Soft Starter for 11kV submersible pump 7

[Loghman_ra]

Hello Fellows

I have a case to discuss,
we have a 11kV submersible motor , 1000kW.
it is asked to use soft starter for starting.
there is 2 limitation:
1- the submersible pump shall reach half nominal speed in less than 5 second which is due to mechanical requirement of the pump,
2- we are trying to restrict the starting current less than 350% or if possible 300%,

the motor/pump data is not finalized yet, some supplier propose VFD instead of Soft starter but we want to realize if SS is possible also or not?

then please share your experience for similar case.
BR

 

[Marke]

Hello Loghman_ra

To engineer the best solution, you need to have all the facts.
"evacuating the floods" - this statement suggests that this is a high flow low head application and commonly served by the use of a turbine pump rather than the more usual centrifugal pump.
A turbine pump has very different characteristics than a centrifugal pump and must be controlled differently.

One of the issues with a turbine pump, is that you must treat it as a positive displacement pump. When you restrict the flow, the shaft power goes up, not down like a centrifugal pump.
If the pump is slowed down, the discharge pressure reduces and it may not have enough pressure to drive the water through the outlet and so it will hydraulically stall. At that point, the shaft torque will increase, not reduce, so you can not use a throttling valve on the discharge to reduce the shaft load in the manner that you would with a centrifugal pump. Neither can you slow the pump down below the speed where the discharge pressure is less than the level required for full flow. You need to get these up to full speed before the discharge line is full, or the pump may stall with major overloading at part speed.

All previous comments are valid for centrifugal pumps, but not necessarily true for turbine pumps. It depends on the actual pump curves, the length and volume of the discharge pipe lines before the highest point etc.
I find that flood pumps are best controlled by soft starters with sufficient current to get them to full speed in 2 - 3 seconds.


Mark Empson
Advanced Motor Control Ltd

 

[edison123]

Mark

As usual, great info. Will VFD's work on turbine/positive displacement pumps?

Muthu

http://www.edison.co.in

 

[Marke]

Hello Muthu

Yes, VFDs can be used on turbine and positive displacement pumps, but you must use them correctly and for the right reason.

If you consider a standard centrifugal pump, then varying the speed of the impellor is essentially the same as varying the diameter of the impellor and there is a set of performance curves head against flow for different impellor diameters or different impellor speeds.
On steep curve pumps, the discharge head varies with flow. By varying the speed, we can compensate for flow variation and keep the discharge pressure constant. If we do this using a valve, then we will have a pressure drop across the valve and a flow through it, so we have a KW loss across the valve. Reducing the speed to reduce the pressure removes the energy loss across the valve, but does replace it with energy loss in the VFD and the harmonic mitigation where fitted. This would be a primary selling feature of a VFD on centrifugal pumps, but if the pump is a correctly engineered flat curve pump, then the speed will be almost constant and the VFD just increases the losses.
Where a centrifugal pump is not engineered correctly and is too large for the application, then the VFD can be used to electronically adjust the size of the pump by setting a different speed.

The VFD can also be used to limit the flow when the discharge pipe is empty and folding the speed down until the pipe is full and fully pressurised is another advantage of a VFD. This can also be achieved by the use of a control valve.

In the case of a turbine pump, a VFD can be used with a fast ramp, to ramp the pump up from zero to full speed provided that the pump does not become hydraulically stalled. This will reduce the start current. Once at full speed and the line is full, then the VFD can be bypassed (requires a special VFD that can be synchronously bypassed) or the VFD can continue to run at full speed. If the pump is too large, it may be able to be slowed by a small amount to reduce the flow, but must not drop the speed to a level where the discharge pressure is insufficient to maintain flow.
In the case where the pump is pumping over a flood wall with the pipe exit well below the top of the flood wall, then syphoning may reduce the effective head pressure requirment and the drive could slow the pump by a greater amount.
I have seen an instance of a 400KW correctly engineered turbine flood pump controlled by a soft starter was replaced by a larger pump and VFD to save energy, but the VFD could not start the pump due to the pump hydraulicaly stalling and severely overloading the VFD.
Centrifugal pumps and turbine pumps behave very differently and some multi stage centrifugal exhibit characteristicss that fall between the two.

Mark Empson
Advanced Motor Control Ltd

 

[edison123]

Thanks, Mark. Never too late to learn.

Muthu

http://www.edison.co.in

 

[FacEngrPE]

I have found it irritating that pump suppliers quotes for centrifugal most often have selections that are too flat for the VFD's often supplied with the same quote being too flat for the VFD's to provide much benefit. There are too many engineers in the AEC field that do not understand (as Mark points out) the magic of VFD can not override physics, and accept these misguided selections.

 

[Marke]

FacEngrPE

There are some who believe that any motor with a VFD installed, will save energy.
I have a saying that I have used for forty odd years, You can only save some of the energy that is being wasted.
A motor that is running at say 93% efficiency, is not wasting much energy, so the energy that is wasted is in the driven load rather than the motor.
I often see it quoted that you must use a VFD for all pumps and fans, but only some will actually benefit from the installation of a VFD.

In the case of a steep curve pump with a very wide range of flow, then reducing the speed at low flow, will reduce the discharge pressure of the pump and the KW consumed is a function of both flow and pressure.
By slowing the pump down, you can eliminate the pressure rise due to the reduced flow. reducing the pressure rise, but keeping the flow constant will save energy, but the amount of energy saved is also a function of the pump efficiency.

There are many instances where the slowed pump if operating at a very low efficiency at that low speed. A different pump design will yield much greater energy savings.
Energy savings is not automatic, and the energy saved has to overcome the extra energy consumed by the VFD and harmonic mitigation.

Some times, a combination of different sized flat curve pumps with soft starters and predictive control, can yield much higher overall efficiency than a single large pump and a VFD.
Check the total efficiency of the installation.

Mark Empson
Advanced Motor Control Ltd

 

[FacEngrPE]

VFD's can be very beneficial for the case where the system differential pressure is proportional to flow rate. This is often the case with cooling tower loops, and HVAC heat transfer systems.

If the required pressure rise is not a function of flow rate as in a city water booster pump, then a steep pump curve and a good understanding of system dynamics is required to get any benefit out of a VFD. Mark covers the reasons and choices on the motor drive side. Choices with the hydraulic controls around a pump are just as important.
[ul]
[li]Do you limit starting power by closing the discharge valve?[/li]
[li]Can the pump throttle to the expected minimum flow or does some provision for low flow need to be taken?[/li]
[li]Does the application require something to keep the suction pressure above a minimum value?[/li]
[li]Is tight pressure control needed or will allowing pressure to follow the pump curve sufficient?[/li]
[li]Ask more questions depending on your use case. This is not a complete list.[/li]
[/ul]

 

[LittleInch]

I think that more than the pump curve, the issue is more with a flat system curve, especially those that have a high static pressure/head to them at 0 flow.

Then the supposed savings from VFDs really don't do what people project for them.

I think once your static head gets to be more than 30-50% of your static flow then a VFD probably isn't for you and you really need to just stick in a control valve.

The other thing everyone forgets is that the VFD units pack out a LOT of heat. Unless you do it very carefully you then also need to seriously beef up your A/C units in the switchgear room before you cook everything and everybody.

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