Modifying System Curve Utilizing Throttling Valve
Modifying System Curve Utilizing Throttling Valve
(OP)
Hi all,
I have been searching for a reasonable solution to a hurdle I have encountered on a recent project. I am designing a pump station that will be delivering a large range of flows to two seperate locations. A system of two pumps will be used to cover the entire range of flows (approximately 200 gpm to 9000 gpm). It is feasible based on intake water levels that the static head between the two systems could vary between 31 feet and 3 feet. We will need to cover the entire range of flows under both conditions. The friction losses further complicate things making our range of total dynamic head varying from nearly 80' (at 9000 gpm and 31' static head) to 3' (200 gpm and 3' of static head).
I am interested in designing a system that will provide adequate, dynamic backpressure that will essentially align the second system curve (to the low elevation outlet) with the first system curve (to the high elevation outlet). I believe that I can accomplish this using an electronically actuated throttling valve (either butterfly, globe or pinch). My confusion lies in how to adequately calculate the minor losses for varying cross sectional areas through the valve. Do the equations at the following link apply, or do they not because the pipe diameter would again increase downstream on the valve (in other words is my A2 the value through the valve, or the value just downstream of the valve)? Any other thoughts?
http://nptel.iitm.ac.in/courses/Webcourse-contents...
Thanks in advance for any responses. I have been a lurker for years and have found this forum to be an excellent resource.
I have been searching for a reasonable solution to a hurdle I have encountered on a recent project. I am designing a pump station that will be delivering a large range of flows to two seperate locations. A system of two pumps will be used to cover the entire range of flows (approximately 200 gpm to 9000 gpm). It is feasible based on intake water levels that the static head between the two systems could vary between 31 feet and 3 feet. We will need to cover the entire range of flows under both conditions. The friction losses further complicate things making our range of total dynamic head varying from nearly 80' (at 9000 gpm and 31' static head) to 3' (200 gpm and 3' of static head).
I am interested in designing a system that will provide adequate, dynamic backpressure that will essentially align the second system curve (to the low elevation outlet) with the first system curve (to the high elevation outlet). I believe that I can accomplish this using an electronically actuated throttling valve (either butterfly, globe or pinch). My confusion lies in how to adequately calculate the minor losses for varying cross sectional areas through the valve. Do the equations at the following link apply, or do they not because the pipe diameter would again increase downstream on the valve (in other words is my A2 the value through the valve, or the value just downstream of the valve)? Any other thoughts?
http://nptel.iitm.ac.in/courses/Webcourse-contents...
Thanks in advance for any responses. I have been a lurker for years and have found this forum to be an excellent resource.





RE: Modifying System Curve Utilizing Throttling Valve
But to address your question: If I understand, you're proposing installing a valve to increase friction in the system to raise the low TDH system curve to the higher TDH system curve. Is this correct? And the valve closes to increase friction in response to lower flows or static head? If I understand right, you'd only need to know the CV for the fully open valve to be sure the valve doesn't add too much friction to the high flow-high TDH scenario. That full open Valve CV should come from the manufacturer, rather than a calculation. In lower flow or static head scenarios, you'd control the valve position to vary the CV all the way down to zero as your conditions dictate. Ypu wouldn't need to pre-calculate the partially open CV - you let your controls set it to whatever it needed to be. I doubt any single valve will have good control characteristics for the full range of flows. Good luck.
RE: Modifying System Curve Utilizing Throttling Valve
RE: Modifying System Curve Utilizing Throttling Valve
RE: Modifying System Curve Utilizing Throttling Valve
RE: Modifying System Curve Utilizing Throttling Valve
If you go to http://www.pumpsystemsmatter.org/content_detail.as... you can down load a cut down version of AFT's Fathom. Its called PSIM? This will enable you to model the various scenarios, adding valves with open% versus Cv etc. Then you can see the range of opening you will encounter. Beware of cavitation if you throttle the valve too much. Valve damage and noise issues are the concern.
You could use Epanet or any other similar software if you are more familiar.
If fluid dynamics are not your area of expertise then get someone else to do it for you.
Once you sort out the steady state you will need to do a waterhammer analysis. Again if this is not something you do regularly get someone who does. If you have to go down that path they could do the steady state evaluation as part of the exercise.
Generally I would not recommend this link to anyone who is not well up on fluid dynamics and you are an unknown quantity. It will same you time and give you a chance to do a thorough review of options.
“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King
http://waterhammer.hopout.com.au/
RE: Modifying System Curve Utilizing Throttling Valve
RE: Modifying System Curve Utilizing Throttling Valve
RE: Modifying System Curve Utilizing Throttling Valve
You need to look carefully at the likely operating conditions, how often at the high flow/ low flow / intermediate flows. It could well be worth considering 1 x 50% pump and 2 x 25% pumps or 3 x 33% or some other combination, don't get hung up on a 2 pump scenario.
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)