Designing an orifice plate to restrict flow
Designing an orifice plate to restrict flow
(OP)
I have designed an underground flow diffuser to recieve bleed off water from a cooling tower. I must restrict the flow of water to the diffuser to less than about 3 gallons per minute. I want to use an orifice plate to do this.
The bleed line is tied into a pressurized pipe. The pressure at the tie in location is 7 psi. The water will be piped through 3/8" vertical copper pipe for a few feet to a 1" horizontal PVC pipe. The PVC drains to the underground flow diffuser.
1. What procedure do I follow to size the orifice?
2. How do I check the velocity at the PVC pipe? I want to make sure that I don't have a jet of water that will damage the pipe or the flow diffuser.
The bleed line is tied into a pressurized pipe. The pressure at the tie in location is 7 psi. The water will be piped through 3/8" vertical copper pipe for a few feet to a 1" horizontal PVC pipe. The PVC drains to the underground flow diffuser.
1. What procedure do I follow to size the orifice?
2. How do I check the velocity at the PVC pipe? I want to make sure that I don't have a jet of water that will damage the pipe or the flow diffuser.





RE: Designing an orifice plate to restrict flow
RE: Designing an orifice plate to restrict flow
I can't use a valve for this application either. The restriction needs to be "hard wired" so to speak. I can't take the risk that the valve will be fully opened by an unknowning technician.
RE: Designing an orifice plate to restrict flow
However, to keep the owner happy, you would need about a 0.20 inch diam (5mm) orifice plate to get your 3 gpm flow rate.
RE: Designing an orifice plate to restrict flow
Would you mind describing how you arrived at the 0.2 inch diameter?
RE: Designing an orifice plate to restrict flow
Q in gpm, d in inches, head loss in feet of water and C is flow coefficient (usually close to 1.0).
In this case head loss (7 psi) is 16 feet. Solving for d2 gives about 0.2 inches.
RE: Designing an orifice plate to restrict flow
RE: Designing an orifice plate to restrict flow
1) Perhaps you can explain something to me that I can't seem to wrap my mind around. It seems common sense that a small hole in an orifice plate will limit the flow through a pipe system. However, the continuity equation states that the flow rate, Q, is the same everywhere in the
pipe. How does the theory reconcile with common sense?
2)The flow will go through the orifice plate and then the vertical 3/8" copper pipe. Will the friction loss in the 3/8" pipe further restrict the flow or does the orifice control?
RE: Designing an orifice plate to restrict flow
Friction loss in the 3/8 pipe will still be there and must be accounted for, whether or not you have fitted an orifice plate.
RE: Designing an orifice plate to restrict flow
Just to expand on what dickon17 has told you. Perhaps you think that "flow rate" and "velocity" mean the same thing ... they do not.
"Flow rate" means either volume/(unit of time) or mass/(unit of time). For example, either ft3/s or m3/s or lb/s or kg/s.
"Velocity" means linear velocity. For example, ft/s or m/s.
Does that help you understand that the linear velocity increases as the fluid flows through the orifice ... even though the flow rate remains constant?
Milton Beychok
(Visit me at www.air-dispersion.com)
.
RE: Designing an orifice plate to restrict flow
So then how is it that an orifice restricts the flow rate? Wouldn't the velocity increase hence maintaining the flow rate?
This is where the formula posted by dickon17 doesn't jive with A1V1=A2V2. I know they are both valid equations but how can they coexist?
RE: Designing an orifice plate to restrict flow
An orifice will restrict flow by creating a significant increase in friction loss into the discharge system. If the system pressure creating the flow remains the same, and friction increases in the discharge pipeline, then the flow will reduce. This is of course a hugely simplified explanation.
The formula comes from ancient published data, Crane (Chapter 3), page 3-5, "flow through nozzles and orifices". There are many such formulae, each specific to the units being used. I just picked the most convenient one. Flow through the orifice is complex and will vary with the Reynolds number and whether the orifice is sharp edged or not. However, this is not rocket science and I used the simplest formula, since we are not looking for answers to the nearest 5 decimal places.
RE: Designing an orifice plate to restrict flow
JasonG:
With all due respect, it appears (at least to me) that you are unfamiliar with the Venturi effect and the Bernoulli Theorem. I strongly suggest the you read this article:
http://en.wikipedia.org/wiki/Orifice_plate
Milton Beychok
(Visit me at www.air-dispersion.com)
.
RE: Designing an orifice plate to restrict flow
mbeychok - thanks for the link. I am familiar with those concepts and formulas. I like to be able to visualize the theories so that I can really understand the numbers and equations that I am working with. I am much more familar with open channel flow which is just similar enough to throw me off when working with pressurized systems.
I think I'm good now. Thank you both for your help.