How does the specific gravity affect the pressure output from a pump?
How does the specific gravity affect the pressure output from a pump?
(OP)
If I have a pump that can put out, deadhead, 100' of water, and I change to another liquid that has the same viscosity but has a 0.7 specific gravity, how would that affect the discharge pressure at the outlet flange of the pump?
Had a discussion with a co-worker on the subject and we seemed to agree that the column of liquid produced by the pump would INCREASE by a certain percentage due to the drop in density, but the outlet pressure of the pump would stay the same because of the equal viscosities. But I am uncertain that this is correct. Is it?
Thanks for your help!
Had a discussion with a co-worker on the subject and we seemed to agree that the column of liquid produced by the pump would INCREASE by a certain percentage due to the drop in density, but the outlet pressure of the pump would stay the same because of the equal viscosities. But I am uncertain that this is correct. Is it?
Thanks for your help!





RE: How does the specific gravity affect the pressure output from a pump?
Johnny Pellin
RE: How does the specific gravity affect the pressure output from a pump?
A centrifugal pump will be rated in feet of head, not feet of water or psi. If you are pumping water, then the two are the same. Centrifugal pumps impart velocity (momentum) to a fluid by spinning it with the impeller. Any mass will rise to the same height, given the same initial upward velocity.
RE: How does the specific gravity affect the pressure output from a pump?
David Simpson, PE
MuleShoe Engineering
Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat
RE: How does the specific gravity affect the pressure output from a pump?
I would argue (conceptually so) with David that the premise was that the viscosity is the same for both liquids, so would this not imply that the density would not adversely affect the capability of the pump? This was one of the sticking points that my co-worker and I were having trouble with was resolving the density difference, taking into account the equation to convert head to pressure and vice versa. It makes sense that a more dense fluid would resist flow more, but is that resistance not due to viscosity? I'm not trying to split hairs here, just trying to understand as best I can.
Thanks,
-Matt
RE: How does the specific gravity affect the pressure output from a pump?
When you mention resistance, you are not thinking like an engineer. How exactly do you define resistance? A one ton mass has greater resistance to moving than a one pound mass. This is due to inertia. There is also rolling or sliding friction. This is equivalent to viscosity. Viscosity is normally neglected when discussing the theory of centrifugal pumps.
RE: How does the specific gravity affect the pressure output from a pump?
David Simpson, PE
MuleShoe Engineering
Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat
RE: How does the specific gravity affect the pressure output from a pump?
RE: How does the specific gravity affect the pressure output from a pump?
RE: How does the specific gravity affect the pressure output from a pump?
At flow rates lower than the BEP, η may drop proportionally more than the increase of the product HQ (assuming constant specific gravity), actually increasing P.
RE: How does the specific gravity affect the pressure output from a pump?
A limitation is obviously the maximum capacity of your electrical motor, if you increase the density, you have to be sure that the motor will not burn.
A small change in the pump specification might be seen due to the slipping of the motor. It is assumed that a motor is turning with a constant speed whatever is the load, what is not exact, and a small difference can be noticed. It is thus this small difference that will increase the pump velocity thus change the HQ specification of your pump if your reduce the density of your flow (thus increase slightly the flow).
RE: How does the specific gravity affect the pressure output from a pump?
It is difficult to sometime comprehend head and pressure, but when you finally get it a lot of pump systems become clearer.
Centrifugal pumps, as has been noted above give out a fixed head for a given flow at a fixed rotational speed regardless of the liquid inside them. Only when you've fully understood and accepted that will the rest become understandable.
For ease of testing and calculation, these generic head / flow charts are commonly quoted in metres or feet of water. when you order a pump the supplier will use the density of the liquid you've specified to obtain power and pressure figures, but the head would be same.
When you change only the specific gravity of the fluid being pumped in a centrifugal pump two things change:
1) The outlet pressure changes in proportion to the change of density, i.e. lighter fluids create less pressure.
2) The amount of power required to keep the pump spinning at the same speed is also proportional to the density of the fluid when comparing one fluid to another, i.e. a heavier fluid will consume more power for the same flow rate.
Viscosity will affect the downstream system flow resistance and hence the flowrate for a system without flow control will change, but this has very little to do with the pump.
Usually the issue is when someone changes a pump designed for one density of liquid and puts a higher density of liquid in it. Then both pressure and power increase if it is a fixed speed pump which can either exceed the pressure rating or overload the motor. Hence the heaviest density fluid that the pump can see should be used for design and if you're changing duties, be careful to examine the impact of the higher density fluid on the pump and motor.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: How does the specific gravity affect the pressure output from a pump?
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: How does the specific gravity affect the pressure output from a pump?
RE: How does the specific gravity affect the pressure output from a pump?
Sort of off topic, but related to the viscosity thing. I think Mythbusters had a guy swim through corn starch and through water. His lap time was essentially the same. Perhaps it's a somewhat odd demonstration of the fact that the viscosity doesn't figure in to pump performance?
RE: How does the specific gravity affect the pressure output from a pump?
Are you saying that viscosity has no effect on pump performance?
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: How does the specific gravity affect the pressure output from a pump?
Don't take this the wrong way, but these days there is no excuse for speculation like this even in general conversation, much less on a technical forum. Take 30 seconds to google it.