Centrifugal Pumps Are In Parallel

This does not seem realistic. If the pumps are in parallel, then they share the same intake and the same discharge configurations. So how can one run dry and the other with liquid? And neither with any valves in the discharge line? I've seen piping mistakes before, but usually not that weird. If liquid and air are mixed, bubbles are guaranteed.

Is this self priming pumps or sump pumps?

This question is not true indeed,centrifugal pump with out dischage v/v never be operate whether in single or parallel,it must be having NRV or isolating v/v at its discharge line.Secondly centrifugal pump never be dry run if it does so it gets ceised due to over heating.

"Secondly centrifugal pump never be dry run if it does so it gets ceised due to over heating"

I will beg to differ on that. Pumps with independent water to the lantern ring or mechanical seal can run dry.

It's either that or I've been hallucinating for 30 years.

I have 13 years experience in thermal power plant,i never saw that pumps having mechanical seal running dry,would u pl.give me some example of dry running cen.pump.So far i know that there is no relative motion whith out friction of heat liberation.That means it definitely need some cooling by air or water.

Independent water supply

As long as water is available to the seals the pump will run OK

And I can over rule you by 22 years

All our pumps had independent clean water to the seals

I think engineering background with 22 years experieced never think unrealistic approach.This will help for becoming optimist.

All our pumps had independent clean water to the seals - then they are not really running "dry". The reason you connected the independent water supply to the seals was to prevent the dry running. This is a neat solution but may not be possible/practical in all situations.

John Crane (and others) have been using dry gas seals on liquid pumps for well over 15 years (since before I stopped dealing with mech seals, 'twas a different world then!). The main issue with them was getting air (or gas) into the pumpage, but no probs with running dry. On certain applications they were fantastic. Try googling it.

Rule of thump, "more pressure? Connect pumps in series. More flow? Connect pumps in parallel".

For this case, you should have (or this is a standard) a non-return valves installed at each of the outlet.

Hope it helps.

Holz,

A very sensible answer, result of a true application of the mind.

One more point for the GA, Holz.

I think that the water will flow through the other pump. Have never tried this.

Thanks. Totally hypothetical, I think? I can't think of why anyone would want to try this, or of any application where it would actually occur, but an interesting question all the same.

Or maybe just a sad and dull reflection of too many examinations! It must have been drummed into many of us from an early age: "Read and answer the question"!

Have to agree - it would be a first if a pump designed for liquid momentum, managed to pump gas at anything like a comparable pressure.

It's just going to sit there, keeping back-flow fluid at bay.

In the absence of a non-return valve, there is a good chance the shaft seal will be wet (depending)

Hoz-

You got there first with the exact answer I was going to enter- The liquid-filled pump will force water into the volute of the air-filled pump and NO air will be injected into the discharge line.

Any air in the discharge line will have come from the liquid-filled pump that MIGHT be running with adequately low NPSH at the inlet to cause some dissolved air in the liquid to be released at the inlet cone and then pressurized back into the discharge as newly entrained air waiting for lower pressure to rise to the top and be released.

Well, your explanation is correct as far as the dry running pump will get fluid from the other pump.

BUT, when the dry pump gets filled with some water, it will start sucking some air from the suction line (Water ring phenomena, like vaccum pumps), and this air will end up in the discharge. It will be pumped into the discharge intermittently!

Therefpre, there is a great possibility that some air will end up in the discharge line. It all depends on the pump type. Even if the pump is not self priming, as long as there is no liquid in the suction line, it will suck air and send it to the discharge. Air will then pass into the discharge.

If the pumps are self-priming, whether liquid-ring type or recirculating type, then yes, they could possibly pump air into the discharge. However, even self-priming pumps will evacuate air only up to a certain pressure in the discharge line, so if the discharge pressure from the liquid pump is above this level (depends on pump type, size, manufacturer, fluid, etc, etc.) then there will still be no air in the common discharge line. Besides which, the OP makes no mention of self-priming pumps, so we should assume they are not, and even with mixed-phase a standard centrif will not get any air into the common discharge.

Let us assume that, in the first place, these two pumps were selected and set to run in parallele, and therefore, must be able to pressurise the discharge the same way: i.e. they are meant to work as such. Otherwise, the whole issue here is a waste of time.

In this context, the dry pump, receiving water from the 2nd pump, will create, at intermittent intervals, enough pressure to suck air and send up to the discharge pipe. Once any air is trapped between two liquids, and there is no flow into the dry pump, downstream, the air must float and migrate upward. The air will not be trapped down unless there is enough flow to push it or keep it down. For this reason, there will allways be air going into the discharge and float up with the flow produced by the 2nd pump!

I am not going to work it with equations because I thing you are able to do it and all it needs is a little though experiment. AND this is the answer the OP is waiting for: Yes there will be Air carryover into the discharge, coming from the dry running pump, no matter what, especially that there are no valves ...

First of all, why bother making assumptions? We are talking about a situation that is, at worst, totally unrealistic, and at best, very unusual. Why not just take the OPs' question at face value and answer it as such? The question, and answer, are very simple until you start introducing unsubstantiated assumptions, in which case there are probably too many variations, and the whole thing becomes meaningless.

How do you work out that the 'dry' pump, will create enough pressure to introduce air into the common discharge? Unless the density of the fluid in the 'dry' pump is the same as that in the 'wet' pump, it will not create the same pressure, therefore there is no chance of any of the discharge from the 'dry' pump entering the common discharge. Even if you had only 1% air, the density of the 'mixed-phase' fluid from the 'dry' pump would be less than that in the 'wet' pump, so the pressure would be less. So the higher pressure from the 'wet' pump would win the battle, and there would be no ingress from the 'dry' pump, but rather flow from the 'wet' pump towards the 'dry' pump. And even if you somehow managed to completely fill the 'dry' pump with liquid and achieved the same differential pressure, it is almost certain that the suction pressure would be less than the 'wet' pump, so the discharge pressure would be less, so the 'wet' pump wins again.

The only possibilities of getting air into the common discharge are:

a) If discharge pressures are relatively low, and good self-priming pumps are used.

b) The air in the 'dry' pump is pressurized to above the discharge pressure of the 'wet' pump.

Since the OP mentions neither of these scenarios I would see them as pointless assumptions, and will stick with my original statement.

I did not assume anything not mentioned by the OP!

Please read properly my statement in the 1st line. The reason of this line being put there was to go back to the OP's submition. He said two identical machines etc. therefore, I only reinforced his statement.

On the rest: Have you tried this set up (as described by the OP), before?

Your confidence about the fluid densities etc... These are also assumptions: You stated that the running pump will fill, even partially, the dry pump. In this case, there will be attempts of pumping by the dry pump. These sporadic actions will continue until there is enough liquids (of the same density as in the 1st pump) between the discharge of the dry pump and the meeting point with the running pump. IF not, where does the leaked fluid go to? Does it pass through the dry pump back into the suction line etc?

The reason there should be a non-return valve is that it will keep that dry line shut and prevent liquid going into it. No one expects the 2nd pump to run dry, but if it did, this valve will still isolate it. [This is beside the question from the OP]

In all honesty, I cannot say to the OP that he will not get air entrainment into the discharge line. IF you can, then good luck to him.

My apologies, you are right. I sort of jumped in there feet first in reaction to your opening "Let us assume...". In fact, we don't have to assume at all. The OP says the pumps are the same specification and configuration.

"Your confidence about the fluid densities etc... These are also assumptions:". No, these are not assumptions, these are facts. If there is even the slightest bit of air in the liquid, the density will be less than 100% liquid. If the density is less, the pressure will be less, and you will not get any flow from the lower pressure discharge into the common discharge.

The only way you could get any flow from the 'dry' pump is if it is totally filled with liquid, and therefore has the same density and the same differential pressure (discharge pressure could still be less if if the air-filled suction line means lower suction pressure) as the 'wet' pump. But in this case there is no air in the pumpage, so still no air into the common discharge. And even if air is subsequently drawn into the pump suction, density and pressure would drop immediately.

I have not tried this set up, and have no intention of doing so. As I've said elsewhere, this set up is extremely unusual, if not downright unrealistic. so I don't see the point. I see the OPs question more as an "I wonder what would happen if..." type thing. So nor will I proffer suggestions as to why it could be so, or what could or should be done. In fact, I cannot even guarantee that there will be no air entrainment. I have more confidence in the empirical ("let's try it and see") approach rather than the theoretical approach which always seems to like to make us look foolish. But the basic principles of operation for centrifs are quite straightforward and well established, so all I am doing is trying to explain what ought to happen.

Points Taken.

I don't think you need equations - just to realize no matter what, the pump with less mass charge cannot 'win' against the line pressure of the fully charged one.

Wouldn't matter if is was back fed by the line and 'self priming', the charge density, so momentum available, will still be less - so it looses.

Unless of course you have a pump 'self primes' at a higher output pressure than it runs at.

I've yet to come across that model. Or one that will self prime with no supply to eventually prime with.

It might happen with a pair of positive displacement pumps - but the OP indicates they're not.

It is enough to say that if enough water is leaked into the dry pump line to enable it perform just for a short time, it will develop the required pressure to send air into the discharge line. This is a possibility since the 2nd pump might not be running at its full maximum pressure, and the dry pump, being the same as the 1st one (OP), can develop the same pressure, even for short time and intermittently...

With all respect to every one contributing here, unless you have had a practical experience of this situation, you cannot just talk theoretical. All the data provided by the op is not enough to corner the issue 100%, therefore, he should be aware that air can and might be entrained.

Holzfeller, I fully agree with your good answer including the first paragraph..

Not wishing to be pedantic but if the pumps do not have a common suction and discharge then they can hardly be described as running in parallel.

The OP mentions a 'common discharge'.

The OP stated "operating in parallel".

Reading further it also says common discharge line!

Exactly, unless the pumps were designed to pump the different fluids independently and then mixed inline above the pumps, but that sounds like a very strange way to mix. The dry running pump would were out after a while due to cavitation, and eventually you'd have one pump running anyways.

YES,

But find out why the one pump is air bound....does it have a pick up screen that is plugged?

I too have had many years (>30) of experience with centrifugal pumps pumping water and sand/water mix.

First, a pump can run dry without damage if it has a labrynth seal . Secondly, a pump is still considered to be running dry if it's seal is cooled/lubricated by liquid via a lantern ring but is otherwise running in air.

In the case in question, as it has been pointed out, liquid from the pump that is working will travel back to the pump running dry through it'd discharge tube. The liquid will attempt to pass the impeller and so that pump will periodically become primed and will force a small quantity of air/liquid mix through it's outlet. This situation will only last for maybe a few miliseconds but will repeat itself frequently.

In my opinion, the question of whether there will be air coming through the common discharge line will depend on the relative heights of the 'dry' pump and the point where the two discharge lines become one.

If the 'dry' pump's discharge is uphill all the way to the junction, then yes, any air that is pumped when the dry pump periodically sends an air/liquid mix into that line, will find it's way up the line and be swept away by the flow from the working pump. And bubbles will appear in the final discharge.

However, if the 'dry' pump is at a high point in the system then no air will make its way to the junction point. In that case any air pumped by the dry pump will immediately be pushed back past the impeller as soon as it has lost its prime. Hence no bubbles in the final discharge.