Flow from pressure gauge

[musleh1975]

For the following system. Water is pumped to atmosphiric tank and there is pressure gauge at the end of the line in a location is very close to pipe exit. Can we use this reading and with pipe diameter to calculate flowrate? By Crane it is suggested to consider k=1 and use velocity head to find flowrate . But i think results is over than actual and i think there will be factor for exit loss.
Any ideas???

 

[Latexman]

Got a level guage/instrument? Measure change in level vs. time. Convert that to volume/time.

Good luck,
Latexman

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

 

[musleh1975]

V=(2gh)^0.5
h we can get from P at the line end
What is the wrong of applying that.?
I think there is somthing missing or factor to be used.

 

[MortenA]

sounds a bit like homework, but for a liquid flowing from a pipe into a tank the loss is equivalent to one velocity height because the fluid is decelerating from its velocity V in the pipe to 0 (=one velocity height). If you can actually measure that (im concerned with your accuracy, the dP will be rather small) is another question

 

[musleh1975]

Actually in feild, their is sometimes need for rough estimate for flow. Many times there is only one reading of pressure . But i beleive there must be coeffecint for K instead of 1 for exit specially if yhere is nozzle at the end of line before exit to atmospheric tank.
The point i want to confirm is there factor or not instead of 1

 

[georgeverghese]

Latexman's suggestion would be easiest. Another way would be to infer level from the reading on the PG. If you know density of this liquid, you'll have an indication of flow.

Guage pressure = rho. g. h for an atmospheric tank
dP/dt = rho. g. dh/dt

Flowrate = F = tank csa . dh/dt
so
dP/dt = rho. g. F/tank csa
or
F= (dP/dt).tank csa/(rho.g)

Here, dP/dt is the rate of press rise at the base of this tank with time. The smaller the full scale range of the PG, the better.

 

[MortenA]

Canr you calibrate it?

Measure dP at a constant flow rate (constant dP) over a given time. Measure the volume increase in your tank. Repeat at some different flow rates

Best regards, Morten

 

[musleh1975]

Actually it is circulation for atmospheric cyclonic tower. So what i want to confirm is the pump flowrate is still on pumping curve or their is deviation from curve. Measuring the level is meaning less in this case because it is circulation pump.
Sorry i didn't explain the system in begining clearly.
In summary what i want to confirm, if only one reading of P is known and because it is atmospheric then P2 also known ( 0 barg).
Then this P equal v^2/2g multiply by some K.
Here to consider K=1 or there is another factor to use.?
Thanks for any idea

 

[apetri]

there is something unclear,
you have a pump curve (pressure vs. flow),
at pumpu discharge there is a short pipe (you know fluid, diameter and lenght then you can calculate dp vs. flow) and finally a static head (h inside tank),
for pump, p out = dp (pipe) + static head (measured by instrument, assuming pipe horizontal, no dH)
to estimate flow from pump curve you need to calculate dp across pipe (could be a table of values)
to estimate flow from pressure loss at end of pipe there are several formulations but probably the simple solution is to create a table of values as already suggested by others

 

[musleh1975]

Thank you. You mention that there is many formulation. What iam looking for the best formulation for pressure loss at pipe exit. That is to consider K=1 or not in velocity head loss

 

[apetri]

I am not sure there is a simple and accurate enough method to estimate pressure losses at end of pipe, normally one assumes 1 (all kinetic energy dissipated) but...
of course you should start from Bernoulli and difference (measured p vs. static p which is that of your instrument when flow = 0 or may be estimated from H and density as suggested by georgeverghese)...
there are many different methods (including differentials across plates, bends etc.) to estimate flow in piping, you may consider alternatives.

 

[don1980]

It's a pretty easy task to calculate a flow rate in a pipe if you know the pressure drop through the pipe and the viscosity of the fluid. The accuracy of your flow calculation depends on the accuracy of the input variables. In your particular case (line routed to an atmospheric tank), you need the pressure at the beginning of the line, but you don't need the pressure at the point where the line connects to the tank. That's because you already know the pressure at the end of the system (atm tank), and I assume you know the liquid height in the tank. This gauge that you refer to, near the end of the line, isn't necessary. Effectively, that gauge pressure is just indicating the static head in the tank, which you already know.

BTW, I'm assuming this line connect near the bottom rather than the top of the tank.