Piston water pumps - why are they better?

[train82499]

Am thinking of going to on demand system, and changing from water storage tank and RV pump to boost pressure. ONe of the posts here covered a number of topics and included an interesting discussion of piston pumps - which I gather are different from displacement (shallow well and maybe some deep well?) and centrifugal pumps (Grundfos eg). It was mentioned that a piston pump is really good in that is uses little power and works well, but that were hard to find. Anyway, I found this link http://earthmovement.net/FMC_Bean_Pumps/FMC_Piston_Pumps/duplex.html which covers a lot of different pumps, including piston pumps and wonder why they would draw less power and whether they are really worth considering in a solar system like mine? Basically I have a battery bank and panels on the roof and an outback 3524 but set for weekend use primarily as it is a cottage.

[mike95490]

piston pumps are fixed displacement, and if you "lessen the load" elevation, you get no increased flow rate, but maybe less of a amp draw.

[train82499]

Interesting. Those pumps speak of putting out 500 to 900 PSI. You could run hydraulics on that! Maybe they are not for solar, but it is a solar type friendly web page.

[Windsun]

Piston pumps are not "better" or "worse", depends on the application.

Piston pumps can pump very long distances and/or to very high elevations. One system we sold a few years ago with a Dankoff pump sent water 1300 verticle feet up the side of a mountain from a small spring.

[russ]

If you need the pressure then piston type are best.

I have never really heard of anyone using piston where a centrifugal pump would do. The centrifugal pump is relatively maintenance free and one (piston type) is far more prone to giving problems.

[peakbagger]

Piston pumps are positive displacement pumps that are tend to be very efficient. Assuming the seals and castings are built heavy duty enough, they can pump at any pressure as long as the driver is big enough. The flow is directly related to pump speed. Centrifugal pumps have to pump to a curve where pressure developed is directly related to flow. Centrifugal pumps are relatively inefficient especially when operating outside of window of ideal flow and pressure. They tend to last longer as there is no internal moving contact except for the shaft seal. A piston pump is going to have constant slding friction between a moving piston and a stationary pump body. There are also going to be some inlet and outlet flow checks that are also potential wear items.

So the trade off is reliability vs pump efficiency.

[russ]

1) Never place a check valve on the inlet of a centrifugal pump. Chances of problems are great and usually lead to a fried pump.

2) An outlet check valve made be required due to the application and piping configuration. The valve (if good quality and correctly installed) should be no problem.

3) It is 100% correct that centrifugal pumps should be carefully selected for the application. Trying to reuse an old pump almost always costs you more power than the pump value very quickly.

Russ

[Blackcherry04]

I replaced a 3/4 hp Centrifugal pump last year with a 5.5 gpm @60 psi Diaphragm Pump. It is a new soft start pulsed pump and only pulls 10 amp @ 12v dc. It has a 5 diaphragm head. With a transducer instead of on/off switch, it's much more efficient.

The pump I replaced cost over $1,000 and pulled 11 amps @ 120 V, 110 + amps dc. While it would deliver 20 gpm @ 40 psi, at 60 psi it dropped to 1.2 gpm. You have to look at the pumping curve for the pump you choose and make sure it fit's what your trying to do.

I find it hard to believe I had a 3000W inverter and maintained a 1,000 amp hr bank of batteries for 20 years to run the wrong pump and there was a $150.00 pump that pulls 10 amp dc to do the same job better.

[Cariboocoot]

1) Never place a check valve on the inlet of a centrifugal pump. Chances of problems are great and usually lead to a fried pump.

In a shallow-well application there will be a check valve on the inlet side. It's called a "foot valve". It's there to keep the water in the line from leaking back into he sump.


2) An outlet check valve made be required due to the application and piping configuration. The valve (if good quality and correctly installed) should be no problem.

True. Not typical for applications, though.


3) It is 100% correct that centrifugal pumps should be carefully selected for the application. Trying to reuse an old pump almost always costs you more power than the pump value very quickly.

Russ

Absolutely! The number of incorrectly sized pumps I've seen over the years ... Whoo-boy! :p

[solarix]

Piston pumps are positive displacement meaning there is no "slosh" in the process making them efficient and you can run them slow as in directly off whatever power you happen to be getting from your PV at the moment. The downside is they have tight clearances, seals, and wear so are subject to damage from dirty water.

[russ]

I don't consider a foot valve as a check valve at the pump inlet I guess (even though it is) Just my strange way to look at it I guess - any restriction in a pump inlet that is not essential for system operation is trouble looking for a place to happen.

I have never used diaphragm pumps except in very dirty water (slurry) applications and even then they were last choice.

Something I need to learn more about.

Russ

[russ]

An excellent site that puts out a news letter about pumps with informative articles is

http://www.lawrencepumps.com/index.htm

Russ

[Blackcherry04]

I have had diaphragm pumps for many years in the Marine area for fresh water systems. The big breakthrough came a couple years ago as the number of chambers was increased to 5 and they became variable speed with a soft start.

With 5 gpm open flow and 3.2 @ 60 psi you no longer get the pulsing and the flow is unbelievable. They are truly a " On Demand " without the need for a pressure storage tank. In fact if you have a limited water supply to draw from you almost need to use a restricter in some outlets. These pumps will allow you to run 3-4 fixtures at the same time.

As was mentioned in the past, the inlet and outlet of these pumps should have a flex line.

http://www.aquatec.com/aquajet-rves-pumps.html


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