Positive displacement return pumps

[Sisterlimonpot]

Why don’t companies make positive displacement return pumps? Imagine the head pressure that they’d produce with very little effort, which in turn will be able to use a low wattage motor?

can you guys provide some cons to this idea?

[Redman88]

need to have a pump supplying that positive displacement pump.

[SGT_York]

Most Positive displacement pumps can create a suction by themselves and woudn't require any other pumps in series. I don't think I've seen any that are not self priming.

They are expensive, they are low GPH pumps in high head situations they'd work but certainly not for a standard in stand sump.

Have you seen any with a ceramic/mag drive? They are used in salt water marina's, but not sure if those are rated for potable use.

[Sisterlimonpot]

I have not seen any for home aquarium use and was thinking that there had to be a reason for that. However I can't think of one that doesn’t have an easy work around. The only thing I can think of is because of the tight tolerances of the pump, if a snail were to make its way into the pump it would jamb the gears but that can be easily fixed by incorporating a screen into the inlet side.

[Coral Dilema]

Positive displacement pumps require a lot of torque, IE a lot of current. A single phase motor would be OK for this but you would be much more efficient with a three phase motor powering one of these so you can maintain torque through the full 360 degrees of rotation. 3 phase power is not readily available in the residential sector, you would need a Variable Frequency Drive capable of taking single phase input and converting it to three phase, and that would present a lot of other possible problems. PWM noise and harmonic noise to start with. Also, if you had a clog in your plumbing, a positive displacement pump would just keep building pressure until something broke loose. . .the clog or the side of the pipe, whichever let loose first.

[asylumdown]

What is a positive displacement pump?

[Redman88]

The ones I am around do 420 gallons a min or more when In use at about 10k psi

[SGT_York]

There are a few smaller ones, that are in our GPH range, they wouldn't require 3 phase the one's Ive seen this size are all DC. I just saw one with a magnetic drive but it didn't say what the shaft was made of. 250 GPH for $115 so relativley the same as small magpul similarly priced. (considering almost no headloss from this type of pump)

Don't think snails would be an issue just some escargot for your fishes.

Maybe the will come but I doubt the will be cheaper than a centrifugal pump for the mainstay of our tanks.

A Positive Displacement Pump is best seen just google and look at the images. It's two impellers that suck and push the water rather than spinning it. Lower GPH (per price and electricity usage) but no headloss from height and pressure.

[Redman88]

The one I deal with are trailer mounted with a deseil engine

[Coral Dilema]

A positive displacement pump is a pump that moves the same amount of liquid per rotation regardless of the speed of the rotation . . . . . . There, clear as skimate?

The 2 main types of pumps are positive displacement and centrifugal. The majority of pumps used in the fish keeping hobby are centrifugal. A centrifugal pump does not have a sealed pumping chamber, it is open. The impeller spins and the blades in the impeller throw the water away from blade by centrifugal force. A centrifugal pump is a variable torque load, meaning that the faster you turn it, the more water it moves with each spin, the more torque is required to spin it.

A positive displacement pump is a constant torque load. Positive displacement pumps have a sealed pumping chamber, such as a piston or a progressive cavity. One full turn of the pump will move the same amount of water each time it spins, regardless of how fast it spins so you need full torque on the motor regardless of the speed. A centrifugal pump will move more water with each spin, as the RPM increases, to a point. Also, a centrifugal pump will only create a certain amount of head pressure, even if the output is deadhead. A positive displacement pump will continue to pressurize against a deadhead until either the pump or the plumbing fails, explosively. You have to provide overcurrent protection for a positive displacement pump to shut it down if a deadhead situation occurs.

Also, if a centrifugal pump is spun too slowly, around 20-25 Hz or below, it will not create enough centrifugal force to move any water.

My guess as to why they are not used more often would be complexity. Positive displacement pumps usually have multiple moving parts are therefore are more likely to have mechanical problems.

[mussel and hate]

PD pumps are prone to wear and failure as they require seals and one way valves. The engine in your car is a PD pump. If you have an aquarium air pump it's a PD pump.

To make a PD pump as reliable as a centrifugal pump would be a very expensive proposition.

I'd rather see someone manufacture a low rpm Archimedes screw pump for aquarium hobbyists.

[SGT_York]

Archimedes screw, that would be an easy adaption, but very large behind your tank.