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Old 01-17-2013, 11:54 PM   #1
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Default Positive displacement pumps

Does anyone have experience using a positive displacement pump? I found a tri-clover, lobe-based PD pump that I'm thinking about purchasing and I think I have an understanding of them, but if anyone can provide practical input or check my knowledge, that would be awesome. It's my understanding that rotary pumps are better at handling air pockets as they provide suction rather than just accelerating the fluid. Any disadvantages in this application, other than the overall complexity/maintenance?

Fluid flow...since they pump at a fixed rate, restricting output would create a mounting pressure, so the common March pump method isn't applicable here. And thanks to to Wikipedia's For Dummies section on NPSH, despite enjoying some beer at the moment, I think I understand why you can't restrict the input (lower pressure that makes fluid flow ineffective). So my conclusion is that, for the practical purpose of including this pump in a brewery, it's fairly difficult to control flow rate, or are there any other methods?

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Old 01-18-2013, 12:03 AM   #2
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Really no need for a positive displacement pump in a home brew application. If you have a valve on the input or output and it's closed, say good bye to your hoses. If you use pipe in lieu of hoses, something bad may happen!

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Old 01-18-2013, 12:25 AM   #3
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Yeah, a PD pump isn't necessary in a typical setup, but I'm building a system with scalability in mind (up to 1 BBL), and considering that I'm using a tri-clover set-up, it's actually cheaper to get these pumps than to get NPT-TC adapters. I can afford high flow rates and have the system designed to accommodate unrestricted flow with safety mechanisms in mind, I'm just more curious about any other practical issues that might arise.

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Old 01-18-2013, 12:36 AM   #4
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To control flow you will need to control the motor speed. Depending on the motor this my be cheap (router speed control) or expensive (VFD/VSD). Do you have any documentation on the pump regarding flowrates, pressures, etc?
The other way to use it would be to set up a grant and use the pump in an on/off mode to enpty the grant when it gets above a certain level.
Also you will want to avoid any bits of grain/etc getting into the pump and prematurly wearing out the lobes/seals.
What size/price are you looking at?

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Old 01-18-2013, 12:57 AM   #5
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The specs are quite important, and unfortunately I have not been able to find them and that's something I'm going to do before committing. I'm going to contact the manufacturer tomorrow when they're open to find those out. It's an actual Tri-Clover product, a PRED series pump. I don't have the actual model number in front of me right now, but it's comparable in physical size to a March pump and the asking price is $125. It's used but is working and comes with a few spare parts.

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Old 01-18-2013, 01:46 AM   #6
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Looks like the PRED needs seal water too.

http://www.twincoinc.com/PDF/Alfa%20.../PRservice.pdf
http://www.cpesystemsinc.com/data/We...tary%5CPRc.pdf

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Old 01-18-2013, 02:07 AM   #7
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If you recirculate the output back to the input you can control the effective flow rate of a positive displacement pump without breaking anything. A 3-way valve on the output with one output leg feeding to a tee back at the input ought to do it...

Cheers!

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Old 01-18-2013, 02:45 AM   #8
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What shape are the lobes in? If they're in rough shape or shot you may have reduced pumping capacity or none if they're bad enough. They may not be cheap to replace either. Does the pump have a motor or speed control as that is the way you will control flow rate rather than a valve on the output like with the march. Even if it needs a rebuild $125 is a steal for any lobe pump that you may be able to pass onto a brewery of you can't utilize it and keep it out of the scrap yard (or I'll trade you a pump for it!).

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Old 01-18-2013, 11:34 PM   #9
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What Day Triper said...

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Old 01-19-2013, 12:22 AM   #10
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Sorry for the delayed response here. Matt, thanks for that spec sheet, that's exactly what I was looking for. I found the service manual but I apparently missed that. I got to talk to the manufacturer to find out some details because their numbering scheme doesn't make sense, despite how meticulously labeled it is. It's a low output pump with ~3 GPM based on the setup that I'm planning to have. That should work out well as a pump for the RIMS tube I want to implement. I'll give one a shot and see how it works out...it's coming with spare parts as well so it'll be a fun experiment. I'm not exactly sure how the lobes are looking, but it doesn't look like replacing them would be a big expense.

I just did this diagram quickly for clarification to see if I understand this correctly. By adjusting how much goes through the loopback vs. the direction I want the liquid to go, I can control the flow rate. Is this correct? It doesn't seem like it would work, but I have little experience with this...

untitled-drawing.jpg  
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