Pipe Flow calculations. 2

[bigsteve39]

In my company we use a 4" mild steel main pipe for delivering fluids to and from tanks.
We are thinking of changing a certain section of it to 6".
will that allow for increased volume flow through the pipe with teh same pump?
Or will the sections that are 4" restrict and give the same flow as if there was all 4" piping?
What calculations would i use to calculate teh volume flow through a certain size of mild steel pipe?

 

[TD2K]

You will get more flow as a result of replacing part of the pipe with 6". How much depends on how much 4" is replaced with 6" and the shape of the pump/resistance curve.

I'd suggest you get a copy of Crane's technical paper, it will give you a good background in quantifying this.

 

[Latexman]

It depends!

If the pump is a fixed speed, positive displacement pump the flow will remain the same.

If the pump is a fixed speed, centrifugal pump the flow will depend where on the pump curve you are. If you are already near the end of the curve, you may not see a measurable increase in flow.

Get Crane's TP 410 like TD2K said.


Good luck,
Latexman

 

[TD2K]

Good point Latexman, why do I always forget about those PD pumps on these type of questions?

 

[HVAC68]

A different thought :

With a larger diameter pipe, can we say that the friction drop is less and hence, with the same pump and motor combination, we can get a higher flow ?

HVAC68

 

[25362]

Rotary pumps, belonging to the displacement pump category, may increase flow as a result of less friction) back pressure, due to reduced internal slip effects, especially on low viscosity fluids.

 

[TBP]

I find it useful to think of the components of piping systems as links in a chain - the outlet pressure from one is the inlet pressure to the next. Change a link, change the chain. Sometimes the change is too small to notice, however. This situation is the reverse of what I find common. I routinely see plants trying to solve capacity problems like the one described by adding boiler/pump/compressor capacity without addressing their piping. Spending a little time getting some background data and doing a few simple calculations can help prevent spending a lot of money to achieve nothing.

 

[carthago]

Basically by changing the pipie diameter you will change the line curve.

If you have a centrifugal pump, and by using a 6" , your line curve will become a "bit" flat.

Pressure drop = function of (1/D^5 ) .. check in text books.

The drawback is you will probably hit the pump curve on the right side..means your pump will work also under another efficiency.

My suggestion is to draw the two line curves (the one with4 and the other with 6"), draw them on the pump curve (of course use same units)..
Then you will see the pump consumption, efficiency ,...

 

[JohnGP]

Not knowing what type of pump is involved makes it difficult to provide detailed information, but for the sake of completeness, assuming it's a centrifugal type, then - as suggested above Crane's Technical Paper No.410 will provide all the info you need to calculate the pressure drop through your system for different assumed flows. Having that you can generate a System Resistance Curve which can be plotted on the pump curve to check where the pump should be operating. (See for example

http://www.cheresources.com/centrifugalpumps5.shtml

for an explanation of the ins and outs of system curves and pump curves.)

Replacing some of the pipe with larger diameter should push the System Resistance Curve to the right, as head loss should be reduced for any given flow. How much depends on how much pipe is replaced, as TD2K pointed out.

 

[rcooper]

Not forgetting, if it is a short section you are changing from 4" to 6", the espansion losses due to the new fittings could increase the head loss of the system, so you could end up pumping less. Using ball park figures, I think you need around 1.5m of 150mm before you will start seeing any improvement in flow.