determining number of parallel pumps

[Prototyp]

A supply system demands 50 to 6000 l/min of emulsion. Head should be adjustable by setpoint between 3.5 and 7.0 bar. Considered are 2+1 (standby) VFD operated centrifugal pumps. Althoug 2+1 seems to be the most common set-up I was wondering how the most appropriate number of pumps could be determinded?

Most appropriate in this case means:
- lowest investment costs (for example is it cheaper to install 3+1 45kW or 4+1 37kW pumps instead of 2+1 55kW pumps?)
- lowest energy costs?
- which mode would be the most sparing for the pumps?

 

[TenPenny]

That's a huge flow range, even if you use 2+1, you're expecting a centrifugal pump to go from 50 to 3000 l/min? I doubt that you'll find something suitable.

 

[SandCounter]

I would approach this by either installing a high flow pump and a low flow pump, or a set of mid-range pumps with an accumulator to assist during high flow periods.

I used to count sand. Now I don't count at all.

 

[Prototyp]

TenPenny, yes I think 50 l/min won't be possible. The minimum flow might be around 500 l/min. To cope with that there is a flow controled minimum flow valve which opens when the flow drops under the limit.

 

[LittleInch]

You really need to have some idea of time or volume required at a number of flow rates.

A 3000 to 500 turndown is 6:1 which is still a big range.

However if this low flow case is only occasional you can accept some sort of min flow recycle, but if it's more common you then need to think about how and where this re-cycle goes as it will start to heat up quite a lot if just going round the pump.

Normally lowest no of pumps is lowest cost and the bigger the pump the lower the cost per m3 of flow.

But the devil is in the detail and how much time is spent at what flow rate.

This is not a simple exercise.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

Typically, VFD gives you speed turndown to 50%. With an aux fan (with dedicated fixed speed electric motor) installed on the motor side for motor cooling, you could go down to less than 10% if need be.

 

[GT-EGR]

Lowest investment cost = the less pumps the better
Lowest energy costs = the more pumps running simultaneously, the less total energy draw due to pump affinity laws
Most sparing for the pumps = more pumps the better, they will have less run-time and last longer.

The answer to your question usually comes down to, what is your minimum system flow, and you want to make sure your pump at it's lowest capable turndown (I'd say with a VFD more like 25-30%) is less than or equal to that. So if your flow range is 500 to 3000 l/min, then you could squeak by with two 1,500 l/min pumps (and then one more on standby), as long as you can turn-down pump speed to 33%. But if you had more pumps, say three 1,000 l/min pumps (and then one more on standby), then you only need a VFD and pump capable of 50% turn-down, and you get the added benefit of less energy use, longer pump life.

More pumps from a design perspective usually calculates better and seems more reliable - but the drawbacks are higher installed costs (pumps, motors, power, pads/supports, piping trim, etc), they take up more space, and the owner has more pumps to maintain.

 

[GroovyGuy]

Hey George,
The max turn-down or 2:1 applies only to constant-torque loads (ie positive displacement pumps). You will have no difficulty in obtaining a 10:1 turn-down on a variable torque load (ie centrifugal pump). This can be accomplished w/o an external blower-fan on the main motor. The external blower fan is only required on constant torque application where the turn-down is > 2:1.
GG


"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[georgeverghese]

Thanks for that, @groovyguy,in fact I was given this thumb rule of 2:1 for a pd pump application.