Pressure Drop across an Orifice Plate
Pressure Drop across an Orifice Plate
(OP)
Hi All,
I have a question regarding non recoverable pressure drop through an orifice plate. Below is my scenario but with simplified (made up) numbers. Any help would be much appreciated.
I have a centrifugal pump, with a minimum continuous flow rate of 10 gpm. I need to design a re-circulation line that will open to protect the pump for when the main process line is shut off. This will ensure that at least 10 gpm of fluid is always being pumped. My main process line uses 6" piping, and my re-circulation line uses 3" piping. My pump discharge pressure is 100 psi while operating at the 10 gpm. The tank the bypass line feeds into has a back pressure of 10 psi on the bypass line. I can design any orifice plate I want, to generate whatever pressure drop is needed to create false head against the pump (so the pump works at the desired 10 gpm).
My question is, what pressure drop do I want out of my orifice plate? Should it be 90 psi? Because I am going from a 6" main line pipe to a 3" pipe in the re-circulation line, the fluid velocity will increase. My pressure should therefore decrease on the inlet side of the orifice. If this is correct, and we assume 3" pipe has an initial 80 psi just before the orifice plate, and 10 psi after the plate, well now my pressure drop across the orifice is only 70 psi.
So which is it? Should the CV of the orifice be designed for a 90 psi pressure drop, or a 70 psi pressure drop?
I think my confusion goes back to not understanding the difference between recoverable and non-recoverable pressure drop.
I have a question regarding non recoverable pressure drop through an orifice plate. Below is my scenario but with simplified (made up) numbers. Any help would be much appreciated.
I have a centrifugal pump, with a minimum continuous flow rate of 10 gpm. I need to design a re-circulation line that will open to protect the pump for when the main process line is shut off. This will ensure that at least 10 gpm of fluid is always being pumped. My main process line uses 6" piping, and my re-circulation line uses 3" piping. My pump discharge pressure is 100 psi while operating at the 10 gpm. The tank the bypass line feeds into has a back pressure of 10 psi on the bypass line. I can design any orifice plate I want, to generate whatever pressure drop is needed to create false head against the pump (so the pump works at the desired 10 gpm).
My question is, what pressure drop do I want out of my orifice plate? Should it be 90 psi? Because I am going from a 6" main line pipe to a 3" pipe in the re-circulation line, the fluid velocity will increase. My pressure should therefore decrease on the inlet side of the orifice. If this is correct, and we assume 3" pipe has an initial 80 psi just before the orifice plate, and 10 psi after the plate, well now my pressure drop across the orifice is only 70 psi.
So which is it? Should the CV of the orifice be designed for a 90 psi pressure drop, or a 70 psi pressure drop?
I think my confusion goes back to not understanding the difference between recoverable and non-recoverable pressure drop.





RE: Pressure Drop across an Orifice Plate
some of that 90 psi pressure drop may be recovered after the orifice. You may want to put a globe valve in there between orifice plate and tank to manually adjust pressure into the tank if necessary.
you must get smarter than the software you're using.
RE: Pressure Drop across an Orifice Plate
I would therefore use a pressure drop of 90 psi. Then the equations you use for the orifice size should take into account what part is not recoverable.
Even if you have sized the orifice too big, creating a flow greater than 10 GPM (wherever the hydraulics balance out) and protecting the pump. But you still might want the globe valve to maintain your max flow from the pump to its destination (i.e. 10 GPM of minimum flow plus your required flow). Under this case the pump head will decrease your dP available will be less than 90 psi and the flow through the kick back line will be less than 10 GPM which is OK. (Unless you have a shutoff valve on the kickback line).
RE: Pressure Drop across an Orifice Plate
RE: Pressure Drop across an Orifice Plate
Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"
RE: Pressure Drop across an Orifice Plate
The other reason might simply be cost of the installation.
There are available automatic recirculation valves which operate on the principle of pressure (against internal spring ARC) which are used commonly for boiler feedwater pumps.
RE: Pressure Drop across an Orifice Plate
RE: Pressure Drop across an Orifice Plate
So with 10 gpm, the pressure drop due to the line going from 6" of flow to 3" of flow is negligible. If we say the minimum continuous flow rate was something higher like 100 gpm, I assume I would need to check the fluid velocity in the 3" line to see what kind of decrease in pressure I will experience when going from a 6" pipe to a 3" pipe. This will give me a new orifice inlet pressure, which I could then use to find my new delta P.
And for those wondering, the orifice plate is being used to save money over a backpressure control valve.
RE: Pressure Drop across an Orifice Plate
I can't see your concern over sudden contraction being a issue at those sorts of relative numbers. If though you were only looking for a pressure drop of say 5 psi, then it might become more relevant - the devil is in the detail.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Pressure Drop across an Orifice Plate
And that cavitation will cut through the pump impeller, and possibly the actual casing, in short order. Continuous cavitation is amazingly destructive.
RE: Pressure Drop across an Orifice Plate
In the returning back lines such as minimum flow for pumps or anti surge lines for compressors the differential pressure across the orifice or control valve is to be considered per presuures in main discharge line and in destination, either suction line or suction drum, at the moment of returning back activation for full flow because the minor flow restriction due to upstream/downstream lines, if any, would be compensated by the overdesign factor has already been considered in the peak of such minimum flow estimation; then 100-10=90 bar is reasonable differential pressure drop for your case...
As ione mentioned, you need a RO with definitely small beta ratio so the pressure recovery after vena contracta should be something negligible.
RE: Pressure Drop across an Orifice Plate
RE: Pressure Drop across an Orifice Plate
(p1-p4)/(p1-p2)=1-Β2
p1=upstream pressure tap
p2=downstream pressure tap
p4=fully recovered downstream pressure
Β=diameter ratio
Β % Pressure Loss
0.1 99
0.3 91
0.5 75
0.75 44
0.9 19
0.99 2
I would say Β < 0.45, which gives about 80% pressure loss, but for a specific case one must really run through the calcs to obtain the required permanent pressure loss.
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: Pressure Drop across an Orifice Plate
If I go with a multi-hole orifice, instead of a single hole orifice, how would this affect the beta ratio? Would I need to convert the equivalent cross sectional area of the multi-hole orifice to a single hole orifice to determine the beta ratio?
RE: Pressure Drop across an Orifice Plate
thread378-205170: Multi Hole Orifice Pressure Drop
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: Pressure Drop across an Orifice Plate
But even if you use 3 plates in series and allow 30 psi per plate the orifice bores only increase to about 5/16" - hardly any more resistant to plugging. The answer is probably to install a strainer before the orifice with a mesh size of 1/16" or less.
Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"
RE: Pressure Drop across an Orifice Plate
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: Pressure Drop across an Orifice Plate
RE: Pressure Drop across an Orifice Plate
velocity about 21m/sec, no idea if it flashes but is a possibility.
I was just pointing out that there are practical issues, potential plugging being one of them and if it happens then your pump isn't doing 10GPM any more it's doing about 2.... I didn't do the calcs, but for that flow rate I agree multi orifices are unlikely to make it any better.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Pressure Drop across an Orifice Plate
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: Pressure Drop across an Orifice Plate
Q = C*Ao*SQRT(2*deltaP/rho/(1-(Ao/A)^2))^0.5
being
Q volumetric flow rate
Ao orifice area
A upstream pipe area
deltaP pressure difference
rho fluid density
Rearranging in order to solve for Ao and then to find the diameter it gives approx 1/4".
RE: Pressure Drop across an Orifice Plate
Notice my disclaimer in the last sentence - "but for a specific case one must really run through the calcs to obtain the required permanent pressure loss".
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: Pressure Drop across an Orifice Plate
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way