7.3.2.1

Bladder Pumps

Bladder pumps (also referred to as gas squeeze pumps) are submersible mechanisms

consisting of a flexible membrane (bladder) enclosed in a rigid (usually stainless steel)

housing. The internal bladder can be compressed and expanded under the influence of gas

(air or nitrogen). A strainer or screen attaches below the bladder to filter any material that

could clog check valves located above and below the bladder. Water enters the bladder

through the lower check valve; compressed gas is injected into the cavity between the housing

and bladder. The sample is transported through the upper check valve and into the discharge

line through compression of the bladder. The upper check valve prevents water from

reentering the bladder. The process is repeated to cycle the water to the surface. Bladder

volumes (e.g., volume per cycle) and sampler geometry can be modified to increase the

sampling abilities of the pump. Automated control systems are available to control gas flow

rates and pressurization cycles.

Bladder pumps prevent contact between the gas and water sample and can be

fabricated entirely of fluorocarbon resin and stainless steel. Pohlmann and Hess (1988)

determined that bladder pumps can be suitable for collecting ground water samples for almost

any given organic or inorganic constituent. Disadvantages of bladder pumps include the large

gas volumes required (especially at depth), and potential bladder rupture. Bladder pumps are

generally recognized as the best overall sampling device for both inorganic and organic

constituents (Barcelona et al., 1985b; Barcelona, 1988b; USEPA 1991a).

7.3.2.2

Helical Rotor Electric Submersible Pumps

The helical rotor electric pump is a submersible pump consisting of a sealed electric

motor that powers a helical rotor. The ground water sample is forced up a discharge line by

an electrically driven rotor stator assembly by centrifugal action. Pumping rates vary

depending upon the depth of the pump. Considerable sample agitation of water in the well

may result from operating the pump at high rates, and this may cause alteration of the sample

chemistry. In addition, high pumping rates can introduce sediments from the formation into

the well that are immobile under ambient ground water flow conditions, resulting in the

collection of unrepresentative samples (Nielsen and Yeates, 1985). Tai et al. (1991) and

Yeskis et al. (1988) indicate that helical rotor submersible pumps perform similarly to bladder

pumps when collecting samples for volatile organics analysis.

7.3.2.3

Gas Drive Piston Pumps

A piston pump uses compressed air to force a piston to raise a sample to the surface.

A typical design consists of a stainless steel chamber between two pistons. The alternating

chamber pressurization activates the piston, which allows water entry during the suction

stroke of the piston, and forces the sample to the surface during the pressure stroke. The

pump is connected to a tubing bundle which contains three tubes, an electric cord, and a

November 1992

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