This blog covers the basic information about procedure for coal tar coating and wrapping of underground pipelines including, minimum requirements for materials cleaning, priming, coating & wrapping, inspection and testing of Coal tar coating to be provided on the field joints of underground steel lines.
Materials to be used for Coal Tar Coating:
All materials used normally conforms to the quality and testing requirement indicated in AWWA C 203-73 or ASTM standards.
Coal Tar Primer
Coal tar Primer usually conforms to the characteristics given below,
The primer consist of processed coal tar pitch and refined coal tar oils only, suitably blended to produce a liquid coating which may be applied by brushing or spraying.
- Curing time to touch at normal humidity at 30 deg C (70%)- Less than an hour
- Maximum boiling point 215 deg. C of solvent
- Penetration of residue 100 gwt 5 sec (ASTM D5-597) Under 7 at 25 deg. C
- Softening point of residue (ASTM D 36-26) 104 deg. C (Min.)
Coal Tar Enamel
Coal Tar enamel conform to the characteristics given below,
Coal tar enamel composed of a specially processed coal tar pitch combined with an inert mineral filler. No asphalt of either petroleum or natural base to be acceptable as part of the ingredients. The enamel have the characteristics conforming to AWWA C-203 or equivalent. Other physical characteristics as follows:
MIN. MAX.
i) Softening Point (ASTM D 36) 105 C _
ii) Filler (ash) ASTM A 271 25% _
iii) Fineness filler, through 200 mesh (ASTM D 546) 90% 35%
iv) Specific gravity at 25 deg. C (ASTM D 71) 1.40 1.60
v) Penetration (ASTM D 5)
- At 77 deg. F – 100g. wt – 5 sec. 10 20
- At 115 deg. F – 50 g. wt – 5 sec 15 35
vi) High temperature test at 162 deg. F (Sag) AWWA C 203 _ 1.6mm
vii) Low temperature test at 20deg. F (cracking) _ None
viii) Peal test No peeling
ix) Spark test 10,000 Volt low amperage None
Fibre Glass Wrapper for inner wrapping
The wrapper conform to the characteristics given below,
Reinforced fibre glass wrapper – 150 mm width, consisting of a uniform bonded mat. The binder to be inert to coal tar enamel.
The fibre glass mat to be thin, flexible & strong. The glass fibre to be longitudinally reinforced by continuous filament glass embedded in the mat at 9.35 mm nominal centers.
- No disbanding of individual glass fibre to occur during or following the embedding process.
- The fibrous glass mat not to give rise to any air gap when applied.
- The wrapper to be sufficiently porous so that it can be embedded in the hot coal tar enamel.
Other physical characteristics to be as follows:
- Weight 45 – 50 gms/sq.m
- Normal thickness 0.5/ 0.1 mm
- Longitudinal tensile strength (20” x 5” wide specimen) 45.36 kg min
- Tear strength in the transverse direction 3.63 kg
- Moisture absorption (by wt. at relative humidity of 95% at 50 deg. C for 24 hours) 5% (max)
Kraft paper for outer wrap
The Kraft paper for outer wrapping to be water proof impregnated with coal tar and conform to the characteristics given below:
When unrolled at temperature from 0 deg. C to 50 deg. C, the wrapper to not crack or stick to such and extent as to cause breaking or tearing.
- Weight 175 gm/sq.m
- Thickness 0.25 mm
- Tensile strength per linear inch 23 kg(min)
- Moisture absorption, (percentage of weight at a relative
- humidity of 95% and 50 deg. C for 24 hours. 5% (max.)
Surface preparation for Coal Tar Coating
The surface to be wrapped and coated to be properly cleaned. Surface to be free of all rust, grease, dirt, weld scales, weld burrs etc.
Surface preparation to be immediately followed by priming.
Priming
The primer to be applied on the pipeline with uniform coating leaving no drips, floods, sags or runs. Any holidays or unprimed places to be reprimed immediately. The surface to be dry at the time of applying primer coat. Primer not to be applied during rain or fog. Drying time to be 24 hours.
Coating by Coal Tar Enamel
Enamel to be placed in a melting kettle and heated at a uniform rate to 240 – 250 deg. C or as recommended by the manufacturer.
Coating material to be stirred properly in the kettles at regular intervals of maximum 15 minutes.
First coat of coal tar enamel to be poured and spread over the primed surface to a maximum thickness of 2.00 mm.
The application to be so manipulated as to achieve a uniform thickness throughout the surface. The variation in thickness not to exceed 0.5mm.
Inner wrapping with glass fibre mat
After the first coat of coal tar, the fibrous glass mat to be applied on the surface while the enamel coating is still tacky. The wrap to be applied with a lap of 25 mm minimum.
Final coat of enamel & Kraft paper wrapping
Immediately after the application of the first inner coating of enamel and wrapping of the mat a final coat of coal tar enamel to be applied.
Along with the final coat of enamel, Kraft paper to be spirally wound round the pipe with a minimum of 25 mm overlap on each spiral.
The construction contractor to furnish and operate at his own cost an electric powered holiday detector of 10,000 volts output for certifying that lining is free from holidays.
Spark testing to be done after minimum of 24 hours after the final coat.
