EFFECTIVE NEW TECHNIQUES

FOR DISCHARGE OF STATIC

ELECTRICITY ON INNER AND

OUTER PIPE SURFACES

 

Sponsored by

Gas Research Institute

GRI Contract No. 5089-271-1877

 

Presented by

Oren Tranbarger

Southwest Research Institute

6220 Culebra Road: Box 28510

San Antonio, Texas 78228-0510

(512) 522-2710

 

Presented at

GRI WORKSHOP ON PLASTIC PIPE RESEARCH RESULTS

September 30 – October 1, 1992

 

GRI Principal Manager

Dr. Michael M Mamoun

8600 West Bryn Mawr Avenue

Chicago, IL 60631

(312) 399-8212

EFFECTIVE NEW TECHNIQUES FOR DISCHARGE OF STATIC ELECTRICITY ON INNER AND OUTER PIPE SURFACES

O. Tranbarger – Southwest Research Institute______________________________________      

ABSTRACT

Whenever high-velocity particulates, such as dust or iron-oxide, hit the interior wall of polyethylene (PE) pipe, electrostatic charge is generated by triboelectrification (friction or contact charging).  The interior charge will reside indefinitely within the pipe because of the high resistivity of PE.  The presence of interior charge produces exterior charge, which may result in a problem in cutting the pipe for repairs.  In both cases, a spark discharge can occur from:  (1) the exterior surfaces to nearby grounded objects; or (2) a grounded cutter penetrating the pipe wall.  A spark discharge in a flammable gas-air mixture produces an explosion or fire and must be avoided at all costs.

            To neutralize charge buildup, the surface resistivity must be lowered to be slightly conductive so that counter charge can flow from ground.  This is easily accomplished externally by wrapping the pipe with conductive material or wetting.  The interior charge problem requires that the pipe wall be penetrated with a grounded metallic cutter and spraying apparatus.

            This project has resulted in materials, equipment, and procedures for neutralizing both the internal and external charge accumulations.  An antistatic PE film wrap is used to replace the wet soapy burlap wrap presently used in the industry.  This is a dry process that eliminates:  (1) the mess in the trench from frequent wetting applications; (2) the freezing problem; and (3) the evaporation problem encountered using the burlap wrap.  For the internal problem, a standard tapping saddle was modified and fitted with a spraying apparatus to coat the interior of the pipe with an antistatic spray.  The antistatic fluid developed for the discharger also can be used on the exterior of the pipe for effective wetting.

INTRODUCTION

Charge Problem

            During leak repairs of PE pipe, serious electrostatic problems can occur because of triboelectrification (frictional charging) that produces charge buildup on the inside and outside surfaces of the pipe.  If the charge buildup is sufficient, a spark discharge can ignite a flammable gas-air mixture, or cause electrical shock to personnel near a charged pipe surface, or produce a pinhole1 in the wall of the pipe.  Spark discharges may have been the cause of ignition in several serious incidents and fatality2 cases that have occurred while repairing PE pipe.  Squeeze-off operations combined with particulates in the gas and contamination within the pipe contribute to the charge problem.  Even under apparently normal operations when the pipe is not being squeezed, pinholing is observable because of high-turbulent flow conditions occurring near TEEs, elbows, etc.  Particulates must always be present in the gas to produce triboelectrification.

Charge Removal Procedures

A common procedure used to reduce the electrostatic hazard is to wet the outside surface of the pipe.  The gas industry currently uses a wet soapy (surfactant) burlap wrap3 or tape to make the pipe slightly conductive to neutralize excess exterior charge. This procedure is beneficial; however, this exterior treatment of the pipe does not reduce the interior charge buildup.  Until the new technology was developed for GRI, no effective method existed for neutralizing charge inside plastic pipe, although this is a more serious problem than exterior surface charge accumulations.  The interior charge problem is evident after gas flow has been cut off, and a defective section of pipe is cut by suing a saw or circular cutter.  When a metal object penetrates the inner wall of a charged pipe, a spark discharge is inevitable.

GRI Project

Because of the serous electrostatic problems of PE pipe, GRI recently initiated a research project to develop methods and techniques for discharging PE pipe to eliminate the spark-discharge hazard.  This project has been a joint effort with the Brooklyn Union Gas Company as a co-funder.

Project Objectives

            The primary objective of the project was to develop a practical prototype electrostatic discharger that can neutralize charge accumulations inside PE pipe used for gas distribution systems.  Secondary objectives included:  (1) developing an improved method for dissipating charge accumulations on the outside surface of PE pipe; (2) developing instrumentation for measuring the electrostatic voltages on the inside and outside surfaces of PE pipe; and (3) investigating methods for improving PE pipe manufacturing to avoid electrostatic charge buildup.

Discussion

            The following discussion presents some of the research results pertaining to: (1) the electrostatic discharge of PE pipe; (2) instrumentation used for assessing charge conditions of PE pipe; (3) the test facilities constructed for charging PE pipe; and (4) laboratory and filed test data.

GAS INDUSTRY NEEDS

            When a repair crew arrives at the scene of leak, the first item needed is an electric fieldmeter to determine if charge is present.  The second need is for an internal discharger to neutralize the charge source.  The third need is a method to neutralize the external charge problem and eliminate problems encountered using the wet, soapy burlap wrap.  Frequent wetting can produce muddy conditions in the trench.  In freezing weather, the soapy solution can freeze.  Evaporation occurs in hot weather.

DISCHARGING PRINCIPLES AND PROBLEMS

Because electrostatic charges are immobile on high-resistivity PE surfaces, the surface resistivity must be lowered so that counter charge can be supplied from a ground path.  By making the surfaces slightly conductive, charges can be redistributed so that high-value point sources are eliminated, and charge mobility is then possible for neutralization from counter charge flowing from ground.

            In the case of internal charge, a surfactant is used to uniformly coat and wet the surface; thereby, lowering the surface resistivity.  Counter charge is provided through the cutting tool that penetrates the pipe, which is grounded.  The antistatic fluid used to coat the inside of the pipe also provides the conductive path between the inside surface and the cutter.

            Exterior discharging must be accomplished through a high-resistivity path to prevent any spark discharges that might ignite a flammable gas-air mixture.  The antistatic PE wrap meets this criterion; and therefore, is sparkless and safe to use for the external problem.

            One method usually suggested for discharging is a wire inserted into the pipe or spiral wrapped externally.  This method is not applicable to the pipe problem because of charge immobility.  Because of the high resistivity of PE and limited surface area of the wire, no current flows between a point charge on the surface and a ground wire. 

            Noncontacting radioactive and X-ray sources can result in charge neutralization but are impractical because of: (1) real or perceived health hazards; (2) size; (3) weight; and (4) cost.

INSTRUMENTATION PRINCIPLES AND PROBLEMS

The theoretical nature of the charge problem makes practical instrumentation for accurately assessing the charge conditions inside a plastic pipe difficult.  Because opposite-polarity charge can accumulate on the inside and outside pipe surfaces, cancellation or the electric filed produces masking effects.

            If the charges inside and outside are equal in magnitude and opposite in polarity, the net external electric field is zero.  However, under these conditions, the electric field within the pipe wall is the highest possible.  Therefore, a zero net electric field does not imply a charge-free condition.

            Although theoretical problems hamper the true assessment of the charge conditions, any electric field detectable external to the pipe implies that a charge problem exits.

            If a charge-free surface can be maintained on the outside pipe surfaces, then a Faraday cage can be used to assess the voltage inside the pipe.  A Faraday cage is a metallic coaxial cylinder (closed shield) around the pipe that has an electric field sensor mounted in the outside wall at the center.

SOLUTIONS

Internal Discharger

            Until this project no internal discharger was available for neutralizing PE gas pipe.  The internal discharger uses a self-tapping saddle fitting to deliver antistatic fluid in the interior of the pipe.  A clamp-on saddle is used for the discharger, which can be mounted and removed very quickly.

External Discharging

            The solution for the external discharging problem is to wrap the pipe using antistatic treated PE film that has a high resistivity but which is conductive enough to dissipated charge effectively through a ground path.  In addition to the PE wrap, the outside of the pipe can be sprayed with the antistatic spray developed on the project, which does not freeze or evaporate.

Instrumentation

            Hand-held electric fieldmeters (voltmeters) are commercially available to meet some of the instrumentation needs.  However, these instruments cannot be used for any quantitative measurements, since induced charge on the exterior of the pipe masks the effects of any internal charge.  Because of masking effects, high electric field conditions can exist in the wall of the pipe although the net external electric field is zero.  Since a zero-field measurement does not necessarily imply a charge-free pipe, a charge problem would exist if any charge is detected.

            Quantitative measurements can be made using a Faraday cage or a closed coaxial cylinder around the pipe, providing the exterior of the pipe is neutral prior to a measurement or test.  The Faraday cage is strictly a laboratory instrument and could not be implemented in a practical fieldable system. 

            A flat plate sensor also can be used for observing any external electric field emanating from a pipe.  This type of instrument is useful in wrap tests.  In operation, it is very similar to the hand-held devices and has the same limitations due to masking effects.

Antistatic Fluid

            One component of the internal discharger is the antistatic fluid that:  (1) is effective down to -20oF; (2) does not evaporate; and (3) is safe for use on PE surfaces without accelerating crack growth.  The antistatic fluid also can be sued for external applications with the PE wrap.

Laboratory And Field Tests

            Numerous laboratory tests and four pilot field tests on actual gas lines were conducted to validate the internal discharger and the techniques developed for complete discharge of plastic gas pipe.

INTERNAL DISHCARGER APPROACH

            Three designs were investigated for the internal discharger; (1) a cutting knife; (2) a heated needle; and (3) a modified tapping fitting.  The modified tapping fitting approach was successful and was recommended for development by gas industry advisors.  Two important requirements of the discharger are that: (1) the unit cost be $2,500 or less; and (2) the weight be 15.9 kg (35 pounds) or less.

Design

            Figure 1 shows the prototype discharger system used in the first three pilot field tests.  For this design, a standard reusable clamp-on self tapping TEE fitting was modified and used in a simple design for the internal discharger.  In the design, the original cutter was replaced by a special cutter that has a recessed side-mounted nozzle near the end.  The discharger cutter cuts a circular plug out of the pipe wall in the same manner as the original cutter.  The discharger cutter is smaller in diameter than the original cutter, since a threaded adapter sleeve is used for protecting the threads on the plastic tapping TEE.  The adapter sleeve also provides a convenient point for grounding the cutter.  After penetrating the pipe from the side and centering the nozzle, pressurized (typically 100 psi) antistatic fluid flows through the center of the cutter body and out through a nozzle. A check valve on the cutter prevents any gas from escaping to the atmosphere during the discharge procedure.   An inline filter within the threaded body of the cutter removes any particles that might clog the nozzle. A toggle (lever) valve at the cutter is used for applying the antistatic spray.  A rubber hose and a quick-disconnect fitting provide the hookup with the pressurized canister (lecture bottle).  The discharger design uses mostly stainless steel components and is estimated to have 8,000-10,000 hours MTBF. 

Operation

            In operation, the self-tapping TEE is bolted on the pipe to be discharged.  The threaded sleeve is grounded through a braided-wire strap connected to a stake screwed into the ground.  The cutter is screwed into the sleeve and turned clockwise (using a box-end wrench) to penetrated the pipe wall.   Although penetration of the pipe wall can be either on the top or side, the preferred orientation is through a sidewall.  After the cutter penetrates the pipe wall, it is further screwed into the sleeve until the nozzle is centered in the pipe.  (The threads are marked to indicate when the nozzle is centered in the pipe.)  The nozzle rotates (continuously) while applying the antistatic spray.  A typical discharge operation requires about 30-60ml (1-2 oz) of antistatic fluid.  Figure 2 shows the discharger mounted on acrylic pipe as the nozzle is being rotated. 

            Generally, gas flow is cut off via squeeze off before discharging.  Spraying from the side occurs by rotating the nozzle in a 0-180 degree upward arc either clockwise or counterclockwise.  As the nozzle rotates, the antistatic spray jet tracks the centerline of the pipe over a ±61 cm (±2 foot) range under no-flow conditions.  After spraying along the centerline of the pipe, fluid then runs down the sides of the pipe and uniformly coats the interior and makes contact with the grounded metallic cutter.  In spraying from the top, the nozzle must rotate 0-360 degrees to cover the inside surface of the pipe, and more fluid is required.

 

 

 

 

Clamp-On Fitting

            After the first three pilot field tests, the discharger system was upgraded to include clamp-on saddle fittings and a purging capability.  Figure 3 shows one of the modified fittings.  This fitting reduces the setup time required to mount the discharger and eliminates an overtorque problem that occurred with the previous fitting.  Purging enhances the safety of the discharge procedures.

EXTERNAL DISCHARGING APPROACH

            After reviewing the technical literature, an antistatic PE film was evaluated for the exterior charge problem.  The material used most extensively throughout the project was static dissipative polyethylene that is available in 2-6 mil thicknesses of various widths.  Its original application is for packaging electronic components, and it is typically manufactured in tubular form (double thickness).  In application, the antistatic PE film is spiral-wrapped on the pipe after grounding the starting end of a roll.  The loose end of the roll is then grounded after completing the final warp.  Grounding and wrapping provide a conductive path for counter charger to flow from ground to neutralize any excess exterior charge.  Application of the antistatic PE film eliminates:  (1) all external electric fields emanating from the pipe; (2) the “solution of dishwasher-type detergent” and associated mess in the trench; (3) frequent wetting applications in hot weather because of evaporation; and (4) freezing problems in cold weather.

            The PE-film wrap contains an internal amine-free, non-blooming organic antistatic agent that is essentially unaffected by relative humidity.  ASTM4 standards for antistatic PE films require a surface resistivity of typically 9 x 1011 ohms per square.  Tests on some antistatic PE-film specimens over 20 years in age show that the resistivity characteristics of the material are unaffected by long shelf life.  Besides the PE-film wrap having an intrinsic resistivity, the wrap is also useful in retaining and preserving antistatic fluid films on the exterior.

INSTRUMENTATION APPROACH

Faraday Cage

For the pipe application, the Faraday cage is a useful laboratory apparatus for determining the interior voltage.  The Faraday cage design for the pipe application is a closed coaxial cylinder around the pipe that provides a shielded chamber (annulus around the pipe) that can be purged with nitrogen for limiting charge leakage on the pipe exterior. 

Figure 4 shows the Faraday cage, which has a length of 61 cm (24 inches) and a diameter of 25 cm (9.875 inches).  The design of the cage includes a circular cutout in the ends for 10-cm (4 inch) pipe.  Adapters can be fitted into the 10-cm (4-inch) cutout for use on other pipe sizes.  An electric field sensor (Monroe Electronics 1019B) mounted on the outside wall at the center of the cylinder measures electric fields emanating from the pipe.  The voltage inside the pipe is given by the expression:

V = kE

Where:             E = electric field (V/m) at the wall of the cylinder;

                        k = geometric factor (m) of the cage; or

                        k = 0.1237 for 10-cm (4-inch) pipe; and

                        k = 0.2039 for 5-cm (2-inch) pipe.

            By purging the cage with nitrogen, good results were usually obtainable in preventing exterior charge accumulations inside the cage, although some tests showed residual charge effects following pipe discharge.

Flat Plate Sensor

Figure 5 shows another useful sensor, which is a grounded flat plate mounted 18 cm (7.2 inches) above the centerline of the pipe.  An electric filed sensor (Monroe Electronics 1019B) at the center of the 41 x 61 cm (16 x 24 inches) plate measured electric fields emanating from the pipe.  The flat plate sensor is applicable only as an uncalibrated instrument for assessing electric fields exterior to the pipe and lacks provisions for preventing charge leakage on the pipe exterior.  The flat plate sensor is ideal for monitoring electric fields during exterior charge neutralization as an antistatic PE film is wrapped around the pipe.

Hand-Held Electric Fieldmeter

            For field applications, the ACL Inc. Model 300B hand-held voltmeter was used to detect the presence of charge on pipe.  Although this instrument can detect charge, it cannot accurately measure voltage on the inside or outside because of the theoretical limitations (described above) resulting from exterior chare masking the interior charge.  Therefore, a zero measurement obtained using this instrument does not necessarily mean that a pipe is charge free.

ANTITSTATIC FLUID APPROACH

            The antistatic fluid for discharging plastic gas pipe is a critical component in the operation of the discharger, since it must not: (1) affect the PE pipe by accelerating crack growth; (2) burn: or (3) product corrosion in gas meters, etc.  Although water can dissipate charge somewhat, it forms beads on slick PE surfaces and does not coat the surfaces uniformly.  A surfactant is necessary to produce a uniform coating on slick PE surfaces.  Generally, antistatic fluids consist mostly of water and will freeze.  To prevent freezing, an antifreeze additive is necessary.

            A special formulation was developed that has proven effective in safely neutralizing static charge accumulations and in meeting the gas industry requirements.  The fluid is a noncorrosive, nontoxic, environmentally benign enzyme solution that can be used with or without an equally benign and environmentally acceptable freezing–point depressant (anti-freeze agent).  The enzyme solution is a commercially available degreasing cleaner, which is offered as a concentrate.  When diluted by 15-30 volumes of water per one volume of enzyme, the resulting solution effectively dissipates static electricity and energy from plastic surfaces.

TEST FACILITIES

            Three closed test loops were constructed from 10-cm (4-inch) pipes for charging various types of PE pipe sections (HD and MD; 10 –cm and 5-cm diameters) to determine the effectiveness of the new discharger technology.  Figure 6 shows a complex piping network that was used for the first test loop.  This loop included several elbows and TEEs to increase charge levels.  The second loop was a large outdoor rectangular (25 x 35 feet) facility using methane gas.  The methane gas atmosphere is not a factor in generating charge or in discharging.  In the final analysis, the test loop configuration is not critical in producing high charge levels.  The third test loop configuration was a simple rectangular loop with sides of 3.4 x 6.7 meters (11 x 22 feet).  Short sections of PE pipe were used to support (and insulate) the loops above ground.  The pipe sections were coupled using rubber hose and hose clamps for easy assembly and disassembly for cleaning.  A 3-hp blower motor also was used in the final test loop.  In designing the test loops, some concern was raised about heating effects resulting from closed circulation.  With the sizes used for the test loops, no perceptible heating effects were observable.

            Circulation of particulates is necessary for generating charge.  Particulates can be almost anything.  Based on experience, an inert-gas atmosphere is necessary to avoid an explosion.  This required purging the system with nitrogen and maintaining a slightly positive pressure in the loop.  Although various particulates were tried, the most effective one was Poly-grit® (blasting material), which is comprised of approximately 30-percent iron oxide.  This material is preferred because of the iron oxide content, which is similar to materials found in gas distribution pipes.  Electric fields approaching 80-percent breakdown (16kV/mm) were generated by using Poly-grit®.  As electric fields increase from charging, corona discharge begins to occur within the pipes that limits the charge levels attainable.  In circulating particulates in PE pipes, it is not always possible to predict the polarity of the charge, since both positive and negative electric fields were observed.  The polarity of the charge is not a factor in the operation of the discharger.

TEST RESULTS

            A total of 50 laboratory tests and four pilot field testes were conducted.  The laboratory tests were accomplished using three different test loops.   The pilot field testes were conducted at three different gas companies:  (1) two at Minnegasco; 92) one at Mountain Fuel Supply Company; and (3) one at Lone Star Gas Company.  Because of the number of laboratory and field tests conducted, only the most significant tests are described in this section.  The tests fall in two categories:  (1) external wrap; and (2) internal discharge.  All laboratory tests conducted using the antistatic PE film showed that exterior charge accumulations are neutralized by counter charge flowing from ground along the conductive surface path of the film.  In applying antistatic spray in the interior of a pipe, unequivocal neutralization occurs when the coating is uniform and counter charge flows from ground, through the metallic cutter, to the interior of the pipe, which has been made conductive.  In conducting the internal discharge tests, some low-level residual electric fields were observed after completing prescribed discharge procedures.   The most probable cause of any residual effects is charge leakage (within the Faraday cage) to the exterior of the pipe, which is difficult to control because of humidity conditions.  Another probable factor for residual effects is space charge5 within the pipe wall, since it has been shown that high electric fields can induce residual space charge effects in PE samples.  Other factors that might affect the outcome of a test include:  (1) initial charge magnitude, polarity, and distribution; and (2) different pipe materials.

            Low-level residual electric fields are not considered significant or hazardous.  In cases where this phenomenon occurred, no charge was ever encountered while cutting a pipe following a discharge test.  If complete discharge is accomplished where both the inner and outer surfaces are treated, the reduction of the exterior resistivity masks any residual electric field effects that otherwise would be observable.

Antistatic Wrap Test

            The flat plate sensor was mounted on 10-cm (4-inch) medium density (MD) pipe and used for evaluating the effectiveness of the antistatic PE-film wrap.  The space between the pipe and the flat plate sensor allows the pipe to be wrapped; therefore, electric field measurements can be made before and after wrapping.

Exterior Charge After Internal Discharge   

            Figure 7 shows an electric field being measured from an unwrapped PE pipe as particulates circulate through the pipe.  In case the hand-held sensor shows a needle deflection all the way to the right on the meter scale.  In Figure 8 where the pipe is wrapped, the sensor needle in centered, or zero plate sensor during the wrapping and unwrapping process.  When the pipe is fully wrapped, no electric field emanates from the pipe.  As the pipe is unwrapped, charge effects are evident again.  These results were repeatable for all tests using antistatic PE wrap.

In one discharge test conducted, the electric fields from the Faraday cage and flat plate sensors were monitored simultaneously.  For this test, the Faraday cage was mounted upstream from the flat plate sensor.  After this test, a residual field was observed at the flat plate sensor because of excess surface charge on the pipe.  The effects of discharging the pipe internally are observable in the data from the flat plate sensor downstream from the Faraday cage.  Excess surface charge of opposite polarity (negative) was left on the pipe.  After discharging the loop internally, outside surface charge was evident by rubbing the test pipe sections.  Discharges were experienced (felt and audible) during this process.  Figure 10 shows the two sensor responses during this test.

            The data from the flat plate sensor show the masking effects of exterior charge.   Although the internal conditions inside the pipe were neutralized (as shown by the Faraday cage data), excess charge accumulated on the pipe exterior.  To an observer in a trench making a repair, the use of the discharger would appear to increase the charge buildup on the pipe exterior.  Without lowering the surface resistivity of the pipe in some manner, the free excess surface charge would greatly increase the hazard of external discharge from the pipe.  Therefore, the discharger should never bus used until proper precautions have been taken to treat the external surface of the pipe.  Treatment of the pipe exterior will involve: (1) applying an antistatic spray (hand spray bottle) or mist to the pipe; or (2) wrapping the pipe with the antistatic film.

First Minnegasco Test

            The first Minnegasco test was conducted on a 10-cm (4-inch) main involving a two-way feed.  A break was simulated by a tapping TEE and venting gas through a vent pipe.  No high-charge conditions were encountered during the discharge procedure.  However, the vent pipe was slightly charged but was neutralized by the antistatic spray in the pipe.  A 1.5-by 2.1 meter (5- by 7-foot) trench approximately 1.8 m (6-feet) deep was excavated to expose the pipe, which was buried at a depth of 1.6 m (62 inches).  Figures 12 and 13 show a diagram of the test site and the shoring box used in the trench.

Prior to discharging, the discharger was checked as shown in Figure 14. 

Squeeze off was performed on each end of exposed pipe to stop gas flow. 

Figure 15 shows the fusion tapping TEE installed on the main fro a 1.6-cm (5/8-inch) PE vent pipe to simulate the leak.  The pie was wrapped using the antistatic PE film.  Figure 16 shows the wrapped section of pipe between the squeeze-off tools.  

The internal discharger was installed on the pipe approximately 41 cm (16 inches) upstream from the tapping TEE.  This point was approximately 76 cm (2.5 feet) downstream from squeeze-off tool #1.  The pipe was penetrated on the side as shown in Figure 17, and the antistatic spray was applied in a 360-degree rotation, starting from the top center of the pipe, spraying across the top of the pipe lengthwise upstream and then downstream.

TEST CONCLUSIONS

            The test conclusions that can be drawn from the laboratory and pilot field tests are listed below.

·            Using developed technology discharger after squeeze off in a no-flow condition proved to be effective and repeatable;

·            Both inner and outer pipe surfaces must be discharged;

·            The interior of the pipe can be successfully discharged using the modified tapping TEE discharger and the antistatic fluid spray;

·            The exterior charge can be neutralized by using the antistatic PE-film wrap;

·            Discharging the pipe interior will increase the apparent hazard of external discharge if the outer pipe surfaces is untreated;

·            Inner discharge is more effective by applying the antistatic spray from the sidewall and rotating the nozzle 0-180 degrees; and

·            Purging is necessary because of possible discharge when pipe wall is penetrated.

FIELD COMMENTS

In conducting the pilot field tests, participating personnel completed a questionnaire on

the equipment and procedures.  The following is a summary of some of the important field comments.

·            The antistatic fluid and PE wrap appear to be superior for treating static problems and have immediate field applications;

·            Field personnel felt the discharger was a good system and would be useful in repairing PE pipe;

·            The antistatic PE-film wrap and antistatic fluid were well received;

·            The discharger penetrates the pie wall easily;

·            Due to space limitations in the trench, a right-angle adapter would be useful; and

·            A quicker method was recommended for mounting the internal discharger.  One alternative might be a tool using a ½ -rotation cam lock lever.

BENEFITS TO THE GAS INDUSTRY

·            The occurrence of electrostatic-related incidents will be eliminated or reduced using the developed technology;

·            The antistatic wrap and developed antistatic fluid are very inexpensive and total field effective;

·            The internal discharger is economical and reusable;

·            Personnel safety will be substantially enhanced;

·            Potential property damage will be significantly reduced;

·            PE pipe repair can be accomplished with confidence knowing that all possible safety precautions are being taken; and

·            The antistatic fluid does not affect the PE pipe or the environment.

FINAL CONCLUSIONS

Charge Problem:  Unavoidable internal charging induces external charge.  The presence of excess charge on either the inner or outer surfaces can result n a spark discharge capable of igniting a flammable gas-air atmosphere;

Need:  Before pipe repair begins, static charge must be eliminated;

Solution:  To eliminate charge problems, the internal and external surface resistivities must be lowered so that counter charge can be produced through a conductive ground path;

Benefits:

·            A practical tool has been developed for the first time to neutralize internal charge;

·            Methods have been found to discharge the external surfaces more effectively than present gas industry practices;

·            Discharging can be accomplished safely; and

·            Electrostatic incidents will be reduced using the new technology.

FUTURE PLANS

·            Manufactures have been solicited to begin commercialization of developed discharger system;

·            The system upgrade has been completed;

·            GRI is selecting prospective vendors;

·            Additional filed tests will be conducted for three months min9ijmum by various participating gas companies;

·            The purpose of the field testes is to optimize and field ruggedize the discharger under blowing –gas conditions; and

·            Commercial systems should be made available to the gas industry by April-May 1993.

REFERENCES

1.            Staker, M., “Static Electric Pinholing Through Polyethelyene Pipe,” American Gas Association Distribution Conference, May 22-24, 1989.

2.            OPSO Report ID #870071, Delta Natural Gas Company, Inc., Corbin, Kentucky, 1987.

3.            American Gas Association (AGA) Plastic Pipe Manual For Gas Service, Catalog No. XR8902, Arlington, VA, February 1989.

4.            ASTM Method D-257-78, DC Resistance or Conductance Of Insulating Materials.

5.            Patsch, R., “Space Charge Phenomena In Polyethylene At High Electric Field,” J. Phys., D:  Appl. Phys., Volume 23, 1990, pp. 1497-1505.

 

 

 

 

 

 

 

 

STEP-BY-STEP FIELD PROCEDURES

FOR

DISCHARGING PLASTIC GAS PIPE

 

            Based on laboratory tests, a preliminary discharge procedure, consisting of a sequence of steps, was devised for the pilot field tests.  These preliminary steps were further revised because of the pilot field test results to include provisions for absolute pinhole protection and for purging.  A single discharge procedure is not entirely feasible for all leak situations because of differing requirements of each gas company.  For example, some gas companies are not concerned with the pinhole problem.  It is believed that cutting the gas off (via squeeze-off) is the highest priority for most gas companies when arriving at the scene of a leak.  Because of different requirements, the following discharge steps are only intended to be a general guide in approaching a field problem.  Any step that is not applicable to a specific field repair situation can be omitted.  It is anticipated that as more experience is gained with the discharger under blowing gas conditions in the commercialization phase of the project, further revisions will occur before the final discharge procedure emerges.  The equipment needed for discharging includes:  (1) a roll of antistatic PE film;  (2) two internal discharger units; an d(3) a spray bottle of antistatic fluid.  The antistatic fluid used for discharging is a special formulation developed on the project that does not freeze, or evaporate, or affect the plastic pipe.

Step 1 - Third Party Damage

            Step 1 involves third party damage such as a gouge or a puncture in the pipe that causes a leak.  At this point in the repair scenario, the pipe is buried, but the hole in the pipe is visible, and the gas is escaping.  A dust cloud probably results from the escaping gas, and it is very likely that high-charge conditions occur around the lip of the hole.

Step 2 - Excavation of Bellholes

            The objective of the step is to shut off the gas flow by digging bellholes (±3 meters, minimum, from leak) and squeezing the pipe on either side of the leak.  In exposing the pipe in these bellholes, exterior charge accumulations will occur if the pipe interior is charged.  An exterior application of antistatic spray using a hand-spray bottle prevents undesirable spark discharges.

Step 3 - Pin hole Prevention At Bellhole #1

            In some regions, pinholing might be a problem in performing squeeze-off operations.  Interior discharging in the squeeze-off area is necessary to prevent pinholing.  However, in performing this step, it will not be known how effective the spray will be unless an estimate is available on the flow conditions within the pipe.  Penetration of the pipe wall with the discharger is between the squeeze-off point and the leak.  The distance from squeeze off to the penetration point is typically five pipe diameters, minimum.  The discharger remains in place after neutralizing the potential pinhole area.  Note: Although the discharger is shown in a vertical position for illustration purposes, the preferred penetration orientation is from the side. 

Step 4 - Squeeze Off At Bellhole #1

            Following interior discharge in the squeeze-off area, the pipe is squeezed off to stop gas flow (at least in one direction).  Gas will continue to escape from the leak until the other side of the pipe is squeezed off; however, at this point, gas flow probably will diminish.

Step 5 - Pinhole Prevention At Bellhole #2

Pinhole prevention also may be necessary in Bellhole #2.  Using the second discharger unit, penetration occurs between the squeeze-off point and the leak at least five pipe diameters from the squeeze-off point.  After discharging the squeeze-off area, the discharger remains in place.  Like Step 3, it may be uncertain how effective the antistatic spray is unless information is available on the flow conditions, including direction of flow.  Note:  Although the discharger is shown in a vertical position for illustration purposes, the preferred penetration orientation is from the side.

Step 6 - Squeeze Off At Bellhole #2

            Squeeze off at Bellhole #2 completes the gas shut off process.  After squeeze off and gas cutoff, the gas cloud disperses so that repair can begin in the trench where the leak exists.

Step 7 - Pipe Excavation

            After the gas cloud disperses, application of antistatic spray (from a hand-spray bottle) neutralizes any high-charge conditions that might exist around the lip of the leak.

Step 8 - Pipe Excavation

            Application of antistatic spray (from a hand-spray bottle) continues in excavating and exposing untreated pipe surfaces in the repair trench.  If the wetted surfaces are grounded, then the antistatic spray treatment will be sufficient in making the exterior of the pipe conductive for neutralizing any exterior chare accumulations.  This is true only for the special antistatic fluid formulation developed on the project that does not evaporate or freeze.  The exterior treatment of antistatic fluid also augments the effectiveness of the antistatic PE film.  The combined fluid treatment and the antistatic PE-film wrap provide two conductive paths for the pipe exterior.  The PE-film wrap also traps the moisture from the spray treatment.

Step 9 - External Discharge

            The external discharge process begins by grounding the free end of a roll of the antistatic PE film or covering the end with dirt.  A piece of double-sided carpet tape (or equivalent) may be necessary for securing the first and last wraps around the pipe.  Spiral wrapping the entire length of pipe in the trench allows counter charge from ground to neutralize the excess exterior charge.  Burying the last-wrap end (cut to length) of the antistatic PE film provides additional grounding for the wrap.

Step 10 - Purge Repair Section

            The penetration holes used for pinhole protection shown in Step 3 and Step 5 are also necessary for purging in the preferred manner shown in Step 10.  The dischargers are replaced in this step by the purging saddles.  Nitrogen is then used for purging simultaneously at both bellholes.  The purging process should continue until the interior of the pipe is sprayed and neutralized near the leak. 

Step 11 - Internal Discharge

            Step 11 shows:  (1) the squeeze-off operation in the bellholes; (2) purging from the bellholes; (3) the antistatic wrap for exterior charge neutralization; and (4) the repair setup for interior discharging.  Grounded dischargers penetrate the pipe on either side of the leak (middle of pipe) and apply antistatic spray in both directions.  Audible arcing during spraying indicates successful interior charge neutralization.  The penetration point is 30-61 cm (1-2 feet) from the leak.  Although the dischargers are penetrating through the top of the pipe in the illustration, the preferred penetration point is through a sidewall.

Step 12 - Remove Dischargers and Wrap

            After accomplishing complete discharge, the dischargers and antistatic PE wrap are removed.  Also at this point, the purging source (nitrogen gas) can be shut off.

Step 13 - Remove Damaged Pipe Section

            Since the pipe is totally neutralized at this point, the repair process proceeds by cutting and removing the defective pipe section.  The pipe to be cut contains the holes from the dischargers; therefore, no repair procedures are necessary for these holes.

Step 14 - Pipe Repair

            The pipe repair is completed by using conventional techniques.  In the bellholes, fusion saddle fittings are installed over the holes made by the dischargers.  The saddle fittings are convenient for purging as required in the next step and for repairing the discharger holes.

Step 15 - Vent New Pipe Section

            Many gas companies do not release air in a line resulting from a repair.  Step 15 shows the right squeeze-off tool slightly released so that the repaired section of pipe can be vented.  The venting process can be accomplished from points 1-3 or 3-1.

Step 16 - Fill Bellholes And Repair Trench

            After venting, both saddle fittings are capped, and both squeeze-off tools are removed to reestablish service.  The final step is to backfill the repair trench and the bellholes.

ALTERNATE PROCEDURE FOR PINHOLE PREVENTION

            When the direction of the gas flow is know, alternative discharge steps can be followed to insure that pinhole protection is achieved in the squeeze-off areas.  In pursing the alternative discharge steps, Steps 1 and 2 would be performed first as described above.  These steps are followed by Step 3A and the succeeding steps described below.

Step 3A - Spray Downstream

            Like Step 3, the pipe is penetrated by the discharger in the left bellhole, and the antistatic spray is directed downstream stream (or with the gas flow) in the area where squeeze-off will occur.

Step 3B - Spray Upstream

            After the squeeze-off area has been sprayed downstream, the pipe is then squeezed off, and the discharger nozzle is directed upstream under no-flow conditions.

Step 3C - Double Squeeze Off

            A second squeeze-off tool is used in the left bellhole in a double squeeze-off operation.

Step 3D - Remove Squeeze-Off Tool

            The first squeeze-off tool is then removed as shown in Step 3D.  This configuration is the same as shown in Step 4, previously described.

Continuation of Discharge Process

            Because of backpressure and escaping gas downstream, the alternative steps (3A-3D) would be repeated in lieu of Step 5 described above.  When the second set of alternative steps is completed, the original steps would be continued, beginning at Step 7.

Alternative Purge Sequence

            The overall discharge procedure can be greatly simplified if there is no concern for pinhole protection.  Figure 38 shows the simplified trench configuration.  This configuration is also compatible with the viewpoint held by most gas companies where the highest priority is to shut the gas off as quickly as possible (via squeeze off) when arriving at the leak site.

            In the simplified procedure, bellholes would be excavated and the gas shut off via squeeze off.  Following this critical step, antistatic spray would then be applied around the lip of the gouge to discharge any high point charges that would probably occur.  Small-diameter plastic tubing would then be inserted into the pipe like a catheter for purging.  These purging tubes would extend up to the squeeze-off points. After the purging tubes are in place, nitrogen would be used to displace any gas-air mixture residing within the pipe.  Purging occurs from within the pipe through the gouge hole as nitrogen fills the interior of the pipe. 

            From this point, the trench is fully excavated for repairing the pipe.  The exposed exterior surfaces are treated by the antistatic spray and wrapped with the antistatic PE film to discharge the exterior surface as shown in Step 9 (figure 27).  Steps 11-13 (Figures 29-31) then follow in the procedure.

            If there is no concern about venting, the squeeze-off tools are removed, and the trench and bellholes are backfilled in restoring service.  If venting is necessary, a tapping saddle must be installed for this purpose.