Internal Coatings – Uses, Advantages & Limitations
The initiative of this post is to provide a brief information about different types of pipe Internal Coatings – Uses, Advantages & Limitations and even for linings, their uses, advantages and limitations.
Purpose of Internal Coatings
- Pipe internal corrosion prevention
- Flow improvement
- To minimize deposit formation
Types of Pipe Internal Coatings / Lining
- Fusion Bonded Epoxy (FBE) Coating
- Glassflake Coatings (Chemflake,Belzona, etc)
- PE (Polyethylene) Lining / ROTO Lining
Pipe External Coatings:
Stopaq, FBE, 3LPE, 3LPP
FBE (Fusion Bonded Epoxy) Coating
FBE (Fusion Bonded Epoxy) Coating
It’s a high performance anti-corrosion powder coating for moderate operating temperatures, generally suitable upto 80 °C.
For higher temperature application special FBE grades such as TK-216 (up to 95 °C) & TK-236 (up to 120 °C) can be qualified subject to Client’s approval.
FBE Coating Application Procedure
- Visual inspection before blast cleaning to check presence of oil, grease, etc.
- The steel surface is thoroughly cleaned by blast cleaning which remove rust, scale, etc and produces a rough surface finish. The roughness value required is 50 – 100 micron.
- Heating : Induction heating or oven heating, usually in the range of 180 to 250 °C
Oven Heating
The application and curing stage:
Internal surfaces of pipes are coated using spray gun, which travel from one end to the other end of the heated pipe at a uniform speed, while the pipe is being rotated in its longitudinal axis. After coming in contact with the hot surface, the powder melts and transforms to a liquid form. This liquid FBE film flows onto the steel surface and soon becomes a solid coating by chemical cross-linking,assisted by heat. This process is known as “Fusion Bonding”.
Standard FBE coating thickness rangesbetween 250 to 500 microns (0.25 to 0.5mm).
Cutback for FBE Coating
Thru-kote Sleeve
Used for joining FBE coated pipes as well as Chemflake/ Belzona coated pipe spools at site.Required for each schedule of pipe.
Thru-kote sleeve piping details
Cross-section of an installed Thru-kote sleeve
FBE Coating of site weld
After field welding of the pipe ends, FBE can be applied on the weld area as well.
FBE Coating Advantages
- Suitable for higher temp. compared to PE/ROTO lining
- Excellent adhesion to steel provides superior long term corrosion resistance
- Can be applied to various pipe diameters from 2” to over 48”.
- Can be applied in a wide range of thicknesses.
- Good chemical resistance under most soil conditions.
- Good abrasion and high impact resistance
Limitations of FBE Coating
- Maximum life of FBE coated pipe is 12 years, and if the plant design life is 25 years, one piping replacement is required.
- Can be applied only in shop
- Applicators not readily available
- Complex design
Internal Glass flake Coating (Chemflake, Belzona, etc)
Internal Glass flake Coating (Chemflake, Belzona, etc)
Chemflake/Belzona coatings are glass flake epoxy coatings, normally suitable upto 93 °C operating temperature (Belzona can be used upto 120 °C by using specific Belzona grades).
Suitable for higher pipe sizes & where Roto lining cannot be carried out because of the weight limitation of pipe spool.
Generally carried out inside a flanged pipe spool, for joining non-flanged pipe spools Thru-kote sleeves are used.
All flanges of Chemflake&Belzona coated piping shall be provided with weld overlay of suitable material prior to dispatch for internal coating.
Application stages:
Cleaning – blast cleaning. The roughness value required is 75 – 130 micron.
Spray – Normal airless spray or two-comp. Airless spray equipment.
Brush – Recommended for stripe coating and small areas. Care must be taken to achieve the specified dry film thickness.
Curing – 1 coat requires around 24 hr to get total dry film.
DFT : 2 x 750 micron (for Chemflake)
1 coat of 1000 micron (for Belzona)
Advantages of Glassflake Coating
- Good chemical resistance,
- Good solvent resistance,
- Good adhesion,
- Low permeability,
- Good weather properties,
- Excellent gloss retention.
Limitations of Glassflake Coatings
- Temperatures & pressure dependent – suitable only up to 10 bar pressure & for non-flowing or less velocity fluids.
- Not suitable for abrasive fluids / slurries
- May cause skin irritation,
- Requires recoating intervals
- Chalking (for epoxy)-Time Consuming
- Very costly
- Used only for less no. of spools, not for the entire project scope
- Design life is questionable, may be upto only 3 – 4 years.
PE & ROTO Lining
PE & ROTO Lining
Used for liquid service with high chloride as well as higher oxygen content.
Maximum operating temperature – 60 °C. Suitable for gas-liquid ratio values upto 300.
A PE liner consists of a number of Polyethylene pipe lengths, which are fused together and inserted into sections of carbon steel pipelines and flowlines. The CS pipe provides pressure containment & the PE liner provides corrosion protection.
PE Lining
PE & ROTO lining is carried out only after the pipe spools are fabricated & hydro tested. No welding is allowed on the pipe spool once the PE or ROTO lining is done. Trunnion member & line stop members shall be welded prior to the lining.
Hydro testing of the spool or pipelines is done before the lining & after the lining also. Therefore, gaskets are required to be considered for each flanged joint for hydro test purpose.
Retainer rings
Retainer rings are used at the flange joint between PE to PE & PE to Roto – for pressure containment & to avoid deformation of PE stub end during service.
Retainer rings are supplied by PE lining vendor.
PE Lining Dimensional Limitations
Longest continuous length of liner depends on diameter and wall thickness. For off-plot piping scope the PE lining can be done for a pipe spool of upto 300 – 400m length because the liner can be put across only one expansion bend with 30D bend radius.
For shop lined piping the maximum length of PE lined pipe spool is considered as 18m because of the transportation limitations. Minimum pipe spool length requirement is 5m (can be as less as 2m if agreed with vendor).
PE lining is applicable only for straight pipe spools. For pipe spools with reducers or branches ROTO lining is used.
PE / ROTO lining cannot be carried out for pipe spools with orifice flanges.
In this case i) use a suitable material (may be DSS) for the upstream & downstream pipe spools & the orifice flanges, or ii) use carrier rings with orifice tapings & orifice plate of the suitable material.
Annulus Vents for PE lined pipe
Every PE lined pipe spool shall have vent points near the flanges for venting.
The purpose of venting is as follows:
- To vent the (ambient) gas from the pipe/annulus during installation.
- To vent the permeated fluids accumulated in the annulus to prevent collapse during service.
- To allow monitoring of the integrity of the PE liner during the service life.
- The vent points to be provided with valves for oil & gas application & without valves for water service application.
ROTO Lining
It’s the method of lining the inside of pipes or other parts with a seamless, one piece inner layer of plastic. Lined spool is produced by heating and rotating a carbon steel spool with a granular polymer placed inside the pipe spool. The polymer melts and forms a liner on the internal surface of the carbon steel pipe. Also, the polymer forms a bond with the metal.
The choice of polymer is based on the chemical resistance properties required of the final pipe spool. PE, PP, PVDF or number of other polymers are used for Roto lining application.
Lining thickness – from 2 mm to 8 mm.
The heavy lining thickness allows post machining of critical surfaces that would not be possible with a thinner lining applied by other methods.
Roto lining Procedure
The loaded pipe /fitting is heated while being rotated about two perpendicular axis.
Flange joint details for PE / ROTO lining piping
Width of Retainer Ring, A = (2 x B + 2 x T) – 3mm
PE/Roto lined flange joint detail
Advantages of ROTOLINING
- Seamless construction with a very smooth interior surface.
- Excellent chemical resistance and an excellent metal to plastic bond.
- Thicker lining & uniform wall thickness can be achieved than electrostatic or spray coating.
- Thicker coating can be repaired by welding if mechanically damaged. Thin coatings must be stripped and recoated, if repairs are not possible.
- Drastically reduces permeation through coating and possible corrosion of the metal substrate.
Disadvantages/Limitations of ROTOLINING
- Requires flanged joint at every directional change.
- Suitable only upto 60 °C
- Factory applied
All these above information covers all basic information about Internal Coatings – Uses Advantages & Limitations. However for further details you can check with coating vendors.
