Which is best starter to avoid water hammer
in pipeline VFD or soft starter?

 

Hello
Water hammer is a sudden pressure change which can cause pipe damage. This can occur if you start a pump DOL into a full pipeline.
Soft starters are cheaper and smaller than VFDs and have the advantage that they can be bypassed with a contactor once the motor is running.
Soft starters have 2 main problems:
1. The motor will have low starting torque.
2. The starting characteristics may vary with varying starting load.
I think you should assume that a water pump starting into a full pipeline will be presented with a heavy starting load and a soft starter may well be useless. Typically in this situation the motor will not get away until the soft starter has ramped up to full voltage at which time the soft starter trips out on overcurrent. If the soft starter is grunty enough it will start the motor at full voltage (DOL).
You may get away with a soft starter if the pipeline is not too long or up hill or if you have a large motor on a small pump.
You can calculate the starting torque you will need from the pump curve and the pipeline data (diameter, length, head at pump). Soft starter data sheets will give you starting torque for a particular sized motor.

VFDs on the other hand are perfectly suited to this application but be generous when you select the VFD model. This is a heavy duty application. Select the next size up from the recommended one for your motor size.

2 things to note when installing VFDs or soft starters.
1. Semiconductor fuses are recommended for the 3 phase supply to the soft starter or VFD.
2. Motor cables will radiate RF noise unless screened. If you have and instrumentation or comms in the area it is best to use screened cable between the VFD/soft starter and motor.

For large motors (>100kW) running from VFDs you should install an earthing brush on the motor shaft to prevent bearing damage from high frequency earth leakage current. This does not apply to motors started with soft starters.

For large soft starters you should install a bypass contactor to avoid wasting power (about 5% is dissipated as heat from the thyristors).

Regards
Jamie Downs
[email protected]

 

I would suggest the VFD since you'll have more control over the upramp/upslope speed vs. time.

Most Soft-start packages I've worked with have a preset or very limited adjustable ramp. The VFD will also have a downslope available by setting the parameters.

 

definately VFD is the best, provided proper setting of accelaration and declaration setting.

 

All is relative to the lenght of empty pipe that
thr pump have to refill. If the time to refill
the pipe is smaller than the ramp time of the soft start, go with it because soft start is cheaper than VFD.

 

My vote is for soft start.
I use to work for A-B selling only control products as well as I have built over a thousand pumping applications, from hydraulics to wast water. Though the VFD can satify your need to reduce hammer and soft start will do the same and for a lot less in cost. Some manufactures even have a pump start option with their unit.

Dan Shoemaker

 

Hi:

I find very interesting your comment, do you know where to find more technical information in relation with the starting torque with a soft starter?

Regards

Humberto Gonzalez