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# Orifice Pressure Drop with Viscosity Change

## Orifice Pressure Drop with Viscosity Change

(OP)
I was working on designing an orifice in a system the other day when I noticed an anomaly that I wanted some insight on. I searched around as best I could to try to find why the system behaved this way but could not find anything so I figured I would try here.

In essence what I am seeing is as I increase viscosity for a fixed pressure drop through an orifice the flow increase along a range (10-1300 cP) then after an inflection point it begins decreasing in flow. My gut feel on viscosity changes is that as viscosity increase the relative pressure drop will increase, thus decreasing flow since the pressure difference across the orifice is fixed. I attached an excel file that shows the data sets I have from 2 different orifice sizes and they both have roughly the same trend.

If anyone could shed some light on this phenomenon I would be greatly appreciative. The only idea I have is at the low ranges the viscosity changes decrease Reynolds number at a faster rate than the higher viscosity increases pressure drop. Then at some point the viscosity outweighs the effect on the Reynolds number and starts to decrease flow.

### RE: Orifice Pressure Drop with Viscosity Change

What are the flow units? What are the units of the orifice diameter? What is the pipe diameter? How are you changing viscosity? Why aren't the damn axes labeled? Why do I have to pull teeth to try to help?

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

### RE: Orifice Pressure Drop with Viscosity Change

Also, what was density doing through this viscosity change? Ditto, the flow coefficient?

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

### RE: Orifice Pressure Drop with Viscosity Change

(OP)
Flow units are lbs/hr
orifice bore in the first set is 0.68 in second set is 1.36 in
pipe diameter is 3 in
x axis is viscosity in cP
y axis is flow in lb/hr

This is assuming all other physical properties stay constant. It is used in a variable process and we were trying to hold everything else constant over the varying viscosity.

### RE: Orifice Pressure Drop with Viscosity Change

How was viscosity changed? How was density held constant during this change? How did Reynolds number change during this? You need to look at all of the variables for orifice flow, including the flow coefficient, not just viscosity. Sounds a little too simplistic based on what little we have been told. Do you know what "they" say about the word assUme? And lastly, are you sure the fluid stayed Newtonian as viscosity went from 10 to 10,000 cP?

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

### RE: Orifice Pressure Drop with Viscosity Change

I assume the graphs are the result of a theoretical calculation where you have simply entered a range of viscosities while keeping the other variables constant. In the equation for calculating the flow rate from the pressure drop the viscosity does not appear explicitly. However, the equation for the discharge coefficient does include the Reynolds Number which of course depends on the viscosity.

The equations are all laid out in the Wikipedia article at https://en.wikipedia.org/wiki/Orifice_plate

However, it is difficult to visualize the behavior from such complicated equations and I have attached the graph of the discharge coefficient against the Beta ratio and Reynolds Number from Coulson & Richardsons "Chemical Engineering" Vol 1. The maximum for the coefficient of discharge occurs at lower Re for small Beta ratios, and this agrees with the results in your graph.

Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

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