Rotary (pos. displacement) pump/system problem

[454Casull]

Right now I've got a rotary pump in a system where the discharge pressure is about 350psi, and the nominal max is 200psi. One of my supervisors tells me that it's bending shafts and whatnot, in addition to delivering poor flow at the end of the system (which is weird, because it's a rotary); now, the pipe is 2" stainless steel and the fluid is something about as viscous as Vaseline. He's considering moving to 3" to relieve the pressure at the pump discharge.

As a cheaper measure to reduce discharge pressure, one of his colleagues suggested doing something like this:

http://www.rootminus1.com/howard/pics/pump.png

where the parallel pipe is the suggested installation. The system resistance would probably drop a little, lowering the discharge pressure, but as far as I know it would be just like upping the cross-sectional area for a part of the pipe.

Can somebody help me get started with the calculations to find the drop in discharge pressure for moving from 2" to 3" pipe? I can get the length of pipe, minor losses, viscosity of fluid, etc. The nominal flow rate of the pump is 50GPM, but it doesn't sound like it's delivering that much.

Pointing me to the right formulas would be great.

 

[quark]

PD pumps give you constant flowrate at varying discharge pressures. However, you are already in unsafe zone if you run the pump at 350psi when you are supposed to run it at 200psi. Just check whether this pump was selected for some other application initially and now you are using it for this viscous fluid. Next step would be to call the manufacturer and take his advice.

To check the new pressure drop for a change in pipe size, use Darcy's pressure drop formula. Keeping the flow rate, friction factor and length of piping constant, the pressure drop is inversely proportional to the 5th power of the diameter. If you want to work out on a parallel piping arrangement, first calculate the total area of both the pipes and then get an average diameter for the sake of calculation.

Most importantly, deduct the static component of the head before attempting Darcy's formula.

 

[454Casull]

I'm sorry if I didn't come off clear, but what I would like is to find the approximate drop in discharge pressure (exit of pump). If there's a way to relate that to the pressure loss (dP), I don't know what it is. I'm looking at the equation for fully-developed Hagen-Poseuille flow, and it seems like I would have to find the pressure at the end of the (new) 3" pipe in order to find the discharge pressure. I don't think that works, so any help would be great.

Very much appreciated,

Howard

 

[GregLamberson]

A couple of things you might want to check, sorry if they seem rather obvious, but you never know.... Are the flow meters & pressure gauges reading right (i.e. column level rising, tank not dropping)? Have you checked NPSH? you mighta lso check the relief valve or the by-pass setting. As quark pointed out, you are at excessive pressure, the sahft may very well be bent.

Have you curved the pump?


Greg Lamberson, BS, MBA
Consultant - Upstream Energy
Website:

http://www.oil-gas-consulting.com

 

[BigInch]

Hagen-Poseuille for laminar flow


f (friction factor) = 64/Nre

Transitional flow begins at Nre = 2100
Fully Turbulent flow begins at Nre = 4000

http://virtualpipeline.spaces.msn.com

 

[BigInch]

Oh sorry, you already have that.

Technically you will have to solve for the looped flow using the diff head vs flow equation for the pump and head loss vs flow equation for the recirculation pipe, but

what I would do first is to just reduce the flow into the downstream piping by the amount of vasoline you want to recycle back to suction. Find the inlet head needed to move the reduced flow into that downstream pipe.

Use that same head as the new discharge head coming off the pump and into the inlet to both the downstream pipe and to the recycle line back to suction.

Size the recycle line diameter for the amount of flow you are bypassing, using that discharge head and the same suction pressure you had before (assuming suction pressure will remain constant, might not be true, but convenient... you can change it if you need to in the iteration process)

Now iterate those steps until any remaining errors are acceptable.

http://virtualpipeline.spaces.msn.com

 

[BigInch]

If you didn't get my obtuse description, I put a spreadsheet for you showing this quick and dirty method I'm talking about at,

http://rapidshare.com/files/32695756/positive_displacement_system_analysis.xls.html

The "secret" to this quick & dirty is that a positive displacement pump will give you whatever pressure you need to get into the downstream piping with that downstream pipe's flow, as long as you have the power available and the pump don't break. With a centrif, you would have a little more work to do with the iteration for the centrif's pump curve, but not too hard in either case.
light control panel

The spreadsheet uses some rough parabolic type approximations for pipe flow curves in the downstream pipe and in the bypass back to suction (ok I'm lazy today), but you can easily refine those to use Hagen-P or any other flow equation you like. Now that I think about it, Hagen is linear... and if I was a lazy guy, I would have used that rather than my parabolics.. oh well, its done now.

http://virtualpipeline.spaces.msn.com