Pump Discharge

The flow rate will be reduced to something like 70% of before. To pin this down more closely, one would need to see the pump curve and know how long the discharge hose is.

It depends upon the shape of the pressure-flowrate curve for the pump, which was published by its manufacturer and is a needy document for proper pump selection at purchase time.

What the curve illustrates is the locus of all the possible operating points of the pump. There is a similar curve, usually unseen, that illustrates the locus of operating points for the system to which the pump is connected. Having either a measured flowrate or a measured discharge pressure gives the location on the pump curve where those two curves intersect. That is why, wherever possible, a discharge pressure gauge is a Very Useful Thing for a pump to have.

In general terms, a 90deg bend can add about 45 diameters of straight pipe to the friction that the pump has to overcome. Reducing the diameter from 2in to 1.5in for the same installation will increase the average velocity in the pipe by a factor of 16/9 so the pipe pressure drop will be increased by a function of that figure, putting the pump at a different place on its operating curve. However if the pump is already operating near the high pressure/low flowrate end of its curve, then the shape and diameter of the pipe may not make a lot of difference to the pump's performance and one would gain more by reducing the height difference between the pump and the destination. Back to that pressure gauge again.

It's something that can only be assessed properly by viewing the actual installation and the pump curve.

my concern is not the flow rate.We can manage the less flow rate.But i am fearful of back pressure or damage to the pump

Provided the pump is somewhere on its operating curve as published by the manufacturer and not off the end of it, then one may expect it to operate without damage.

'....I am an electrical engineer so i am asking a basic question....'

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_{snicker, snicker, snicker...}

As an electrical engineer, you know that efficiency can be measured electrically. Check the pump label, it shows normal operating amperage. Measure the pump running free in the air, running with no hose attached at the sump location, and running in your setup. Those numbers should give you the picture. Then you can make changes and test the amp load again, to see if you have made things better or worse. One way to improve pump efficiency is to use long smooth curves (long sweep elbows) on the discharge.

Another thing you can do is to route the discharge to the lowest place possible. The water going downhill will act as a syphon and reduce pump effort.

Wait....why are you suggesting the pump be run dry?

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Aside from some pumps and packing not appreciating that, what information is provided by knowing the the current draw when the pump is 'running free in the air'???

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Additionally, what use is noting the current draw without having any data on the flow rate and change in head?

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One more: putting the discharge in the lowest place will only make a difference if that place is at or above the surface of the volume of liquid to which it is being discharged . If the discharge is not into air (above the surface to which it pumps) then lowering the discharge won't make a difference.

I just wanted to give the engineer as much data to work with as possible. A quick run dry will just let him know how the pump compares to the label, give a base measurement.

He didn't say if it was a well pump or a sump pump for basements, and his discharge may be old fire hose. He just wanted to know if he was abusing the pump.

Actually, pumps are quite forgiving, some don't mind being deadheaded for a little bit, and a little back pressure can actually help, by reducing cavitation and such.

Finally, he didn't mention changing head or flow rate. By noting amps after changes, and comparing to flows, (naturally) efficiency can be determined.

The answer to your question is a simple "Yes, it will make a difference." The question at this point is "How much difference and what kind of difference?" There are varying characteristics by which every pump is governed. Since yours is a submersible pump, I will go out on a limb and assume that it is a centrfugal pump. So, my analysis will be based on that assumption only. Any centrifugal pump will be bound by the Laws of Affinity. If the discharge head increases, the flow will decrease and vice versa. The Laws of Affinity describe, among other things, the amount of energy the pump will expend at various combinations of head (pressure) and flow. And this is where it gets sticky. If the head (pressure) increases, the amount of energy required by the pump motor will decrease as well. If the head (pressure) decreases, the energy requirement of the centrifugal pump will increase. Therefore, it becomes evident, then, that, contrary to popular opinion, if the discharge hose is completely shut off, there will be no damage to the driving motor. The motor will continue to "snore" without exerting much energy at all. The damage occurs with the advent of other extraneous forces of which heat is the most damaging. All that being said, the difference you will experience is a lessening of the amount of flow you will get through the pipe. You won't damage the pump. You won't use more power. You will be less efficient because the pump will not be able to operate at its optimum point on its characteristic curve. I hope this helps. You, being an electrical engineer, can comprehend the value of efficient operation of a pump.

"If the head (pressure) increases, the amount of energy required by the pump motor will decrease as well. If the head (pressure) decreases, the energy requirement of the centrifugal pump will increase."

Is this correct, or did you get your words mixed up? Otherwise, good post.

For centrifugal pumps, that is correct. The change in flow more than compensates for the change in head.

You've really lost me this time. I consider an increase in head pressure to mean an increase in back pressure, like the restriction the op mentions. That means more work for the motor, more load.

A pump working against a lower head should draw less power.

Horsepower is loosley defined as "the power to lift 550 pounds to a height of one foot in one second against the Earth's atmosphere." Of course that definition is tempered by such language as "at STP" (standard temperature and pressure) and so on, but that is the essence of the definition. That being the case, when a centrifugal pump is operating it is bound by the characteristics of its curve. The delivery of the impeller will always be somewhere on that curve. The curve, from left to right has a general movement from high on the left to lower on the right, the vertical being head (pressure) and the horizontal being flow. If the head increases, the flow will decrease. And, conversely, if the head decreases, the flow will increase. To cause the head and the flow to increase at the same time, the tip speed of the impeller must change. When the flow changess, the head will change "exponentially". That is not just an expression. The flow of the impeller is directly proportional to the tip speed at which it is turning, however, the head changes as the square of the speed change (hence the use of the word exponentially). The point is, as the head decreases and the pump will deliver more liquid, the horsepower goes up since water has mass (and weight) and, by definition, this mass has to be lifted. That's why I prefaced my last post by saying that it would "...get sticky." This concept is hard for a lot of people to comprehend. It is the common perception that lifting higher requires more power, and, all things being equal that is true. But all things are not equal here. Consider you are lifting a 5 gallon bucket with a simple rope block and tackle, and, as you lift the bucket it is leaking. And say you are lifting it to a height of 20 feet. And further, that the rate of leakage makes it such that the bucket is empty by the time it reaches 20 feet. How much power is expended in the final foot? Not much, I would say. This is a very simplistic offering, laden with possibly, more questions than answers, but it serves to illustrate the concept of a centrifugal impeller. Hope it wasn't too laborious.

Thanks, guys. This sure has been a lesson to me. For another project I have been working on: if I were to put an amprobe on my home pump, as the pump approaches cut-off pressure the amps should actually go down?

Exactly. However, small pumps of that sort operate in a very narrow range of amperes so it won't be a lot.