Robert B. Grove and William H. Pugh
Allegheny Process Equipment

Polypropylene Process Equipment
in the Steel Industry

INTRODUCTION
HISTORY
ENGINEERING
STRENGTH OF POLYPROPYLENE
TYPES OF POLYPROPYLENE
LIMITATIONS OF POLYPROPYLENE
PROCESS EQUIPMENT CONSTRUCTION
COST EFFECTIVENESS
APPLICATIONS FOR POLYPROPYLENE EQUIPMENT
CONCLUSION

INTRODUCTION

With the current demand for new and rebuilt process lines there has been an explosion of growth in the plastic process equipment industry. Polypropylene equipment has gained acceptance around the world and is being specified as the material of choice for many applications previously considered inappropriate for its use. This interest in plastic equipment is driven by economics. Not only by the original equipment price, but, also, by the long term cost advantages. The USA is the world leader in the design and use of heavy wall polypropylene fabrications. Other countries around the world are looking here for the latest technology in this equipment.

HISTORY

Traditionally most equipment that was used for containing corrosive solutions and fumes was fabricated from steel with a coated exterior and with a rubber-lined interior. An additional acid resistant brick lining was often used to protect the rubber lining and to act as thermal insulation. This construction requires maintenance of all four components with four different crafts and takes a considerable amount of time for major repairs. Also, for brick lined equipment, the installation and replacement time is significantly long.

In the mid 60’s polypropylene was introduced to the steel industry. It offered many advantages over traditional materials and it quickly gained acceptance for relative light duty fabrications and, then, for larger tanks, etc.

Since there were no plastic design and engineering standards for polypropylene fabrications, most of the engineering was "trial and error" rather than scientific stress analysis.

Breaking the Barriers

While there was some resistance to the introduction of polypropylene in the metal processing industry, the obvious advantages of this material caught the attention of many engineering, production, and maintenance people. New designs were developed and tried with the collaboration of mill people and plastic designers. Many designs were successful and resulted in the advance of polypropylene as a viable alternative to traditional materials.

However, problems with some poorly designed and fabricated equipment made it difficult for many to believe that polypropylene could be used for pickling tanks, vertical plating cells, storage tanks, and other large fabrications. Their major concerns centered on temperature limitations, the expansion rate of plastic, and mechanical damage to polypropylene equipment.

Successful designs caught the imagination of innovative people in the metals and plastic industries and fueled the desire to develop an engineering criterion that could permit trustworthy stress analysis of new designs. Before dependable, large, mill duty equipment could be built, the necessary engineering standards needed to be in place.

From Guessing to Understanding
In general, the practice had been to design polypropylene process equipment by a "gut-feeling" process, partially based on extrapolating previous designs that worked or by "beefing-up" designs that appeared marginal. Recently, this approach has changed to a more scientific or engineered approach. Early engineered designs were based on research on the strength of polypropylene pressure piping. More recently, polypropylene process equipment structural designs have been in accordance with the DVS 2205, Part I issued by Deutscher Verlag Fur Schweisstechnik (a German design standard).

Today’s Design Criteria
          The most complete reference for the structural process equipment design engineer is the DVS-2205, Part I. "Design calculations for containers and apparatus made from thermoplastics characteristic values." This design reference addresses the recommended design values for material strength and creep modulus and how those design values vary with type of welding during construction, chemical exposure, design life, and type of loading, i.e. static or cyclic.
          Creep is a phenomenon in which at a constant stress level, the deformation continues to increase as a function of time. This is seen to occur in metals at elevated temperatures but occurs in polypropylene at all temperatures. The practical approach has been to consider creep effects in design by using the ten-year creep modulus to conservatively estimate deformations due to load.
          A successful polypropylene equipment design must not only address the allowable stresses of the material but must also consider the deformations under load and the thermal expansions experienced by the equipment. The thermal expansion of polypropylene can be expected to be nearly ten times that of carbon steel. This makes thermal expansion a significant concern to be addressed in the design of polypropylene process equipment.
          Satisfactory polypropylene process equipment designs have been executed following the DVS 2205, Part I stress limits associated with the ten year creep modulus for estimating deflections.

The Value of FEA
          The design of polypropylene process equipment, using the appropriate design parameters and material properties can be completed with a combination of hand calculations and computer run Finite Element Analysis (FEA).
          FEA becomes a necessity when the structure becomes statically indeterminate. That is, the load or stress distribution in the structure is dependent upon the deformed shape. FEA is also necessary to determine peak stresses at discontinuities, corner joints, and to verify strain limits etc. For a complete FEA analysis to be executed for polypropylene process equipment, a non-linear analysis should be accomplished. However, this is usually impractical due to time, lack of detailed material strength design data, and the relatively low allowable design stresses used. Satisfactory designs have been consistently accomplished by using a linear elastic FEA approach and by using a constant ten year creep modulus value for the design allowable stresses.
          Measurements of fabricated polypropylene structures indicate the linear elastic approach to be a viable and relatively accurate prediction of equipment deformations.

The Effects of Temperature
          Polypropylene strength will be significantly reduced with temperature. Polypropylene process tanks have been built that consistently operate in the 200 to 210^o F range with satisfactory service life. In addition to thermal effects on the mechanical properties of polypropylene, an increase in temperature is associated with an increase in the corrosive or oxidation effect of the tank contents.
          In the design of polypropylene process equipment it is necessary to be aware, of not only the material properties of the polypropylene, but also of how various construction details can be used to your advantage to provide a successful design for even the harshest environment.

STRENGTH OF POLYPROPYLENE

The strengths of polypropylene as a material for the construction of process equipment include:

• Thermal insulating properties
• Electrical insulating properties
• Readily thermo-formed
• Readily welded
• Can be cut to size with wood-working tools
• High corrosion resistance
• Long life span
• Not susceptible to damage by slight surface abrasion as a coated surface
• Low density and light weight (as compared with other materials)
• High chemical resistance

TYPES OF POLYPROPYLENE

Polypropylene materials of interest to the process equipment designer are generally limited to homopolymer and copolymer polypropylene.

Homopolymers are generally stronger and stiffer than copolymers. Process equipment provided to the steel industry must meet the requirements of heavy steel mill service. Copolymer polypropylene is the material of choice for applications that must stand substantial mechanical abuse. Typical homopolymer polypropylene has an Izod impact strength of 2 ft-lb/in at 73^oF. Copolymer polypropylene has an Izod impact strength of 8 ft-lb/in at 73^oF. This means that copolymer material is less brittle and more suitable for some applications. At extreme cold temperatures, homopolymer polypropylene becomes brittle and is susceptible to crack initiation and propagation from notches and sharp corners. At 0^oF copolymer polypropylene has the same Izod impact strength that homopolymer polypropylene has at 73^oF.

Flame retardant grades of polypropylene are generally used for covers, hoods, exhaust ductwork, and fume scrubbers for corrosive applications. Polypropylene sheet material is available with the Underwriters Laboratory rating of V0. Recently, at considerably more cost, a polypropylene Class A building material has been developed with a 25 feet per minute flame spread rating. Also available is a laminated sheet with two flame retardant polypropylene skins and a copolymer polypropylene core.

Temperature Extremes
          For process equipment, polypropylene has usable temperature range from 40^oF up to 210^oF. At low temperatures brittleness becomes a design consideration. At elevated temperatures, loss of strength and stiffness is a consideration.
          For low temperature service, a copolymer polypropylene material should be used. In addition to the design and maximum service temperature for process equipment, the minimum ambient temperature must be considered in selecting fabrication details, designing lift points and in the development of handling procedures.

Effects of Strong Oxidizers

Polypropylene process equipment has successfully been used to contain strong oxidizers such as:
          20% Hydrochloric Acid up to 210^oF
          15% Nitric + 5% Hydrofluoric up to 160^oF
         
          The DVS 2205, Part I has a reduced allowable stress for these special applications. In addition, the attention to details to minimize the amount of exposed hand and extrusion welding to extend service life.
          The affect of the strong oxidizers is to attack the polypropylene sheet, especially hand and extrusion weld areas, causing a chalky oxide surface. With time, the depth of the oxide attack increases. In addition to a general reduction in overall integrity, the oxide can make repairs difficult.

Process equipment can be fabricated from standard, readily available polypropylene forms. Polypropylene is available in various sizes of sheet, solid rod, pipe, and some molded shapes. Many small tank appurtenances can be fabricated by machining a solid rod.

Welding of Polypropylene
Welding of polypropylene is accomplished by three methods:

• Hot gas hand welding
• Hot gas extrusion welding
• Heated tool fusion butt welding

The DVS 2205, Part I recommends various design efficiencies for the three methods of welding. These factors presuppose complete mastery of the relevant welding procedure and that is executed by qualified, tested personnel.

Bending and Forming
Within limits, polypropylene sheet can be elastically deformed into a curved or arched configuration. Unlike steel, polypropylene does not have a well-defined stress-strain diagram. Therefore, to plastically deform the polypropylene material to yield is not practical at ambient temperature. For large radius to thickness ratios, elastic forming is possible. When elastic forming is not possible, polypropylene can be shaped by thermoforming. To thermoform polypropylene material, it must be heated to the point where it becomes soft and pliable, almost rubbery, then clamped into a forming fixture to cool. Radius corners for tanks, nozzle necks, etc. can be fabricated by thermoforming. Often square, sharp corners are required for process equipment. A bent corner can be made using a sheet bender. A bent corner is much stronger than a welded corner and can be fabricated in a fraction of the time of a welded corner. To make a bent corner, the bender has a heated sword that melts a V-notch. After this V-notch is made the sheet is bent and held until the two melted surfaces are fused.

Machining
Polypropylene is readily machined using metal cutting lathes, milling machines, drill presses or band saws with some modification to the cutting tools. By machining, more complicated geometries can be obtained.

Reinforcement
As the sizes of the polypropylene equipment and the design temperature increases, reinforcement becomes vital to maintaining the structural integrity. The type of reinforcement depends on the loading, tank geometry, plant space envelope limitations, and/or installation handling requirements. Reinforcement may be as simple as a polypropylene bar welded to another surface or to a more complex welded steel reinforcement frame encapsulated with polypropylene. Steel reinforcing can be any geometry necessary to support the structural requirements of the equipment. A word of caution, when using encapsulated steel, thermal expansion of the polypropylene material must be considered in working out fabrication details.

COST EFFECTIVENESS

The initial cost of process equipment fabricated from polypropylene is usually lower than traditional materials. However, this is a function of the complexity, the loads, the operating temperature, and the type of materials being replaced. For instance, polypropylene equipment is cost effective when replacing exotic metals or with tanks with carbon block linings.

Even when polypropylene fabrications are not initially cost competitive, there are long term advantages that favor its application for many types of process equipment. The total cost savings over the lifetime of the equipment generally outweighs any initial difference in price.

The installation cost is lower because the installation is simplified and much less time consuming, especially for cases where there is no longer a need for brick lining of tanks.

Maintenance costs are reduced since polypropylene is a monolithic material that has the same corrosion resistance throughout. There is no need for external maintenance since corrosion resistant coatings are not required. Also, internal maintenance is minimal and it can be accomplished quickly. Simple welding techniques are used to make repairs and modifications with out extensive preparation. There is no time lost for repairing and curing the rubber lining or bricking a tank. There are many reports of the significant reduction of maintenance costs through the use of plastic construction.

Due to the light weight of polypropylene equipment, the support structure is less expensive. It is also much easier to handle during installation and removal.

There is also a significant time savings when replacing tanks. For many pickling tank replacements this has often been accomplished over a weekend.

Because of the electrical insulating qualities of polypropylene, there are no current losses through the tank structure. There have been reports of 40% less current requirements on several tank replacement projects. The thermal qualities provide better insulation than tanks with brick linings.

APPLICATIONS FOR POLYPROPYLENE EQUIPMENT

As design and engineering capabilities have progressed, the applications for plastic process equipment have grown. Polypropylene fabrications are now fully engineered products that are being used for the following major applications:

• Strip tank covers – This is one of the first products made from polypropylene. Some of the original covers fabricated in the 60’s are still in service.

• Electrolytic cells – Because of the excellent insulating properties and corrosion resistance of polypropylene, this is an obvious choice for electrolytic cleaning.

• Tin plating trays – This was another early application that took advantage of the insulating and corrosion resistant properties of polypropylene.

• Batch tanks – These tanks are used for pickling and rinsing of tubing, pipe, bars, plate, rod, wire, shapes, etc. They have been fabricated in sizes up to 12’ deep, up to 72’ long, and up to 14’ wide.

• Fume exhaust systems – Polypropylene’s monolithic structure and the availability of flame-retardant grades makes this an excellent material for hoods, ductwork, and fume exhaust scrubbers.

• Primary and secondary containment tanks – Waste acid sumps up to 110,000 gallons capacity have been fabricated from polypropylene.

• Storage and circulation tanks – Vertical cylindrical and rectangular tanks are used for many aggressive hazardous solutions.

• Pit and trench linings – An excellent polypropylene lining material has been developed with molded anchors that lock the material into the concrete substrate.

• Strip cleaning tanks for galvanizing lines – These tanks have internal surge return channels for high strip speeds.

• Strip tanks for stainless pickling lines – Tanks for mixed nitric and hydrofluoric acids have been developed. They offer reasonable life and low cost. Several styles of turbulent pickling tanks are currently in use.

• Electrolytic ESS, electrolytic sulfuric, and electrolytic nitric tanks for stainless lines – Electrolytic pickling tanks have been designed to support up to 74,000 pounds of electrodes and to operate at temperatures up to 195 ^o F.

• Vertical plating cells – Recent designs have been developed for vertical polypropylene tanks for tin plating with methane sulfonic acid, nickel plating, electrolytic caustic cleaning, pickling, and rinsing.

• Fumeless wire, strip, and tube pickling lines – These systems permit cleaning, pickling, rinsing, and plating in equipment that does not require a fume exhaust system.

• Brush scrubber and cascade rinse tanks – Polypropylene bodies for this equipment extends the life and eliminates concerns about corrosion.

• Tanks for HCl pickling lines – All-polypropylene tanks for continuous, catenary type pickling lines have been installed at lengths up to 300’.

CONCLUSION

Polypropylene design and fabrication has advanced from the trial-and-error stage to the fully engineered stage that allows for the supply of trustworthy, mill duty equipment. The impact that polypropylene process equipment has made in the steel industry is the direct result of the development of an engineering discipline.

Present engineering and fabricating technology has led to a breakthrough into areas previously thought to be beyond the capabilities of polypropylene equipment. Future pickling, electrolytic cleaning, plating, and other process lines will use this material to its best advantages. They will operate at lower cost, with less maintenance, and with less environmental concerns.