Weld Joint Coating on Pipelines Coated with FBE

Technical Comparison between FBE Powder and Heat Shrink Sleeves

1 - Introduction

     Both joint coatings are used in the field and both have their advantages and disadvantages with regard to materials and application methods.

 

1.1 - Weld joint coating with epoxy powder

     The application process is the following:

     a) clean the exposed steel by sand or grit blasting to a degree of cleanliness of SA3 (or better)

     b) provide, by the same operation and the selection of the abrasive, a surface roughness of 80 to 160 microns (as required by the epoxy powder supplier) in order to create an adequate anchor pattern for the epoxy layer to adhere to the steel surface. 

     c) eliminate all sharp edges with a file or mechanic tool (all thin film coatings need this special care as part of the surface preparation).

     d) clean both edges of the adjacent line coating with a  solvent, to remove all traces of dirt, debris, oil, grease or other contamination.

     e) preheat the joint, with due consideration to the ambient temperature and the thermal inertia of the steel mass, up to 220C, to obtain a perfect fusion of the epoxy powder particles amongst themselves and with the steel surface. 

     f) apply the epoxy powder with an electrostatic spray gun (or flocker)

     g) allow to cure

     h) check the result

     i) paint holidays in the joint coating with liquid epoxy or FBE repair sticks

 

1.2 - Heat shrinkable sleeves

     The application is the following:

     a) clean the exposed steel by wire brushing to a cleanliness of ST3 or alternatively SA2 or SA 2 1/2 (some products require more surface preparation than others).
     b) clean superficially the two edges of the line coating.
     c) preheate the joint area to 80-160C (depending on the manufacturer's recommendation)
     d) if applicable, precoat the bare metal and adjacent FBE with epoxy primer
     e) wrap sleeve and secure closure
     f) shrink the sleeve on the pipe
     g) inspect

2 - Comparison
     Both processes give excellent results, provided they are applied in accordance with all applicable instructions.  However, all site operations made to date with the epoxy powder spraying method have shown a major drawback:  the process is complicated and not really appropriate for some field conditions. 

2.1 - Methods
     Fusion bonded epoxy powder needs to be applied under specific conditions and using specialized equipment that can only be properly controlled in a plant and at a very complex technical level.  It is not at all easy to ensure this same performance level under field conditions.

2.2 - Equipment

2.2.1 - The equipment required for the fusion bonded epoxy powder joint coating is comprised of the following:
     - a refrigerated storage container (the shelf and storage life of the epoxy powder decreases rapidly at temperatures exceeding 20C)
     - a mobile grit / sand blasting unit, that can achieve the required anchor pattern.
     - a solvent storage outfit with fire extinguishers
     - a screening, recycling and conditioning installation (fluidized bed) plus a primary electric power source.
     - an induction pipe preheating system, including a special generator set (single phase, 800 or more cycles, special cables and an induction coil per pipe size.
     - powder spray equipment with electrostatic spray gun
     - control equipment for constant monitoring of temperature and gelling
Most of the above equipment utilizes electronic components that suffer from voltage variations, temperature excursions and vibration (coming from the generator sets on the same frame). 

2.2.2 - The equipment required for heat shrink sleeve installation is:
     - electric or pneumatic wire brushes, or a simple sandblasting outfit
     - propane torches with propane bottles, or a simple induction heating system

2.3 - Skill of operators
     The complex application procedure of fusion bonded epoxy powder requires highly qualified technicians for both, powder application and equipment maintenance.  The application of heat shrink sleeves is a matter of a few hours of training. 

2.4 - Necessary manpower
     All the various activities involved with the storage / feeding / reconditioning and application of the epoxy powder obviously require much more manpower than the simple shrinking of sleeves.  Even when labor is inexpensive, this may become an important issue.  Very often, sites are in remote locations and only limited space is available in the living quarters. 

2.5 - Ease of creating supplementary crews
     If, during the construction period, more application crews become necessary, it is very easy to put together another crew of skilled heat shrink installers, as there is virtually no supplementary equipment needed.  On the other hand, every supplementary crew for epoxy powder needs new highly skilled specialists and a complete set of the equipment described in 2.2

2.6 - Life time of raw materials
     Epoxy powder has a limited life time, depending very much on storage and handling conditions.  A heat shrink sleeve can be kept in store for an unlimited period of time and at temperatures up to 55C

2.7 - Preheating temperatures
     The preheating temperatures for epoxy powder are in the range of 220C.  To obtain a uniform thickness, a uniform preheating temperature is an absolute must.
     Preheating temperatures for heat shrink sleeves are in the range of 80-160C, depending on the adhesives used.  In case of a liquid epoxy first layer, the preheating needs only to be at 60C for all adhesives.  Preheat temperatures up to 100C can be easily achieved with propane torches under all field conditions.

2.8 - Tie-ins
     For the epoxy powder, every tie-in crew also needs a complete set of equipment.  The equipment for tie-ins needs to be even more mobile than the standard joint-protection set. 

2.9 - Back-up equipment
     For the epoxy powder process at least one complete spare set of equipment needs to be kept on hand to avoid risking the complete shut down of the job.