Mechanical Bladder Pump
What is MBP and how does it work? What kind of flow rate can I get if I use one? Do I need a compressor and pump controller to operate it? Will it work for low-flow sampling? And most importantly will the samples be any good (representative) once collected ??? Let’s go over the answers ... one at a time.
So
most, if not all of you, are familiar with conventional pneumatic bladder
pumps. You know ... a generator to run the compressor, a pump controller
to regulate the air flow from the compressor to operate the pump downhole
... lines, cables, tubing, hoses, noises, leaks ... ugh! All this so the
compressed air can squeeze a small Teflon® bladder to push water slowly
up a small tube to the surface. Just how many 
horsepower
are used to pump that water to the surface so we can collect two 40 mL
vials for VOC analysis? Amazing inefficiency, isn’t it? While the
mechanical bladder pump still uses a bladder, it operates a little differently
... but more efficiently.
What
is the Mechanical Bladder Pump and How Does It Work?
To answer part of this question just remember you don’t need a generator,
a compressor, or a pneumatic pump controller to operate it. The mechanical
bladder pump (MBP) is very simple. It has about a dozen parts and uses
a corrugated Teflon® bladder. A set of concentric tubes extends from
the top of the pump to the surface. The outer tube (HDPE) threads directly
into the body of the pump and the inner tube (usually FEP Teflon®)
threads onto an adapter that connects to the corrugated bladder. At the
surface, the outer tube is held in place while the inner tube is alternately
raised and lowered about four inches (100 mm). This alternately raises
(expands) and lowers (compresses) the corrugated bladder. There are check
balls above and below the bladder which regulate fluid movement up the
inner tube to the surface. A spring above the bladder helps to compress
it and pull the inner tube back down during the sample stroke (as the
engineers say “it’s hard to push a string”).
Now
you ask, “Do I actually hold the outer and inner tubes with my hands
to operate the pump?”. You can do it that way. It’s tedious
but it definitely works. Another option is a simple set of hand grips
(Manual Actuator Kit, MB7000) that fits on the inner and outer tubes to
make holding them easier. The manual actuator makes it easy on your hands
and also on your bank account! If you’re going to be pumping for
more than 10 or 15 minutes per sample, you may want to consider the Mechanical
Actuator (MB6000). It can be mounted on your probe rods or conventional
PVC casing up to nominal two-inch (50 mm) diameter. The actuator has a
poly compression fitting that holds the outer tube in place and another
that holds the inner tube to the slider block and arm. The crank arm on
the MB6000 can be used to operate the pump using a circular stroke. The
pump is easy and comfortable to operate with the circular stroke and can
provide flow rates of up to several hunderd milliliters per 
minute.
The mechanical actuator can alsobe operated with a vertical stroke.
How
About Flow Rates?
Next question ... what about flow rates? No, you will not get five gallons
of water per minute from this little pump (remember, it’s less than
0.5 inch [only 12 mm]) in diameter. But when compared to other same-sized
bladder pumps, you can get good flow rates.
Earlier
this spring I installed a closed stand pipe to a depth of 70 feet (21.3
m) so we could conduct some flow tests with the MB470. A series of tests
were conducted as the pump was lowered on 10-foot (3 m) increments and
the water level was maintained 5 feet (1.5 m) above the pump intake during
each test in the stand pipe. A peristaltic pump was used to add water
during the flow tests to maintain the correct water level. Using the mechanical
actuator with circular strokes, the maximum flow rate achieved was almost
850 mL/min with the pump intake at 10 feet, water level at 5 feet, and
pump speed of 150 cycles per minute (shown in the graph on the right).
When a vertical stroke was used to actuate the pump under the same conditions,
the maximum flow rate approached 950 mL/min. The flow rates that can be
obtained are very useful for the low-flow minimal drawdown sampling protocol
recommended
by the U.S. EPA and many state regulatory agencies for water quality sampling.
Even with the pump intake at 70 feet and water level at 65 feet, you can
get a flow rate around 100 mL/min with a circular stroke at 100 cycles
per minute ... not bad for low-flow protocol in small wells.
Last,
But Definitely Not Least, What About Sample Quality?
Obviously,
for environmental investigations of groundwater contamination, getting
representative samples is an absolute must. So we went directly to the
U.S. EPA for help in evaluating the sample quality obtained from the Mechanical
Bladder Pump. The EPA’s Environmental Technology
Verification (ETV) Program was planning a verification study of small
diameter groundwater sampling devices that could be used in small diameter
wells. Geoprobe® Systems participated in the ETV project to verify
that the MBP would provide representative water quality samples. There
were two phases to the verification testing (see
related story). The first phase of testing was conducted at the USGS
Hydrologic Instrumentation Facility (HIF) in southwestern Mississippi,
and the second phase was completed at Tyndall Air Force Base in Florida.
To keep the story brief, I’ll focus on the VOC results from the HIF
standpipe tests.
The HIF includes a 100-foot tall, 5.0-inch diameter stainless steel standpipe inside the former rocket hangar (talk about really BIG garage doors!!). Two stainless steel mixing tanks at the top level were used to prepare spike solutions of either volatile organic compounds (VOCs) or several cations. The standpipe is equipped with sampling ports at various depths that are accessible from one of the six floors in the building. The pumps were installed sequentially at 17 ft, 35 ft, and 76 ft below the top of casing, corresponding to three of the sampling ports on the standpipe. Two liters of water were purged through the pump at each level prior to sampling. Then a sample was collected simultaneously from the pump sample tube and from the port on the standpipe at the corresponding depth.
Results
for VOC samples from the MB470 and corresponding port samples are plotted
on the graph shown to the left. The results between the MB470 correlate
well with the results from the port samples and lie close to the 1:1 line
plotted on the graph. Some of the basic statistics for the paired data
sets are shown on the graph. The correlation coefficient (r2) is 0.999
for the data set, and the regression line has a slope just greater than
1.0 and an intercept value of only 0.011. These results indicate that
the mechanical bladder pump provides samples representative of water collected
from a well.
Another graph (right) shows how the MB470 compared to the small, 0.5-inch Geoprobe® Pneumatic Bladder Pump also included in this ETV verification program. Again, both the correlation coefficient and slope of the regression line are very close to 1.0 and the intercept is 0.00, nearly a perfect match.
So,
what does all this mean? It means that the mechanical bladder pump will
provide representative samples for the tested VOCs, essentially equivalent
to those obtained with a pneumatic bladder pump, when the low-flow sampling
protocol is followed.
Additional details of the ETV results for the verification study are available either by obtaining a copy of the formal EPA reports online at www.epa.gov (request document EPA/600/R-03/086 for the Mechanical Bladder Pump report or document EPA/600/R-03/085 for the Pneumatic Bladder Pump report). Geoprobe® Systems can provide you with a copy of the ETV Programs Verification Statement for either of the bladder pumps, and also has additional information on the use and specifications of both pumps.
(This article was featured in the Fall 2003 edition of the Probing Times. Request the printed edition of this publication with our online Literature Request Form.)