The 'big push' for submersible pumps

A "Technically Speaking" feature by John Hartmann.

The concept of placing a fuel pump in the storage tank (instead of at the point of use) arose to fill a critical need during World War II. Gene Mittemier, engineering veteran, describes the situation. German fighter planes outmaneuvered their British oppo-nents by climbing higher and at steeped angles, and flying inverted for longer periods of time, without loss of power. How the Germans accomplished this was of greatest concern – the control of the skies over Great Britain hung in the balance. British engineers eventually salvaged enough of a downed German fighter to determine that the Germans had placed their fighter plane’s fuel pumps in-side their fuel tanks; the fuel was “pushed” to the engine, rather than drawn by vacuum pump. Loss of fuel during extreme maneuvers caused the British fighters to lose power due to fuel vapor lock and the limitations of suction pumps. The British adopted “submersible pumps,” and the results are history.

Pushing the product
This insight into the advantages of pushing, rather than pulling, motor fuel was used to good advantage by petroleum marketers following the war, as well. Use of higher octane fuels, larger service stations, and more dispensers caused many of the same problems expe-rienced by the British airplanes, including diminished flow rates and vapor lock.

The solution was to place the pump in the under-ground tanks; and, by the mid-1950s, petroleum mar-keters in the U.S. began utilizing submersible pumps in all but the smallest volume locations. They real-ized the advantage of having one pump per product, rather than one in each dispenser. In addition to lower costs, pushing the product eliminated vapor lock on hot days and provided more predictable delivery rates at the nozzles – despite the greater distances between tanks and dispensers, larger diameter tanks and deeper burial depths.

#1. Product intake #2. Centrifugal pump impellers #3. Motor #4. Column pipe #5. Discharge #6. Pump housing

UL tests it out
Some authorities doubted the wisdom of having 230-volt electric motors submerged in gasoline; however, testing and third-party certification of the safety of the systems by Underwriters’ Laboratories put them at ease. (UL 79, Power Operated Pumps for Petroleum Dispensing Systems; and UL 674, Motors and Generators for Use in Hazardous Locations). The safety records for submersible pumps have been very good. Technology evolved to incorporate tank siphoning, line pressure leak detection, and compatibility with ever changing fuel chemistry.

Submerging the pump motor and centrifugal pump impellers has not only proven to be safe, but also efficient. Pumps can be sized to improve delivery rates; piping and tank connections are minimized; vapor locking is eliminated; and the pump motors are cooled by the product.

What goes where
Components of the lower section of the typical submersible pump include the product intake, centrifugal impellers and defusers, and the motor. This replaceable pumping assembly is connected to the tank through the pump housing and column pipe. The pump housing serves as the product outlet, and provides a means of disconnecting the pump motor wiring and removing the lower section. The casting also houses an air eliminator, check valve with expansion relief, siphon valve and, in some cases, a line pressure leak detector. Service on all components can be performed without removal of the housing and riser from the tank.

How it works
The check valve holds pressure in the piping when the pump is not running. An expansion relief valve allows small amounts of fuel to bypass the check valve to prevent the buildup of unacceptably high pressure in the product piping. This buildup usually results from thermal expansion. In some installations, a siphon valve establishes a vacuum in the tank manifold piping to start the flow of product between tanks. The siphon valve creates the vacuum by diverting a small amount of pumped fuel back into the tank. Impellers, valves, seats, etc. must be compatible with the products pumped, including motor fuels containing high proportions of ethanol and methanol, and additives such as MTBE, ETBE or TAME.

In summary...
Submersible pumps are the heart of most pumping systems. Leak detection, variable speed and higher pressure pumps and monitoring systems will be covered in future issues.

#7. Electrical disconnect #8. Air eliminator, syphon nozzle, check valve and venturi #9. Check valve with expansion relief #10. Manifold assembly leakage return to tank

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