Pump Curve

You have 10m of suction head?

Understanding Net Positive Suction Head - Pump School

Do you have a data sheet/pump curve for this pump, or are you just reciting some numbers?

Maybe a model number?

Have you ever worked with pumps before? Never mind. I just looked at your history. You should talk to the pump supplier.

ITT B&G curves shows a good position for 1150RPM (60hz). Alternately, water bypass thru volume control valve is a common practice for an existing pump

Probably maximum restriction in your circuit is 10m of H2O column at 60 m3/h.

You can increase the restriction by adding orifice(s) so that the restriction increases to achieve desired flow.

Probably existing maximum restriction in your circuit is 10m of H2O column at 60 m3/h.

You can increase the restriction by adding orifice(s) so that the restriction increases to achieve desired flow.

Typically, the pump curve represents the practical operating range for the pump. Your info is unclear, because you do not define your term "available head". If you mean static head, you need to add friction head, a function of flowrate. (Plot on your pump curve, total dynamic head (TDH), ie, static plus friction head, at various flow rates and where this "system curve" intersects the pump curve, is the expected duty point.) Based on guessing at what you mean, the duty point is probably off curve, and if so, you risk motor burnout because the impeller will very probably demand more power than the standard motor for this pump will deliver.

At shut off, no flow , pump operates at maximum head. Pump cannot be operated at this level; Hence minimum flow is required. For pump safety and energy efficiency pumps shall be operated within the operating range w.r.t pump curve.

The problem with pumping at heads below the pump characteristic curve is that the pump may well cavitate and become less efficient. Cavitation, caused by the creation of bubbles of vacuum which collapse upon the edges and trailing faces of the impeller, may thereby result in noise and damage to the impeller.

As someone already has said, throttling the discharge, though inefficient, can be a temporary solution but "wiredrawing" may cause excessive wear to the valve.

Geof,

I think you're confused.

The inlet of a pump should never be throttled down, for the reason you suggest.

Always restrict the outlet of a pump to control flow.

Restricting the outlet will do two things. 1. Decrease the load on the motor. (Restricting the inlet will do this too, but may cause cavitation).

2. Over-restriction will cause excessive heat build-up in the volute and may/will cause failure of the seal or housing , over time.

He should get the correct pump.

No Lyn, I am not in the least confused. I won't comment on the want of courtesy.

If the pump discharge is throttled to the desired flow, the throughput of water is likely to suffice for cooling. But that's totally inefficient.

If the pump is allowed to pump against only 10m head the impellers will be in a state of stall and the flow will be very approximately 80 - 90 m³/hr depending on how much of the nominal 10m is friction - beyond the operating range the curve is likely to drop off sharply. The actual head at the pump is likely to be higher than the 10m nominal.

If the discharge is throttled to give a total head of about 55m there will be about 45m loss across the throttle. There will be no 'churning' within the pump, because 60m³/hr is within the pump range. With that throttling, the pump will operate 'on curve".

Energy recovery is an idea, but that's whole new chapter!

Geof,

Sorry, my apologies.

You never mentioned throttling the intake, and I incorrectly assumed that we were discussing an air handler in another thread, where restricting the intake was discussed.

You are right, of course.

Dear Mr.Abusaleem,

Your data is not conveying clear message. You referred "10 M available" Does this mean the Level above the Pump or the Water is below the Pump.

If it is below the pump definitely you will face problem as the safe Suction -Static Height is 7.9 Metres (26 Feet) i.e water surface to Pump Shaft Level and Friction Head is to be added to this for Suction side. Probably less than 0.2 Metre(assumed as Max.) totalling to 8.1 Metres.

Your case is 10 M hence there is a Problem on account of Higher Suction Head. Since the pump has very high Head , for a given discharge, while you run at low Head, the discharge will be much more higher than the rated output, because the Pump Equation is W x H/75 = H.P (THEORITICAL) and W x H/ (75 x Eff.) = HP (Actual)

Hence you can see if H is reduced the W will increase INVERSELY.

But you referred the discharge is less than 20 M^3/Hr. This suggests that there is a probem in the Suction side, this is to be checked. Was there any noise from the pump while running while discharge is less than 20M^3/Hr.? If so it will be due to CAVITATION, and definitely the Discharge will suffer.

You should not compare this performance to the Pum curve, which is for a definite working head and discharge.

DHAYANANDHAN.S

These curves are purely hypothetical but they may illustrate the problem.

Dear All Thanks for response, My situaton as following: 1- Client requirements as per his data 60m3/h and 60m head (pump supply and installatio) 2- The actual pump data installed as per above mentioned data (14-61m3/h and 54-80m) 3- The pump above the underground tank (3m from the bottom of the tank up to the impeller) 4- The outlet pipe is 10 m height 4" size, that is what I mentioned as available head,

NPSH - Wikipedia, the free encyclopedia
NPSH should be figured from the top of the solution in the tank.
What you want to do is fraught with danger.
Use a foot valve and hope, or a submersible pump.

 5- Now the total head is around 15 m (including friction losses calculation) 6- The NPSH required for the pump is 5m 7- My question: the losses is around 15m which is less than 54m (the minimum head of the available pump), is right when the total head is out of curve the pump shall deliver less capacity. 8- We had already call the manufacturer and he said your total head is out of curve

"7- My question: the losses is around 15m which is less than 54m (the minimum head of the available pump), is right when the total head is out of curve the pump shall deliver less capacity."

No, the pump will attempt to deliver MORE flow than you expect, because the pump can deliver much more head (pressure) than you need.

As the flow goes UP, the friction and other losses will also go up (above 15m.) until the pump head balances the system head. Without seeing the curves, it is hard to predict what the flow rate will be.

Are you sure you have included all losses in your 15m? Footvalve, friction losses, elevation difference, (inlet and outlet,) bend losses, valves, etc?

If you allowed the pump to operate with the outlet valve fully open, it would deliver more than 61 m3/hr and, as discussed before, it will not be "happy". Some mentioned the drive motor, but also the pump, because it is not operating near its point of maximum efficiency, will experience unbalanced forces on the impeller. This causes vibration and premature bearing failure.

There are lot of contradictions in your statements. Why don't you discuss the revised requirement with pump OEM and get solution from them.

To me, it appears that the pump is over disigned for your present requirement. so, the possible solutions could be

1. Replace the pump with correct one.

2. Install a VFD, if feasible to change pump speed to suit your head and flow requirement. (it will also save power & pay off the investment).

3. Trim the impeller size to reduce head and power (if requirement is not going to change).