ROTOLINING COMES OF AGE

A SEAMLESS LINING TECHNIQUE FOR PIPES AND VESSELS

Brian Gardiner · RMB Products, Inc. · 1201 RMB Court · Fountain, CO 80817

ABSTRACT

Rotolining is a unique process that has recently seen significant advantages in part complexity and material performance.  This technique allows for a seamless polymer lining to be applied to the interior surfaces of metal structures.  The use of high performance thermoplastics in rotolining assures that this process will continue to grow and be accepted as an industry standard.

INTRODUCTION

Rotational molding has traditionally been used for molding large, simple objects. Typical items would be agricultural tanks, refuse containers, toys and recreational products.  Historically, the process has been limited to a small menu of materials, primarily polyethylene and vinyl plastisol.  Rotational molding has been an under utilized technique for processing advanced, high performance plastics.

Several processors have used a variation of rotational molding to line fittings and vessels with various polymers.  Rotational lining is an ideal method to line the internal surfaces of complex structures.  Until recently, rotolining has been limited to relatively small items due to the size of equipment available.

DESCRIPTION OF THE PROCESS

The process is simply a method of applying a uniform layering of material to the interior surfaces of a metal structure.  In the case of a free standing plastic part this would be the mold.  For a lined fitting or vessel, the actual metal component would provide the internal cavity.

The various materials used require different surface preparation techniques.  When properly prepared, a rotolined part will have excellent plastic to metal adhesion.  Full vacuum service at elevated temperatures is possible.

After preparing the surfaces to be lined, the fitting or vessel is mounted to the molding machine.  The polymer, which is in a granular form, is placed in the fitting.  The loaded fitting is then heated while being rotated about two perpendicular axis simultaneously (Figure 1).  Heating the part is usually accomplished by cycling a differential mechanism, with the fitting attached, through a forced air oven.

During the heating cycle, the polymer particles begin to stick to the hot metal substrate.  A skin is formed.  This skin gradually forms a homogenous layer of uniform thickness.  Ultimate wall thickness is determined by the amount of material that is initially placed into the cavity.

Theoretically, all surfaces of the cavity should be in contact with the material mass an equal amount of time.  Rotolining is a tumbling and coating process, there is no centrifugal force or "throwing" of the material.  The viscosity of the molten polymer is such that it never becomes liquid, there is no "sloshing".

The process itself introduces no force or shear to the material.  The result is a relatively stress free lining.  Rotolined parts are completely seamless and weld free.

After a predetermined time at a specific temperature, all of the plastic is distributed over the surface of the vessel.  The fitting is then cooled by a combination of forced air and water mist.

The part is then removed from the machine and surfaces such as flange faces and "O" ring sealing areas are machined into the plastic.  Linings are spark and ultrasonically tested to insure liner integrity.

Because it is desirable to form a uniform lining thickness, there are a number of variables that must be controlled.  Rotation ratio, time/temperature parameters, thermal conductivity of the metal and part geometry all have an effect on lining thickness consistency.

BENEFITS OF ROTOLINING

Recent developments in rotational molding technology have increased the size capacity for rotocast pipe and vessel lining.  Columns, vessels and fittings up to seven feet (2.1m) in diameter and twenty one feet (6.3m) in length can now be lined.

Figure 2 shows a typical structure that has been rotolined.  This absorber column was lined with 0.20 inch (5.1mm) ETFE fluoropolymer.  The main body section is 22 inches (559mm) in diameter and approximately 12 feet (3,658mm) in length.  Material of construction is carbon steel with #300 ANSI flanges.

One of the unique features of rotolining is the ability to apply a relatively uniform layering of polymer to a very complex interior pipe geometry.  Figures 3 and 4 show examples of complex shapes that have been lined using this process.  This allows for a substantial reduction in the number of flanged connections when compared to other construction methods.

Thicker linings can be applied using the rotolining process than by any other technique.  The rotational molding process allows a seamless lining up to 0.5 inch (13mm) thick to be applied to the interior surfaces and gasket faces of metal structures.  The heavy lining thickness allows post machining of critical surfaces that would not be possible with a thinner lining applied by other methods.  Figure 5 shows a pump casing that has been extensively machined after the lining process.

Figure 6 is a 42 inch (1,067mm) diameter, long radius elbow that was lined with 0.5 inch (13mm) thick cross linked polyethylene.  This is the largest fitting that has been rotolined to date, weighing over four thousand pounds and having a 72 inch (1,829mm) center to face dimension.  Three hundred and seventy-five pounds of polymer were applied to this fitting.

This hybrid approach of combining plastic and metal construction has many advantages for the customer.  The corrosion resistance, purity and cleanability of plastic are combined with the structural reliability and dimensional repeatability of metal.  Lined tanks and vessels can be built to ASME Code.  Figure 7 shows several code vessels that were lined with ETFE fluoropolymer.

Virtually any type of metal weldment or casting can be rotolined.  Carbon steel, aluminum, 304 and 316 stainless steel and various castings and forgings are common.  Carbon steel welded fittings with flanges up to #1,500 ANSI have been lined using this process.

Typical structures that would be rotolined are columns, tanks, filter housings, large fittings and complex weldments.  Figure 8 shows the largest rotolined structure to date.  This is a packed column sixty-five feet (19.8m) in length and twenty inches (508mm) in diameter.  This column was built in thirteen sections and was lined with over 1,200 pounds of ETFE fluoropolymer, having a nominal thickness of 0.20 inch (5.1mm).

LINING MATERIALS

Polymer linings have excellent chemical resistance, relatively high temperature performance and an excellent plastic to metal bond.  Available materials include Cross Linked Polyethylene, PVDF, ECTFE and ETFE.  Each of the various polymers that can be used for rotolining are useful in different applications.  There is a wide price differential between the low cost polyolefins and the higher performance fluoropolymers.

Typically, Polyethylene is used for low temperature, moderately corrosive service.  This would include sea water piping, some mild acids, caustics and brines at temperatures up to 180F (82C).

Fluoropolymer linings of PVDF, ECTFE and ETFE offer higher chemical resistance and higher temperature service in specific applications.  Depending on the chemical service, fluoropolymer linings can be used up to 300F. Please refer to material manufacturers published data for temperature and chemical compatibility information.

ROTOLINING COMPARED TO OTHER LINING METHODS

There are a number of advantages which rotolining has relative to other coating and lining processes.

When compared to electrostatic spray coating or dispersion coatings, rotolining allows for deposition of a much thicker material build up.  In addition, the rotolining process enables a lining to be applied to the interior of complex parts that would be inaccessible to an electrostatic or spray coater.

Because the material is evenly distributed during the rotolining process, a much more uniform wall thickness can be achieved compared to spray on methods.  Both electrostatic and spray coatings are manually applied by an operator who has limited control over how much material is applied to the entire surface of a part.

One advantage that a spray on coating has is the ability to line the exterior of a part.  Rotolining is limited to interior surfaces only.

Compared to sheet lining, rotolining offers several advantages. Sheet lining involves bonding an extruded plastic sheet to the interior surfaces of a tank or vessel.  This sheet stock often has a fabric backing fused to one side to enhance the adhesive bond.  After the tank is lined, the seams are welded and sealed with a cap strip.

Sheet lining is a very operator sensitive process, the bonding and welding steps require very skilled technicians.  It is difficult to sheet line complex surfaces such as elbows and reducers.  The sheet must be shaped to conform to the pipe interior prior to bonding.  Rotolining naturally conforms to virtually any interior geometry without the need to work and possibly stress the polymer.

Sheet lining can be applied to large vessels that exceed the size limitations of the rotocast equipment that currently exists.  However sheet lining requires that the fabricator have access to all interior surfaces to install the sheet and weld the seams.

SUMMARY

Within the size limits, rotolining is unique in its ability to:

1. Apply a relatively uniform lining to extremely complex shapes.

2. Provide a bonded liner without the use of adhesives.

3. Line fairly large structures without welds or flared flanges.

4. Apply thicker linings than can normally be installed with other methods.

Although not universally known in the corrosion control industry, rotolining promises to continue its advancement and growth.

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