Pump Selection

The flow rate and duty cycle are a good start.
Del

Don't forget the delivery pressure requirement!

This can be a quite complex question.

Start with how much flow you need, and at what pressure. Many/most combinations of these will lead to a centrifugal pump selection. [High pressure moderate flow may require multi-stage centrifugal, piston, progressive cavity, regenerative turbine, or other special types; high flow low pressure might lead to an axial (propeller) pump choice.]

If solids are present in the water, the passages of the impeller must be large enough to allow the solids to pass through, or else the pump must have a way to chop the solids to small enough pieces. This applies especially to sewage or sump pumps.

The shaft seal can be either a packing gland (small leakage) or a mechanical seal (no leakage unless the seal fails).

And this is only a start....

Here is the big question

Ineed 15 l/h to 30000 l/h at 40 meter dinamic pressure.

im going to use a variable speed ac drive to get a stable flow and pressure

Call three different pump suppliers. Tell them you need a pump, but you do not know how to select it. Answer the questions they ask. Then buy what they recommend.

For that range of flow, speed control will be highly inefficient from an energy perspective and will create significant instability for control at the lower volumes.

We have water supply systems for towns and portions of the city that do experience that sort of variation at that sort of pressure and we achieve this with pumps capable of maximum demand volume (actually at least 200% capability) and a reservoir. We maintain reservoir between 90 and 95% full. That way, pumps get to be run at efficient speeds, relative pressure change is less than 1m head, and low flows are easily supported.

For instance, a centrifugal pump capable of 30000l/hr would have material backflow through the impeller when speeds are low enough to supply 15l/hr.

Not favouring any particular pump suppliers, but brands like AJAX, ITT Flyght and ABS come to mind in recent transactions.

"When in doubt ask"

Call a reputable pump manufacturer's engineer and ask him to select the pump for your required application. When he does that then ask him how? then ask him to show you? Before he leaves take his pump catalog indicating the capacity & sizing chart.

write all . go home review all what you have written, work out on different examples and you will learn.

If you cannot learn then do not bother yourself and then rely on the pump engineer.

Is the head fixed, at 40 meters?

A variable speed AC drive is the most efficient way to get the flow and head you require. However, it will not come anywhere near to the flow range you are looking for. It is adviseable to operate a centrifugal pump within the range of 20%-140% of BEP (Best Efficiency Point). It is very difficult to maintain stable flow and pressure below about 20% as most curves are quite flat (or even humped) at this point, and ultimately it can damage the pump. With a single pump and speed control you might, at best, get a 15:1 turndown ratio, and you are looking for 2,000:1.

If you cannot use a reservoir system, as proposed by Just an Engineer, then there are a few other options:

1) External recirculation (from pump discharge to source) of a proportion of the pump flow (up to 99.25%!), possibly in conjuction with control valves. This is the least efficient method at low flowrates, but possibly lowest initial outlay.

2) Use three seperate pumps, with speed control (and/or control valves), to cover the range.

3) Any combination of the above.

Dear Mr.Claudio,

Your question is General in nature, and lot of explanations are available.

Pump Selection depends upon Flow Required, Head, Viscosity of fluid, Temperature of fluid, NPSH (If very hot fluids are to be pumped) Type of Suction - Flooded suction or Negative suction, etc. shut-off pressure and leak-off connection size etc.

Pl. describe your requirement and you will get good response from CR4 MEMBERS.

Alternatively - search in this forum itself.

Rajeswari.