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Diffrential Pressure Switch
Need help on the Differential pressure switch installation and working.
By Hamad Sultan on 19 December, 2009 - 2:30 am

HI Friends,

I am working in oil and gas department. Here gas turbine are operated to compress the gas and then transmit it. Here in the control of turbine the FLAME OUT protection is used also through the logic. But The input from that protection in to PLC is differential pressure switch which is connected as under.

The LOW side of the switch is directly connected to the Compressor discharge pressure and the HIGH side of the switch is connected with the same line but a orifice is installed between high side and the input of the Compressor discharge pressure. So in short A PCD LINE DIRECTLY CONNECTED WITH LOW SIDE AND VIA ORIFICE CONNECTED TO HIGH SIDE.

The size of the orifice is 0.009 inch or 0.023MM.

And the switch rating is 6 PSID INCREASING and 4 PSID decreasing.

how it works and what is the effect if pcd is 60 PSi? Help me in that issue

Thanking you i will remain

Friends please help me on this issue

A picture is worth a thousand words. I tried to sketch it out but I did not understand exactly what the connections were or where the orifice is.

Please sketch/draw the mechanical configuration and post it somewhere publicly accessible, like, and put a link here, so that we can see what the exact layout is.

I've started to reply a couple of times, but I can't really understand where this switch is mounted. Is it on the gas turbine axial compressor or on the gas compressor which is driven by the gas turbine?

I can tell you that when flame is lost, the gas turbine axial compressor discharge pressure (called PCD in older GE-design heavy duty gas turbine control systems) drops very quickly. That's because there is a pressure rise in the combustor when flame is present, against which the axial compressor has to operate. When flame is lost the pressure in the combustors decreases and axial compressor discharge pressure also decreases because of the decrease in "back-pressure" in the combustor.

There are gas turbine manufacturers that don't use flame detectors and use a combination of axial compressor discharge pressure monitoring and exhaust temperature monitoring to determine if flame is present.

Is there a device number assigned to this differential pressure switch by the packager of the turbine/gas compressor?

The only thing I can think of would be a scenario where if there was a sudden loss of gas turbine axial compressor discharge pressure due to loss of flame that the orifice in the sensing line of the high side of the pressure switch would not allow the pressure on the high side of the switch to decrease as quickly as the pressure on the low side of the switch. This would cause the switch to actuate when the pressure increased above 6 psid.

Eventually the pressure "trapped" between the orifice and the high side of the switch would decrease on sudden loss of pressure, but during that time the switch would be actuated (presuming that the dp was greater than 6 psid on loss of flame/pressure drop) and then the switch would be deactuated again (when the differential pressure dropped below 4 psid ).

So, during starting and acceleration the gas turbine axial compressor discharge pressure increases relatively smoothly and the pressure on the high side of the switch, passing through the orifice, will not be significantly less than the pressure on the low side (because the orifice is large enough to allow sufficient flow to keep the pressures on the high- and low sides of the switch relatively equal as long as the pressure changes are occurring smoothly).

However, when flame is lost in the gas turbine, the gas turbine axial compressor discharge pressure will drop very suddenly and the orifice in the high side leg would not allow the pressure on the high side of the switch to decrease as quickly as the pressure on the low side of the switch, thereby actuating the switch. Depending on the sizes of the tubing and the orifice diameter, the pressure on the high side of the switch would eventually decrease and the switch would be deactuated.

If this is the way this works, it's likely that the switch is only actuated for a brief period of time (a couple of seconds or less!) on loss of flame.

If this is the way this works, and you are having problems with this switch not working correctly, then it might be that there is rust or some other foreign object blocking the orifice, especially if the tubing isn't oriented correctly (if there are low spots in the tubing in which moisture/condensate can collect).

But, this is just my SWAG (Scientific Wild-Arsed Guess) and without a drawing and more clarification that's about all I can think of.

HI Friend;

Hope your health is good. First thanks for the detail answer. I agree with you. The detail as under in our system.

we have gas turbines from solar turbines INC; the model is centaur 47. there is a protection of FLAME OUT DETECT for two possible reason one is high fuel flow and other is high fuel signal.

the first on that is high fuel flow is controlled by that differential pressure switch. u can see the image how its connect on the following URL




Well, I wouldn't say we're friends just yet. Acquaintances, yes; colleagues, likely. Friends, well, don't you think that's rushing things?

In any case, I'm now more confused than before. So, as for your concern for my health, I have something of a bigger headache now, thank you very much.

What am I missing, because you said there is a flame out detect (is this the same as "loss of flame"?) for two possible reasons, one being high fuel flow, and the other being high fuel signal. In my experience, the only way high fuel flow can extinguish flame is by exceeding the fuel-air ratio in which there's too much fuel and not enough air and from what I know of gas turbines that's extremely difficult to do since they usually have multiples of excess air.

Could this be some kind of low emissions combustor which can be flamed out easily with too much fuel which requires this kind of switch?

As for a high fuel signal causing a flame out, well, I don't know how that could happen either, unless they are equating a high signal with possibly extinguishing the flame due to exceeding the fuel-air ratio.

I'm sorry; I'm not familiar with the Solar turbines. I tried to take a stab at how the switch might have been intended to work, but that's all I've got. I think my explanation of how the switch can be actuated and de-actuated was pretty good, if I say so myself. But, I think you're looking for more than that, as in the theory and reasoning behind why this switch was installed and/or is necessary, not just how the switch can be actuated and de-actuated based on its connections which are a little unusual.

Have you read your manuals on this subject? Have you asked the manufacturer? Have you asked the plant design engineer(s)?

Perhaps now that you've said what kind of turbine you are working on there might be someone on this forum who can provide some information to help you. It's best to provide as much information as you possibly can in your original post to get a concise reply in the quickest possible time. This may be why you didn't get a quick response in the first place. And why you didn't get the concise response you wanted in the second place, either.

You still haven't said what kind of specific problem you are having with this switch. Is it actuating when it shouldn't? Or not actuating when it should? If you tell us what problem(s) you're having, and what you've done to troubleshoot the problem, we can be more help. But, and I'm sure English is not your first language, but it's just not clear what problem you're having and how this switch is part of the problem and what you've done to try to rectify the situation.

Most of the gas turbine-related questions on this forum are related to GE-design heavy duty gas turbines and Speedtronic turbine control systems.

Lastly, remember that the the help you receive from this forum is free. We get paid just as much as you did to post this question on this forum. Which is: Nothing. We're just trying to help others, including you, and we're not on your schedule and we can't know everything about your site and installation and problem(s) except what you tell us.

Are we still friends now?

First of all i answer your last question that is we are friends if you want.

Presently i have no problem regarding to flame out. i just want to learn what is the story behind that switch and working principal of that switch to sense the flame out detection on high fuel.

and for the other that is flame out by the fuel signal is very simple that if fuel signal is greater than 95% it trips the turbine on flame out detect.

I understand that if the air (PCD) is less then the flame in the combustion is red hot than the blue one. and the temperature of the combustion rises but it takes some time to sense by the thermocouple to trip the turbine than that switch. am i correct?

secondly solar turbines are low exhaust emission with solonox lean premixed combustion technology.

Hope fully now you help me on this regard.
Can we become friends?


Our definitions of flame out detection have to be radically different. To me, flame out detection means a determination that flame has been lost in a running turbine and that's typically done with a some kind of flame detector failing to see flame when it should be seeing flame.

My definition of flame out also means that flame was lost in a turbine before any other trip condition was detected, such as low-low lube oil pressure, or high-high vibration. The usual cause for flame out is a loss of fuel flow-rate, due to loss of supply pressure or a blockage or a failed shut-off valve (failed closed), etc.

Not all Solar turbines are equipped with low emissions combustors.

I suggest you speak with your Solar service provider about what protection this switch provides.

I can add nothing further to this discussion.

Good luck with your quest, friend.

By Rotating Equipment guy on 12 April, 2017 - 1:58 am

Dear Sir CSA,

I am partially understand the configuration of this PDS_1500. But still learning to understand the design intent, the working principal. At this point of time, I am aware this arrangement is to detect loss of flame. May be this provide a different view. I am very keen to understand this feature. I am trying to look for the patent document but couldn't find it until today. [The PDS references the turbine compressor discharge pressure and monitors the rate of change in differential pressure. The flow orifice keeps one side of the PDS high and as there is a drop in pressure, the low side of the PDS detects this and will shut down.]

Looking forward to a fruitful discussion.