Bureau of Mines Report of Investigations/l986 

Tri boelectric Effects on Polyethylene 

Methane Drainage Pipelines 

By A. A. Campoli, J. Cervik, and R. L. King 

UNITED STATES DEPARTMENT OF THE INTERIOR 
UNITED STATES DEPARTMENT OF THE INTERIOR 

Donald Paul Hodel, Secretary 

BUREAU OF MINES 

Robert C. Horton, Director 
Library of Congress Cataloging in Publication Data: 

j 

Campoli, A, A, (Alan A,) 

Triboelectric effects on polyethylene methane drainage pipelines. 

(Report of investigations ; 9017) 

Bibliography: p. 13- 14. 

Supt. of Docs. no.: 1 28.21: 9017. 

1. Coalbed methane drainage. 2. Gas-Pipe lines. 3. Polyethylene. 4. Mine dusts. 5. Electric charge. 1. Lervik, Joseph. 11. 

King, Roger I*. 111. Title. IV. Series: Keport of investigations 

(United States. Bureau of Mines) ; 9017. 

TN23,U43 [TN844,5] 622s [622'.334] 86-600003 

h 
CONTENTS 

Acknowledgments ................................................................ Static electricity and gas-piping system ...................................... Laboratory equipment ........................................................... Electrostatic test procedures .................................................. ~aborator~ results ............................................................. ~ield tests .................................................................... summary and conclusions ........................................................ 

ILLUSTRATIONS 

Pitot tube being used to measure gas-stream velocity ...................... Relative humidity and temperature being measured with psychrometers ....... Humidity chamber .......................................................... Rock-dust chamber ......................................................... Multirange electrostatic voltmeter ........................................ General configuration of probe ............................................ Averaging data for three electrostatic tests .............................. Model of capacitance of measurement system ................................ Comparison of pipe data ................................................... Effect of velocity variation .............................................. Effect of exposure-time variation ......................................... Effect of dust-size variation ............................................. Comparison of Pittsburgh and Coalburg Coalbeds ............................ Configuration for elbow test .............................................. Results of elbow test ..................................................... Effect of humidity variations ............................................. Results of grounding evaluation ........................................... Results of rock-dusting test .............................................. Real-time aerosol monitor ................................................. Single-range electrostatic voltmeter ...................................... 

TABLES 

Effect of wall thickness on reduction of internal charge using wet-rag 

technique ................................................................ Coal analysis ............................................................. Particle-size distribution of Pittsburgh Coalbed samples .................. Rock dust analysis ........................................................ Dust contamination data for methane drainage pipelines .................... Results of real-time evaluation of methane drainage pipelines ............. 
UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 

centimeter 

degree Celsius 

angular degree 

degree Fahrenheit 

foot 

foot per minute 

cubic foot per day 

cubic foot per minute 

gram 

gram per minute 

horsepower 

inch 

kilogram 

kilo\.olt 

kilopascal 

kilowatt 

lb pound weight 

lbf/in2 pound force per square inch 

m meter 

Pm micrometer 

mg/m3 milligram per cubic meter 

min minute 

mL milliliter 

mm 

MMF t 

PlFlft3/d 

m3 

m3/s 

PCt 

rad 

v 

millimeter 

mill-ion cubic feet 

million cubic feet per day 

cubic meter 

cubic meter per second 

percent 

radian 

volt 
I TRIBOELECTRIC EFFECTS ON POLYETHYLENE METHANE DRAINAGE PIPELINES I 

By A. A. Campoli,' J. ~ervik,~ and R. L. in^^ 

ABSTRACT 

The Bureau of Mines performed laboratory experiments to evaluate the 

triboelectric effect of particulate-laden gas streams on polyethylene 

pipe at various velocity, humidity, dust-size, and dust-load conditions. 

Charges in excess of 10 kV were produced on the outside and inside 

surfaces of laboratory pipe samples, when exposed to 40 pct relative humidity (RH) gas stream containing coal dust. However, no charges were 

produced in laboratory tests that duplicated the over 75 pct RH and 

dust-free gas streams found in working underground pipelines. Bare copper wire, which is wound around the polyethylene pipe in a 1-ft (31-cm) 

spacing and grounded, eliminated the charge on the outside surface of 

the pipe, but not on the inside of the pipe surface, 

 inin in^ engineer. 

2~upervisory geophysicist. 

3~esearch supervisor. 

I Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. 
INTRODUCTION 

The Bureau of Mines estimates that 

as much as 766 trillion cubic feet 

( 7.12~ 10 ' Om3) of coalbed gas, predominantly methane, is contained in the coalbeds of the United States (1).4 This gas 

is both a natural resource and a hindrance to mining. Methane drainage systems have been employed in most major 

coal regions of the United States to remove gas from the coalbed prior to mining 

and/or capture the coalbed gas for fuel. 

Methane drainage systems employ horizontal, vertical, and cross-measure boreholes. Vertical borehole systems (2) 

consist of holes drilled from the surface 

into the gas-bearing coal strata. The 

boreholes are cased with metal pipe, 

which provides the conduit for the flow 

of gas from the gas-bearing strata to 

the surface. Horizontal (3) and crossmeasure (4) systeos require the drilling 

of borehoies into the coalbed or associated strata from underground locations. 

The boreholes are cnnnected to an underground pipeline that carries the gas to 

the surface. 

Methane, which is piped through mine 

passageways in quantities sufficient to 

contaminate large areas of the mine, justifies caution and concern in the design, 

installation, protection, and maintenance 

of the underground pipeline. Both the 

Bureau (5) and the Mine Safety and Health 

~dministration (MSHA) , U. S . Department of 

Labor (6), outlined procedures for the 

safe operation of underground methane 

drainage pipelines. These reports should 

be consulted for a general overview of 

pipeline protection requirements. 

The Bureau study (5) - provided the basis 

for the pipeline material selection. 

Polyethylene pipe best meets the design 

criterion because of its superior resistance to corrosion and impact forces. It 

also is attractive to mine operators because it is lightweight, easy to install, 

and significantly cheaper than metal 

piping. 

In an attempt to negate any hazard 

produced by the triboelectrification of 

polyethylene pipe, MSHA recommends that a 

grounded bare copper wire be spirally 

wound around all polyethylene piping. 

The recommendation was incorporated in 

many of the methane drainage systems now 

in place across the United States. The 

objectives of this study were to obtain a 

better understanding of the triboelectrification process as it relates to the underground polyethylene methane drainage 

pipeline and to evaluate the recommended 

grounding safety procedure. 

ACKNOWLEDGMENTS 

The authors wish to thank Donald Duvall 

of Plexco Pipe, John Merideth of Phillips 

Drisco Pipe, and Joseph Uhler of the 

Equitable Gas Co. for the pipe samples 

they provided for laboratory testing. 

Jack Workman of Workman Developments 

and Robert Reif, formerly of the Battelle 

Memorial Institute, donated much appreciated technical assistance on polyethylene 

pipe manufacture and static charge testing, respectively. 

The efforts of John Stevenson of Jim 

Walter Resources Inc., Dave Fitzpatrick 

of Beckley Mining Co., and Dan Weaver of 

the BethEnergy Mines Inc. were instrument a l i n the performance of methane drainage pipeline evaluations in the field. 

STATIC ELECTRICITY AND GAS-PIPING SYSTEMS 

To the average person the words, "stat- walking across a carpeted floor, noise in 

ic electricity," mean the shock experi- a radio receiver, or the tendency for 

enced when touching a doorknob after clothes to cling or stick tightly. Near- - 

ly everyone recognizes that these phenom4~nderlined numbers in parentheses re- ena are more intense when weather is dry. 

fer to items in the list of references at In most situations, static electricity is 

the end of this report. simply an annoyance. 
The term "electricity" is derived from 

the ancient Greek word "electron" meaning 

"amber," because it was with this substance that the phenomenon of electrification was first observed. For centuries, the term meant only what is now 

called static electricity, the property 

exhibited by some substances after being 

rubbed with material like silk or wool 

of being able to attract or repel lightweight objects. Mechanization produced 

stronger electrification that was first 

observed about 300 years ago (7), and in 

comparatively recent times, tGe properties of "flowing" electricity were discovered; the word "static" then came into 

use as a means of distinguishing the two 

forms. The implication that static electricity is always at rest is erroneous, 

because such electricity causes the most 

concern when it ceases to rest. 

For the sake of simplicity, one may 

imagine electricity to be a weightless 

and indestructible fluid that can move 

freely through some substances, such as 

metals, that are called conductors, but 

can flow with difficulty or not at all 

through or over the surface of a class of 

substances called nonconductors or insulators. This latter group includes 

gases, glass, rubber, amber, resin, sulfur, paraffin, most dry petroleum oils, 

and many plastic materials such as polyethylene pipe. When electricity is present on the surface of a nonconductive 

body, where it is prevented from escaping, it is called static electricity. 

Electricity on a conducting body that is 

in contact only with insulators is also 

prevented from escaping and is therefore 

nonmobile or static. In either case, the 

body on which this electricity is evident 

is said to be charged (7). 

Static charges on polyethylene gas distribution pipelines have long been a concern to natural gas utilities and pipe 

manufacturers (8-9). Of primary concern -- is the "bell hole" fire caused by static 

electricity. A "bell hole" is dug around 

a damaged section of a buried gas transmission pipe to provide access for repair. Sparks from the pipe can ignite 

explosive mixtures of the narural gas and 

air when the worker enters the "bell 

hole" with a tool to squeeze off the 

plastic pipe. 

In the preceding example, charges were 

generated by the extremely high velocity 

gas flow through a rupture. Charges also 

occur at lower velocities by the addition 

of a particulate matter (9). Whether 

generated by rupture leaks or dust-laden 

gas flows, the static charges are the result of triboelectrification. Gas molecules and particulates vigorously rub the 

surface of the pipe causing a transfer of 

electrons to the nonconductive pipe. 

Conductive pipe used in the same application would not change the charging process; however, it would provide a path 

for the triboelectricity to ground. 

The "wet-rag" technique is accepted as 

the basic safety procedure for removal 

of static electricity prior to repair of 

polyethylene gas pipelines. Wet rags are 

wrapped around the pipe to increase the 

surface conductivity. This increase in 

conductivity caused by the addition of 

moisture does not eliminate the chargegeneration mechanism; it only provides a 

path to ground for the static elecricity. 

The technique is quite effective in removing the charge from the outside surface (9). However, its effectiveness in 

removiZg charges from the inside surface 

is adversely effected by pipe wall thickness. Table 1 shows that as wall thickness increases, the wet-rag technique becomes less effective in removing static 

charges from the inside surface of the 

pipe (9). Most methane drainage pipelines are 4 in (10 cm) or larger. Although no data are presented on the 4 in 

TABLE 1. - Effect of wall thickness 

on reduction of internal charge 

using wet-rag technique 

I Wall I Internal 

Pipe size, in I thickness, 1 discharge, 

B................ 1 .76 NA 

NA Not available. 
relative humidity and temperature were 

measured with psychrometers (fig. 2) . Pipe samples were connected to the pump 

with 6-in (15-cm) flexible suction tubing. Particulates were hand-fed into the 

inlet end of the pipe samples. 

Four types of medium-density polyethylene pipe used for gas transission were 

tested: DuPont type 2, Drisco type 2, 

Drisco type 3, and Plexco type 3. Type 2 

pipe is orange. Type 3 is black owing to 

the addition of carbon black, which prevents the development of brittleness when 

the pipe is exposed to ultraviolet light. 

Thus, care must be taken in the storage 

of the type 2 pipe because structural 

degradation may result. Owing to the 

proprietary nature of the chemical consist of polyethylene pipe, no further information could be obtained regarding the 

composition of the pipe samples. 

The particulates used in the laboratory 

tests were crushed coal from the Pittsburgh and Coalburg Coalbeds, limestone 

rock dust. iron filings. and sand. Table 

-. 

2 shows the proximate and ultimate analy- FIGURE 1. - Pitot tube being used to measure gassis of the two crushed coal samples. stream velocity. 
Scale, ft inlet 

FIGURE 2. - Relative humidity and temperature being measured with psychrometers. 

FIGURE 3. - Humidity chamber. 

, Rock dust and 

air inlet 0 I 

pump 

FIGURE 4. - Rock-dust chamber. 
wide band. The heads were then glued 

into 4-in (10-cm) long sections of 2-in 

(5-cm) round solid polyvinyl chloride. 

An insulated wire, recommended by the 

voltmeter manufacturer, was connected 

directly from the probe head to the 

grounded voltmeter. 

of the pipe sample were "wet ragged," and 

a charge measurement was made to insure 

that no charge remained on the pipe prior 

to the next test. 

All tests were repeated three times, 

and the results were then averaged. Figure 7 shows three separate tests conducted under identical conditions. The 

static charge generally decreases from 

the inlet to the exhaust end of the pipe 

for each test. Variations between test 

4-in-wide aluminum 

sheet on copperwire frame, 

2-in solid PVC 

pipe handle 

FIGURE 5. - Multirange electrostatic voltmeter. FIGURE 6. - General configuration of probe. 
10 1 I 1 I I 

0 

8- o Trial I 

o Trial 2 

A Trial 3 

I I 1 I I I I I 1.1 

0 2 4 6 8 10 12 14 16 18 

DISTANCE FROM INLET, ft 

FIGURE 7. - Averaging data for three electrostatic 

tests. Velocity - 5,000 ft/min, temperature = 65' F, 

relative humidity = 48 pct, Pittsburgh Coalbed dust, 

DuPont type 2 pipe, exposuretime - 3 rnin, dust concentration = 5,500 mg/m3, dust size = minus 200 

mesh. 

runs range to 4 kV; the causes for those 

variations are not known. 

Measuring electrostatic charge on an 

object is a difficult task due to the effects of the capacitance of the measurement equipment. It is a well-known scientific principle that the electrostatic 

charge is stored in the electric field of 

the system's capacitance (C). Therefore, 

when a measuring device is brought into 

the vicinity of the object of interest, 

an effect on the electric field can be 

detected. This effect is a function of 

the relative size of the capacitances and 

the configuration of the electric circuit 

after the measuring device is connected. 

In the case of measuring the electrostatic voltage on a methane drainage 

pipeline, the following circuit relationships can be derived from the test setup. 

Assuming that the pipeline is isolated 

from ground, the actual voltage to which 

the pipeline is charged is as follows: 

where V, = the actual pipeline voltage, 

Qo = the charge stored in the 

pipeline's capacitance, 

and Cp = the capacitance of the 

pipeline. 

To make an actual measurement, the 

pipeline is isolated and a quantity of 

FIGURE 8. - Model of capacitance of measurement 

system. 

particulate material is driven through 

the pipeline by a certain velocity airstream. The measuring system is then 

brought into contact with the inside surface of the pipeline to make a measurement. Figure 8 shows that the measuring 

circuit contains two capacitors, one for 

the capacitance of the probe (C') and the 

other for the capacitance of the meter 

(C) Because the charge will redistrik 

ute itself between the three capacitors 

when the switch is closed (i .e. , a measurement taken), the equation relating 

the reading on the meter (V,) with the 

actual voltage (V,) that was initially on 

the pipeline is as follows: 

This equation shows that if the capacitance of the object being measured (Cp) 

is much greater than the capacitance of 

the measuring circuit (C' and Cm), the 

meter reading is equivalent to the actual 

voltage (V,). Second, if the measuring 

circuit does have an effect on the actual 

reading, the error will be such that the 

recorded reading is less than the actual 

reading. Therefore, if all of the capacitances were either measured or calculated, a means to correct the readings 

for the effect of the measuring circuit 

could be made. However, because the capacitance of the pipeline was large with 

respect to the measuring circuit and 
because the purpose of the project measuring circuit was deemed suitable 

was not to ascertain actual voltage num- for the project. 

bers but relative ones, the fabricated 

LABORATORY RESULTS 

Clean, dry air equivalent to 1 MMft3/d 

(0.33 m3/s) was put through a 2-in-diam 

(5-cm) Dupont type 2 pipe sample at a velocity of 33,000 ft/min (168 m/s). This 

velocity is 22 times the maximum velocity 

measured in underground methane pipelines. The temperature and relative humidity of the gas stream were 63" F 

(17" C) and 22 pct, respectively. While 

the temperature is normal for an underground coal mine, the humidity is very 

low. No charge was detected over a 3-h 

operating period., Thus, even at low humidities, gas streams free of particulate 

matter have no static charge potential. 

Six-foot (1.8-m) long sections of 6-in 

(15-cm) diam Dupont type 2, Drisco type 

2, Drisco type 3, and Plexco type 3 were 

compared for their charge susceptibility. 

Pittsburgh Coalbed dust, minus 28 mesh 

(600 um) (table 3), was fed into a 5,000- 

ft/min (25-1s) airstream. Each sample 

was exposed to a 5, 500-mg/m3 dust concentration for 2 mine The temperature and 

the relative humidity of the gas stream 

were 59" F (15" C) and 40 pct, respectively. Figure 9 shows that the type 2 

pipe charged to a greater voltage than 

did the type 3 pipe, which contains carbon black. 

o Plexco type 3 v Drisco type 3 

The effect of gas velocity on static 

charge generation was determined by feeding 2.6 lb (1,200 g) of dust into a pipe 

for 3 min. Figure 10 shows that a 5,000 

ft/min (25 m/s) , a maximum charge of 3.2 

kV is generated close to the inlet of the 

pipe and decreases thereafter. At 2,500 

ft/min (13 m/s), the charge builds to a 

maximum of 2.0 kV about 5 ft (1.5 m) from 

the inlet, and at 1,000 ft/min (5 m/s) a 

maximum charge of about 1.6 kV occurs 

about 11 ft (3.4 m) from the inlet. No 

measurable charge is detected along the 

first 11 ft (3.4 m) of the pipe when the 

gas velocity is 1,000 f t/min (5 m/s) . Apparently at 1,000 ft/min (5 m/s) and to 

a lesser extent at 2,500 ft/min (13 m/s), 

the dust particles require more time to 

reach the velocity of the gas stream, and 

TABLE 3. - Particle-size distribution 

of Pittsburgh Coalbed samples, 

percent 

Minus 

28 mesh 

NAP 

NAP 

1.6 

64.6 

26.7 

7.1 

Praticle size, 

mesh 

-318 in, +8.. . -8, +28....... 

-28, +48.. ..a 

-48, +loo..... 

-100, +200.. e. 

-200. . l . . . . . . . 

Minus 

200 mesh 

NAP 

NAP 

NAP 

NAP 

NAP 

100.0 

Minus 

318 in 

31.0 

39.7 

13.4 

7.8 

4.3 

3.8 

NAp Not applicable. 

DISTANCE FROM INLET, ft DISTANCE FROM INLET, fi 

FIGURE 9. - Comparison of pipe data. Velocity FIGURE 10. - Effect of velocity variation. Tem- 

- 5,000 ft'min, temperature = 59-' F, relative hu- perature = 67O F, relative humidity = 60 pct, Pittsmidity = 40 pct, Pittsburgh Coa lbed dust, exposure burgh Coalbed dust, DuPont type 2 pipe, exposure 

time = 2 min, dust concentration = 5,500 mg/m 3, time = 3 min, dust load - 400 g/min, dust size = midust size = minus 28 mesh. nus 28 mesh. 
consequently the charging process occurs 

further from the inlet. 

Figure 11 shows that the magnitude of 

the static charge on the pipe increases 

as the exposure time to particles increases. Only about 13 ft (4.0 m) of the 

20-ft (6.1-m) length of pipe is charged. 

Note that the pipe is not charged uniformly along its length. The charge is 

zero near the inlet, increases to a maximum at about 5 ft (1.5 m), and then decreases to zero. This is consistent with 

the 2,500-ft/min (131n/s) velocity test 

(fig. 10). 

Particle size has a marked effect on 

the magnitude of the static charge generated on polyethylene pipe (table 3). 

Figure 12 shows that minus 200 mesh (75 

ym) dust produces a higher charge than 

does minus 28-mesh (600 ym) dust. , Apparently, the finer dust has a larger surface area from which charges can be 

stripped. The charging characteristics 

DISTANCE FROM INLET, ft 

FIGURE 11. - Effect of exposure-time variation. 

Velocityl - 2,50Oft/min, temperature = 60° F, relative humidity = 55 pct, Pittsburgh Coalbed dust, 

DuPont type 2 pipe, dust concentration = 32,000 

mg/m3, dust size - minus 28 mesh. 

of the Pittsburgh and Coalburg Coalbeds 

(table 2) were compared using minus 200- 

mesh (75 ym) dust. Figure 13 shows that 

the Coalburg Coalbed dust imparted a 

greater charge than did the Pittsburgh 

Coalbed dust over about the first 6 ft 

(1.8 m) of pipe downstream of the inlet 

and a lower charge downstream of the 9-ft 

(2.7-m) measurement point. Apparently, 

the difference in charging characteristics is due to the higher ash content of 

the Coalburg dust (table 2). Both tests 

compared equal weight dust samples. 

Plexco type 3 pipe, 4-in (10-cm) diam, 

was assembled into a 24-ft (7.3-m) long, 

double 90" (1.57 rad) bend test section 

(fig. 14). The circled numbers represent 

the eight static charge measurement 

points. Pittsburgh Coalbed dust [minus 

10 I I I I I I I I 

- 

KEY 

a Pittsburgh 

o Coalbura 

2 4 6 8 10 12 14 16 18 

DISTANCE FROM INLET, ft 

FlGlJRE 13. - Comparison of Pittsburgh and Coalburg Coa Ibeds. Velocity = 5,000 ft/min, temperature 

= 65" F, relative humidity = 48 pct, DuPont type 2 

pipe, exposure time = 3 min, dust concentration = 

5,500 mg/m3, dust size = minus 200 mesh. 

KEY 

Measurement point 

4-in pipe 

DISTANCE FROM INLET, ft I 

FIGURE 12. - Effect of dust-size variation. Ve- -- 

dust ~nlet 1 

i 

locity = 5,000 ftlmin, temperature = 65O F, relative L- II' humid ity = 48 pct, Pittsburgh Coa lbed dust, DuPont 

type 2 pi pel exposure time = 3 mint dust concentra- FIGURE 14. - Configuration for elbow test. (Plexo 

tion = 5,500 mg/m3. type 3 pipe.) 
28 mesh (600 urn)], degreased iron filings 

[minus 40 mesh (425 urn)], and sand [minus 

12 mesh (1.4 mm)] were fed to a 10,000- 

ft/min (51-m/s) velocity airstream. A 

200-mL volume of coal dust, sand, and 

iron filings or 0.44, 0.60, and 1.12 lb 

(200, 270, and 510 g), respectively, was 

added over separate 3 min exposure time 

tests. Figure 15 shows that for coal 

dust, the maximum static charge occurred 

near the inlet (stations 1 and 2) and decreased toward station 3. At the elbows, 

the charge on the pipe increased (stations 4 and 7). Apparently, the elbows 

cause coal-particle degradation which exposes new surfaces to the electron exchange process. For the iron and sand 

particles, no charge occurred on the initial straight portion of the pipe (stations 1, 2, and 3) until the particles 

impacted the pipe at the first elbow 

(station 4). The charge was greatest at 

the elbow, and then decreased along the 

straight portion of the pipe (stations 5 

and 6) until the next elbow was reached 

where the charge increased again (station 

7). The sand and iron particles did not 

charge the first 10 ft (3.1 m) of the 

pipeline (stations 1, 2, and 3). These 

are much heavier than coal particles and 

may require a longer pipe length to reach 

the speed of the airstream on a much 

higher velocity airstream. 

Experience shows that when the relative humidity is low inside a building, 

walking across a carpeted floor can 

generate static charges of considerable 

I I I I I 1 

Elbow I Elbow 2 

A Pittsburgh Coal 

magnitude, but when the relative humidity 

is high, the static charge annoyance is 

minimal or eliminated. Similar results 

have been observed for polyethylene pipe. 

A 20-ft (6.1-m) length of pipe was placed 

in a chamber (fig. 2), and steam was injected to control humidity. Figure 16 

shows that when the relative humidity of 

the airstream is increased from 38 to 

80 pct, the static charge on the pipe decreased dramatically. Thus, the relative 

humidity of the airstream is an important 

factor controlling the static charge on 

the pipe. 

Generally, bare copper wire is spirally 

wound on polyethylene pipe, and at intervals of about 500 ft (152 m), the copper 

wire is connected to a buried copper 

ground rod. Laboratory tests were conducted to evaluate the effectiveness of 

this grounding procedure on the static 

charges on the inside and the outside of 

the pipe. Figure 17 shows that when no 

ground wire is present, the charge on the 

inside surface of the pipe ranges from 

about 10 kV near the inlet to 6 kV about 

7 ft (2.1 m) from the inlet. When the 

copper wire is spirally wound in 1-ft 

(30-cm) or 2-ft (61-cm) spacing, the 

static charge on the inside surface is 

reduced but not eliminated. On the outside surface of the pipe, no charge is 

measured when the pipe is spirally wound 

with bare copper wire in a 1-ft (30-cm) 

spacing. For a 2-ft (61-cm) spacing, a 

static charge of about 2 kV is measured 

between the winds of copper wire. Thus, 

KEY 

o 38 pct RH(gas stream and ambient) 

A 80pct RH (gas stream and ambient) 3 - 6- o 8Opct RH gas stream and 37 pct 

W 

(3 

rx 

a 4- I 

U 

2 - 

C 

0 

2 4 6 8 10 12 14 16 18 

I 2 3 4 5 6 7 8 DISTANCE FROM INLET, ft 

MEASUREMENT STATION FIGURE 16. - Effect of humidity variations. VeFIGURE 15. - Results of elbow test. Velocity = locity = 5,000 ft/min, temperature = 69' F, Coalburg 

10,000 ft/min, temperature = 68' F, re lat ive humid- Coalbed dust, DuPont type 2 pipe, exposure time = 

ity = 46 pct, exposure time = 2 min, contaminate 2 min, dust concentration = 11,000 mg/m3, dust size 

volume = 200 mL. = minus 200 mesh. 
KEY 

5 U No qround wire 

a Ground wire, 2-ft spaced spiral 

0 Ground wire, I-ft spaced spiral 

1 I I I I 1 

2 3 4 5 6 7 

DISTANCE FROM INLET, ft 

FIGURE 17. - Results of grounding evaluation. Velocity = 5,00Oft/min, temperature = 62' F, relative humidity = 40 pct, Coalburg Coalbed dust, DuPont type 

2 pipe, exposure time - 2 min, dust concentration = 

11,000 mg/m 3, dust s ize -- minus 200 mesh. 

a spirally wound, grounded copper wire is 

effective in reducing the charge on the 

outside and inside surfaces of the pipe. 

Underground methane pipelines are located in return airways, which are rock 

dusted periodically using forced air to 

blow the rock dust into the entry. This 

procedure was simulated in the laboratory 

to determine the static-charge effect of 

the rock dust particles on the pipeline 

(fig. 4). Air velocity around the pipe 

in the dust chamber was about 200 ft/min 

(1 m/s) and over a 5-min period, 25 lb 

2 3 4 5 6 7 

DISTANCE FROM INLET, ft 

FIGURE 18. - Results of rock-dusting test. Velocity = 210 ft/rnin, temperature = 57O F, relative humidity - 30 pct, limestone dust, dust weight = 25 Ib, 

DuPont type 2 pipe, feed time = 5 min. 

TABLE 4. - Rock dust analysis, percent 

(11 kg) of rock dust was drawn over the 

pipe (table 4). Rock dust coated the 

pipe, and no measurable static charge was 

detected on the outside surface of the 

pipe. However, a static charge of about 

2 kV was measured on the inside surface 

of the pipe (fig. 18). The relative humidity during the test was about 30 pct, 

which is low compared with an underground 

environment and may have contributed to 

the generation of static charge. 

CaO .......... 91.5 

S102......... 4.8 

d7O3........ 1.3 

FIELD TESTS 

Fe202........ 1.2 

MgO.......... 1.1 

S............ .1 

Three underground pipelines were discoloration, scarring, or deposits of 

dismantled and examined for contamina- dust or dirt were found. One of the 

tion. Table 5 shows that the volume of pipelines was used in a cross-measure 

methane transported by these pipelines system, which is more prone to produce 

ranged from 100 million to 400 million dust because of strata movement. 

ft3 (2.8 million to 11.3 million m3). No 

TABLE 5. - Dust contamination data for methane drainage pipelines 

Site Drainage 

system 

trans- Evidence of 

material Pipe I ported, 1 contamination 

Beckley Mining Co. : Beckley Mine 

None 

None 

BethEnergy Mines Inc. : 

Cambria 33 Hne.. .............. Marianna Mine.................. 

Horizontal.. . cross-measure 

Horizontal... 

Polyethylene 

.. .do.. ..... 1 J-J-i / 

Metal....... 

~Mft3 

200 None 
- -' 

- *A%- . FIGURE 19. - Real-time aerosol monitor. 

To further investigate the dust levels 

in working methane drainage systems, a 

cross-measure surface borehole and two 

vertical gob boreholes were inspected. A 

real-time aerosol monitor (fig. 19) was 

used to measure the concentration of 

minus 20-um dust particles in the 

methane-air gas flows from the three 

boreholes. The measured dust levels were 

comparable to levels in a clean air environment, which produced no static charge 

on the pipe in the laboratory tests even 

though the velocity of the gas stream was 

33,000 ft/min (168 m/s) and relative humidity was low (22 pct) . 

Static-charge measurements were conducted on two underground methane drainage pipelines (table 6). A 4-in (10-cm) 

wide band of aluminum was wrapped around 

each pipeline to collect static charges 

and a 0- to 300-V electrostatic voltmeter 

was used to measure the static charge 

(fig. 20) . For both pipelines, the relative humidity of the gas stream and the 

entry containing the pipeline were 78 pct 

or greater. Dust concentrations in the 

gas streams were comparable to a clean 

air environment. No static charge was 

detected on the two pipelines. 

Gas stream.. .......... Entry................. 

Methane conc.. .... .pct. . Temperature.. ...... .OF'. . Charge measured.. ... .V.. 

100 

9 8 

100 

7 7 

0 

78 

83 

9 7 

63 

0 
SUMMARY AND CONCLUSIONS 

Velocities up to 33,000 ft/min (168 

m/s) in polyethylene pipe produced no 

static charges when the gas stream was 

free of particulate matter. When particulate matter is present, the magnitude of 

static charge is directly proportional to 

the gas-stream velocity and dust concentration and inversely proportional to 

size of the dust particles and relative 

humidity. 

Type 2 polyethylene pipe charged to a 

greater magnitude than did type 3, which 

contains carbon black and is presumably 

more conductive than type 2. 

Field measurements showed that the relative humidity of the gas transported in 

underground methane drainage pipelines is 

78 pct or greater, and the dust concentration is negligible (0.06 mg/m3). No 

static charge was measurable on the underground pipelines under these conditions. In laboratory tests where the 

relative humidity was 78 pct or greater 

and dust concentrations were 11,000 mg/ 

m3, a static charge of only 2 kV was generated. Where relative humidity was 22 

pct and particulate concentrations were 

negligible, no measurable static charge 

was generated even though gas-stream velocity was as high as 33,000 ft/min (168 

m/s). Thus, relative humidity and dust 

be prevented by wrapping bare copper wire 

around the pipe in a 1-ft (30-cm) spacing. When a 2-ft (61-cm) spiral wrapping 

is used, static charges exist between the 

wraps. However, the static charge on the 

inside pipe surface is reduced but not 

eliminated for both the 1-ft (30-cm) and 

2-ft (61-cm) spacings. 

The low-velocity flow [200 ft/min (1 

m/s)J of rock dust around polyethylene 

pipe generated a static charge of about 

2 kV on the inside surface and no measurable charge on the outside surface, 

which was coated with rock dust. The 

relative humidity during the test was 30 

pct compared with field measurement at 

78 pct or greater, which would lower the 

static charge on the inside surface 

considerably. 

Field measurements show that the relative humidities in the mine entry and the 

gas stream are high (78 pct or greater), 

and the level of particulate matter in 

the gas stream is comparable to that of a 

clean air environment. These conditions 

are not conducive to the development of 

static charges on underground polyethylene pipelines. 

Because of the possibility of staticcharge buildup on underground pipelines, 

the wet-rag technique should be employed 

concentration conditions would have to to remove static charges from the outside 

change radically for the development of pipe surface and to reduce the static 

sizable static charges on underground charge on the inside surface before a 

methane pipelines. polyethylene pipeline is repaired or 

The buildup of static charges on the dismantled. 

outside surface of polyethylene pipe can 

REFERENCES 

1. Diamond, We P. Site-Specific and 

Regional Geologic Considerations for 

Coalbed Gas Drainage. BuMines IC 8898, 

1982, 24 pp. 

2. Maksimovic, S. D., and F. N. Kissell. Three Coal Mine Gob Degasification 

Studies Using Surface Boreholes and a 

Bleeder System. BuMines RI 8459, 1980, 

10 PP. 

3. Cervik, J. Experience With Methane 

Control From Horizontal Boreholes. Paper in Proceedings of the Second International Mine Ventilation Congress, Reno, 

NV, Nov. 4-8, 1979. Soc. Min. Eng. AIME, 

1980, pp. 257-264. 

4. Campoli, A.A., J. Cervik, and 

S. J. Schatzel. Control of Longwall Gob 

Gas With Cross-Methane Boreholes (upper 
Kittanning Coalbed). BuMines RI 8841, 

1983, 17 pp. 

5. Energy Application, Inc. Design 

and Recommended Specifications for a Safe 

Methane Gas Piping System (contract 

50155145). BuMines OFR 109-76, 1977, 80 

pp.; NTIS PB 2593408s. 

6. Tisdale, J. E., D. W. Mitchell, 

R. A. Elam, M. J. Lawless, and B. E. Taylor. Piping Methane in Underground Coal 

Mines. MSHA IR 1094, 1978, 34 pp. 

7. National Fire Protection Associat ion. Recommended Practice on Static 

Electricity. ANsI/NFPA 77, Dec. 7, 1982, 

58 PP. 

8. Davis, G. W., and I. K. DeBlieu. 

Static Electricity and Lightning Effects 

on Plastic Pipe. Paper in Proc. AGA 

Distr. Conf., Los Angeles, CA, May 1975. 

AGA, 1975, pp. 175-177. 

9. Grenier, G., and R. Caldwell. 

Static Electricity on Polyethylene Pipe. 

Paper in Proc. 4th AGA Plastic Pipe 

Symp., Denver, CO, Nov. 1972. AGA, 1972, 

pp. 84-86. 

10. Allegheny Mineral Corp. Private 

communications, 1985; available upon request from A. A. Campoli, BuMines, Pittsburgh, PA. 

U S. GOVERNMENT PRINTING OFFICE: 1986-605-017140,022