6 replies to this topic

[Mahesh@A&M]

I need to decrease transfer time from a pressurised tank to another tank which will be at 7 psi initially. Pressurised tank is at 75 psi. Fluid is a polymer with avisocsity of 3500 cp.

I suggested to increase transfer line size from 3" to 4" in order to increase flow rate and decrease transfer time. Attached are my calculations. Plz help me out.

When I increased pipe size to 4", velocity increased. Is this right? usually with increase in pipe size velocity shhould drop.

Thanks a million

Attached Files

•  3_inch_transfer_pipe.xls   20KB   61 downloads
•  4_inch_transfer_pipeline.xls   19KB   47 downloads

[Mahesh@A&M]

here is the PFD.

We have two similar systems. For one system, pipe length is 50 feet and it takes 30 min to transfer.
and the sytem of concern is 100 feet pipe line with equal fittings as previous system. this takes 55 min to transfer.

I feel with increase in pipe size u will have more area for the fluid to pass through. Head is also same on both the systems. I cant decrease the pipe length on this system due to constraints in pipe racks.

[pawan]

Earlier I got confused with a pumping system. Anyway

1. DP is fixed for transfer due to fixed pressure of supply & reciever tanks.
2. If DP is fixed Then V is proportional to D^2 for laminar flow.
3. Also U can not alter DP at different segments improportionately It will be changed by a common factor.
4. Effect of pressure change should be considered if any.

However if pressures are constant then simply DP = f * V^2 / D I have omitted constant terms. Now for laminar f = const/Re = mu / (D * v * rho) = const / Dv

So DP ~ V^2 / D * const / (D * V)
~ V / D^2

I have used ~ sign bcoz they are equatiosn with some other constants which are omitted here.

[Art Montemayor]

Chemroopa:

You say you have two “similar” systems. The important question is: are they identical systems – as far as the line size, the pumping flow rate, and the pressure drop across from the onset to the target? Do you have two identical pumps, each feeding its own system?

You state, “When I increased pipe size to 4", velocity increased. Is this right? Usually with increase in pipe size, velocity should drop.” NO! When you increase the pipe diameter and maintain the same flow rate, the velocity decreases. You are right in the second sentence. Are you doubting yourself?

We received no PFD, contrary to what you say. What is really needed is a system sketch, preferably as an isometric view, showing both systems and the piping and fitting details. It does not have to be to scale. You can easily do this on an Excel spreadsheet.

Basically, you have to identify the flow rate – and this is obtainable from the pump(s) furnishing the same. We also need the isometric sketch with the pipe sizes and lengths as well as fittings in order to calculate the pressure drop. I assume we are dealing with a Newtonian fluid. Is that correct?

So you see, you haven’t furnished enough basic data to resolve the reason(s) you are getting slower flow in one system than in the other. It might be simply that you don’t have enough driving force and too much frictional resistance through the second system. We need all the basic data – especially the driving force generators – the pump(s) – and the existing pressures on both the start and ending points.

[joerd]

Pawan's analysis is right, this is the Hagen-Poiseuille equation. Look at this page (found with Google image search), about half way down the page (item 3. Pressure loss due to friction in a pipeline.)
You can see that the flow profile is a parabola, for laminar flow. So, if the pipe is bigger (larger diameter), the center velocity can be higher for the same fluid, and therefore also the superficial velocity can be higher.
Somewhere near the bottom of the page, they derive that
,
which is what pawan (almost) showed above. So, the mean superficial velocity u_m is proportional to the available pressure drop, and to the diameter of the pipe squared.

[pawan]

Thanks Joerd
Without going into complexity, I prefer simple routes be it problem solving or deriving any equation.

Roopa
The problem in your calculation is that U have different DP variation between same segment of lines for 3" & 4" which is not possible. The changes in DP will reduce or increase by the same factor in a fixed DP system.
SO U need to recalculate the flow where OR simply it will increase by 16/9 ratio for changing from 3" to 4". So for ex your flow of 900 Kg/hr will increase to 1600 Kg/hr.

[Mahesh@A&M]

Thanks Pawan. Yeah I just noticed that DP is different coz I missed one pipe segment in 4" line calcs.

Thank You

Roopa