VFD

1. Energy efficiency- adds up every time you start and hence depends upon your number of starts

2. Better spped torque capacity at start - do-

3. The mechanical stress on components are much lesser due to more uniform speed pick up

4. variable speed (and hence variable flow rate)

5. possibility to have regenarative VFD too (but may not be much useful in pumping operation where the flywheel effect will be low)

VFD if you do not need to have pump run at full capacity all the time. If the work load the pump does varies with time.

Soft starter if it does run full capacity all the time as you have no need to vary the work the pump does.

VFD will do the job of a soft starter in addition to its job of varying the frequency applied to the motor. A soft starter would do only soft starting. For apump application, there is tremendous scope for energy savings with a VFD, if your flow requirement is variable.

Post #3 is the best answer really. But the key is ONLY IF YOUR FLOW REQUIREMENT IS VARIABLE. If your flow requirement is constant, a VFD will WASTE energy for you. Also just to be clear, this ONLY applies to Centrifugal pumps, not to Positive Displacement pumps. Your statement indicating VT means it will apply to you, I only mention it for the benefit of others who may read this in the future.

Forget the anonymous comment on start-stop issues, there is NO energy savings issue associated with that, it's a common misconception. Both the soft starter and the VFD will soft start the motor which is good for everything, but will not make a difference in energy related cost. Soft starting saves on long term wear and tear on the mechanical components and is a good idea from that standpoint, and both technologies will accomplish that just fine.

the ONLY difference is as stated above; IF you have a variable flow situation. If you do, and you are using a valve to reduce flow, then reducing flow by slowing the motor with a VFD will reduce the losses associated with the valve and that will save you SIGNIFICANT energy. But of your flow is fixed and does not vary in your process, then there are no savings associated with a VFD and because the VFD consumes a small amount of energy due to internal losses (2-3%), the VFD will actually cost MORE to operate than a properly sized pump motor AND PROPERLY TRIMMED IMPELLER.

Thanks, Raef! But, OP's VT Pump could mean Vertical Turbine Pump and not necessarily Variable Torque. Isn't it?

"But, OP's VT Pump could mean Vertical Turbine Pump and not necessarily Variable Torque. Isn't it?"

D'oh!

But then again, most Vertical Turbine pumps will be variable torque as well.

There is a good article here which provides a good insight and comparison.

As far the efficiency issue, I will like to mention the principle of Variable Voltage variable Frequency operation (VFD) Vs Constant Frequency variable Voltage Operation (Soft Start)