PUMP QUESTION AND ANSWER 2

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 counter balanced 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 startup. 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 shall be measured to an accuracy of one-quarter of one percent. The time interval from the collection period shall 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 flows are permitted during the test.

The liquid levels in a reservoir shall 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 shall also be measured to an accuracy of one-quarter of one percent.

There are innumerable other method that can also be used, but the manufacturer of each should be asked if there are any limits to viscosity for there use.

Q. Many articles are being written about life cycle cost as applied to the purchase of pumps. How much of this information is also applicable to existing pump installations?

A. The Europump and Hydraulic Institute publication, “Pump Life Cycle Costs” addresses this question in detail in Chapter 3. The following is a condensed version of that chapter:

1. Assemble a complete document inventory of the items in the pumping system. These items typically include the source and destination sumps or tanks, individual pipelines, pumps, system components, valves and items that control the rate or direction of flow. The inventory should include typical nameplate information such as the item name or identifier, manufacturer, model, size, and any other information that describes the item.

2. Determine the rates of flow required for each load in the system. Once the rates of flow for each load have been determined, they can be compared to the value needed in the system load requirement. Any load with an excessive rate of flow is an excellent candidate for energy savings.

3. Balance the system to meet the required rates of flow to each load. Systems with flows feeding a variety of paths in parallel, such as HVAC chilled water and service cooling water systems, are typically much more

difficult to analyze than single loads.

4. Minimize system losses. Identify those fittings which might be causing unusually high head losses due to friction. Globe valves are a good example since they cause high losses even when wide open. Control valves are prime targets, and if practical, they might be replaced with variable speed drives which eliminate the loss due to controls.

5. Effect changes to the pump to minimize excessive pump head. Conservative system design often results in the use of oversized pumps which waste power. This can be corrected by reducing the diameter of the impeller or replacing the pump with a smaller one.

6. Identify pumps with high maintenance costs. Once identified, take corrective action to eliminate the cause of failures or replace the pump with one more suited to the application.

Q. What is discharge recirculation and what damage does it do to pumps? Can changes to the pump be made to correct this problem?

A. When centrifugal pumps are operated at rates of flow below about fifty percent of the best efficiency flow, some of the flow from the impeller is blocked from leaving the pump casing and is forced to recirculate back into the impeller. This recirculating flow is unstable and causes erratic vortices resulting in cavitations and consequent pitting of the pressure side of the impeller vane at the discharge end.

In addition, the recirculating flow quickly alternates from one side of the impeller to the other causing an oscillating axial pressure unbalance on the impeller which imposes oscillating axial movement of the impeller and high thrust forces on the thrust bearing. Cracking or failure of the impeller shrouds near the discharge may occur as well as damage to the casing volute tongue.

One step towards correcting the problem is to add a bypass line to divert some flow back to the source. This will increase the pump flow until the pump is operating above onset of recirculation.

Another approach to dealing with the problem is to replace system control valves with variable speed drives. Variable speed controls the flow by reducing the pump speed instead of throttling the pump flow. As the pump speed is reduced, the flow related to discharge recirculation is also reduced resulting in a lower flow before the problem begins. The pump also operates at a lower energy level which helps reduce damage.

An alternate impeller may also be available for the pump. If so, the alternate

impeller should be designed for a lower rate of flow with a corresponding lower flow before the onset of recirculation.

Q. How is the performance of air operated diaphragm pumps affected by the viscosity and specific gravity of the liquid?

A. The manufacturer’s published data is normally based on testing with water. The supplied air pressure, pump flow rate and NPSHA, and the pumping system determine the discharge pressures for a given air-operated diaphragm/bellows pump.

Liquids with viscosity below 500 centipoise do not generally affect manufacturers’ published pump performance data. As viscosity increases above this value, the possibility of liquid cavitation increases, and pressure drops across pump components, particularly the suction check valve(s), raise NPSHR significantly.

Specific gravity can affect the pump suction performance. A liquid with a high specific gravity will reduce the manufacturer’s published data on suction lift capabilities.

Viscous liquids tend to impede efficient check valve operation, which can result in a reduction of flow rate. This condition is caused by delayed check seating and reverse liquid flow.

Care should be taken to determine the nature of the liquid being pumped. Non-Newtonian or shear-sensitive liquids may have pumping characteristics unrelated to those observed with Newtonian liquids of similar quiescent viscosity. Apparent viscosity for an application using a non-Newtonian liquid can be adjusted based on flow rate conditions for the application by consulting the material manufacturer’s shear rate vs. shear stress diagram for the specific material.