8 replies to this topic

[jaychemical]

Flow rate through an existng 300' of 1" sch 80 pipe is 25 gpm. In order to get 35 gpm, if I want to increase atleast 150' of 1" pipe to 1.5", how can I calculate the flow rate? With new configuration, I calculated k for each pipe size and fitting and also head loss. In the flow rate equation, what value of diameter(ID) should I consider? Inlet pressure is 65psig. fluid - DMAc. Pressure drop across a control v/v in this line is 3 psi and pump is operating at its design head. Pressure drop across 1" line is 63 psig

Any help/advice is valuable for me.

[Art Montemayor]

What is "DMAc" - Dimethyl Acetate? What are the fluid viscosities, inlet pressure and temperature as well as outlet conditions. Is the flow isothermal or adiabatic?

The calculation is pretty straight forward and should be accurate (within 15%) using the Darcy equation. I would use the 3-K method (from Darby) if I needed the maximum accuracy.

But we need the fluid full identification, conditions, and properties. We also don't have the fittings, valves, and iso sketch.

For water, a 25 gpm flow in a 1", sched. 80 pipe for 300 feet will yield a pressure drop of 180 feet of water = 78 psi. This is using the Hydraulic Institute Tables, which are derived from the Darcy equation.

[jaychemical]

I posted this topic while I am half asleep. Fluid is acetone enereing at 90 deg F. I can use 3-K method to find K values. and I can estimate head loss through pipe using bernouli's and Darcy equation.

To calculate flow rate after estimating hL and K in the equation

q = 0.0438*d^2(sqrt(hL/K))

What value of d should I use in this kind of series pipe connection. Please help me out.

Thank You
Jay

[jaychemical]

Can anyone forward me an excel sheet to do iterative calculation on flow rate estimation ina series pipe connection please


Thank You

[Emetovweke Ochuko]

My Dear,
From my little experience, you need the two diameters for your calculation. Also needed are the inlet and outlet conditions of the fluid as was rightly stated by Art Montemayor.
Thanks
ochuko E.

[latexman]

jaychemical,

Since you will have a single flow system with two different pipe diameters to increase the flow rate, I’d recommend you not use the q equation approach, because it does not allow for two different diameters. If you bust your system into the two logical pieces with different pipe diameters and sum up pressure drops (dP) that each diameter of pipe, fittings, and CV will give you, the following equation will facilitate two different pipe diameters:

dP = 0.00001799.K.rho.Q²/d⁴

Be sure to look at your pump curve to find the new pressure drop available at the new flow rate. If it’s a centrifugal pump, there will probably be less pressure drop available at 35 gpm than at 25 gpm.

Dig into the details of your control valve and understand where it normally runs and how much capacity it actually has or doesn’t have. I.e. get the vendor catalog and data out. You'll need Cv versus % open. Most vendors provide it. I usually do not include the pressure drop of the CV in the total system pressure drop, but rather determine by difference how much pressure drop is left for the CV and figure out what % open it will run at and then make a judgment if that is acceptable or not. I don't like to see a CV run less than 20% open or more than 80% open. If not acceptable, that means more 1” pipe has to be replaced with 1.5” pipe. It could be you need bigger trim or even a bigger CV. Or maybe not, it depends on how the numbers come out.

In essence what you are doing by the approach above is summing and plotting the new system curve on the existing pump curve until a satisfactory solution at 35 gpm is found.

[jaychemical]

Thanks Guys.

I have calculated pressure at each node of the pipeline for exisiting 1" for 25gpm. Now I increased the flow rate to 35 gpm and kept changing certain amount of pipe length to 2" until the discharge pressure is greater that 3 psi( Discharge goes in to a tank which has 2.5 psig at the connection point) I still need to fin the economical dia and length to fit in.

I have one more question. While I am calculating pressure at each node( exisitng system), I got -2.0 psig pressure at about 220 feet of length which is at 22 feet elevation. The discharge elevation is 2.5'. With negative pressure I still went ahead to calculate pressure at the end node which is 3 psig.

Is it common in pipelines or have I done any error?

[latexman]

It is not uncommon for a pipeline to run at a vacuum at it's highest point. You have to decide if that is okay. Will the acetone start boiling at this point on a hot summer day? Is the CV nearby and will the acetone cavitate in the CV. All valid concerns you need to think about and address if they are real and detrimental.

[jaychemical]

Latexman,

Appreciate all your suggestions. I used galvanized steel instead of stainless steel pipe. With galvanized steel, f = 0.032 which gave me vacuum at the highest point. I rectified my material selection mistake now with stainless steel. This has a f = 0.024. I got positive pressures at each node and complies with the exisitng pipeline. Now It is easy for me to key in larger pipe size and increase flow rate by monitoring pressure drop across the line as wellas control v/v.

I changed the control v/v from cv = 5.2 to a spare control v/v with cv = 13.8 but it did not help. Only option is change the pipe size. As you suggested, at the new flow conditions I will check the system point of this centrifugal pump.

Thank You