To keep moving your flow through a pipe you need some deltaP.An orifice is an obstacle to the flow, it turns some of the available deltaP into heat; with less available deltaP(flow) you get weaker flow.This relationship is however not linear so that at some ratio deltaP(orifice): deltaP(total)an increse of deltaP(total)exerts only very little effect on deltaP(flow).
m777182
1) Please anyone can explain more for the topic. How can restrict orifice limit the flow rate? I mean when it is installed to the pipe the flow rate will constant regardless of upstream or downstream pressure, is that right or not?
2)And when sizing the restriction orifice, is it used the same equation for sizing orifice plate?
1) Yes, The volumetric flow rate through a pipe is constant.
When a restriction (orific)is placed within the pipe, it impedes that mass flow, to maintain the volumetric flow constant the velocity of the fluid across the restriction increases.
This increase in fluid velocity produces a presure differential across the orifice, which is measured and is propotional to the volumetric flow rate.
2) Each orifice plate manufactue provides with their plate a "orifice coefficient" that is unique for that particlar plate.
This coeffiecient is used in the following equation:
q = C SRQ(dP/p)
q = flow rate
C = orifice coefficient
SRQ = square root
dP = pressure drop across orifice
p = fluid density
The flow of fluid through a pipe is governed by Bernoulli's equation. In layman's language, this means that the sum of the velocity head (i.e., kinetic energy on account of the fluid velocity), pressure head and level head (i.e., potential energy on account of the level of the fluid above a reference datum)is constant.
Now, when there is a restriction in the pipeline (such as is produced by the orifice), the velocity head increases at the orifice (because the fluid is constrained to flow through a lesser area. As a result, the pressure head will have to decrease (assuming, a horizontal pipe, to simplify things - so that the level head remains unaltered).
Moreover, the decrease in pressure head will be equal to the increase in velocity head.
Now, the flowrate is the velocity times the area of cross-section of the orifice.
When you solve the equation, you will find in the end that the flowrate in the pipeline will decrease as the orifice bore is decreased more and more.
And I always thought Flange-Heads had odd views of electricity.
The continutity equation says that mass flow rate past any point in a system where mass is neither added nor removed is constant. Volume-flow-rate will be constant as long as density remains constant in a system where the continuity equation applies. Bernoulli's equation doesn't "govern" anything, but it does describe some idealized flows with reasonable approximations.
A restriction orifice in a flow stream does not violate the continuity equation, instead it results in slowing the upstream mass flow rate. You can see this by looking at the volume flow rate (for a constant density system)--upstream of the orifice the product of the (small) velocity and (large) area exactly equal the product of the (large) velocity and (small) area of the orifice.
The key to understanding restriction orifices is the continuity equation. The effect cannot be localized to the area around the orifice so you have to think in terms of the entire flow stream slowing down to make up for the reduced flow area of the orifice (with its high velocity).
I never try to follow zdas04 and others when speaking in terms of conservation of energy etc. I liked one word, velocity. You can search the web for stuff on "choked flow" to help with the idea. As the velocity increases the pressure drop increases exponentially.
You can size a restriction orifice based upon the flow orifice equations - but not for flange taps. Look for the input to select pipe taps to calculate the permanent pressure loss instead of the temporary loss across the plate with flange taps.
By the way, this is all well and good through about Class 300. If you are dropping high pressure gas, consider using a choke.
With my unsophisticated input complete, perhaps David and others can straighten things out.
jsummerfield,
The original question dealt with liquid flow which is incompressible and will not exhibit choked-flow behaviour.
Liquid is both simpler and more difficult than gases. It is simpler in that it rarely experiences significant changes in density. It is more difficult in that you don't get to take advantage of a choked-flow condition in sizing restriction orifices or measurement equipment (a "sonic flow" nozzle that is used as a meter prover just doesn't have the same capability in liquid).
