Q. We need to increase flow through our system and plan on adding a second pump in parallel to the existing pump. Are there any pitfalls in doing this and how can they be avoided?
A. Operating rotodynamic pumps in parallel is quite common and usually works well. The most common concern is that both pumps should have the same total head at shut off or zero flow. Next, the total head required for the higher rate of flow will be higher and must be within the capability of both pumps.
It is suggested that you construct a head versus rate of flow curve for the two pump operation as shown in the figure below.
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Pumps in Parallel
The system head curve must be determined and drawn on the graph. The head curve for the original pump is shown. The head curve for the new pump is then added. The head curve for two pumps in parallel must be determined by adding the rates of flow at each head and adding it to the graph. If both pumps have the same curve, the rate of flow contribution of either pump 1 or 2 when operating in parallel is determined by observing the rate of flow which either pump develops at the total head C. This rate of flow is illustrated at D. Note that the rate of flow of each pump is less than that of a single pump operating in the system. Verify for each pump that the NPSHA is adequate for single pump operation and that the driver for the original pump is capable of delivering the required horsepower at the new maximum rate of flow condition.
The rate of flow of the second pump can be either greater or less than the first, but the shut off head must be the same. For reliable pump operation and optimum energy savings, both pumps must be operated near or at their Best Efficiency Point (BEP).
Q. The shaft of our vertical turbine well pump sometimes rises up and disconnects from the top of the driver. What causes this and how can it be corrected?
A. When the pump is operating, the water entering each impeller is turned from a vertical to a horizontal radial direction. This change in direction imposes a momentum force on each impeller in the upward direction. Normally this momentum force is counterbalanced by the weight of the rotor and a downward force from the discharge pressure acting on the back (top) of each impeller. However, when the pump is started, the discharge pressure is typically zero and the rate of flow is at its maximum, resulting in a net upward force. The result is a momentary movement upward which disconnects the driver shaft at the drive pins in the coupling.
Most vertical well pump motors are built with single acting tapered roller bearings, which are designed to carry the heavy down thrust of VTP rotors. Such bearings have no thrust carrying ability in the upward direction since it is usually not necessary. The disconnect drive pins are provided to protect the pump against operation in this mode, which could cause damage to the pump or motor. This problem occurs more frequently in high flow, low head pumps.
Several actions can be taken to overcome this problem. One is to add a foot valve at the bottom of the pump inlet. This keeps the column pipe full and maintains sufficient discharge pressure on the pump at start-up. Another is to start the pump more slowly. However, this will require a change in the motor starter. Finally, a thrust collar can be added to the pump to carry the momentary up thrust. This will probably require some design help from pump manufacturer.
Q. What is the best method for the measurement of rate of flow of viscous liquids?
A. The Hydraulic Institute standard ANSI/HI 3.6 Rotary Pump Test recommends measurement by weight. This is done by measuring the change in weight of a tank during a measured period of time using a liquid of known and consistent specific gravity. The tank can be located on the inlet or discharge side of the pump, and all flow in or out of the tank must pass through the pump.
Measurement of rate of flow by weight depends upon the accuracy of the scales used and the accuracy of the measurement of time. Scale weight readings should be measured to an accuracy of one-quarter of one percent. The time interval from the collection period should also be measured to an accuracy of one-quarter of one percent.
Another method is measurement by volume. This is done by measuring the change in liquid level in a tank or reservoir of known volume during a measured period of time. The tank or reservoir can be located on the inlet or discharge side of the pump, and all flow in or out of the tank or reservoir must pass through the pump. No other flows into or out are permitted during the test.
The liquid levels in a reservoir should be measured by means such as hook gauges, floats and vertical or inclined gauge glasses. Consideration should be given to the geometric regularity (flatness, parallelism, roundness, etc.) of the reservoir surfaces and its stability under changing hydraulic loads or dimensional changes due to thermal effects.
Positive Displacement Meters provide direct reading volume measurement.
The time interval for the collection period should also be measured to an accuracy of one-quarter of one percent.
There are innumerable other methods that can also be used, but the manufacturer of each should be asked if there are any limits to viscosity for their use.
Pumps & Systems, November 2007