stainless steel cable. The tubes convey the gases to and from the pump; the electric cable

powers the water level indicator, and a steel cable supports the downhole assembly. Flow

rate can be controlled by adjusting the driving pressure to the pump. The piston pump

provides continuous sample withdrawal at depths that are greater than most other devices.

The pump can be constructed of materials that minimize the possibility of chemical alteration

of the sample.

The bulk of associated equipment may reduce the portability of the pump. The

valving mechanism may cause a series of pressure drops in the sample that could cause

sample degassing and pH changes. The tubing bundles may be difficult to decontaminate

between wells. The pump intake should be filtered so that particulate matter does not damage

the pump's valving. A study by Yeskis et al. (1988) indicates that gas drive piston pumps

perform similarly to bladder pumps when collecting samples for volatile organics analysis.

7.3.2.4

Gear Drive Electric Submersible Pumps

Gear drive submersible pumps are designed to be portable and easily serviceable in the

field. A gear drive pump operates using a small high efficiency electric motor that is located

within the pump housing. The electric motor rotates a set of PTFE gears from an intake

screen at the top of the pump. The water is drawn through the gears and driven to a

discharge line that transports the water to the surface. The pumps have self contained power

sources, however, external sources may be used. Flow rates cannot be controlled on

conventional gear drive submersible pumps. Wells that have high levels of suspended solids

may cause the gears to require frequent replacement.

7.3.2.5

Centrifugal Pumps

Centrifugal (also called impeller) pumps transport fluid by accelerating it radially

outward. Specifically, a motor shaft rotates an impeller that is contained within a casing.

Water that is directed into the center of the rotating impeller is picked up by the impeller

vanes, accelerated by the rotation of the impeller, and discharged by centrifugal force into the

casing. A collection chamber within the casing converts much of the kinetic energy into head

or pressure. Certain submersible centrifugal pumps are constructed for ground water

monitoring purposes. These pumps are fabricated of stainless steel and PTFE, and can be

adjusted to achieve flow rates as low as 0.1 L/min. Studies conducted by Gass et al. (1991)

concluded that low flow rate submersible centrifugal pumps can deliver "representative"

ground water samples. A study conducted by Paul and Puls (1992) comparing a low flow 

rate submersible centrifugal pump, a bladder pump, and a peristaltic pump concluded that the

low flow rate submersible centrifugal pump produced the least negative impacts when trying

to obtain representative and reproducible ground water samples at the particular site and wells

investigated. Research performed by Yeskis et al. (1988) indicates that submersible impeller

pumps perform similarly to bladder pumps when collecting samples for volatile organics

analysis.

November 1992

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