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

Centrifugal Submersible Pumps

In the past, centrifugal submersible pumps, or impeller-driven pumps, were primarily designed for use in water supply wells. Recently, manufacturers have offered several models that work well for both the purging and sampling of 2-inch diameter monitoring wells. Centrifugal submersible pumps are categorized as a positive displacement device.

Design and Materials

A centrifugal submersible pump consists of impellers or vanes that are spun or rotated by a sealed electric motor. Pumps designed for 2-inch monitoring wells are usually cooled and lubricated with water rather than with hydrocarbon-based coolants and lubricants that could contaminate groundwater samples.

These pumps can be fabricated of stainless steel, PTFE (Teflon®), Viton® and other non-sorptive materials appropriate for collecting VOCs and other sensitive parameters. Pumps are now available that can achieve variable flow rates and flow rates as low as 100 ml/minute (0.1 L/min or 0.03 gpm). Several commercially available pumps capable of achieving low-flow rates are available for 2-inch monitoring wells.

Operation

Centrifugal submersible pumps operate by spinning or rotating an impeller or series of impellers that cause water to be accelerated outward and then upward into the pump's discharge line. Figure 12 illustrates the movement of the impellers and water in this type of pump. The higher the pumping rate, the greater the potential for sample alteration by sample agitation, increased turbulence and pressure changes in the sample. Consider using a variable-speed pump when purging and sampling monitoring wells.

A centrifugal submersible pump is usually suspended in a monitoring well by its water discharge line, a support cable, or both. These pumps can be dedicated to above ground or flush mount wells (see Figure 12), thus eliminating the need to transport, set-up and decontaminate the pump. Dedicated pump systems also eliminate the need to collect quality assurance field blank samples.

Low-flow centrifugal submersible pumps appear to perform similarly to low-flow bladder pumps in preserving sample integrity during the well purging and sampling process.

Figure 12: Centrifugal submersible pump: a) functional diagram, left (Courtesy of Grundfos Pumps Corp.); b) stand pipe and flush mount systems, right (Parker et al, 1992).

Advantages of centrifugal submersible pumps

When low-flow pumping rates are used, these pumps consistently collect high quality samples.
Motor is sealed from impellers thus protecting against contaminating the samples.
Capable of variable flow rate (typ. 0.1 - 30 L/min or 0.03 - 8 gpm).
May collect low turbidity samples (< 5 NTUs) when low pumping rates used.
Moderate to high lift capability, approximately 300 feet (90 meters).
Initial high capital cost may be recovered if dedicated pumps are used.
Priming is not necessary.
Models are available that are constructed of relatively inert materials.
Allows for easy, direct in-line filtration of samples.

Limitations of centrifugal submersible pumps

Models not capable of low-flow rates are not suited for collecting gas-sensitive and VOC samples.
Requires external power source - portable systems usually require a heavy generator typically powered by gasoline (potential extraneous contaminant source).
Some variable speed models must be started at high flow rates initially.
Purging and sampling from deep wells may be slow.
Relatively time consuming to disassemble and decontaminate.
Portable but may be bulky, heavy and difficult to transport over long distances and over rugged terrain.
Portable systems may freeze up in winter during sampling and decontamination.
Transport, setup and decontamination time is high compared to bailers if the pump is not dedicated to the well.
Motor may slightly heat the samples.