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Triboelectric Effects on Polyethylene Methane Drainage Pipelines

Triboelectric Effects on Polyethylene Methane Drainage Pipelines

Bureau of Mines Report of Investigations/l986

Tri boelectric Effects on Drainage Pipelines

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

UNITED STATES DEPARTMENT OF THE INTERIOR Report of Investigations 9017

Tri boelectric Effects on Polyethylene Methane Drainage Pipelines

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

UNITED STATES DEPARTMENT OF THE INTERIOR Donald Paul Hodel, Secretary

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

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. Polyethy- lene. 4. Mine dusts. 5. . 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

j h CONTENTS

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

ILLUSTRATIONS

Pitot tube being used to measure gas-stream velocity ...... Relative 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 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 lb pound weight degree Celsius lbf/in2 pound force per square inch angular degree m meter degree Fahrenheit Pm micrometer foot mg/m3 milligram per cubic meter foot per minute min minute cubic foot per day mL milliliter cubic foot per minute mm millimeter gram MMF t mill-ion cubic feet gram per minute PlFlft3/d million cubic feet per day horsepower m3 cubic meter inch m3/s cubic meter per second kilogram PCt percent kilo\.olt rad radian kilopascal v kilowatt 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 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 hu- midity (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 cop- per 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~upervisorygeophysicist. 3~esearchsupervisor. I Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. INTRODUCTION

The Bureau of Mines estimates that of the underground pipeline. Both the as much as 766 trillion cubic feet Bureau (5) and the Mine Safety and Health ( 7.12~10 ' Om3) of coalbed gas, predomi- ~dministration(MSHA) , U. S . Department of nantly methane, is contained in the coal- Labor (6), outlined procedures for the beds of the United States (1).4 This gas safe operation of underground methane is both a natural resource and a hin- drainage pipelines. These reports should drance to mining. Methane drainage sys- be consulted for a general overview of tems have been employed in most major pipeline protection requirements. coal regions of the United States to re- The Bureau study (5)- provided the basis move gas from the coalbed prior to mining for the pipeline material selection. and/or capture the coalbed gas for fuel. Polyethylene pipe best meets the design Methane drainage systems employ hori- criterion because of its superior resist- zontal, vertical, and cross-measure bore- ance to corrosion and impact forces. It holes. Vertical borehole systems (2) also is attractive to mine operators be- consist of holes drilled from the surface cause it is lightweight, easy to install, into the gas-bearing coal strata. The and significantly cheaper than metal boreholes are cased with metal pipe, piping. which provides the conduit for the flow In an attempt to negate any hazard of gas from the gas-bearing strata to produced by the triboelectrification of the surface. Horizontal (3) and cross- polyethylene pipe, MSHA recommends that a measure (4) systeos require the drilling grounded bare wire be spirally of borehoies into the coalbed or associ- wound around all polyethylene piping. ated strata from underground locations. The recommendation was incorporated in The boreholes are cnnnected to an under- many of the methane drainage systems now pipeline that carries the gas to in place across the United States. The the surface. objectives of this study were to obtain a Methane, which is piped through mine better understanding of the triboelectri- passageways in quantities sufficient to fication process as it relates to the un- contaminate large areas of the mine, jus- derground polyethylene methane drainage tifies caution and concern in the design, pipeline and to evaluate the recommended installation, protection, and maintenance grounding safety procedure.

ACKNOWLEDGMENTS

The authors wish to thank Donald Duvall pipe manufacture and static charge test- of Plexco Pipe, John Merideth of Phillips ing, respectively. Drisco Pipe, and Joseph Uhler of the The efforts of John Stevenson of Jim Equitable Gas Co. for the pipe samples Walter Resources Inc., Dave Fitzpatrick they provided for laboratory testing. of Beckley Mining Co., and Dan Weaver of Jack Workman of Workman Developments the BethEnergy Mines Inc. were instrumen- and Robert Reif, formerly of the Battelle tal in the performance of methane drain- Memorial Institute, donated much appreci- age pipeline evaluations in the field. ated technical assistance on polyethylene

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 phenom- 4~nderlinednumbers in parentheses re- ena are more intense when weather is dry. fer to items in the list of references at In most situations, is the end of this report. simply an annoyance. The term "electricity" is derived from hole" with a tool to squeeze off the the ancient Greek word "" meaning plastic pipe. "," because it was with this sub- In the preceding example, charges were stance that the phenomenon of electri- generated by the extremely high velocity fication was first observed. For cen- gas flow through a rupture. Charges also turies, the term meant only what is now occur at lower velocities by the addition called static electricity, the property of a particulate matter (9). Whether exhibited by some substances after being generated by rupture leaks or dust-laden rubbed with material like or gas flows, the static charges are the re- of being able to attract or repel light- sult of triboelectrification. Gas mole- weight objects. Mechanization produced cules and particulates vigorously rub the stronger electrification that was first surface of the pipe causing a transfer of observed about 300 years ago (7), and in to the nonconductive pipe. comparatively recent times, tGe proper- Conductive pipe used in the same applica- ties of "flowing" electricity were dis- tion would not change the charging pro- covered; the word "static" then came into cess; however, it would provide a path use as a means of distinguishing the two for the triboelectricity to ground. forms. The implication that static elec- The "wet-rag" technique is accepted as tricity is always at rest is erroneous, the basic safety procedure for removal because such electricity causes the most of static electricity prior to repair of concern when it ceases to rest. polyethylene gas pipelines. Wet rags are For the sake of simplicity, one may wrapped around the pipe to increase the imagine electricity to be a weightless surface conductivity. This increase in and indestructible fluid that can move conductivity caused by the addition of freely through some substances, such as moisture does not eliminate the charge- metals, that are called conductors, but generation mechanism; it only provides a can flow with difficulty or not at all path to ground for the static elecricity. through or over the surface of a class of The technique is quite effective in re- substances called nonconductors or in- moving the charge from the outside sur- sulators. This latter group includes face (9). However, its effectiveness in gases, , rubber, amber, , sul- removiZg charges from the inside surface , paraffin, most dry petroleum oils, is adversely effected by pipe wall thick- and many plastic materials such as poly- ness. Table 1 shows that as wall thick- ethylene pipe. When electricity is pres- ness increases, the wet-rag technique be- ent on the surface of a nonconductive comes less effective in removing static body, where it is prevented from escap- charges from the inside surface of the ing, it is called static electricity. pipe (9). Most methane drainage pipe- Electricity on a conducting body that is lines are 4 in (10 cm) or larger. Al- in contact only with insulators is also though no data are presented on the 4 in prevented from escaping and is therefore nonmobile or static. In either case, the TABLE 1. - Effect of wall thickness body on which this electricity is evident on reduction of internal charge is said to be charged (7). using wet-rag technique Static charges on polyethylene gas dis- tribution pipelines have long been a con- I Wall I Internal cern to natural gas utilities and pipe Pipe size, in I thickness, 1 discharge, 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 trans- mission pipe to provide access for re- pair. Sparks from the pipe can ignite explosive mixtures of the narural gas and B...... 1 .76 NA air when the worker enters the "bell NA Not available. (10 cm) or larger pipe sizes, the general leaks exist in natural gas systems. The trend is a reduction in effectiveness of particular matter of concern in natural outside grounding in removing inside sur- gas systems is rust scale that is car- f ace charges. ried from metal lines into polyethylene The polyethylene pipe employed in coal lines. Rust is not a concern in coalbed mine methane drainage is the same as methane drainage because the pipe is all the pipe used by the natural gas indus- plastic. However, coal dust from hori- try; however, the gas flow velocities zontal boreholes or rock particles from and pipeline environment are different. cross-measure boreholes are a possibil- Coalbed methane drainage systems utilize ity. Finally, the polyethylene pipe is the natural coalbed pressure to move the buried in natural gas systems and is ex- gas, and pipeline pressures are generally posed in methane drainage applications. 5 lbf/in2 (34 kPa) or less compared with Thus, electrostatic hazards are paramount more than 30 lbf/in2 (207 kPa) in natural in repair situations only in natural gas gas transmission systems. Consequently, systems, but could be a concern through- much higher velocities and the possibil- out the life of a pipeline used for coal- ity of extreme velocities due to pinhole bed me thane drainage. LABORATORY EQUIPMENT Laboratory apparatus was assembled to A rectangular wooden frame, 8 by 8 by simulate a variety of velocity, dust con- 24 ft (2.4 by 2.4 by 7.3 m), was covered centration, and humidity conditions in with clear plastic to provide for a and around polyethylene pipe samples. A variable humidity environment (fig. 3). 50-hp (37-kW) , 7404t3/min (0.35-m3/s) water-ring vacuum pump was used to gen- erate the airstream through the pipe sam- ples. Gas-stream velocities were mea- sured with a pitot tube (fig. l), and relative humidity and temperature were measured with psychrometers (fig. 2) . Pipe samples were connected to the pump with 6-in (15-cm) flexible suction tub- ing. Particulates were hand-fed into the inlet end of the pipe samples. Four types of medium-density polyethy- lene 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 pre- vents 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 con- sist of polyethylene pipe, no further in- formation could be obtained regarding the composition of the pipe samples. The particulates used in the laboratory tests were crushed coal from the Pitts- burgh 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 gas- sis of the two crushed coal samples. stream velocity. Steam and air were drawn into the chamber TABLE 2. - Coal analysis, percent by the partial vacuum generated by the pump. The steam and air intake for the Analysis Coalburg Pittsburgh chamber were located at the far corner Proximate: from the pipe sample intake. The humid- Moisture...... 3.2 2.9 ity and temperature inside and surround- Volatile matter.. 27.0 35.7 ing the pipe sample were identical dur- Fixed carbon..... 39.4 53.7 ing operation (fig. 3). Ash...... 30.4 7.7 The rock dust is blown into return air- Total...... 100.0 100.0 ways in coal mines to render loose coal Ultimate: dust incombustible. To determine if rock ...... 4.0 5.3 dusting procedures could impart a static Carbon...... 54.5 75.5 charge to a polyethylene methane drainage Nitrogen...... 8 1.5 pipeline, a 2- by 2- by 8-ft (0.6- by ...... 1 1.1 0.6- by 2.4-m) chamber was constructed to Oxygen...... 10.2 8.9 simulate such procedures (fig. 4). The AS h ...... 30.4 7.7 chamber provided for the movement of rock Total...... 100.0 100.0 dust over and around the pipeline. Ve- locity within the chamber was measured chamber (not shown on figure 4) to pro- with a vane anemometer. A door was lo- vide access to the chamber for surface cated on the top panel of the rock dust charge measurement. Static charge mea- surements were made with a high-impedance (5x1015-ohm) voltmeter (fig. 5). The me- ter is operated.

inlet Scale, ft

FIGURE 3. - Humidity chamber.

,Rock dust and air inlet 0 I

pump FIGURE 2. - Relative humidity and temperature be- FIGURE 4. - Rock-dust chamber. ing measured with psychrometers. ELECTROSTATIC TEST PROCEDURES

Even though the electrostatic volt- A similar probe was built for inter- meter consumed very little current due nal charge measurement. The probe was to its high impedance, a charge collec- constructed of the same material as the tion device was required to provide suf- probe described earlier (fig. 6) and con- ficient charge to activate the instru- tacted a 4-in-wide half cylinder on the ment. Initially, tests were performed inside surface of the 6-in-diameter pipe with a 4-in (10-cm) wide, 1164-in (0.4- samples. mm) thick aluminum band wrapped around The general procedure for the labora- the pipe (fig. 5). Aluminum was selected tory experiments was initiated by estab- for its low cost and ease of handling; lishing the gas-stream velocity, along however, copper or any other highly con- with the temperature and relative humid- ductive metal could have been used. Dur- ity of the gas stream and of the ambient ing the testing, we found that the bands air. Dust was then hand-fed into the in- behaved as charge sinks. They gave erro- let of the pipe sample as consistently as neously high readings at the point of possible over a predetermined time of ex- measurement. Thus, a probe was put in posure. Measurement of the static charge contact with the pipe sample after it was was accomplished by placing the probe charged (fig. 6). An insulated handle at head on the pipe immediately after the least 3 ft (0.9 m) long insured that the flow was shut off. The probe was removed person taking the reading did not come from the pipe and grounded to eliminate close enough to t-ie pipe to disturb the any bias on subsequent measurements. . The probe heads were Subsequent measurements were made indi- fashioned from aluminum sheeting, 1/64 in vidually. The total elapsed time between (0.4 m) thick, and solid copper wire, the final measurement and when the fan 3/16 in (4.8 mm) in diameter. They were was shut off was no more than 2 min for shaped to cover one-half of the circum- all the tests. At the conclusion of each ference of the pipe over a 4-in (10-cm) test, the internal and outside surfaces wide band. The heads were then glued of the pipe sample were "wet ragged," and into 4-in (10-cm) long sections of 2-in a charge measurement was made to insure (5-cm) round solid . that no charge remained on the pipe prior An insulated wire, recommended by the to the next test. voltmeter manufacturer, was connected All tests were repeated three times, directly from the probe head to the and the results were then averaged. Fig- grounded voltmeter. ure 7 shows three separate tests con- ducted 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 copper- wire 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 con- centration = 5,500 mg/m3, dust size = minus 200 FIGURE 8. - Model of capacitance of measurement mesh. system. runs range to 4 kV; the causes for those particulate material is driven through variations are not known. the pipeline by a certain velocity air- Measuring electrostatic charge on an stream. The measuring system is then object is a difficult task due to the ef- brought into contact with the inside sur- fects of the capacitance of the measure- face of the pipeline to make a measure- ment equipment. It is a well-known sci- ment. Figure 8 shows that the measuring entific principle that the electrostatic circuit contains two , one for charge is stored in the electric field of the capacitance of the probe (C') and the the system's capacitance (C). Therefore, other for the capacitance of the meter when a measuring device is brought into (C) Because the charge will redistrik the vicinity of the object of interest, ute itself between the three capacitors an effect on the electric field can be when the switch is closed (i.e. , a mea- detected. This effect is a function of surement taken), the equation relating the relative size of the and the reading on the meter (V,) with the the configuration of the electric circuit actual voltage (V,) that was initially on after the measuring device is connected. the pipeline is as follows: In the case of measuring the electro- static voltage on a methane drainage pipeline, the following circuit relation- ships can be derived from the test setup. Assuming that the pipeline is isolated This equation shows that if the capaci- from ground, the actual voltage to which tance of the object being measured (Cp) the pipeline is charged is as follows: 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 where V, = the actual pipeline voltage, circuit does have an effect on the actual reading, the error will be such that the Qo = the charge stored in the recorded reading is less than the actual pipeline's capacitance, reading. Therefore, if all of the capac- itances were either measured or calcu- and Cp = the capacitance of the lated, a means to correct the readings pipeline. for the effect of the measuring circuit could be made. However, because the ca- To make an actual measurement, the pacitance of the pipeline was large with pipeline is isolated and a quantity of 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 The effect of gas velocity on static (0.33 m3/s) was put through a 2-in-diam charge generation was determined by feed- (5-cm) Dupont type 2 pipe sample at a ve- ing 2.6 lb (1,200 g) of dust into a pipe locity of 33,000 ft/min (168 m/s). This for 3 min. Figure 10 shows that a 5,000 velocity is 22 times the maximum velocity ft/min (25 m/s) , a maximum charge of 3.2 measured in underground methane pipe- kV is generated close to the inlet of the lines. The temperature and relative hu- pipe and decreases thereafter. At 2,500 midity of the gas stream were 63" F ft/min (13 m/s), the charge builds to a (17" C) and 22 pct, respectively. While maximum of 2.0 kV about 5 ft (1.5 m) from the temperature is normal for an under- the inlet, and at 1,000 ft/min (5 m/s) a ground coal mine, the humidity is very maximum charge of about 1.6 kV occurs low. No charge was detected over a 3-h about 11 ft (3.4 m) from the inlet. No operating period., Thus, even at low hu- measurable charge is detected along the midities, gas streams free of particulate first 11 ft (3.4 m) of the pipe when the matter have no static charge potential. gas velocity is 1,000 f t/min (5 m/s) . Six-foot (1.8-m) long sections of 6-in Apparently at 1,000 ft/min (5 m/s) and to (15-cm) diam Dupont type 2, Drisco type a lesser extent at 2,500 ft/min (13 m/s), 2, Drisco type 3, and Plexco type 3 were the dust particles require more time to compared for their charge susceptibility. reach the velocity of the gas stream, and Pittsburgh Coalbed dust, minus 28 mesh (600 um) (table 3), was fed into a 5,000- TABLE 3. - Particle-size distribution ft/min (25-1s) airstream. Each sample of Pittsburgh Coalbed samples, was exposed to a 5, 500-mg/m3 dust concen- percent tration for 2 mine The temperature and the relative humidity of the gas stream Praticle size, Minus Minus Minus were 59" F (15" C) and 40 pct, respec- mesh 318 in 28 mesh 200 mesh tively. Figure 9 shows that the type 2 -318 in, +8.. . 31.0 NAP NAP pipe charged to a greater voltage than -8, +28...... 39.7 NAP NAP did the type 3 pipe, which contains car- -28, +48.. ..a 13.4 1.6 NAP bon black. -48, +loo..... 7.8 64.6 NAP -100, +200.. e. 4.3 26.7 NAP -200. .l ...... 3.8 7.1 100.0 NAp Not applicable.

o Plexco type 3 v Drisco type 3

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, Pitts- midity = 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 = mi- dust size = minus 28 mesh. nus 28 mesh. consequently the charging process occurs of the Pittsburgh and Coalburg Coalbeds further from the inlet. (table 2) were compared using minus 200- Figure 11 shows that the magnitude of mesh (75 ym) dust. Figure 13 shows that the static charge on the pipe increases the Coalburg Coalbed dust imparted a as the exposure time to particles in- greater charge than did the Pittsburgh creases. Only about 13 ft (4.0 m) of the Coalbed dust over about the first 6 ft 20-ft (6.1-m) length of pipe is charged. (1.8 m) of pipe downstream of the inlet Note that the pipe is not charged uni- and a lower charge downstream of the 9-ft formly along its length. The charge is (2.7-m) measurement point. Apparently, zero near the inlet, increases to a maxi- the difference in charging characteris- mum at about 5 ft (1.5 m), and then de- tics is due to the higher ash content of creases to zero. This is consistent with the Coalburg dust (table 2). Both tests the 2,500-ft/min (131n/s) velocity test compared equal weight dust samples. (fig. 10). Plexco type 3 pipe, 4-in (10-cm) diam, Particle size has a marked effect on was assembled into a 24-ft (7.3-m) long, the magnitude of the static charge gen- double 90" (1.57 rad) bend test section erated on polyethylene pipe (table 3). (fig. 14). The circled numbers represent Figure 12 shows that minus 200 mesh (75 the eight static charge measurement ym) dust produces a higher charge than points. Pittsburgh Coalbed dust [minus does minus 28-mesh (600 ym) dust. , Appar- ently, the finer dust has a larger sur- face area from which charges can be 10 I I I I I I I I stripped. The charging characteristics - KEY Pittsburgh a o Coalbura

2 4 6 8 10 12 14 16 18 DISTANCE FROM INLET, ft DISTANCE FROM INLET, ft FlGlJRE 13. - Comparison of Pittsburgh and Coal- FIGURE 11. - Effect of exposure-time variation. burg Coa Ibeds. Velocity = 5,000 ft/min, temperature Velocityl - 2,50Oft/min, temperature = 60° F, rela- = 65" F, relative humidity = 48 pct, DuPont type 2 tive humidity = 55 pct, Pittsburgh Coalbed dust, pipe, exposure time = 3 min, dust concentration = DuPont type 2 pipe, dust concentration = 32,000 5,500 mg/m3, dust size = minus 200 mesh. mg/m3, dust size - minus 28 mesh.

KEY Measurement point

4-in pipe DISTANCE FROM INLET, ft I i FIGURE 12. - Effect of dust-size variation. Ve- -- dust ~nlet 1 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 magnitude, but when the relative humidity [minus 40 mesh (425 urn)], and sand [minus is high, the static charge annoyance is 12 mesh (1.4 mm)] were fed to a 10,000- minimal or eliminated. Similar results ft/min (51-m/s) velocity airstream. A have been observed for polyethylene pipe. 200-mL volume of coal dust, sand, and A 20-ft (6.1-m) length of pipe was placed iron filings or 0.44, 0.60, and 1.12 lb in a chamber (fig. 2), and steam was in- (200, 270, and 510 g), respectively, was jected to control humidity. Figure 16 added over separate 3 min exposure time shows that when the relative humidity of tests. Figure 15 shows that for coal the airstream is increased from 38 to dust, the maximum static charge occurred 80 pct, the static charge on the pipe de- near the inlet (stations 1 and 2) and de- creased dramatically. Thus, the relative creased toward station 3. At the elbows, humidity of the airstream is an important the charge on the pipe increased (sta- factor controlling the static charge on tions 4 and 7). Apparently, the elbows the pipe. cause coal-particle degradation which ex- Generally, bare copper wire is spirally poses new surfaces to the electron ex- wound on polyethylene pipe, and at inter- change process. For the iron and sand vals of about 500 ft (152 m), the copper particles, no charge occurred on the ini- wire is connected to a buried copper tial straight portion of the pipe (sta- ground rod. Laboratory tests were con- tions 1, 2, and 3) until the particles ducted to evaluate the effectiveness of impacted the pipe at the first elbow this grounding procedure on the static (station 4). The charge was greatest at charges on the inside and the outside of the elbow, and then decreased along the the pipe. Figure 17 shows that when no straight portion of the pipe (stations 5 ground wire is present, the charge on the and 6) until the next elbow was reached inside surface of the pipe ranges from where the charge increased again (station about 10 kV near the inlet to 6 kV about 7). The sand and iron particles did not 7 ft (2.1 m) from the inlet. When the charge the first 10 ft (3.1 m) of the copper wire is spirally wound in 1-ft pipeline (stations 1, 2, and 3). These (30-cm) or 2-ft (61-cm) spacing, the are much heavier than coal particles and static charge on the inside surface is may require a longer pipe length to reach reduced but not eliminated. On the out- the speed of the airstream on a much side surface of the pipe, no charge is higher velocity airstream. measured when the pipe is spirally wound Experience shows that when the rela- with bare copper wire in a 1-ft (30-cm) tive humidity is low inside a building, spacing. For a 2-ft (61-cm) spacing, a walking across a carpeted floor can static charge of about 2 kV is measured generate static charges of considerable between the winds of copper wire. Thus,

I I I I I 1 KEY Elbow I Elbow 2 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

A Pittsburgh Coal 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. Ve- FIGURE 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 2 3 4 5 6 7 5 U No qround wire a Ground wire, 2-ft spaced spiral DISTANCE FROM INLET, ft 0 Ground wire, I-ft spaced spiral 1 I I I I 1 FIGURE 18. - Results of rock-dusting test. Veloc- 2 3 4 5 6 7 ity = 210 ft/rnin, temperature = 57O F, relative humid- DISTANCE FROM INLET, ft ity - 30 pct, limestone dust, dust weight = 25 Ib, DuPont type 2 pipe, feed time = 5 min. FIGURE 17. - Results of grounding evaluation. Ve- locity = 5,00Oft/min, temperature = 62' F, relative hu- midity = 40 pct, Coalburg Coalbed dust, DuPont type 2 pipe, exposure time - 2 min, dust concentration = TABLE 4. - Rock dust analysis, percent 11,000 mg/m 3, dust s ize -- minus 200 mesh. CaO ...... 91.5 Fe202...... 1.2 S102...... 4.8 MgO...... 1.1 a spirally wound, grounded copper wire is d7O3...... 1.3 S...... 1 effective in reducing the charge on the outside and inside surfaces of the pipe. (11 kg) of rock dust was drawn over the Underground methane pipelines are lo- pipe (table 4). Rock dust coated the cated in return airways, which are rock pipe, and no measurable static charge was dusted periodically using forced air to detected on the outside surface of the blow the rock dust into the entry. This pipe. However, a static charge of about procedure was simulated in the laboratory 2 kV was measured on the inside surface to determine the static-charge effect of of the pipe (fig. 18). The relative hu- the rock dust particles on the pipeline midity during the test was about 30 pct, (fig. 4). Air velocity around the pipe which is low compared with an underground in the dust chamber was about 200 ft/min environment and may have contributed to (1 m/s) and over a 5-min period, 25 lb the generation of static charge.

FIELD TESTS

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 trans- Evidence of system materialPipe I ported, 1 contamination ~Mft3 Beckley Mining Co. : Beckley Mine Horizontal.. . Polyethylene 200 None BethEnergy Mines Inc. : Cambria 33 Hne...... cross-measure .. .do...... 1 J-J-i / None Marianna Mine...... Horizontal... Metal...... None FIGURE 20. - Single-range electrostatic voltmeter.

TABLE 6. - Results of real-time evaluation of methane drainage pipelines

Jim Walter Beckley No. 5 Mine Mine Gas stream: Velocity...... f t/min.. 1,450 1,040 Dust conc...... mg/m3.. 0.06 0.05 Relative humidity, pct: Gas stream...... 100 78 Entry...... 98 83 -' Methane conc...... pct. . 100 97 - - *A%- .. Temperature...... OF'. . 77 63 FIGURE 19. - Real-time aerosol monitor. Charge measured...... V.. 0 0

To further investigate the dust levels Static-charge measurements were con- in working methane drainage systems, a ducted on two underground methane drain- cross-measure surface borehole and two age pipelines (table 6). A 4-in (10-cm) vertical gob boreholes were inspected. A wide band of aluminum was wrapped around real-time aerosol monitor (fig. 19) was each pipeline to collect static charges used to measure the concentration of and a 0- to 300-V electrostatic voltmeter minus 20-um dust particles in the was used to measure the static charge methane-air gas flows from the three (fig. 20) . For both pipelines, the rela- boreholes. The measured dust levels were tive humidity of the gas stream and the comparable to levels in a clean air envi- entry containing the pipeline were 78 pct ronment, which produced no static charge or greater. Dust concentrations in the on the pipe in the laboratory tests even gas streams were comparable to a clean though the velocity of the gas stream was air environment. No static charge was 33,000 ft/min (168 m/s) and relative hu- detected on the two pipelines. midity was low (22 pct) . SUMMARY AND CONCLUSIONS

Velocities up to 33,000 ft/min (168 be prevented by wrapping bare copper wire m/s) in polyethylene pipe produced no around the pipe in a 1-ft (30-cm) spac- static charges when the gas stream was ing. When a 2-ft (61-cm) spiral wrapping free of particulate matter. When partic- is used, static charges exist between the ulate matter is present, the magnitude of wraps. However, the static charge on the static charge is directly proportional to inside pipe surface is reduced but not the gas-stream velocity and dust concen- eliminated for both the 1-ft (30-cm) and tration and inversely proportional to 2-ft (61-cm) spacings. size of the dust particles and relative The low-velocity flow [200 ft/min (1 humidity. m/s)J of rock dust around polyethylene Type 2 polyethylene pipe charged to a pipe generated a static charge of about greater magnitude than did type 3, which 2 kV on the inside surface and no mea- contains carbon black and is presumably surable charge on the outside surface, more conductive than type 2. which was coated with rock dust. The Field measurements showed that the rel- relative humidity during the test was 30 ative humidity of the gas transported in pct compared with field measurement at underground methane drainage pipelines is 78 pct or greater, which would lower the 78 pct or greater, and the dust concen- static charge on the inside surface tration is negligible (0.06 mg/m3). No considerably. static charge was measurable on the un- Field measurements show that the rela- derground pipelines under these condi- tive in the mine entry and the tions. In laboratory tests where the gas stream are high (78 pct or greater), relative humidity was 78 pct or greater and the level of particulate matter in and dust concentrations were 11,000 mg/ the gas stream is comparable to that of a m3, a static charge of only 2 kV was gen- clean air environment. These conditions erated. Where relative humidity was 22 are not conducive to the development of pct and particulate concentrations were static charges on underground polyethy- negligible, no measurable static charge lene pipelines. was generated even though gas-stream ve- Because of the possibility of static- locity was as high as 33,000 ft/min (168 charge buildup on underground pipelines, m/s). Thus, relative humidity and dust 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 3. Cervik, J. Experience With Methane Regional Geologic Considerations for Control From Horizontal Boreholes. Pa- Coalbed Gas Drainage. BuMines IC 8898, per in Proceedings of the Second Interna- 1982, 24 pp. tional Mine Ventilation Congress, Reno, 2. Maksimovic, S. D., and F. N. Kis- NV, Nov. 4-8, 1979. Soc. Min. Eng. AIME, sell. Three Coal Mine Gob Degasification 1980, pp. 257-264. Studies Using Surface Boreholes and a 4. Campoli, A.A., J. Cervik, and Bleeder System. BuMines RI 8459, 1980, S. J. Schatzel. Control of Longwall Gob 10 PP. Gas With Cross-Methane Boreholes (upper Kittanning Coalbed). BuMines RI 8841, 8. Davis, G. W., and I. K. DeBlieu. 1983, 17 pp. Static Electricity and Effects 5. Application, Inc. Design on Plastic Pipe. in Proc. AGA and Recommended Specifications for a Safe Distr. Conf., Los Angeles, CA, May 1975. Methane Gas Piping System (contract AGA, 1975, pp. 175-177. 50155145). BuMines OFR 109-76, 1977, 80 9. Grenier, G., and R. Caldwell. pp.; NTIS PB 2593408s. Static Electricity on Polyethylene Pipe. 6. Tisdale, J. E., D. W. Mitchell, Paper in Proc. 4th AGA Plastic Pipe R. A. Elam, M. J. Lawless, and B. E. Tay- Symp., Denver, CO, Nov. 1972. AGA, 1972, lor. Piping Methane in Underground Coal pp. 84-86. Mines. MSHA IR 1094, 1978, 34 pp. 10. Allegheny Mineral Corp. Private 7. National Fire Protection Associa- communications, 1985; available upon re- t ion. Recommended Practice on Static quest from A. A. Campoli, BuMines, Pitts- Electricity. ANsI/NFPA 77, Dec. 7, 1982, burgh, PA. 58 PP.

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