Some Help with Water Pump Issues

I could be wrong...but 'too big' sound fine to me...

If you must restrict it, restrict the outlet rather than the inlet.
Or maybe just run it with a 50% duty cycle, say 5minutes on, 5minutes off ?

Feel free to shoot me down guys... I'm just a Cat.

If you need assistance to drink beer and lie by the pool I'll happilly oblige.
('Lie' by the pool... 'Of course I did those chores Dear' )

Del

Running a pump with restrictions on the intake may cause cavitation. Check the impeller at the intake (with a beer at hand to cool it down - In SA we go for Lion {NO17}).

ah, the joy of experiment...

Well, I hooked it up, Holy cow! like the fountain at bellagio!

restricting the inlet:

the motor never pulled more than 7 amps (full load current 10A) the suction line collapsed, (cheap vinyl hose) and ampere draw went down to 2A

restricting the outlet:

let me draw you a picture...pump was sitting on deck, discharge aimed towards pool - and neighbors back yard..

I put my hand over discharge, to restrict flow, and "aim" restricted discharge into pool

lets just say I have bad aim, and my neighbor has clean siding (and thankfully a good sense of humor!)

blocking intake or discharge resulted in lower ampere draw... so I think I am OK - well, I'm not OK, but my pool should be.

Why restrict the pump? Let it run unfettered, that way it will use less juice......and be more fun!!!

Not so, my friend. Restricting the inlet or discharge of a centrifugal pump will cause the pump to deliver less water. Delivering less water uses less energy. That is evidenced by the fact that the writer stated that the FLA of the motor was 10 amps and the pump was drawing only 7 amps. There are other problems that can occur if the suction is restricted, but restricting the discharge is an acceptable solution to the pump delivery question. However, I do agree that the pump would probably work very well if it were allowed to operate unrestricted. It would probably run quieter.

Simply not true.

Take the example of blocking the exit hole completely, with your scenario you would have us believe that it will take no load, I would maintain that it will now take the maximum load.......leaving it "open and free" will allow the motor current to reach a low, but shift the maximum amount of water.

Reducing the input could cause cavitation, that will quickly wear away the impeller, but may show a lower load while so doing....it depends on the design....

Take the example of blocking the exit hole completely, with your scenario you would have us believe that it will take no load, I would maintain that it will now take the maximum load

I don't think it will take max load (assuming it's a centrifugal pump...as obviously a piston or diaphragm pump would stall)...it will merely be rotating a fixed body of water within the casing, with no flow there would presumably be few losses. The reported observation of current draw contradicts yor assertion...maybe someone will do a test??

Interesting discussion.

Del

Del, only one current was measured, the other was read off the plate....!

BOTH need to be measured and that will then show that a blocked exit takes the higher current as against no restrictions, all other points being equal!!

If your exit gets blocked I am sure that you would draw a higher current too.....

If your exit gets blocked I am sure that you would draw a higher current too.....

Yea for Andy...he puts it so everyone understands! And nicely too.

One further item for consideration. Go back and look at reply #3. The writer states as follows:

"the motor never pulled more than 7 amps (full load current 10A) the suction line collapsed, (cheap vinyl hose) and ampere draw went down to 2A"

Notice what happened when the suction line colapsed. The flow went to zero and the amp draw went down to 2. I believe, as johnntbravo stated in that thread:

"ah, the joy of experiment..."

He has proved my point!!

I agree entirely with the effects when the suction side is blocked, that was never the question I had, as of course the load drops and therefore the load current, simple 1st year physics.....

If you had read FULLY (and understood) what I had written, you would have seen there was no point in your comments, except to signal that you did not read my comments fully!

The question I had (please refer back to my original post for verification if you wish!) was to do with the output being blocked!

My understanding was that the load current would rise, this (at least mentioned so by other posters) appears not to be the case....I must also add that I am personally not yet convinced.

THOUGH I DID UNDERSTAND THE OTHER POSTER'S POINT COMPLETELY ON THE MATTER!!! Having been MOST careful in reading it through fully as he could well be right!!

You are closer!! Blocking the discharge completely would, indeed, reduce the load on the motor. It would not go to "no load" because it still takes energy to run the motor, but the lowest attainable load wouold be experienced at that condition. The laws of affinity would govern this action by the centrifugal pump. This law describes the action and load requirements of a centrifugal pump as follows:

Flow (in GPM) x Total Dynamic Head (in feet) / 3960 x pump hydraulic efficiency = BHP (break horsepower)

I don't know the head and flow of this pump, but as the flow decreases, so, too, does the load on the motor. Therefore, the load would be less for a pump with a restricted discharge. If you were to look at any pump manufacturer's characteristic curves for a centrifugal pump, you will see that as the head (pressure) increases, the flow decreases. The curve will show horsepower required as well. You will see that it decreases as the flow decrease as well.

I have looked around on the web for anything to substantiate your comments on blocking the outlet. I have found nothing that supports you. Please be so kind as to supply web links to such material for me.

Here is a web link that supports what I said:-

Centrfugal Pump Design

I would like to draw you attention to a paragraph in this .pdf document, it reads so:-

Centrifugal pumps have a required minimum flow below which the pump can incur severe operational problems and damage. If the primary pump discharge-path can be blocked, a recirculation (kickback) line is required to accommodate the pump's minimum flow.

This seems to support my thoughts completely on the subject!

Waiting on your web links!

This statement seems directed to the well known cavitation problem and not a closed suction.

If it helps, the the primary pump discharge-path is the outlet!!!! Not the inlet(suction!!)

The query is to do with motor load and a closed outlet or discharge, not a closed inlet!!!

Of course, a closed outlet will result in cavitation and heating of the fluid, only to be expected, but what is the load on the motor???? That is the $64,000 question!!

Well, you kind of put me on the spot here. In the interest of time, the only pertinent link I could come up was the one below. On page 14 of that docement, you will see the line that reads as follows:

"Starting the pump with the discharge valve closed allows the motor to start unloaded, as no liquid is flowing. Once the motor is running the outlet valve is gradually opened and the motor gradually loaded."

Now, I don't know about you, but to me when it states the motor starts unloaded, that means...unloaded - no load - low amp draw.

http://www.lifetime-reliability.com/Training/PGT006%20Centrifugal%20pumps%20intro.pdf

Now, I'm off to start up two centrifugal pumps. I'll let you know how it goes later this afternoon when I return.

I'm back. The start-up went well. The two pumps were 7½ HP, 1/230/60, 1750 RPM sewage pumps. The full load amps of the pumps is 37.5 amps each. With the discharge plug valve closed, the pumps drew about 9.2 amps. With the valves wide open, the amperage was 19.3. They were reaching their system head at that point, so no further amperage would be expended. The rating of each of these pumps is 140 GPM @ 51' TDH. Thank you all for waiting.

I feel that you did not put much time into reading through the excellent document that you found. But I did......many good examples were there to read.....many thanks.

Here are a few of the sentences that you appear to have missed from the same document that would support my theory perfectly. All are taken from different paragraphs, but basically say the same thing, pressure is highest, load is highest, pump explosion possibility also highest....:-

1) From the curves you can see that when the discharge, or outlet, valve is closed (known as 'dead-head') there is no flow and the pressure is highest.

2) Dead Heading - occurs when the discharge of the pump is closed off by a closed valve or a blockage. This condition must be especially avoided for too long as the liquid will heat up and boil. Pumps have exploded from the super heated pressure caused by running a pump with the discharge valve accidentally shut.

3)Don't run pumps with a closed discharge valve, as this causes over-heating and will damage the pump and hurt people when the pump explodes when the liquid boils inside the pump.

In answer to your own italics, it is actually not a good idea to start a pump with no liquid as mentioned elsewhere and can only be done at the max for a few seconds, because the seals will get damaged, but that was not the situation that I mentioned anyway:-

4)Centrifugal pumps are not self-priming and must be primed by filling them with liquid, otherwise pumping will stop, and the shaft seal will probably burn out. The
purpose of priming is to remove air from the pump and suction line to permit
atmospheric pressure and flooding pressure to cause liquid to flow into the pump.

Furthermore, the question was not on starting them up, it was running them and closing off the outlet, a slightly different set of circumstances!!!!

You have given me/us an excellent document that proves that with the outlet closed, pressure is highest and therefore motor load will also be highest, so I really should thank you most kindly for helping me out in this way..........

Andy, I am back in the office now. Not to belabor the subject, but yours is a common misconception. It is hard for people to understand the the pump uses less power when it is pumping against a dead-end. It goes against what most people conceive as common sense. However, let me finish by responding to your last post.

The link you cited does not support what you said. The contents of the link do not address the condition where the discharge is blocked, I agree. It simply wasn't adddressed. The comment stating that there are significant problems with a blocked discharge is absolutely true. Problems can occur. However, those problems do not include overload or higher work for the motor. The problems concern themselves with mostly the heat generated by the fact that the pump impeller is constantly spinning in the same water and nothing is moving. That small amount of water gets hotter and hotter the longer it is "beat" by the impeller. This can cause, as the link I provided states, explosions in the volute of the pump. This can occur especially in the case of pumps with rotative speeds that are in the range of 3500 rpm or so.

The writer of the original question was concerned about restricting the flow, not closing it altogether. Restricting the flow by PARTIALLY closing the valve on the discharge is a completely safe mode of operation for this type pump, and it uses less energy to do so. I hope I have sufficiently explained my position on this matter. It is not my opinion, it is fact. And I thank you for the mental exercise. Lord knows I needed it. Thank you.

wow, its nice to know it was puzzling to y'all too!

just to reiterate:

nameplate amps 10A

blocked inlet: drops to 2A

blocked outlet: current draw dropped (sorry Andy!), but I couldn't stop the flow completely - yelling from next door, and I was laughing too hard!

my assumption was the centrifugal pump would not be moving water, so the load would decrease - that was definitely the case. (1 point for Del and Commoner!)

another assumption:

if the water is being whirled around in the housing (volute?) the water would increase in temp - I would assume a major increase in temp would damage pump and/or seals, thus the terse warnings from Andy's link

I already considered a bypass loop to keep a small flow of water moving thru the pump housing. but, because the pump is so big, i figure I will only run it 3x30 minutes a day... (built a really cool timer a few years back)

since the original post, I needed to fly to an other city. (not sure about you guys, but gotta pay the bills!)

hopefully this weekend the pump will be installed - water is turning green!

so, now I need to block the discharge just to prove our discussions.

I welcome your discussion - life is a series of discoveries, eh?

Nameplate amps are not necessarily the actual amps drawn. This depends on the mechanical load!!! They are an indication of the maximum safe amps that the motor can handle, no more no less....in different jobs, different currents will be measured....

Have you measured the amps drawn with a blocked outlet? Yes or no please. If yes, what were they? Up to now, you have not, or at least not told us!!!

I can only assume that you are not an Engineer, or at least this is not your area of expertise. Its not mine either, but I would have measured the amps......that is the simple way out.....

As of this moment, neither theory has been proved......

If one can relate a centrifugal fan (air instead of water for example), which is built into many vacuum cleaners, every child has at some point blocked the suction and heard the motor increase in speed, and blocking the outlet reduces the speed.....and in a normal working condition, the working speed is somewhere between the two.

So why is it thought that a centrifugal pump with water is any different?

as stated in post,

amps did drop, but laughter (and spraying water) prevented precise measurement

I'll check this weekend and report back.

your homework assignment: (I would do this right now but I am in a hotel room)

plug the outlet of your vacuum cleaner - my bet is the motor will increase in speed, not as much as blocking inlet, but if the "pump" is not moving air, it will have less load.

although on vacuum motors, some of the air is used to cool the motor as well... maybe this will lead to false readings?

freundlichen GruBe

Johnny

For anyone who cares, a follow up

blocked discharge = 4.7 amps

blocked inlet = 4.3 amps

thanks for the lively discussion

johnny

Many thanks for the final correction, I now feel to be fully vindicated in my at first, mostly theoretical musings on the subject!!

I am not surprised which was finally the highest and lowest, only that they were relatively closer to each other than I had thought possible. I had expected a larger "gap"....

The similar relationship to a centrifugal fan operation is quite uncanny.

Johnnybravo, Thanks for the update. It appears that my post #8 states the facts as they have been borne out by your experimentation. Thank you for the validation.

You previously wrote (completely wrongly as it turns out!) the following:-

Andy, I am back in the office now. Not to belabor the subject, but yours is a common misconception. It is hard for people to understand the the pump uses less power when it is pumping against a dead-end. It goes against what most people conceive as common sense.

That is NOT what the results gave us, in fact you were 100% wrong!!!

In fact my common sense was completely right, pumping against a blocked outlet takes a higher current than pumping with a blocked inlet!!! As expected!!

Have a great day in spite of you being wrong this time my friend, it can happen to anyone.....

Andy, Andy, Andy. For heaven's sake, give it up. Read the information presented by johnnybravo:

Pump Full Load Amps 10.0 amps

Pump draw when running with full open discharge 7.0 amps

Pump draw with closed discharge 4.7 amps

How can you deduce anything other than the pump uses less power when the discharge is closed? Look at the facts, they are there for you.

In your response #25 you asked if johnnybravo had measured the amps with the discharge blocked. Now he has, and you choose to ignore it. Further in that thread, you state that this is not your area of expertise. Well it is mine. I have been working with centrifugal pumps since 1964, non-stop. This has been my livliehood and my only professional endeavor for well over forty years and I still do it daily. I have learned many things about centrifugal pumps from the theory of operation to the manufacture, application and maintenance of them. Actually, if you were located here in the United States and were active in the field of centrifugal pumps you would know that already because you would recognize my name if it were given to you. Your attack on my knowledge and expertise in this matter is ludicrous, though you don't know it. But what is more ludicrous is your unwillingness to admit you are wrong. Shame on you , Andy.

And to all those who share this forum, I apologize for this reaction, but it is the second time in as many months that I have been attacked personally by a subscriber . The first was simply an outburst by one who shared a differing opinion than I, and he and I came to an understanding and agreed to disagree. Now this where I am being chastised by one who knows nothing of what he is speaking. I must defend myself.

Read the final results and try to understand that you were totally wrong!!!!

The pump uses more power when pumping to a closed outlet than when pumping from a closed inlet. This would and should be apparent to anyone with a little understanding of being an Engineer....exactly as I said it would be......!

You based your wrong expectations on a failure to measure (but reading the motor plate) of what the motor was ACTUALLY taking at these two points. The original 10 amps was from the plate and was the maximum current that the motor would be allowed to take ever under any circumstance, IT WAS NOT THE CURRENT TAKEN WHEN THE PUMP OUTLET WAS CLOSED OR OPEN.

Try to read and understand that you were WRONG WRONG WRONG WRONG WRONG and apologize!!!!

The pump takes more current hen the outlet is close than when the inlet is close.

Game, set and match!!!!!

But I have the impression that you both did not understand the results and did not WANT to understand the results......eg. a very bad and childish loser.......!!!

A really good loser would just simply admit he was wrong....Remember, you are not the only person reading all these posts.....

Andy, this will be my last reply to anything dealing with this question. If I fail to convince you that what I have said is correct, I will bow out gracefully and accept the fact that I have not adequately explained my position and the facts as I know them to be true. I have seen your replies to many other threades and I have always been impressed with your knowledge and articulation. I felt you were wrong on this one and simply tried to state that as tactfully and truthfully as I could. This will be the final attempt to do that. If I fail, we will remain friends of the forum and I will see you in the future on other threads. Here goes:

The original question was not the subject of my argument. I replied in response to your reply #7. You said:

"Why restrict the pump? Let it run unfettered, that way it will use less juice......and be more fun!!!"

The itallics and boldface are mine for emphasis.

If you will again read my response #8 to your #7, I stated that:

"Restricting the inlet or discharge of a centrifugal pump will cause the pump to deliver less water. Delivering less water uses less energy."

Nothing more, nothing less. Everything that I have written or advanced in this forum from then until now has been in defense of that proposition. Your contention that "...it will use less juice..." is a total fallicy. It will not use less juice, it will use more. I have demonstrated that point over and over in this forum. It makes no difference how you cause the pump to deliver less water. You can restrict the inlet or the discharge, but the fact remains that either action will use less "juice" than will letting the pump operate unrestricted. Period.

This final reply does not require a response, unless, of course, I have now reached you and you can tell me "I now see what you are saying and I agree." For that, I would be very proud of you and myself for finally getting the information to you in a way that convinces you. As I said, I will not respond to any further replies to this thread. I thank you for the lively discourse, and I truly respect your opinions. See you on another one!!

I know I'll hate myself for this, but it had to be said

"...it will use less juice..."

is ...TRUE!

IF...

the pump is circulating juice!.. (orange/apple/mango, etc.)

now you know why I am not an engineer... Academia and I did not play well together.

c'mon guys, lighten up! - the pools open, water is clear, got a few burgers on the grill, neighbor is talking to me again... hmmm... My wife got a new swimsuit! damn!

High school sweethearts and she can still make my heart skip a beat.

CR4 is a very good escape from stresses of my job. I apologize for any insult my wise cracks might produce. Thanks to all for your help and opinions.

Johnny B

You screwed up big time here, why not admit it like a man?

Must be time to put this one to bed!

Good Night Gentlemen.

Like a man that posts anonymously.

It is unambiguous, for anyone who cares to read it.

The confusion has arrisen by the argument changing from ..

a)Load pumping water into unrestricted pipe vs pumping with blocked outlet.

to

b)Load with blocked inlet vs Load with blocked outlet.

Re-read the whole thread and make your own mind up...but don't post anonymous tripe.

Del