)check for the K63FLY (TD)value, it should be at least 3 Secs.

I hope one of the above things would be of some aid to you. Write back your experience.

 

Low GT fuel pressure tripping is provided to avoid high pressure fuel pump starvation. Normally control valves are used in both naphtha and backup fuel pipelines to maintain steady pressure before three way valve. Naphtha (running fuel) control valve is opened more when compared to that of backup fuel as it is supplying fuel to GT. After initiation of fuel changeover command, back up fuel flow will start increasing and naphtha fuel flow will start decreasing. The sudden increase in back up fuel flow will result in lesser pressure before on base fuel pump. The control valve in backup fuel line will try to open more to maintain the pressure. If the control valve is sluggish then GT might trip on low fuel pressure. GT tripping can be avoided by doing any of the given below actions

1) The control valve in backup fuel might be made to act more fast. Then the control valve will react fast leading to control valve hunting. Momentary high pressures will reset the low fuel pressure timer. The low fuel pressure is normally set above 10 Sec.

2) Using the fuel flow to trip the GT instead of pressure will avoid the tripping. Our main concern here is to protect the starvation of pumps due to absence of fuel. Low flow below a minimum set point with time delay can be used to trip GT.

3) Assume the Naphtha control valve is opened 50% and backup fuel control valve is opened 20%. Normally the control valves are controlled through DCS. During changeover, by using suitable logics in DCS, open the backup fuel control valve automatically to 50%. By opening the backup control valve to 50%, we can maintain the required pressure. After the changeover is complete, the operator can take the control valve in manual mode and do the necessary things to normalise the situation. This has been done by my friend in a newly commissioned GT and is working well.