Soft starters meet changing cooling needs

Maintenance costs of over GBP 25,000 per year were being incurred due to the absence of any control when starting-up the cooling fans.

Damhead Creek is an 800MW combined cycle gas-fired power station with two gas turbines and one steam turbine and it's owned by Scottish Power.

A bank of cooling fans regulates the temperature of the main power station cooling water circuit and the fans are switched in one at a time, as required, usually as a result of changes in the ambient air temperature.

In cold weather fewer than half the fans are required and in hot summer weather all will generally be run.

The shift from night to daytime temperatures also varies the demand for cooling air.

With a total of 20 of these 30kW belt-driven fans, Plant Engineer Ian Carter was suffering costly and time consuming maintenance problems caused by the torque and current surges when starting the fan motors direct-on-line.

Vee-belts were experiencing excessive wear, would jump along the pulleys and have to be repositioned and the motor contactors suffered shortened life due to the high starting currents.

This problem was exacerbated by off-line fans "windmilling", which further increased the starting torque and motor current.

All-in-all, maintenance costs of over GBP 25,000 per year were being incurred due to the absence of any control when starting-up the cooling fans.

Ian Carter said, "We desperately needed to exercise some form of control over the starting of these fans but didn't need a variable speed system as the fans simply run up to full speed so soft-starters seemed the ideal answer".

"Softstart recommended their compact RVS series of digital starters with integrated bypass control, installed in IP65 stainless steel enclosure that we could mount outdoors right next to the fan unit drive motors".

"Installation was pretty much a three-wire in/out exercise so the retrofit costs were pretty modest".

The units interfaced easily with the existing distributed control system and each soft starter provides an 'up-to speed' signal that confirms that its motor is up and running.

If this signal isn't received within 20 seconds, an alarm is flagged up.

Once the motor is up to speed, the soft starter automatically bypasses the motor to direct-on-line and switches itself out of circuit.

This not only saves energy but lengthens the life of the soft-start units.

"Softstart UK gave us great service from start to finish" said Mr Carter " Despite the fact we needed IP65 stainless steel enclosures, delivery was very prompt".

"I had been concerned about the windmilling fans but Softstart reassured us and indeed the RVS units catch the contra-rotating motors very well".

"The bypass feature gives us additional efficiency and with all 20 units installed and fully functional, the maintenance costs associated with these fans have been slashed".

"Overall, we estimate our total investment was recovered in under 12 months".

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