7 replies to this topic

[Amin Va]

Hello everyone,

 

I have a question about minimum flow requirement for a progressive cavity (screw) pump.

 

As you can see in the attached schematic the normal fixed flow is 4.2 m3/hr (the pump runs at fixed speed). The concern is when the XV valve is shut or when there is a restriction in the downstream package we do not want to  deadhead the pump or pop the PSV.  To solve this problem we think that using a minimum flow will help. Does any one have experience of having a minimum flow for a PD pump and how this should be set up. As you know the pressure through the liquid rises so fast and the recommended scheme for minimum flow should be able to relieve the pressure very fast that's why I doubt any sort of valve (eg. PCV) (or even shutdown system) will help.

 

Thanks

Attached Files

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Edited by Amin Va, 17 July 2015 - 03:04 PM.

[fallah]

Amin,

 

Your concern isn't so clear...anyway you can use a modulating pilot operated PSV or a self regulating pressure control valve for that purpose...

[latexman]

 . . . or switch to a centrifugal pump with a minimum flow orifice back to tank and delete the PSV.

[Amin Va]

@Fallah

Thanks Mr. Fallah for your comment. My concern in regards to PCV is that whether it can act fast enough to relieve the pressure in this system or not. As you see there is an XV valve (1” solenoid) downstream that can close very fast which might cause problem due to several reasons:

 

1-The liquids are incompressible and pressure can transfer through the liquid medium very fast. The moment that the XV valve closes, the pressure wave travels to the PCV in a fraction of a second. Is PCV able to give a fast response to the change in pressure?

 

2-The liquid volume inside the pipes is very small and it won’t help in dampening the pressure (as opposed to a large reservoir)

 

3-The regulator (PCV) reaction time is much slower in liquid compared to gas service  due to larger viscosity of liquids and hence larger friction.

 

All in all, as PCV is probably the cheapest option, I would like to know whether its function will  be compromised due to any of the above reasons in this system or not.

 

@Latexman

Thanks  Mr. Latexman for your comment, unfortunately the pump is bought and hence existing  which rules out this option for me (usually its hard to find centrifugal pump for small flows like this).

Edited by Amin Va, 19 July 2015 - 03:48 AM.

[breizh]

Amin,

Will you consider to install a 3 way valve  ?

I've installed similar equipment ( pig tail pump ) at the bottom of a thickener to feed a centrifuge .

A PSH hard wired should be considered to stop the pump .

Hope this helps

 

Breizh

Edited by breizh, 19 July 2015 - 04:27 PM.

[thorium90]

Hardwire the pump to trip if the XV valve is closed. A few additional auxiliary relays tied to the positioner should do the trick.

Or, you can add one more flow switch, dry contact will do. This will also be able to trip the pump if the valve is closed.

The flow switch will probably be your cheapest option.

You can do both functions within your PLC, or add additional relays to do the job.

Edited by thorium90, 20 July 2015 - 07:52 AM.

[curious_cat]

 . . . or switch to a centrifugal pump with a minimum flow orifice back to tank and delete the PSV.

 

What happens if a orifice is put on a return line back to tank in the current arrangement. Would that not work?

[latexman]

Ah, existing pump.  I thought that may be so.  Several vendors make "low flow" centrifugal pumps which may have worked had they been considered, but . . .it is a moot point now.

Edited by latexman, 19 July 2015 - 09:00 AM.