Centrifugal pump question

I think it depends on how much the flow is restricted. If you try to measure at the output of the pump and the water is just going into the air right at the output of the pump, then the pressure would be zero.

If you block the output of the pump I think it would be 90. So I think you could get zero to 90 , depending on how constricted the output is.

This is my guess.

Dan

My guess is "no." The pump creates pressure from slinging a *mass* of water. I don't think it matters what the pressure is entering the pump.

To clarify that, I think that a typical 10 psi centrifugal pump that has 80 psi at the inlet will have something like 80 psi at the outlet.

Check Harbor Freight for a pump that boosts water pressure.

Hul

snipped-for-privacy@whidbey.com wrote:

I guess i could do that and see what kind of pump it is.

It would, but unless that pump is designed as a pressure-boosting pump, the likelihood of the shaft seal being able to contain such pressures is not high. On a cheap pump, even the housing might fail.

IF the volume of water remains constant the pressure coming out will be at most 80 psi. if the pump is designed to produce 10 psi. It may be lower depending on the size of the housing and the restriction the impeller creates. Say your input side is 2" and the pump can produce 10 psi. at zero head pressure out of a 1.5" outlet.

Feed that pump with an 80 psi head pressure and the pump won't add any pressure because it cannot pump faster than the water is already flowing through it.

This is a common question in the fire service. Now if the pump is rated for a higher pressure and flow it could boost the pressure. That is how a 2 stage high rise pump operates. Those may pull in 90 psi hydrant water through a 5", feed that through the first stage and boost it to

125 psi. That is then fed into the second stage which can boost it to a higher pressure. Our old two stage could put out 300 psi through a 4" line. with 70 psi at the 5" inlet. BUT if the inlet pressure dropped the outlet dropped much farther.

Currently our biggest pump can lift water 20' and put out 2,075 gpm at

150 psi and keep pumping that way until she runs out of fuel. That's with 2 6" suction lines

That's exactly what I thought, but Jim's reference to multi-stage pumps threw me. Since water isn't compressible, I don't see how the multi-stage pumps work. For gas, no problem, but I don't get it for liquids.

That reference showed it could be done, but not how. I was hoping it would lead someone who knows more than I do about fluid dynamics to a better description.

The pumps in series business is confusing the issue. The output of the first (centrifugal) pump in the chain is far from an ideal pressure source. The original question was about a pump with a constant inlet pressure, either 0 psig or 80 psig. Flow wasn't specified, but as long as the flow is constant for both inlet conditions, the delta P across the pump will be the same. In other words, the pump will increase the pressure by 10 psi in both cases.

Consider connecting the pump to the bottom of a 180 foot tall (approx

80 psi head) tank. Will the pump be capable of pumping, at the specified flow, to a height of 23 feet (10 psi head) or 203 feet?

Practical matters of seal design aside.

Thanks Ned and everyone else who posted. I am going to put in a hot water recirculating setup in my house. I was going to buy the pump, valves, and pressure switch but I got lucky when a friend gave me a new Grundfos pump that was slated for in floor heating. I was pretty sure the pump would work but the Grundfos web site didn't mention what type of hot water circulating system it would work for. Eric

Why do you think there is something called "two-stage" or "dual-stage" air compressor?

The second stage will use the output of the first stage as its intake to add more pressure to it.

Water is actually compressible. The compressed volume doesn't change as much as gas would.

"The low compressibility of non-gases, and of water in particular, leads to their often being assumed as incompressible. The low compressibility of water means that even in the deep oceans at 4 km depth, where pressures are 40 MPa, there is only a 1.8% decrease in volume."

threw me. Since water isn't compressible, I don't see how the multi-stage pumps work. For gas, no problem, but I don't get it for liquids.

Thanks. I suspect that most of the people here know that. The pressure/volu me relationship, though, isn't in agreement with Boyle's law. Gases approxi mate it. It's easy to imagine a multi-stage non-positive-displacement compr essor that keeps building pressure in a material that obeys Boyle's law. It 's much harder to imagine it with liquids.

measure at the output of the pump and the water is just going into the air right at the output of the pump, then the pressure would be zero.

nk you could get zero to 90 , depending on how constricted the output is.

water. I don't think it matters what the pressure is entering the pump.

s 80 psi at the inlet will have something like 80 psi at the outlet.

Because it has two stages.

With a gas, no problem. With a liquid, the problem is more complex.

snipped-for-privacy@gmail.com wrote on 5/27/2017 11:08 PM:

ps threw me. Since water isn't compressible, I don't see how the multi-st age pumps work. For gas, no problem, but I don't get it for liquids.

volume relationship, though, isn't in agreement with Boyle's law. Gases a pproximate it. It's easy to imagine a multi-stage non-positive-displaceme nt compressor that keeps building pressure in a material that obeys Boyle 's law. It's much harder to imagine it with liquids.

Water does compress. It takes a lot of pressure to compress water just a

little bit, but that little bit reduction in volume is what gives the compressed water the "stored pressure".

If you daisy-chain two power-washers, theoretically you should get a more powerful water jet than using just one power-washer.

snipped-for-privacy@gmail.com wrote on 5/27/2017 11:08 PM:

ps threw me. Since water isn't compressible, I don't see how the multi-st age pumps work. For gas, no problem, but I don't get it for liquids.

volume relationship, though, isn't in agreement with Boyle's law. Gases a pproximate it. It's easy to imagine a multi-stage non-positive-displaceme nt compressor that keeps building pressure in a material that obeys Boyle 's law. It's much harder to imagine it with liquids.

The idea is the same. You use pressure to reduce the volume of a fluid. The gas/water restores it original volume after losing the pressure.

The graph in the link below shows that 200-bar of pressure (2900 psi)

On Sunday, May 28, 2017 at 2:17:58 AM UTC-4, HJART?? ? ?????? ? ?? ?????? ??lBYWJ wr ote:

ps threw me. Since water isn't compressible, I don't see how the multi-stag e pumps work. For gas, no problem, but I don't get it for liquids.

volume relationship, though, isn't in agreement with Boyle's law. Gases app roximate it. It's easy to imagine a multi-stage non-positive-displacement c ompressor that keeps building pressure in a material that obeys Boyle's law . It's much harder to imagine it with liquids.

I don't think so. Centrifugal pumps are very lossy machines. They couldn't hold pressure that way.

From a physics point of view, I think the answer lies in sorting out the ki netic aspects of a turbo pump (velocity) and the potential aspects (pressur e). A turbine pump that's pumping a liquid must be producing potential ener gy from kinetic energy.

I'd need to see a good, expert explanation to understand it. I see no way t hat an ordinary turbine pump can hold the pressure generated by a previous stage, unless the entire thing is kinetic, which we're then measuring as po tential energy (pressure).

No turbine pump could hold that 0.7% compression.