Design and Evaluation of a Flow Pump System for Column Testing

Patrick L. Redmond I and Charles D. Shackelfor~

Design and Evaluation of a Flow Pump System for Column Testing

REFERENCE: Redmond, E L. and Shackelford, C. D., "Design and Evaluation of a Flow Pump System for Column Testing," Geotechnical Testing Journal, GTJODJ, Vol. 17, No. 3, September 1994, pp. 269-281. 2. ABSTRACT" The design of the equipment components of a flow pump system for the measurement of solute (effluent) breakthrough curves from soil columns is described. In addition, the performance of the system in terms of the effects of equipment components and temperature on measured differential pressure fluctuations across soil specimens is illustrated. The flow pump for the system provides continuous flow of permeant liquid to the specimen, which is essential to produce the several pore volumes of flow required for measurement of solute (effluent) breakthrough curves for laboratory column testing. Differential pressure fluctuations due to effects of high back pressures, imperfections in the flow pump equipment, the sampling of effluent, and temperature fluctuations within the laboratory are illustrated. While the magnitude of these differential pressure fluctuations can be large (e.g., as much as 6.9 kPa in some cases), the influence of these fluctuations did not affect significantly the measured hydraulic conductivity of a processed kaolin specimen at a flow rate of 2.65 × 10-4 cm/s. However, an evaluation of these effects is recommended before such testing commences since the ability to accurately test soil specimens at low flow rates is one of the reported advantages of flow pump systems. An electrical conductivity cell used to measure the electrical conductance (EC) of the effluent while providing for continuous permeant flow during sampling is described. The cell provides a quick method for determining the electrical conductance and pH of the effluent, and sampling of effluent from the cell produced only minor differential pressure fluctuations across the soil specimen. Test results indicate good agreement between the EC values measured within the constructed probe and EC values measured at a later time using potentiometric methods on effluent samples recovered from bladder accumulators. However, the pH values measured in Cell EC2 were about 1.5 pH units lower than the values measured on recovered effluent samples presumably due to release of CO~ gas from the back-pressure saturated specimen after sampling.

3.

4. 5.

6.

1989; Olsen 1966, 1969, 1972, 1985; Olsen et al. 1985, 1988; Morin and Olsen 1987; Morin et al. 1989; Pane et al. 1983; and van Zyl 1984). To determine the compatibility of natural clay deposits permeated with liquid hydrocarbons (Fernandez and Quigley 1985). To measure the coefficient of consolidation and specific storage of soil (Morin and Olsen 1987; Morin et al. 1989; Olsen et al. 1988). To measure the unsaturated flow properties of soil (Znidarcic et al. 1991). To determine the effects of coupled flow processes (e.g., osmosis and electro-osmosis) on hydraulically driven flow through soil (Olsen 1965, 1969, 1972, and 1985). To control drainage during triaxial shear strength testing of soil (Olsen et al. 1988).

The reported advantages of flow pump systems are that a continuous reading of the head (pressure) loss across the specimen is made allowing for accurate and relatively rapid determination of the permeability of the soil, and extremely low and accurate flow rates can be produced resulting in induced hydraulic gradients across the specimen which generally are more typical of those found in nature (Olsen 1966). In spite of these advantages over the more traditional hydraulic systems, utilization of the flow pump has been limited, for the most part, to research. Widespread commercial use of the flow pump has not yet been evident, probably due to the relatively high initial cost of the systems, general unfamiliarity with the flow pump testing, and/or lack of standardized test equipment. However, the potential uses of the flow pump system undeniably are enormous. As evidenced above, multiple uses for flow pump systems are being developed continuously resulting in increased flexibility of flow pump systems for geotechnical testing purposes. In general, individual flow pump systems which differ in design and appearance have been developed on a case by case basis due, in part, to a lack of widespread use and general unfamiliarity with the testing apparatus. Nonetheless, the underlying principle of the flow pump system (i.e., the use of a pump and syringe to generate constant rates of flow) is inherent in all flow pump systems regardless of individual system differences. In this paper, the design and evaluation of a flow pump system used to perform column tests for the measurement of the transport properties of a NaC1 solution with processed kaolin is described (Shackelford and Redmond 1995). This flow pump system also has been used to measure the permeability of fly ash-soil mixtures (Shackelford and Glade 1994). The description of this particular

KEYWORDS: permeability, columns, hydraulic conductivity, column testing, flow pump, pH, electrical conductance, laboratory testing

Flow pump systems differ from more traditional hydraulic systems (e.g., constant-head or falling-head systems) in that the flow pump maintains a constant volumetric flow rate regardless of changes in the hydraulic conductivity (permeability) of the soil and/or changes in the induced hydraulic gradient. Flow pump systems have been used for the following purposes: 1. To measure the permeability of soil (e.g., Aiban and Znidarcic ~Geotechnical engineer, ESA Consultants, 215 Mendenhall, Suite C l, Bozeman, MT 59715. 2Associate professor, Department of Civil Engineering, Colorado State University, Fort Collins, CO 80523. © 1994 by the American Society for Testing and Materials 269

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270

GEOTECHNICALTESTING JOURNAL

flow pump system should assist in the future development of other flow pump systems.

Flow Pump System The flow pump system described in this paper is shown in Fig. 1. The essential features of the flow pump System are shown in Fig. 2. The equipment can be subdivided into five main components with reference to Fig. 2: 1. 2. 3. 4. 5.

Flow-pump equipment (A). Flexible-wall permeameter (B). Pressure regulation equipment (C). Permeant supply and storage equipment (D). Effluent monitoring equipment (E).

Aside from the flow-pump equipment (Component A), the remaining components may be considered typical for some of the other, more traditional test apparatus. However, the ability of flow pump systems to measure small fluctuations in pressure which occur during testing results in a sensitivity which generally is not common with other types of hydraulic systems. The remainder of this paper describes the design considerations for the flow pump system illustrated in Figs. 1 and 2. In addition, the effects of system components as well as temperature on measured differential pressures are evaluated.

Flow Pump Equipment The term "flow pump" as used herein refers to a system of three primary components, viz. (1) a syringe pump, (2) two stainless steel syringes, and (3) the plumbing which allows one syringe to refill while the second syringe simultaneously infuses permeants through the specimen. A schematic of the syringe pump, syringes, and associated plumbing is shown in Fig. 3. Syringe Pump

The syringe pump drives the syringes to produce the constant rate of flow. A Harvard Apparatus Model 944 infusion/withdrawal

pump was selected for this system to provide a capability of operating two syringes simultaneously to provide uninterrupted flow at a constant rate for prolonged periods. Continuous flow for a prolonged period is required for the measurement of solute (effluent) breakthrough curves used to determine transport properties, the primary purpose for the development of this particular flow pump system (see Shackelford and Redmond 1995). A manually operated switch on the unit determines whether both syringes travel in the same direction (parallel motion) or in the opposite direction (reciprocal motion). Therefore, the pump is placed into reciprocalmotion mode for column tests, allowing one syringe to infuse permeant liquid while the second syringe is refilled with permeant liquid. A two-way switch is used to reverse the direction of travel of both syringes. A gear box on the Model 944 syringe pump containing twelve gear selections provides a linear displacement rate over a 5000 to 1 gear ratio (i.e., the syringe will travel approximately 5000 times faster in Gear 1 relative to Gear 12). In addition, the range in syringe speeds can be extended to 50 000 by use of a variable speed control (e.g., from 10 to 100% of each range can be selected). The flow pump should be calibrated before each test to determine the relationship between the linear displacement rates and the resulting volumetric flow rates. The calibrated volumetric flow rates produced with the flow pump used in this study ranged from 2.65 × 10 -5 cm3/s tO 0.133 cm3/s when the variable speed control was maintained at 100%. The syringe pump from the factory is equipped with a bronze half nut riding on a lead screw which drives the syringe plunger (see Fig. 3). This arrangement was found to result in erratic flow rates at high back pressures apparently due to the half nut attempting to ride up the threads on the lead screw. The differential pressure output obtained at a back pressure of 690 kPa (100 psi) using the half nut is illustrated in Fig. 4. The erratic readings were eliminated when the half nut was replaced by a solid saddle. The resulting test apparatus has been used successfully to test the permeability of a compacted clay at a back pressure of 828 kPa (120 psi). Variations in the differential pressure across the specimen also were observed during infusion of permeant through the specimen.

FIG. l--Pictorial view of flow pump system.

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REDMOND AND SHACKELFORD ON FLOW PUMP SYSTEM

271

VACUUM SOURCE CONFINING PRESSURE CEIL 2

C V2

I

L..--

I

BACK PRESSURE B

E

vc4

BLADDER ACCUMULATORS

"rcl

A VF1

VC6 ~'~

FLEXIBLEWALL

CELL 1

I SOIL I FLOW TD

PUMP

VDI

VD2

"IF I

VB7 ~

I

t

VB6 VB5

_ _ VF6

FIG. 2--Schematic of flow pump system for column testing.

2-WAYVALVE

NOTTOSCALE

Q 3-WAYVALVE TO DEAIRED PERMEANT

TO SOIL

SPECIMEN~ LEAD SCREW SOLID SADDLE

I APPARAaVS I MODEL944 I DUALACTING I RINGEPUMP

SYRINGE PLU~GER

$~LY

~')

@

: ,= ZZ[ZZZZZ:Z: Id

I

\

SYRINGE SYRINGE CAP

BODY

FIG. 3--Profile of flow pump components.

These variations became more apparent at the lower flow rates. For example, a record of the differential pressure across a specimen while infusing at a rate of 2.65 × 10 -5 cm3/s is shown in Fig. 5. Points A and B on the trace are systematic and occur once per revolution of the syringe lead screw with the solid saddle replacement. Therefore, the systematic variations are assumed to represent

mechanical imperfections in the drive mechanism for the syringe pump. The hydraulic gradients are determined for an average differential pressure located approximately equidistant between the extreme values on the trace. Although the deviations appear to be large, the error induced by selecting the approximate mean

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272

GEOTECHNICALTESTING JOURNAL

M

m a_ w

@

11t

Q a_ D

m

m

C @

m

12

0.0

0.5

1.0

1.5

2.0

ZO

12.0

Elapsed Time (hours) FIG. 4--Differential pressure fluctuations typically resulting when a bronze half nut is used to drive the syringe plunger in conjunction with high back pressures.

differential pressure is small. For the example illustrated in Fig. 5, the computed hydraulic conductivity of the specimen was between 4.6 × 10 -8 and 5.6 X 10 -8 cm/s, with an average of 5.1 X 10 -s cm/s. The deviations were not noticeable when the flow rates exceeded approximately 2.65 x 10-4 cm3/s.

Syringes

The syringe design is shown schematically in Fig. 6. Fluid is ejected from the syringe by forcing the syringe plunger through the syringe body, thereby displacing the permeant liquid within the syringe body. The syringes for the flow pump were machined from 3.81-cm (1.5-in.) Grade 316 stainless steel for durability and resistance to chemical attack. The syringe body was bored to a 1.59-cm (0.625in.) diameter, and two openings for fluids were drilled to intercept the bore. The openings are located at the opposite ends of the bore to facilitate filling the syringe with minimal air entrapment. The syringe cap was bored to 1.27 cm (0.500 in.) and fitted with internal O-rings to seat around the plunger and an external O-ring to seal the cap to the end of the syringe body. The cap and syringe body are bolted together, and the cap is tapped to ailow mounting of the syringe to the syringe pump. The plunger was machined from Grade 316 stainless steel to a diameter of 1.267 cm (0.499 in.) and polished to a No. 16 finish.

Syringe Pump Plumbing

The plumbing for this flow pump, illustrated in Fig. 2 (Section A), allows one syringe to fill at pressures less than atmospheric, while the second syringe simultaneously infuses permeant liquid through the specimen under relatively high back pressure. This capability allows deaired permeant liquid to be cycled to the specimen without inducing differential pressure fluctuations across the specimen. All fittings for the flow pump were manufactured by Swagelok and were constructed with stainless steel to minimize corrosion. The fittings were connected with 0.318-cm OD, 0.231-cm ID (0.125-in. OD, 0.091-in. ID) stainless steel tubing to minimize corrosion, escape of air, and volume changes. The valves are stainless steel, zero displacement ball valves, manufactured by Whitey Co., equipped with Swagelok fittings and sealed with Teflon packing. Flexible-Wall Permeameter A flexible-wall permeameter is used with this flow pump system to reduce the potential for side-wail leakage, allow for back-pressure saturation of the specimen, and allow for control of effective stresses. The design of the flexible-wail permeameter, which allows for a 10.16-cm (4-in.) diameter specimen, is the same as that described by Daniel et al. (1984). All permeant lines leading from the flow pump to the base of the permeameter as well as from the permeameter to the differential

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REDMOND AND SHACKELFORD ON FLOW PUMP SYSTEM

Trace A

273

Trace B

2.0

A

a_ 4)

14 a.

"~

1.0

, m

c e o

D

0,0

0

1

2 Elapsed Time (hours)

FIG. 5--Differential pressure fluctuations resulting from imperfections on the lead screw for the syringe pump drive mechanism.

pressure transducer are made of 0.318-cm OD, 0.231-cm ID (0.125in. OD, 0.091-in. ID) stainless steel tubing as previously described. The effluent lines consisted of 0.318-cm (0.125-in.) OD polyethylene tubing to allow for ease of placement of the top cap on the soil specimen. The polyethylene tubing is nonreactive to most inorganic chemicals, and compliance is not a concern on the effluent end of the specimen since the back pressure is regulated to a constant value. The lengths of the effluent lines are made as short as possible to minimize mechanical mixing of solute within the effluent lines prior to sampling. Pressure Regulation Equipment

Electronic pressure transducers are used to determine the Bparameter during the back-pressure stage of testing and to provide a continuous measure of the fluid pressures as well as the differential pressure across the specimen. Dmck, Inc., Model PDCR 810 pressure transducers with a pressure range of 0 to 690 kPa (0 to 100 psi) were used to measure confining, in fluent, and back pressures. The locations of these pressure transducers are indicated by TB, TC, and TF in Fig. 2. All signals to and from the Druck pressure transducers were channeled through a Vishay switch box and into a Budd Company Model P-250 strain indicator for direct observation in units of pressure. A schematic representation of the pressure sensing and monitoring system is shown in Fig. 7. A Validyne Model DPI 5 differential pressure transducer .is used

to measure the pressure differential across the specimen during flow. The differential pressure is used to calculate the hydraulic conductivity of the specimen in accordance with the following form of Darcy's law k-

QL~/w AAu

(i)

where k is the hydraulic conductivity (permeability) of the specimen, Au is the differential pressure loss across the test specimen, Q is the volumetric flow rate, L is the length of the specimen, and 7~ is the unit weight of the water. At steady-state flow, volume change within the soil specimen is zero and, therefore, Q, Au, and k are constants. The transducer is used in conjunction with a Validyne Model CD223 carrier demodulator to convert the output directly into units of pressure. In addition, the differential pressure also is recorded on a Soltec 1243 strip chart recorder to provide a continuous and permanent record. A schematic of the differential pressure sensing and recording equipment is shown in Fig. 8. The location of the differential pressure transducer is indicated as TD in Fig. 2 (Section B). The pressure regulation equipment used in this study is shown in Fig. 9. In this system, a second air tank placed downstream from an initial pressure regulator is used to dampen the pressure pulses caused by the compressor. Pressure regulation is accomplished in stages. First, a two-stage compressor with a 0.23-m 3

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274

GEOTECHNICALTESTING JOURNAL STAINLESS

STEEL SYRINGE

CAP MOUNTING BOREHOLES / x .

o"

"b

@

o

DEAIRING/REFII "

IJNGPORT

. . . . . . " I x .....

1 /

............................................................

GROOW-S~ , L- . . . . . . . . . - . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

:'---F z " ' /

/

o . .

......

END VIEW

INFLOWIO~W

PORT

SIDE VIEW

STAINLESS STEEL PLUNGER

SYRINGE

CAP

CAP MOUNTING BOREHOLES

SYRINGEPLLrNGER r O BOP~a~OL~ ~

_-"t~D i ~_ I

SYRINGMOUNTIN~ E ' 0"~ ~00 l

O-RINGS

"lllf . . . . . . . 111/-"

° 1 . . ....... . . SIDE VIEW

END VIEW

FIG. 6----Syringe design for flow pump.

(60-gal) tank provides the compressed air. The compressed air in this tank is regulated to between 862 and 966 kPa (125 and 140 psi), corresponding to the pressures at which the compressor activates and shuts off, respectively. Second, the pressurized air passes through a filter and regulator (RA1) which reduces the pressure to 759 kPa (1 I0 psi), i.e., the pressure in the second tank. The purpose of the second tank is to buffer the pressure fluctuations not eliminated by RA1. Third, Pressure Regulator RA2 reduces the air pressure to 690 kPa (100 psi). Finally, Regulators RA3 and RA4 provide the required regulated pressure for the confining and back pressures, respectively. Valves VA1 and VA2 serve to isolate sections of the supply system in case of mechanical problems. Filters F2 and F3 prevent accidental transport of chemical solutions into the pressure regulators. This relatively complex system was DRUCK, INC. MODEL PDCR 8 I0 PRESSURE TRANSDUCER

V/SHAY MODEL 1012 SWITCH AND BALANCE UNIT

I

2

3

4

5

6

NOT TO SCALE

BUDD COMPANY MODF_.I.,P-350 STRAIN INDICATOR

F-% O

O

O

essentially maintenance free and provides supply pressures with long-term stability and no short-term fluctuations. Separate pressure regulation was required for the confining pressure and the back pressure on the specimen. The influent pressure on the specimen is based on the hydraulic conductivity of the soil and the induced flow rate in accordance with the following equation Ui =

where ui is the influent pore water pressure, ue is the effluent pore water pressure, and Au ( > 0) is given by Eq 1. The regulated back pressure supply lines are connected to the bladder accumulators which provide an air-permeant interface. Two bladder accumulators are required to allow uninterrupted back pressure application to the soil specimen; i.e., one accumulator is used to provide back pressure to the specimen while the effluent solution is being sampled from the other bladder accumulator. A schematic of the bladder accumulator system in this flow pump

VALIDYINE MODELDP15 DIFFERENTIAL PRESSURE TRANSDUCER

O

POWER SUPLLY

FIG. 7--Schematic of pressure sensing and monitoring equipment for confining, influent, and back pressures.

(2)

U e "~- A n

VALIDYNE MODEL223 CARRIER DEMODULATOR AND TRANSDUCER INDICATOR

SOLTEC MODEL1243 STRIPCHART RE~ORDER

NOT TO SCALE FIG. 8--Schematic of sensing and recording equipment for differential

pressure measurements.

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REDMOND AND SHACKELFORD ON FLOW PUMP SYSTEM

[~

PG4

FAIRCHIIJ3 MODEL 10 PRESSURE REGULATOR 1~ IN-LINE AIR FILTER

TO CELL 2 RA3

TWO-WAY VALVE O

275

ANALOG PRESSURE GAUGE

1~32

?

0.23 m 3

• 0.03 m3 (7 gal)

IKI1

(60 gal) Two-Stage Air

Air Tank

PG3

?

VA2

Compressor VAI

RA1

862 kPa 025 psi)

759 kPa (II0 psi)

RA2

PG5

, ~ - - ~ T O BLADDER ~ ACCUMULATORS RA4

~Minimum

Minimum FIG. 9--Pressure supply and regulation equipment for flow pump system.

system is shown in Fig. 10. The permeant back pressure is recorded by Pressure Transducer TB (see Fig. 2, Section E). The regulated confining pressures are applied to the top of Cell 2, which serves as a storage reservoir for application of confining pressures to the specimen (see Fig. 2, Section D). An outlet on the base of the cell transfers the confining fluid to Cell 1. Cell pressures are recorded with Pressure Transducer TC (see Fig. 2).

THREE,-WAY VALVE

As an example of the use of the flow pump system and Eqs 1 and 2, the hydraulic conductivity (k), pressure loss (Au), and temperatures (T) resulting from a column test performed on a processed kaolin specimen at a flow rate of 2.65 × 10 -4 cm3/s are shown in Fig. 11. The specimen was permeated first with distilled water to flush the soluble salts from the pore water of the soil before permeating with 0.01 M NaC1 solution. Since the column stage of the test begins with the permeation of the NaC1 solution, the data in Fig. 11 are plotted versus the net pore volumes of flow, PVnet, defined as follows PV,et = P V -

TO PERMEANT SUPPLY

Prow

(3)

or

EFFLUENT SAMPLING

•

rosom ~

SPECIMENL - ~

A

PERMEANT

@ VITON MEMBRANE

PERMEANT

./---< AIR

TO VACUUM SUPPLY

AIR

II

T O REGULATED_~ BACK PRESSUR

II

where PV is the total of pore volumes since the start of permeation with distilled water, and PVow is the total number of pore volumes for permeation only with distilled water (PVow = 2.75 pore volumes of flow in this case). The hydraulic resistance of the specimen during permeation with the distilled water is reflected by gradual increase in differential pressure (Au) across the specimen. A slight decrease in Au is noticeable immediately after introduction of the NaC1 solution. This reduction occurs when the flow is momentarily stopped due to switching from one syringe to the other syringe and is followed by a slight increase in differential pressure upon reestablishment of steady-state flow conditions (see Trace C, Fig. 12). The average hydraulic conductivity values of the specimen based on permeation with DW and the NaCI solution are 6.7 × 10-8 cm/s and 6.3 × 10-8 cm/s, respectively. Further details of the test conditions and results are provided by Shackelford and Redmond (1995).

Permeant Liquid Supply and Storage Equipment NOTTOSCALE FIG. lO--Bladder accumulator system for collecting permeant effluent and providing an airlperrneant interface for the transfer of regulated back pressure.

The flow pump system described herein was developed for inorganic permeant liquids. As a result, the three 11.43-cm (4.5in.)-diameter cells (Cell 3, Cell 4, and Cell 5) used to store permeant

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GEOTECHNICAL TESTING JOURNAL

Hydraulic Conductivity

I - t -

U - Pressure Loss ~

Temperature 25

T

1 0 "~

U

=

NaCI Solution

Distilled Water

+mr

L.

10 "s

e~ @

i!1 mmnu~n

{a =

m~ n~nninn~num.m

-11-41

15

I I I

IIJ

@

tll

10-9-

10

-4

-2

0

2

4

6

8

Net Pore V o l u m e s o f F l o w , P V n e t FIG. 11--Results from a test performed on process kaolin at a flow rate of 2.65 × 10 -+ cm3/s.

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