Varying pressure drop for constant flow rate

[ports394]

I'm working with a cross flow membrane at the moment.. what I've noticed is that at lower TMP's, the dP across the membrane is higher, to give me X gpm.

At higher TMP's the dP is lower to give me the same X gpm.

I have 2 flow meters in the line checking the flow, and the X gpm rate is correct. It's also been "bucket tested".

Any ideas?

I'm wonder if it's just gauge error at low/high values on the gauges. We're pumping water, so it's not a compressible liquid at V < 0.3M

 

[Latexman]

What are the different TMP's and how much does the dP change at constant flow rate? Throw us some data to look at.

Good luck,
Latexman

 

[ports394]

Under 15 psi, a dP of 4-5 gets me 8 gpm. Closer to 40 psi, a dP of 1-2 psi gets me 8 gpm.

 

[Latexman]

Not hydraulically full? Higher pressure compresses a trapped vapor space and makes more wetted flow channel for the same fluid flow.

Good luck,
Latexman

 

[ports394]

I think it is hydraulically full. or at least the conditions should be similar, since the flow rate is constant. As far as vapor pressure, there should be no vapor in this section of the system.

I think it comes down to gauge error for this, but I can't be sure without new gauges for a finer low range.

I'm also wondering if there is a flexing of the membrane. It's a tubular type of module. Basically a paper tube, coated with a membrane...it'd be easy to stress/stretch it IMO.

Have you worked with membranes before Latexman?