How to Determine Flow Rate from a Pressure Reading 1

[PotashMechy]

Hi All,

I am trying to determine the flow rate through a 2'' pipe based on a pressure reading taken on that branch. The pressure reading was 40psi. The piping up to that point is very complex and there is no way to know for certain how long it is.

Is there any way to calculate the flow rate through the 2'' line based on the factors that I do know? My known factors are: pressure reading at that location and pipe diameter.

Thanks in advance.

 

[JGard1985]

No....to obtain flow from pressure (single reading) you need more information. Flow from pressure is derived from measuring the differential pressure across a known flow element installed in the piping system


Jeff
Pipe Stress Analysis
Finite Element Analysis

http://www.xceed-eng.com

 

[PotashMechy]

Thanks Jeff. Do you have any recommendations on what further information I should gather? My 2'' line is a short branch that is coming off of a main line which is also 2''.

 

[3DDave]

Still not enough. Pressure is an independent variable from flow - One could easily have any amount of pressure with any amount of flow. JGard1985 indicated flow is potentially measured as a difference in pressures when that difference is due only to the flow conditions. So a pressure at the top of a tank < pressure at the bottom of a tank does not mean that any fluid is flowing in the tank.

 

[SlideRuleEra]

...any recommendations on what further information I should gather? My 2" line is a short branch that is coming off of a main line which is also 2".

Measure pressure at another point on the 2" line that is as far from the the first reading as practical. Knowing the distance between the two pressure readings (measured along the pipe line), the number and type pipe fittings in that line (located between the two pressure readings), and the pipe's inside diameter, you can make an assumption on the pipe's interior roughness and get an idea (probably a first order approximation) of the flow rate.

http://www.SlideRuleEra.net

 

[georgeverghese]

The simplest flow meter I've come across is an elbow meter, where a dp reading is taken over an elbow. Presumably this may have been in a clean noncorrosive environment. Else look for a suitable flange break where a flow element can be installed.

 

[katmar]

It isn't clear whether you want to calculate the flow through the main 2" line or the 2" branch. If you want to know the flow through the branch then you need to make a dimensioned sketch of the branch and measure the pressure at the end of the branch (if it does not discharge to atmosphere). You also need to specify the fluid that is flowing. If you want the flow through the main line before the branch you need a whole lot more information.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[MJCronin]

How about one of these?

Portable Clamp-on Transit Time Ultrasonic Flow Meter

https://www.onicon.com/products/f-4400-portable-ultrasonic-flow-meter/

How much entrained air. debris etc do you have in your flow stream ?

How much accuracy do you need in your flow measurements ???

https://www.badgermeter.com/busines...n/dxn-portable-hybrid-ultrasonic-flow-meters/

Please respect us....do not be like most of the Newbie OPs ..... Come back, complete the thread tell us about your final solution ....

MJCronin
Sr. Process Engineer

 

[SandCounter]

I was going to suggest running a calc on what's down stream of the measurement point instead of what's up stream. However SlideRuleEra has the best answer. Measure the difference in pressures and length between the measurement points. Convert to head and extrapolate what it would be over 100 ft and then (assuming it's water) go to a friction table like Cameron Hydraulic data (p. 3-16), find the head loss over 100ft that matches and look at the flow column.

I used to count sand. Now I don't count at all.

 

[MJCronin]

Sandcounter.....

I disagree ....

Clamp on ultrasonic flow meters do not require opening the system up and installing another pressure indicator.

They are cheap, can be used all over the plant, have been around for decades and have more accuracy than the solution you advocate...

MJCronin
Sr. Process Engineer

 

[SandCounter]

MJCronin, your approach may be the most accurate, but it does not answer the question. The question was how to get flow from a pressure reading. As for opening the system, Potash may have a second gauge already installed or a test port or pet cock available.

I didn't mean to step on your toes, just trying to answer the question as posed.

I used to count sand. Now I don't count at all.

 

[PotashMechy]

Hi All,

Thank you for all of your replies! I actually found a method as recommended in the NFPA 291 guideline. They recommend opening the line and using a pitot tube to gauge the pressure, and calculate the flow by using the formula:
Q=29.84*c*d^2*sqrtP

where:Q-flow
c=coefficient of discharge
d=diameter of the outlet
P=pitot pressure

 

[LittleInch]

This method is for measuring flow in the open air coming out of a hydrant.

It is not designed to measure flow in a pipe.

Use a normal pitot tube and calculate flow that way.

Or use any other type of simple insertion meter

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

Another alternative would be to measure the dp across an existing full open valve with known Cv on this 2inch branch.