Proper Pump to Motor Alignment Techniques: Part 5 of 6
Consider and Adjust for Thermal Growth
The vast majority of pump to driver alignments are performed in the cold condition, prior to flooding the pump with water, and certainly prior to operation. Depending upon the design and application of the pump, it may or may not be necessary to account for thermal growth.
With respect to pump design, the key criterion for determining if a thermal growth adjustment is necessary is the location of the pump feet. If the pump feet are located directly below the pump, as shown in the figures below, a thermal growth adjustment may be required. For other pump designs with “centerline” mounted pump feet, a thermal growth adjustment usually is not required, unless the motor is operating at extreme temperatures.
The second criterion to consider when determining the need for a thermal growth adjustment is application. Depending upon the pumping application, the pump may be operating at a significantly higher temperature when compared to a non-operating cold state. For example, heater drain pumps in power plants typically operate at temperatures in excess of 400°F. In this example, during operation, the pump is expected to grow considerably in the vertical direction. It is therefore recommended that the cold alignment settings be adjusted to reflect the final operational position of the pump.
The O&M manual supplied with the purchase of the pump should specify what thermal growth adjustment, if any, is required for your specific pump and application. If this data is not available, the amount of thermal growth that the pump/driver will encounter during operation can be calculated utilizing the formula below.
ΔL = α × L × ΔT
Where,
ΔL = Thermal Growth
α = Linear coefficient of thermal expansion for material (0.0000059 in for cast iron)
L = Height of centerline to base of machine
ΔT = Change in temperature from ambient to operation