Another Pulsation Damping question / metering pump recommendation request

[mel39]

Hello,

I'm designing a bench-top water treatment membrane testing system, which requires me vary flow rates (low: ~0.5 - 2 L/min) and minimize flow pulsation at low pressures (it's an open system). An economical, all-in-one lab pump system run from single-phase power seems like the right solution, and my questions are the following:
1) If I get a simplex diaphragm pump such as

http://www.knfusa.com/labliqpumps.htm,

a single damper will not smooth flow enough for my purposes, especially when operating at lower flow rates. Would you suggest I try a second damper in series? Or perhaps some other type of flow-smoothing device?
2) If this route of just trying to damp highly pulsing flow doesn't seem practical, does anyone out there know a lab/metering pump brand they like that makes what I'm looking for and charges less than ~$3000-4000/pump? I've done extensive searching on the web and have seem many models (FMI/Masterflex/Hydracell/March/etc) that come close but seem sub-optimal in one way or another.

Thanks for any input you can give.

 

[mel39]

Hmmm. So, is this a dumb question, or a just hard-to-answer question?

 

[ccfowler]

If you need to minimize pulsations, why do you want to use a pump with a strong pulsation characteristic? Why not use a pump with a minimal pulsation characteristic and meter the flow to maintain the required flow rate(s)? A 4:1turndown ratio on a decent metering system is hardly remarkable in a laboratory situation.

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.

 

[mel39]

Thanks for the response ccfowler. My guess is my follow-up will reveal even more of my ignorance, but here goes:
I thought flow meters only measured flow rates, but you imply there are models that control flow rate? Or are you saying I should simply design a bypass for a constant-flow pump with an adjustable valve, and use a flow meter to monitor flow in the relevant pipe branch?

To answer your first question, I would certainly prefer a pump with minimal pulsation characteristic, but in my search for all-in-one adjustable flow lab pumps in the size that I want, I've only found peristaltic and diaphragm pumps. If someone can point me to a manufacturer who makes something better I'm all ears.

 

[ccfowler]

Pumping, metering, and controlling are distinct functions usually best handled by distinct devices in a situation such as I am assuming you are trying to arrange. I would not want to just use a metering pump without some independent way of verifying that it actually performs as expected. Since this is a bench top system, I assume that it is continuously attended when in operation, so manual control of the flow with a globe or needle valve should be adequate. For metering, my first guess is that a variable area meter (Rotameter is one brand name) should work well, and some of these include a suitable needle valve for control

As far as the pump is concerned, there should be little difficulty finding a suitable small centrifugal pump. Again, since this is apparently an experimental bench top system, neither efficiency nor durability are likely to be controlling considerations, so availability, suitability, and cost will be the dominant elements in the actual choice.

Just so there can be no misunderstanding, safety must be the first consideration in all choices of all components and in your system design.

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.