Centrifugal Pump Suction Conditions
Centrifugal Pump Suction Conditions
(OP)
We are installing a centrifugal pump downstream of a close-packed hydrocyclone. The hydrocyclone vendor was concerned that the pump would draw on the cyclone and reduce cyclone performance.
It is my understanding that centrifugal pumps don't "suck." Are there any conditions that would cause the pump to affect upstream conditions?
John
It is my understanding that centrifugal pumps don't "suck." Are there any conditions that would cause the pump to affect upstream conditions?
John





RE: Centrifugal Pump Suction Conditions
There are a number of good references on calculating pump and piping hydraulics, including Cameron Hydraulic Data, published by Ingersoll-Rand, and the Hydraulic Institute website, www.pumps.org among others.
RE: Centrifugal Pump Suction Conditions
Here's how I thought a centrifugal pump worked. The pump takes liquid entrained in the impellers and throws it out the discharge, thereby allowing more liquid to be pushed into the pump. If the pump doesn't move liquid out of the suction, then there is no flow into the suction. The pressure at the suction is that which would normally be there if the pump were there or not (the pressure as a result of friction loss, elevation change, etc.).
A positive displacement pump, I believe, DOES suck, since it displaces air as well as liquid, creating a stronger and stronger vacuum (to a point), thereby pulling liquid into the pump.
Thanks for the Cameron Hydraulic Data reference, I will certainly check it out. If my logic above is hosed, please feel free to un-hose it.
Thanks,
John
RE: Centrifugal Pump Suction Conditions
RE: Centrifugal Pump Suction Conditions
It is the system pressure that acts on the liquid to move it into the pump. Not enough pressure in the system, and the pump does not get any more liquid. There must be a net positive head available on the suction greater than the pumps requirement.
Using the baby bottle analogy, where does the "push" in the bottle come from. It comes from air pressure, either added through the nipple when the baby pauses in a glass bottle, or by the plastic bag collapsing. Put that bottle in a vacuum, and no milk to the baby.
Another good reference with more information on this, and probably better descriptions than mine is at www.pumpline.com.
I still maintain that the pump can affect the upstream conditions. You have a dynamic situation with velocity and mass flow, versus a static situation with the pump not in operation. This can change the conditions in the system upstream of the pump.
RE: Centrifugal Pump Suction Conditions
The reason why the pressure is lower at the suction of a pump is due to the Bernouli equation. Remember, you have water moving through a pipe and all of that water has to go through the eye of the impeller (which is much smaller than the pipe cross section) and thus, the fluid velocity must increase.
When the velocity of the fluid increases at the impeller eye, its pressure decreases. When its pressure decreases, more fluid is forced into it from the suction supply line (since it is at a higher pressure). There will be a pressure gradiant from the impeller eye to the suction flange and, in many cases, even further upstream of the suction flange (this is one reason why you don't want an elbow bolted directly to the suction unless the pump is designed for a suction elbow).
The pressure of the fluid is lowest at the suction eye and increases as the fluid travels up the vane, strikes the casing (in a volute pump) and transferrs the kinetic energy into pressure energy. The pressure is now higher than the pressure at the impeller eye and the water is now forced out of the pump at a pressure that matches the downstream system pressure.
No where in this discussion does a centrifugal pump ever cause a volume displacement to "suck" any water into the impeller eye - the pump is a rigid structure with a fixed volume that does not change (as opposed to a diaphragm pump).
If you do not have adequate suction pressure to force the water into the impeller eye, then the pump will just sit there and churn and put out much less water than expected...this is called cavitation or one the more scientific terms such as: "ain't got enough umpf" "my water's too hot" "how'd rocks get in my water?" or the other dreaded phrase: "Who closed that suction valve?"
Tim
RE: Centrifugal Pump Suction Conditions
"Are there any conditions that would cause the pump to affect upstream conditions?"
In a fluid system, flow and pressure are interrelated. You change one and you affect the other. Putting a pump, any kind of pump, in your system will likely have SOME effect, both upstream and downstream. You may need to do some kind of hydraulic analysis to determine the extent of that effect.
RE: Centrifugal Pump Suction Conditions
Getting down to brass tacks, to answer the question, you need to advise the piping conditions, details of the pump unit that you intend to use and you need to find out how much impact via the suction conditions of the installed pump the hydrocyclone can handle with out detrementally affecting the performance. I am assuming that the hydrocyclone is positively fed. Assuming that the correct flow passes through the hydrocyclone, you would have positive head available to your downstream pump. Using NPSH calculations, it is possible to calculate the effect of the pump. Using pressure gauges in the line, I believe that this problem can be solved. It may even involve throttling the suction of the downstream pump but there are many factors to take into consideration. You cannot expect a clear answer to such a vague question. Why don't you put up as much information of the application as you can. The two people I mentioned earlier do know exactly what they are talking about but cannot solve the problem for you if you just argue over definitions. jhelm's advice regarding the Cameron Hydraulic Data Book is excellent. I work in the mining industry in Kalgoorlie, Western Australia and find it to be an excellent reference.
Regards,
Andrew
RE: Centrifugal Pump Suction Conditions
RE: Centrifugal Pump Suction Conditions
RE: Centrifugal Pump Suction Conditions
RE: Centrifugal Pump Suction Conditions
If you work in Absolute pressure, there is no suction.
If you work in Gauge pressure, relative to atmospheric, anything below atmospheric is "suction" or negative.
It makes no differnce on how the reduction on pressure is achieved, the pump sees NPSH, or absolute pressure at suction less the vapor pressure of the liquid.
As for effect on the hydrocyclone? If the pump is sufficiently downstream that the prerotation of the liquid into the pump does not affect the hydrocyclone, then the only issue is flow rate in a steady state flow condition. Dynamic flow during stop and start of the pump will affect the system due to inertia of the fluid.
RE: Centrifugal Pump Suction Conditions
RE: Centrifugal Pump Suction Conditions
The only way it will is if its in a closed system and pushed by the pumps output or the output of another pump or gravity.
With that said ... nothing in the world "sucks". There is only the difference of PRESSURE!
RE: Centrifugal Pump Suction Conditions
RE: Centrifugal Pump Suction Conditions
RE: Centrifugal Pump Suction Conditions
RE: Centrifugal Pump Suction Conditions
Methinks much much more information is needed before you start this job - which normally requires hiring someone to spec it all out if you don't have the necessary "know-how" yourself. Try posting this as a new thread with more info.
Patricia Lougheed
RE: Centrifugal Pump Suction Conditions
RE: Centrifugal Pump Suction Conditions
RE: Centrifugal Pump Suction Conditions
DJMiner: The suction bell is to overcome a suction pipe inlet problem, most likely minimum submergence. If you can handle the suction losses of a 12” x 24” reducer you can probably just use one of those.
ynot3737: Lets say your pump is 2.31’ below the surface level of your pool. Lets also say your pump can produce 25 psi. Finally lets say your filter is really dirty and you have 20 psi differential pressure at your pumps recommended minimum flow. Not taking into account any friction losses if the filter is on the discharge side you would get your minimum flow. If the filter was on the suction side you would not get minimum flow. If you were at sea level you would have approximately 14.7 psi + 1 psi suction head or 15.7 psi available (not taking friction losses into account). That will not over come the 20 psi differential pressure from the filter at the pumps minimum flow.
Regards, checman