Flow Control Valves - Parallel Operation
Flow Control Valves - Parallel Operation
(OP)
I'm looking at an oil export system that has two valves in parallel. Only one at a time should be controlling the flowrate in a cascade level onto flow control.
Due to piping anomalies, I'm being asked whether the two valves can be operated with one in manual (@35% open) and the other in auto. I think this won't work because as the one valve closes, the flow will increase through the other one. (Which is now behaving like a fixed orifice)
However, I can't find any formulae to describe the interaction so I'm struggling to convince anyone. Upstream of the two valves are a couple of BIG (2x7MW) centrifugal pumps so I don't want things to go wrong...
Due to piping anomalies, I'm being asked whether the two valves can be operated with one in manual (@35% open) and the other in auto. I think this won't work because as the one valve closes, the flow will increase through the other one. (Which is now behaving like a fixed orifice)
However, I can't find any formulae to describe the interaction so I'm struggling to convince anyone. Upstream of the two valves are a couple of BIG (2x7MW) centrifugal pumps so I don't want things to go wrong...





RE: Flow Control Valves - Parallel Operation
However, since these are both control valves, why not change the control technique to allow both valves to open (like split range?)
RE: Flow Control Valves - Parallel Operation
RE: Flow Control Valves - Parallel Operation
Traditionally for an application like this the small valve is sized to take 1/3 of the system capacity, and the larger valve is twice as big (1/3:2/3 split)
COntrol the valves with split range* and the large valve will transition to control the flow once the smaller valve exceeds its capacity.
* 4 milliamps: Both valves closed. 12 ma, smallest valve fully open, larger valve just begins to crack off the seat. 20 milliamps: both valves fully open.
RE: Flow Control Valves - Parallel Operation
Assuming you have one flow meter common to both valves.
The position control loop uses the small valves position (PID output) as it's CV and holds the small valve within a good control range by Open/Closing the large valve. This loop is set to react slower than the flow control loop.
To reiterate.
As the small flow control valve opens the large valve opens to bring the small valve back.
RE: Flow Control Valves - Parallel Operation
roydm - I have made this recommendation already. This kind of control is also known as 'gap' control and uses two valves; one with a Cv << than the other - maybe a fifth of the size. We have a massive valve (Cv = 1140) and a huge valve (Cv = 870) in place so a new trim would be required (Cv = 200?). This means spending money and shutting down production... You can imagine how well that went down!
JLseagull - I didn't mention that there is also a maximum overall flowrate limit. Leaving one valve in manual exposes it to downstream pressure changes. If someone else down the pipe shuts down, the pressure drops at my end. Flow is a function of dP at a particular Cv so the flow through the massive valve goes up. The FT is common to both valves and measures total flow so it shuts in the huge valve running the pumps up their curves (as noted by JimCasey) which puts even more through the massive valve.
Steamdog's idea is great, but I also neglected to include the fact that they run with one valve at max 35% open because of pipework vibration. D'oh!
Honestly, I really don't get paid enough for this crap
If we use the analogy of two resitors in parallel, what is the equivalent formula for valves?
Thanks again folks.
RE: Flow Control Valves - Parallel Operation
We do this so the big valve is never controlling close to its seat, giving good control and long lasting service.
RE: Flow Control Valves - Parallel Operation
RE: Flow Control Valves - Parallel Operation
In my mind:
A) The answer to the original question is yes, under certain conditions.
B) The answer to additional questions are: it depends on the flow conditions.
As any flow and pressure regulating valve has to be dimensioned on the actual flow and pressure conditions before and after the valve, and vice versa: the actual performance can only be calculated when flow, Cvs and restrictions and conditions and losses upstream and downstream are known, and you in addition have a parallell/split range regulation, there is simply not enough data available to give a sensible answer.
If you gave all possible variations of upstream input flow from pumps (including worst case and performance on PRVs), and all details on valves and downstream pipe dimensions and restriction/losses you could perhaps come a step nearer, but only if you are for safety reasons sure you have taken into consideration all possible scenarios.
... and yes, you are probably not paid enough
RE: Flow Control Valves - Parallel Operation
I was trawling through 'Liptak' (Chapter 6 of process control and optimization) and found examples of 'Gap' or 'Floating' control, 'Small/Large' control and split ranging techniques with hints from Shinskey about how to make them work. Trouble is, they all require a re-trim to work which, as I mentioned, is not a popular solution. A quote for a Cv 200 valve re-trim is 58,000GBP BEFORE the production hit is taken into consideration (1 - 2 days @ 230,000 BOPD!).
I've done a 'toy' valve simulator using excel which shows that, as the valve controlling the flow closes - the pump moves up its curve slightly which increases the DP which increases the flow through the fixed valve. The flow controller reacts by closing in the controlled valve even more and so on. It does reach a steady state eventually, but there is the potential for big flow rate oscillations which would probably cause a production shutdown. Not good. Not good at all...
Studies using Unisim (HiSys) back this theory up too. I only have Matlab left to try.
I guess, at the end of the day, it's their valves and they're entilited to run them any way they like. My A**e is covered. And hey, if it works, I'll post it here for reference!
ATB.
Loki
RE: Flow Control Valves - Parallel Operation
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/