How to Size an Orifice Plate for Flow Restriction? 2

[1Sponge]

I am trying to use an orifice plate for flow restriction in a Soda Ash line and am having trouble working out the hydraulics.

The system is set up as follows:

Soda ash is being pumped through a pipe loop. So….a pump pulls the soda ash out of a tank, pumps it around the site through a PVC pipe loop, and then discharges it back into the tank. There is a secondary process which periodically draws soda ash off of the recirculation line. I want to restrict the flow to the secondary process using an orifice plate. For example if the flow in my recirc. line is 40 GPM I want to limit the flow to the secondary process at 10 gpm. So when the secondary process is active I want 10 gpm to go to my secondary process and 30 gpm to remain in the recirc. lne. I figure an eccentric orifice (installed in the lower 1/2 of the pipe) is needed in this application to prevent soda ash settlement in the pipe. How do I size my orifice plate to achieve this? Any suggestions on what equations should I use? Note: I do not have the luxury of being able to purchase modeling software, thus I need to be able to perform these calcs by hand.

I’ve looked at the following orifice equation but it appears to apply to instances of free discharge only so I can’t use it:

Q=CA SQRT (2gH)

Where
Q= dischage
C=orifice coefifcent
A= area of oririce
g= 32.2 ft/s^2
H= depth of water above the orifice

And the continuity equation does not work either because my Qs are not constant.

Q=AV, A1Vi==A2V2; == means not equal

I am currently trying to back into the orifice size by iteratively adjusting the size of the orifice in order to get the losses through the orifice plate down to ¼ of the recirc line losses but have not reached a solution yet.

Any help you can offer on this topic much appreciated.

Thanks for reading.

 

[bimr]

Here is an online calculator for orifice plates:

http://www.pipeflowcalculations.com/orifice/

http://en.wikipedia.org/wiki/Orifice_plate

 

[psmart]

Reminds me of a test I took once! How about this:
1) Determine the head loss in the recirc loop from the orifice location to the end of the recirc loop, calculated at 30gpm. (not 40) Assuming that the recirc loop ends at atmospheric pressure, this is the pressure at the orifice inlet when the orifice is flowing at the design rate of 10gpm.
2) Now size the orifice to yield 10gpm at the calculated inlet head. This assumes the orifice discharges to atmospheric pressure. Otherwise, subtract the secondary system pressure from the head when sizing the orifice.
Depending on the flow accuracy you need, oversize the orifice and insert a control valve at the orifice so you can fine-tune the flow.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[beej67]

If I were you, and those GPMs you listed need to be pretty accurate, I would get a gate valve or ball valve instead of an orifice plate, so you could vary your opening and fine tune your flows. Any mathematical solution is going to be close but not completely accurate, since there's assumptions that might not be exactly right.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[1Sponge]

Thanks for the feedback all.

psmart...can I use the standard orifice eqn to find discharge [Q=CA SQRT (2gH)] in this application? If I do use the std eqn. my H in the eauation will become headloss NOT the "depth of water above the orifice". I haven't seen anything in the literature to support the use of this equation for piped flow...I do not do hydraulic calcs on a regular basis so I am a little rusty. Do you know of a good techincal resource I can consult about this?

 

[psmart]

Yes, the standard orifice equation would apply, using the head loss (differential) between the two sides of the orifice. If you're discharging to atmospheric pressure, this is the same as the "depth of water above the orifice".

You may also need to consider the losses in the orifice piping. As beej67 says, there are a lot of variables, so you really should install a valve so you can adjust the flow.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[1Sponge]

There are a couple of existing valves on the line to the secondary process right now but they are not helpful in regards to what we are trying to do (i.e. open/close diaphram)... the addition of a ball valve instead of an orifice may not be a bad idea...Thanks again all.

 

[bimr]

Here is an easier method for your calculations:

http://www.mcnallyinstitute.com/13-html/13-12.htm

 

[bimr]

Do you have an orifice in your pipe loop? For this application to work properly, it will probably be necessary to have around 40 psi in the loop to provide back pressure.

 

[1Sponge]

No existing orifice plate on the recirc line right now. It is a free disharge to atmosphere (into a tank). Concerning an orifice for back pressure...you are suggesting this from a pump perfomance standpoint correct?

 

[bimr]

Actually, you need what is called a back pressure regulator to control the pressure in your recirculating loop.

http://www.equilibar.com/back-pressure-regulator/liquid-applications.asp

Assume you do not have a back pressure regulator on the recirculating loop. When you install a new flow restriction on the takeoff from the loop, the liquid will than follow the path of least resistance which is back to the tank (instead of through the orifice).

If you have a steady pressure in your recirculating loop, then you can expect to control the flow with an orifice plate.

Otherwise, it will be like a hose with 2 outlets. Restrict one outlet and the flow comes out the other outlet.

 

[stanier]

You could download PSIM and model it.

http://www.pumpsystemsmatter.org/content_detail.aspx?id=110

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His Holiness the Dalai Lama.

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