Information from www.dnr.state.wi.us/org/water/dwg/gw/dsk-3a.htm

Bladder Pumps (gas-operated squeeze or diaphragm pumps)

Bladder pumps are categorized as a positive displacement device. Bladder pumps are commonly used for purging and sampling monitoring wells for a wide variety of parameters, including VOCs and trace metals. They are typically considered among the best devices for collecting samples of VOCs, trace metals and other substances and parameters (Tai et al., 1991; Barcelona et al., 1984; Unwin and Maltby, 1988, Imbriogiotta et al., 1988; and Houghton and Berger, 1984). However, Yeskis et al., (1988) found that the bladder pump was one of the most difficult devices to decontaminate in the field.

Design and Materials

A bladder pump consists of a flexible, squeezable bladder encased in a rigid outer casing. Bladder pumps are designed so that the gas that squeezes the outside of the bladder does not come into contact with the samples. Figure 14 shows the design of a common bladder pump.

Figure 14: Bladder pump: a) cut-away diagram, upper left (Pohlmann et al., 1990); b) Isomega bladder pump, right (Courtesy of TIMCOTM); and c) functional diagram, bottom.

Bladder pumps are commonly constructed of a stainless steel body and a flexible bladder. Bladder pumps are available for 2-inch diameter monitoring wells. As with centrifugal submersible pumps, bladder pumps can be dedicated to above ground wells or flush mount wells (see Figure 12), thus eliminating the need to transport, set-up and decontaminate the pump and collect quality assurance field blank samples.

Operation

A bladder pump operates much like a plastic squeeze bottle that has a liquid in it. Figure 14 illustrates the fill and discharge cycles of a common bladder pump. After you lower the device into the well's water column, water enters the bottom of the bladder under hydrostatic pressure through a check valve at the bottom of the pump. When the bladder is full, a check valve seals its bottom. A controller box at the well surface injects gas into the space between the pump casing and the outside bladder wall, thus squeezing the bladder. This squeezing causes the water to rise out of the bladder and up the sample tubing. When the bladder is nearly empty, the controller box releases the gas pressure and the bladder fills up again with water. A check valve at the top of the pump ensures that the water in the sample tubing does not re-enter the bladder. In some models, the water and air chambers are reversed.

A pneumatic controller box at the surface controls the gas injection and pressure release cycles that drive the pump. The controller box adjusts the purging and sampling flow rates by adjusting the injection and exhaustion cycles of gas in and out of the space between the outer casing and the bladder. The pump's lift capabilities are directly related to the pressure rating of the bladder and tubing and the ability of the pressure source (e.g., air compressor or compressed gas) and controller box to apply a sufficient force of gas at depth.

Advantages of bladder pumps

When low-flow pumping rates are used, these pumps consistently collect high quality samples.
Sample does not contact compression gas or mechanical parts of pump.
Flexible bladder may be constructed of relatively inert materials.
Capable of variable flow rate and low-flow rates.
Capable of collecting very low turbidity samples (< 5 NTUs).
Allows for easy, direct in-line filtration of samples.
Very high lift capacity (1000 feet or 305 meters for some models).
Initial high capital cost may be recovered if dedicated pumps are used.
Pump is not damaged if run dry.
Easily repaired in the field and very reliable.
Lends itself to permanent dedication to a well.

Limitations of bladder pumps

Portable but may be bulky, heavy and difficult to transport long distances or over rugged terrain.
Requires compressed gas and controller box.
Purging and sampling from deep wells may be slow.
Depending on design, may be time consuming to disassemble and decontaminate.
Bladder may rupture when used in deep wells.
Portable systems may freeze up in winter during sampling and decontamination.
Transport, set-up and tear-down time is high compared to bailers if the pump is not dedicated to the well.