Natural Gas Pipeline Co. of America recently employed a new pipeline-coating system during rehabilitation of a 25.4-mile, 26-in. pipeline section in South Texas.
Major components of the system were designed for CRC-Evans Pipeline International Inc., Tulsa, by Incal Inc., Houston. After manufacture in CRC-Evans' Tulsa facility, Incal tested and commissioned the unit in the field and provided project management and on-site services during the job.
The line-travel multicoating system automatically applies a rapid-curing, two-component polyurethane tar coating to pipelines in the field. The system, designed and built at the request of NGPL, is engineered to apply any plural component coating material on 24 to 30-in. pipe at rates up to 1 mile/day.
CRC-Evans began the detail design of the equipment in late March 1989 with completion scheduled for mid-July. The system was used between Aug. 21 and Oct. 4 on-site between Refugio and Victoria, Tex.
The 26-in. pipeline NGPL elected to rehabilitate was originally constructed in the early 1950s and coated with coal-tar enamel.
NGPL selected Protegol UT 32-10, a 4:1 polyurethane tar coating system touted for having excellent abrasion and impact properties and an ability to withstand high operating temperatures. Also, the pipe could be handled within minutes after coating without damage, due to the rapid curing of the material.
Coating application averaged 5,089 ft/day and a coating thickness of 23.5 mils for 11 operating days.
Material loss over the 11-day working period was 25.1% including drum loss of 5.3%.
Total material applied was 20,000 gal.
The system was able to coat in winds of up to 25 mph with losses estimated at less than 30%.
COMPONENTS
The line-travel multicoating system consists of a line-travel coater capable of applying a uniform coating of 20-24 mils at the rate of 1 mile/day, an equipment skid towed behind the side boom cradling the line travel coater, a pipeline heater, and a maintenance shop.
Because of the complexity of the system and the very short (30 sec) pot life of the material, a Toshiba EX40H programmable controller was selected to control the entire coating system.
The line-travel coater includes eight air-operated spray guns located around the pipe, a spray-tip reversing mechanism to permit the tips to be rotated 180 during operation to clear any tip blockage, and a mixing gun with static mixing tub for mixing the two material components.
Also included in the line-travel coater are a distribution manifold for dividing the mixed material into eight equal streams feeding the spray guns; a variable-speed travel motor; and an air blower, blow shoe, and brush to remove loose material from the line.
The coater unit can be opened for removal and installation on the line at any point.
The equipment skid consists of an operator control panel, a programmable controller, two hydraulically operated metering pumps with high mass line heaters to heat the material to spraying temperature, two Component A preheat tanks, pressurized Component B feed tank, an ambient Component A drum and feed pump, and an ambient Component B drum and feed pump.
Also part of the equipment skid are a solvent tank and solvent pump, a 75-kw generator, an air compressor and refrigeration dryer, an hydraulic power unit, and a crane for transferring material.
An umbilical cable connects the equipment skid to the line-travel coater and contains Component A supply and return lines, Component B supply and return lines, a solvent line, a control cable, and power cables for drive motor and blower.
UNIT OPERATION
Component A material (resin) is transferred from the ambient drum to the component preheat tanks where the material temperature is gradually raised to 140 F.
Material transfer between tanks is automatically controlled.
Component B material (isocyanate) is transferred to the pressurized feed tank. Component A and B materials are fed to the hydraulically operated metering pumps.
The metering pumps pump the Component A and B materials at the required ratio of 4 to 1.
The metered Component A and B materials are heated to the spraying temperature with the in-line high mass heaters.
Component A and B materials are mixed at the mixing gun located on the line-travel coater.
The mixed material is then broken down into eight equal streams at the distribution manifold and fed to the eight spray guns where it is sprayed on the line.
At the completion of a run, the mixer gun is turned off, and a solvent is pumped through the mixing gun, distribution manifold, and spray guns to purge all mixed material from the system.
A hand-operated spray gun is available. The gun allows for touching up holidays caused by tip clean-out operation, applying a second coat to square longitudinal seam found on pipe manufactured by A.O. Smith Co., applying a second coat to girth welds and to minor pipe imperfections such as small delaminations raised by shot blasting, and to hand spray the ends of the pipe not coated by the line-travel coater.
The hand gun is fed by the second metering pump and is independent of the first system.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.