Control valve downstream pressure drop

[Talgat]

Hello,
I have abnormal situation on a site, there are installed 3 valves in parallel (cascade) in Fuel gas skid, upstream pressure of the valve is 30 bar, downstream pressure 25-26 bar, when 2 GTG starts up to load 40 MWt (100 %) the downstream pressure of the control valves decreses to 21 bar (this affect to GTG to decrease load), upstream pressure is stable 30 bar, What can be the reasone of incresing of ΔP? one option that design upstream pressure is 37 bar (dew to some reazons we cannot reach that 37 bar) Is this anyhow affect to the valve's ΔP; and at nearest time GTS's will increase load up to 120 MWt.
Please help to identify the problem!

 

[S_Happens]

Against my better judgement I'm going to reply. I'm assuming that English is not your first language and hopefully that is the reason you've left so much out.

First, you haven't given enough information so that people, like me, who are capable of answering but not intimate with power generation can answer you.

Second, to answer your question... it depends. Fuel gas pressure is going to be regulated somewhere. Are these valves there to regulate the fuel pressure on their downstream side (supply directly to turbine) or simply to supply fuel gas to the turbine based on another variable (meaning that the fuel gas pressure upstream of the valves is regulated independently)? Cascade is not a synonym for parallel, but refers to a type of control scheme where one controller outputs a setpoint to another controller (knowing it would possibly let me answer your question).

If the valves are there to regulate the downstream pressure (simply Differential Pressure controllers), then there is a restriction (or mechanical stop in the valve, or since the supply pressure is lower they are not being operated as designed). If they are being used for something else, say to control turbine rpm, then I don't have enough information to answer your question.

If the designed upstream pressure is supposed to be 37 bar, then those valves were probably designed as such and may not be able to provide enough flow at that supply pressure.

Again, I am not intimated with power generation systems, but very familiar with process control. If you want my help you can supply the necessary details, but I am not going to try to read your mind just to answer.

 

[Talgat]

I beg pardon for my poor English, and many thanks for your reply and notes!

The fuel supply is going through two PCV-001A/B, PCV-001C is a stand by valve, cascade is that when valve A became fully open valve B starts open,
2 gas turbines was working with their initial load, around 10 MWt, Valve A was open 70%, after increasing of the load up to 40 MWt is started to use more fuel gas, that is flow rate is also started increasing, Valve B started open fully. From now appered the problem: after reaching the load 40 MWt the downstream pressure of PCV became 21 bar (LowLow Level of FG for turbines is 21.5 bar), and also after 2 month it is planed to start another two GTG with additionan 40 MWt through our two PCV valves, which have problem with existing 2 GTG,
I think there are some probable problems:
1) Cv of the valve was not clearly identified during design,
2) Valve was designed to work with upsteram pressure 37 bar
and working condition 30 bar interact of inscreasing of deltaP
up to now i am trying to find out some valve curves delta P vs flow rate, at different upstream pressures.
the valve manufacture Masoneilan, 41000 type globe, 8 inch pipe line
instrumentation on site is working properly
Is anybody have any ideas, data sheets or curves? Please help.

 

[S_Happens]

Okay, PCV means it's a pressure control valve. So it's controlling the pressure downstream of it, on the inlet to the turbines?

You're still not using cascade correctly. In this case you seem to have a split range controller, but that is not important. Cascade would be like a pressure controller outputting a setpoint to another controller, like a flow controller which then outputs to a valve. (this paragraph was not important to the actual problem)

It is still difficult to get exactly what I need, but I'll post my understanding/assumptions below.
-2 turbines running initial load, valve A is 70% open (valve B closed?)
-load increased, valve A and B both 100% open?
-fuel gas pressure downstream of valves drops to 21 bar low low (pressure) activates whatever it is that it activates (shutdown, I dunno, I can't read your mind)

By valve C being a standby valve do you mean it is simply there to be replace A or B when they are out of service?

If my assumptions are correct, then you have a restriction at the valves you are talking about (or somewhere in between the valves and where the turbine inlet pressure is being measured).
Without being able to read your mind and know the exact situation, it could be any of these things.
-Valves themselves cannot pass enough flow at 30 bar upstream pressure because they're designed for 37 bar (most likely to me)
-Valves have a mechanical stop, are improperly adjusted, or malfunctioning and not opening fully (need to be checked in field)
-Restriction other than the valves (piping diameter change, etc) in between valves and turbine

Your options if this is the case
-Replace valves A and B with valves designed for 30 bar supply pressure
-Place valve C in service along with A and B (this removes the backup, but could potentially get operation going while proper valves/solution are ordered/implemented)

I do not have information on the valves you are using. Why don't you? If you don't, then call the manufacturer (can you do anything yourself?). I also don't have the specific information, so I can't be sure of what's going on and my recommendations are based on assumptions. This is in no way a proper diagnosis and it's up to you to pursue these avenues.

 

[jim hardy]

Here's a link to the MasonNielan brochure for that valve product line. It gives Cv for valves .
If link doesn't work, just search on phrase 'Masoneilan, 41000 type globe'

http://www.google.com/url?sa=t&rct=...27CECA&usg=AFQjCNFfXvg5g_QOczwt-yNCdrIdhZFPLQ
S Happens is right . Compressible flow is sensitive to inlet pressure because at lower density the velocity is higher and ΔP is velocity^2
so there's a √P involved and it's working against you.
That 30 vs 37 bars is a problem but not necessarily the only one.I see it's a pilot operated valve.
So you know where the stem is but not necessarily the plug.

Get good readings of mass flow rate and pressures and stem position, check your Cv , probably the MasonNielan rep will be happy to help . If Cv isn't what's expected you need to look inside valve.

Shappens mebtioned an internal block limiting valve travel.
It is not uncommon to install an oversized valve but set its stroke less than full travel to meet a specific Cv. If you're lucky you can just re-stroke the valve but be aware that'll change gain of control system and you'll probably have to tweak a controller's proportional adjustment. Hopefully it's electronic.

If you're real lucky you'll find an upstream manual valve that's not fully open.

S Happens appears quite well versed , I'd say you're in good hands there.

Best of luck !

 

[Talgat]

thanks a lot for your supports! I found them very usefull!