Two Diaphragm pumps in parallel

Hill_Country · January 2019

I am working on a water system that will be pulling from a spring at ground level and will be serving about 9 fixtures, which equates to about 9 gpm (using the rule of thumb of 1gpm per fixture). The distance from the the spring to the fixtures is about 250 feet horizontal, with an elevation gain of about 15 feet vertical. We are looking to obtain household water pressures of about 40-60 psi and flow rates of at least 8-9gpm.

Is it ok to use two diaphragm pumps in parallel, to achieve the necessary household water pressures along with sufficient flow rates mentioned above? It seems that two diaphragm pumps in parallel would be much cheaper and smaller to install than a larger, single pump capable of these pressures and flow rates. I found the Grundfos SCALA2 pump, but at $650+, it seems somewhat expensive and it seems to have some quality control issues. We will be grid-tied, but still value efficient pumps.

Thoughts?

BB. · January 2019

Can you pump to a cistern? Either a midpoint--With a separate pump(s) to pressurize your home. Or a tank >40 PSI higher than the point of use (~15 PSI per 30 feet elevation)? That could allow you to get away with one lift pump from the spring.

I would suggest check valves on the output of each pump to lessen the surge from the other pump pulsing back into the the second pump (I think--I am not a pump engineer). I worry about the pumps "beating" against each other (vibrations from the pumps in/out of phase pumping, or slightly different RPMs).

In general, the suction sides of the pumps are sources of problems... Need screens to filter out sand and such, but screens plug up and cause restrictions (which can damage pumps by causing cavitation). Also, air leaks from pipe threads, etc. pulled into the intake. Possible need of foot valves to prevent drainback/loss of prime.

Creating a settling pond/tank at water level to reduce fouling.

Hopefully, some other folks have good experience/information and can help more.

-Bill

Hill_Country · January 2019

Thanks Bill, I appreciate it. I contacted a representative from a pump company that sells RV/Marine diaphragm pumps and they said it would be fine if the two pumps that are paralleled are not the exact same. The two pumps they recommend are 1) variable speed diaphragm pump, and 2) diaphragm pump with an outlet check valve (to prevent back flow). They also said that each inlet pipe should be separate to the source (i.e. do not have a single inlet pipe to feed the two diaphragm pumps). Does this sound reasonable? This would be the cheapest/most straightforward solution for us...

BB. · January 2019

Yes, I agree two separate feeds (one for each pump) is the correct way to go. The suction side of pumps are the most sensitive/critical side for problems.

-Bill

Horsefly · January 2019

I've always wanted to try a Ram Pump, but never had a situation that it fit. I don't think Bill's question about a cistern was really answered, but if you were going to a cistern via a Ram Pump you could use a pretty small on-demand pump for the household pressure.

jonr · January 2019

Typically, you can count on not all fixtures being in use at the same time. Or even if so, a pressure tank will usually cover the draw long enough that a smaller (or single) pump is fine. Slow pumping also reduces dynamic head.

Brushed pumps have limited life.

Hill_Country · January 2019

We will not be doing a cistern, as we don't have much room around our structure for a tank. I'd like to keep the amount of buried water lines to a minimum as well. I'm hoping that the buried inlet water pipe from the spring house to the house will be the only water infrastructure that I'll need to have outside of the spring house. I'm trying to use "off-the-shelf" diaphragm pumps so that if something goes awry, I can simply swap one out and replace it with another one with minimal effort. All of this equipment, plus filtration, will be located inside of small (5ft x 5ft x 5ft high) spring house that will have 120v AC lines run to it. Freezing is also a concern and am trying to make sure that we can survive colder temps (0 - 32 deg. F); however most of the sensitive equipment will be in this springhouse and will be insulated (but not "heated").

My concern is that if we have a spigot running (goat shed) and someone is drawing a bath that we'll quickly exceed the capacity of a single 5-stage diaphragm pump at normal household water pressures (say 40-60psi) that is pushing 2-3gpm. Even with a pressure tank (say 40 - 60 gallons that have a drawdown of 11-14 gallons) we'd still draw that down as well. Thoughts?

Please know that, despite my signature line, we will be having grid-tie solar, so while power usage is very important (we may still try and go off-grid in the future), it is not critical right now.

Estragon · January 2019

With temps down to 0°f, I'd heat trace any supply lines not buried below the frost line. Self-regulating types only use power when needed to prevent freezing.

jonr · January 2019

Insulated but not heated will eventually be as cold as outside. You need some heat (with great air sealing and insulation, very little). Ground heat and/or heat from the water might be enough, but some backup electrical is a good idea.

cow_rancher · January 2019

Hill_Country said:

Is it ok to use two diaphragm pumps in parallel, to achieve the necessary household water pressures along with sufficient flow rates mentioned above?

Two pumps in parallel will not increase the pressure, they will almost double the flow rate, it helps if they are a matched pair.

Think of in terms of Batteries, 2 batteries will double the amperage (flow), and 2 batteries in series will double the voltage (pressure).
I use 2 booster type pumps in parallel to achieve the flow rate I need, I plan to work up a plan with an additional pressure switch to switch on out for low flow periods, i.e. have one switch on at 40 psi, and if the demand is greater, have the second one switch on at 30 psi. I would then use an alternating relay to swap them back and forth so that one is not carrying all the load (run time). I have a flow meter plumbed in to both outputs to make sure that they are both working, along with check valves on the output of each one.

Rancher

Hill_Country · January 2019

Thanks y'all! @cow_rancher , what diaphragm booster pumps are you running in parallel?

Two Diaphragm pumps in parallel

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