Pressure drop through strainers

[iainuts]

I need a conical strainer (wire mesh cloth possibly reinforced using perforated metal sheet) installed in a line between flanges. The strainer manufacturers I've talked to don't seem have a decent method of determining pressure drop.

Things that are known about the filter material are open area (percentage of total area) and mesh size.

Also can determine total conical area from dimensions of assembly.

What equations can be used to determine pressure drop for a gas? (helium).

Thanks for the help.

 

[zdas04]

Use whatever equation you want, then multiply the answer times zero. I've flowed a bunch of gas through fairly small Witches Hat strainers and never been able to measure a dP on a clean strainer. Typically I put a dP gauge across them (calibrated in inches of water) and they always read zero till they read very high (usually off scale high on a 0-25 inch scale). One thing that I've had compressor fabricators fail to understand is that conical strainers need to flow inside out, they will always collapse and make a real mess if you flow them outside in and ever get them clogged.

David

 

[iainuts]

Hi David,
That's good information. Thanks...

I need to know within about 0.1 psid what the strainer dP is and the flow rate is very high. An 8" double wafer check produces about 1/2 psid assuming it is wide open - which requires a very low spring load. dP is everything in this system.

That said, you've given me an idea. I suppose I could measure the dP once the system is set up. If I do that however, I'll need to come up with an alternate plan to mitigate the risk if I find the strainer dP is too high.

This isn't as easy as saying there's very little dP so it can be neglected, because even 1/2 psi dP will be a concern.

 

[zdas04]

What is your flow rate and upstream pressure? I've flowed 4 MMCF/d through a conical strainer in a 6-inch line at 5 psig on the suction side of a screw compressor with no measurable dP (on a 0 to 25 inch H2O scale) while the strainer is clean. Of course a tighter scale would have showed some dP or there wouldn't be any flow, but it is a really little number, way under your 0.5 psid (13.8 in H2O).

David

 

[iainuts]

Thanks David. I've had a closer look at this strainer and I just realized it has a flange on it that reduces the 8" pipe ID (8.329") to 7.375". That alone results in a roughly 1 psi drop in my application. <sigh>

Thanks for your help.

 

[zdas04]

How much gas are you flowing????? The I.D. of Std 8-inch is 7.981 in, and if I look at 1 psid with the beta ratio of 0.92 at 20 psig upstream I get around 17 MMCF/day. Using your 8.329" ID (0.88 Beta) I still get 15 MMCF/d.

I don't know of any reason that you couldn't fabricate a conical strainer with an ID that matches your pipe more closely.

David

 

[shahyar]

There is a diagram in Perry's handbook for this.

 

[iainuts]

hi zdas,
Flow is roughly 4.5 MMSCFH helium at about -445 F. Pressure ~ 5 psi. I'm actually taking it off a process from a 6" pipe, opening it up to 8" for a short distance and eventually to 24". I can put the strainer in the 24" section, though the gas temperature is likely to rise significantly by then. Still, the 24" section looks much more promising.

shahyar, thanks for the tip. I'll take a look at Perry's soon.