Centrifugal Pump Suction Conditions 3

[Enginator]

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

 

[jhelm]

You are correct that centrifugal pumps don't "suck," however, they can affect upstream conditions. What the pump does is to reduce the pressure at the inlet to the pump, allowing the system pressure to "push" more liquid into the pump. What you have to be concerned about is what the reduction in pressure and the velocity of the liquid in the suction piping will do to the system.

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,

http://www.pumps.org

among others.

 

[Enginator]

Thanks for the reply. However, your answer seems to contradict itself. You agree that pumps don't "suck," yet you say they reduce the pressure at the inlet allowing system pressure to "push" more liquid into the pump. But if the system HAS pressure, then why does the pump need to reduce the pressure at the inlet. That sounds like "suction" to me. A baby sucks a bottle by "reducing" the pressure at the nipple to vacuum (by drawing back his tongue), thereby allowing bottle (system) pressure to push liquid into his mouth.

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

 

[butelja]

A centrifugal pump will indeed suck, providing that it is adequately primed and has adequate NPSH.

 

[jhelm]

Maybe it was my answer that sucked. It is a confusing topic, as evidenced by your explanation, which also used the term "push" as well as mine. I should probably avoid the terms "suck" and "push" entirely, as they are not very clear. The reduction in pressure at the pump inlet is a relative term. It is less pressure in the pump inlet than upstream on the suction side. It is true that a centrifugal pump can "lift" liquid from below the pump centerline. The key here is what our friend butelja said: as long as the pump has adequate NPSH or Net Positive Suction Head. A centrifugal pump can "lift" water to a maximum of the available NPSH in the system before it begins to cavitate. The practical maximum suction lift for any centrifugal pump is about 28 feet of water. This is based on atmospheric pressure, which is required to act on the surface of the liquid and move it into the pump suction once the imeller vanes have accelerated the liquid that was in the pump inlet and moved it away. NPSH is expressed in absolute terms, so in a closed system it is the pressure available in the suction piping and system, less the vapor pressure of the liquid.

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

http://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.

 

[tstead]

First, and foremost, centrifugal pumps DO NOT suck. When something is "sucked", pressure is reduced in cavity "A" via displacement of volume thereby lowering the presure in that cavity and allowing the fluid in cavity "B" to flow into it because it is now at a higher pressure.

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

 

[packdad]

You wrote:
"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.

 

[supplier]

I think that maybe everyone is getting off track here argueing about weather pumps suck or draw. tstead and jhelm are technically correct from an engineering perspective though the terms suck and draw as specified in these responses, in common day english tend to mean the same thing.
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

 

[Enginator]

Here are details requested by Andrew &quot;supplier&quot; above. Cyclone inlet pressure: 15psig; cyclone inlet/outlet datum elevation: 0.00; cyclone outlet pressure: 1.00 psig; pump suction datum elevation: -4 ft; line size between cyclone and pump: 6-inch sched 40 stainless; qty of 90 deg bw ells: 5; straight pipe length between cyclone and pump: 75ft; flowrate: 290 usgpm; SpG: 1.01 (water with 10% solids <=40 micron-sized particle slurry); fluid temp: 60degF. Note that pump is basically gravity fed in this configuration. The pump boosts pressure to 135psig on the discharge. The cyclone vendor is concerned that the pump will &quot;draw&quot; on the cyclone, but cannot quantify the exact conditions that are undesireable. I think he's concerned that more flow will exit the overflow than the cyclone is sized for.

 

[kenelms]

Sorry for coming in late on this question -which hopefully by now has been correctly resolved.The pump suction and fluid flow entry to the eye of the impeller are vital notably in the prevention of cavitation.Centrifugal pumps have both a required npsh value [based on the pump design ,geometry and working fluid] and a system available npsh value. The temperatures too are important. With a postive suction head to the pump inlet then no serious problems should occur. A variable suction head can be tolerated provided that the minimum required npsh is not passed.The pump manufacturer is the best party to contact for your application.

 

[electricpete]

This is a good thread. I thought jhelm did a good job of providing a reasonable bridge between technical terms and commonplace terms. I give him a star for his comical approach (1st sentence of 2/22/01 post)

 

[troubleshooter]

Suck or push?

If you work in Absolute pressure, there is no suction.
If you work in Gauge pressure, relative to atmospheric, anything below atmospheric is &quot;suction&quot; 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.

 

[steveb2]

First time on this forum, but trying to answer the concerns, about the pump having a flow rate greater than the optimum for the hydrocyclone. A centrifugal pump will operate on its curve, provided the conditions NPSHa and Discharge head are suitably within the pumps rating. In this operation, it may or may not be possible to size a pump that will operate at exactly the flow rate desired. If system conditions vary to any degree, inlet or outlet of the centrifugal pump, the flow rate will change because the the pump, not being an intelligent piece of equipment, will simply move to the point on the curve were the system curve and pump curve intersect. If it is important that the flow rate remain constant or that a constant pressure be maintained at the hydrocyclone outlet, a control system utilizing a variable speed drive (or other means, such as a flow control valve on the pump dischare), on the centrifugal pump may solve the problem. The use of a variable speed drive will also allow the pump to operate at a minimum RPM to maintain the desired condition. Operating at a minimum RPM would also have energy and maintenance advantages.

 

[SteveKW]

NO pump can &quot;suck&quot;, they can only push. Take any pump and put it on the moon with NO atmosphere and NO atmospheric pressure and the water WILL NOT go into the pump.

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 &quot;sucks&quot;. There is only the difference of PRESSURE!

 

[TBP]

Nothing in the world sucks? You obviously haven't had the misfortune of working for a couple of the companies that I have

 

[SteveKW]

No offense, but maybe it was the &quot;pressure&quot; of those jobs! As you can see it is still the &quot;differential&quot; of two perspectives, you and the job.

 

[djminer]

We are looking to install a suction bell on a water discharge pump with a 12 inch suction pipe. A vendor quoted a $3700 cast iron bell inlet. Surely we can fabricate a metal one cheaper. What dimensions would the bell require. Theirs only specified 24 inch ID with no overall dimentions.

 

[vpl]

djMiner

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 &quot;know-how&quot; yourself. Try posting this as a new thread with more info. Patricia Lougheed

 

[25362]

If sucking means to produce a vacuum, then PDP are the equipment to be considered.

 

[ynot3737]

i need a simple answer...why does my swimming pool filter PUMPS the water through the filter and does not SUCK it through? Would it make any difference in the waterflow or energy consumption? I think, the water pressure in the inlet pipe is same as in the outlet pipe ( friction loss neglected).