How Pumps and Motors are Affected by Cycle Stop Valves
Increasing backpressure does not make pumps work harder. One horsepower is the measure of power it takes to lift 33,000 pounds of weight one foot in one minute. Gallons and weight are the same thing to a pump. Restricting the flow from the pump with a valve, backpressure will increase. As backpressure increases, gallons or weight decreases. As the weight or gallons of the water being lifted by the pump decreases, so does power consumption, amps, or horsepower. Excess backpressure is a free by-product of horsepower. Backpressure makes pumps pull less amperage not more. Less amperage, means motors run cooler, use less electricity, and last longer.
While Cycle Stop Valves will increase backpressure on pumps when needed, they will never let the backpressure increase to complete shut off head. The Cycle Stop Valve can never completely close. There is always 5 GPM flowing through the valve even when in the fully closed position. This flow is derived from the minimum cooling requirements of a pump and motor. Large submersible motors can operate with much smaller flows than (.5 feet per second). Flow charts for motors running at FULL LOAD AMPERAGE are not relevant for motors pulling an average 60% of full load. As backpressure increases until the pump is only pumping minimum flow, amperage decreases, derating the motor. Pulling only 50 to 60% of full load, the derated motor can safely pump hot water up to 140 degrees according to the charts. If a derated motor can safely pump any amount of 140 degree water, then a tiny amount of cool water (86 degrees or less) will easily prevent the motor from overheating. Minimum cooling charts for derated motors, have not been made available by the motor manufacturer. Years of experience has proven many times over that motors such as a 50 HP sub will drop from 77 amps to about 40 amps when the pump is restricted to 5 GPM flow. This 5 GPM flow of 70 degree water going past the motor will increase in temperature to 78 degrees. 78 degrees is not even close to 131 degree water that the charts say can safely cool a 50 HP motor when derated by 40%. Full speed turbines and centrifugal pumps can operate at even lower minimum flows as their motors are cooled by air. Motor and cooling fan are still spinning at full RPM, which will keep a motor that is only pulling 60% of full load amps very cool.
The load on thrust bearings will increase as the backpressure increases the K factor on the impellers. Bearings in air-cooled motors must be properly lubricated and designed strong enough to handle the maximum K factor of the pump. The fan in an air-cooled motor will still be spinning at full RPM, which helps to keep the bearings cool. At low flow the power required by the pump is reduced and the windings in the motor produce less heat. With the fan still spinning at full RPM this helps reduce the heat of the entire motor, which includes the thrust and radial bearings. With water-cooled motors such as submersibles, heat produced is also reduced because of the low amperage produced at lower flow rates. Less flow is required to cool the thrust bearings as heat from the windings at low flow is reduced, leaving more of the cooling flow available for the bearings.
Radial Deflection increases on centrifugal pumps as the flow rate decreases. The impellers on pumps with a single volute tend to push at odd angles to the cutwater vane as flow is decreased. As long as the shaft and radial bearing are strong enough to handle the load, very little if any deflection is possible. Pump companies should not manufacture pumps with shafts that are too slender and bearings too weak to handle this deflection, unless the pump is designed with planned obsolescence as a primary objective. Pumps with a diffuser ring such as submersibles, turbines, and multistage centrifugal pumps, will equally distribute the discharge from the impeller. This makes deflection to any one particular side impossible and radial deflection a non-issue.
Resting Pumps and Motors?
Pumps and motors are designed for continuous operation and do not need to "rest". This means they will last longer if they run continuously than if they "cycle" off and on. Motors that are coasting along at low amperage 24 hours a day, will use less electricity than the same motor pulling full load and cycling on and off every 10 minutes or so. Most motor and pump failures occur during start up. Starting current can be six times normal running amperage. Starting test every component of the pump and motor. Windings, bearings, shafts, impellers, splines, couplings, panels, even the generator at the power company are all tested each time a pump starts. All of these problems go away once the motor is up and running. Common sense would suggest that the fewer times it starts and stops, the longer a motor and pump will last.
Soft Start Equipment?
Cycle Stop Valves will completely eliminate water hammer with or without soft start equipment. The main reason to use soft start equipment is to reduce the end rush of electricity on pump start up. This will reduce the electric bill if a demand charge is included. Some electric companies require soft starts on larger horsepower systems. When soft start controls are needed, an auto-transformer type soft start will do an excellent job of reducing end rush currents. These auto-transformer soft starters are mechanical devices that will eliminate the problems associated with troublesome electronic soft starters and variable speed drives.
Restricting the discharge from a pump with any valve will decrease the NPSH required. The NPSH available will increase as the flow rate decreases. Increasing the NPSHA and/or decreasing the NPSHR reduces the chance of cavitation. Recirculating water from the outlet to the inlet of an impeller can occur at low flow. The 5 GPM bypass exiting the Cycle Stop Valve will keep this recirculating from heating up the pump. Cavitation like wear can occur if the pump chosen has a recirculating problem such as with a loose-fitting wear ring. Pumps that are made of materials with a high tensile strength are more resistant to wear from cavitation. When equipped with an additional pressure sustain pilot the Cycle Stop Valve can also control cavitation at high flow rates by limiting the maximum flow from the pump.