-Plastics However, it weighed only 1600 pounds as compared t o the 7940 pounds for steel. Shipping costs and erection costs were much lower for the plastic tank. Likewise, the plastic tank required no maintenance for its surface because of its good chemical resistance. The low thermal conductivity of the plastic tank led to a further saving by cutting down loss of gas ( 1 5 ) . SUMMARY
1
Plastics are relatively new, untried materials on the chemical industry construction scene. I n recognition of this, t h e intent of this presentation has been to provide information which will help the designer orient his thinking about plastics and what they can do so t h a t he may make a sound decision as t o whether he should consider plastic material or one of the conventional materials in a given application. If the decision is to use plastics then he should consider which of the various generic groups will best perform the required function. The final step of choosing a particular formulation or composition and developing the economics is one t h a t is necessarily worked out with the plastics material supplier and the fabricator. REFERENCES (1) Adams, C. H., Am. Soc. Memo to Sec. F, Sub. 1953.
(8) Ibid.. KO.6, p. 91. (9) Carswell, T. S., and
Nason, H. K., Am. Soc. Testing Materials, Philadelphia, Pa., Special Tech. Publ. 59, p. 31, 1944.
(10) ( 1 1) (12) (13) (14)
Ibid., Ibid., Ibid., Ib(d.,
p. 35. p. 335. pp. 336-7. p. 341.
Carswell, T. S., Telfair, D., and Haslanger. R. U., M o d e r n Plastics, 20, NO. 6,79-82 (1943). (15) Chrm. Eng.. 59, No.12,218 (1952).
(16) Ibid., p. 219. (17) Chem. E n g . News,32, No. 22, 2180 (1054). (18) Ibid., No. 27, p. 2696. (19) Dietz, A. G. H., presented at annual -4S;CIE meeting, December 1953. (20) Glick, S. E., I n d i a Rubber WorZd, 125, 192-4 (1951). (21) Grimm, G. O., Brit. Plastics, 19, No. 212, 22-30 (1947). (22) Larson, W. S., Product Eng., 15, 845-8 (1944). (23) Mason, J. P., and Manning, J. F., “Technology of Plastics and Resins,” p. 53, Van Nostrand, New York, 1945. (24) Ibid., p. 54. (25) Ibid., p. 115. (26) Manufacturing Chemists’ A4ssociation, Washington, D. C . , “Technical Data Book on Plastics,” October 1952. (27) M o d e r n Plastics, 31, No. 77, 73 (1954). (28) hIoore, H. E., and Moore, M. B., “Textbook of Materials of Engineering,” McGraw-Hill, New York, 1941 (29) Reinhart, F. W., SPE N e w , 4, 3 (1948). (30) Rose, K., Materials & Methods, 37, No. 1, 87 (1953). (311 Sasso, J., “Plastics Handbook for Product Engineers,” pp. 10252, McGraw-Hill, New York, 1946. I
Testing Materials, Philadelphia, Pa., VI, ASTM Committee D-20, August
(2) Ibi$.;Special Tech. Publ. 132, 1952. (3) Adams, C. H., and Taylor, J. R., I b i d . , Preprint 54-SA-68,1954. (4) Adams, C . H.. Findley, W. N., and Stockton, F. D., Dresented (5)
Construction 1Materials
a t annual ASME meeting, December 1954. Bruner, W. M., and Waynes, P. J., Chem. Eng., 61, No. 7, 38,
f1953). ‘6) Ibid., p.’ 194. 7 Campbell, J. B., Materials & Methods, 38, No. 5, 89 (1953).
(32) Staff, C . E., Quackenbos, H. M., end Hill, J. M., M o d e r n Plasfics, 27, NO.6, 93-6 (1950). (33) Underwriter’s Laboratories, New York, N. Y., “Standards for (34)
Thermoplastic Insulated Wires,” 1954. Yustein, 5. E., Winans, R. R., and Stark, H. J., A S T M Bull.,
196, 29, 1954. RECEITEDfor review September 17, 1954.
ACCEPTEDM u o h 21, 1Q5&
Engineering Plastics into Chemical Plants Maximum economic utilization of plastics as engineering materials cannot be effected until the design engineer, the construction engineer, and the maintenance and safety engineers have more complete and specific information. Engineers require measured data that will permit design and construction of equipment with assurance that such equipment will perform adequately under process conditions. Data such as chemical resistivity, stress to rupture, creep strength, and fatigue strength should be available for the entire range of allowable working temperatures. The materials employed should be of standard quality and dimensions. Standard methods of fabrication, erection, and maintenance must be developed. “Areas of ignorance” are emphasized in order to encourage development of fundamental data to keep pace with the growing application of plastics.
H. E. ATKINSON E. I . d u Pont de Nemours 6% Co., Wilntington, Del.
P
LASTICS occupy the headlines today in prominence and
profusion The uninformed reader may easily conclude that a line of new engineering materials is fully developed to solve the corrosion problems of the chemical industry or t o effect cost reduction. However, before maximum economic utilization of plastics can be effected, engineers must have more complete and specific information. The purpose of this article is t o show the
July 1955
needs of design engineers, construction engineers, and maintenance and safety engineers. A review of the sequence of events in engineering plastics into chemical plants will also point out the important role of research chemists, resin manufacturers, molders or extruders, fabricators, and the engineer himself in satisfying the needs. It is unreasonable to expect t h a t all data required on plastics
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“
Figure 1.
Plastic flange guards
will be forthcoming immediately in such a young fast-growing industry. There are so many plastics, so many required properties, and so many potential uses, that only time and directed effort will suffice t o provide the data If research is t o be guided, however, and if necessary development and standardization programs are t o be complete, engineering requirements must be understood. Engineers require measured data t h a t wiII permit design and construction of equipment with assurance t h a t such equipment will perform adequately under process conditions. Calculated or estimated data are insufficient for a truly engineered structure. Engineers cannot afford t o take undue risks in such a complicated industry as chemical manufacture with its continuous processe~, complicated equipment, and large investment. “Areas of ignorance” and the precautions believed vital to the best engineering practice with plastics are emphasized here not t o discourage the use of plastics but t o encourage the development of fundamental data t o keep pace with the growing application of plastics. The train of events in engineering plastics into useful equipment is complicated. The principal phases are material selection, equipment design, material specification, fabrication, installation, and maintenance. Safety and economic considerations pervade each of these phases. Many problems cannot be answered adequately a t t h e present stage of development in plastics technology. There are many data, but they are scattered and generally cover only a narrow range of the variables. I n reviewing the sequence of engineering events, the following terminology is applied t o plastics: “Type” refers t o the basic material, such as polyethylene or polyvinyl chloride. “Class” refers t o the general composition, such as nonplasticized, plasticized, or pigmented. “Grade” refers t o identification of a specific material by such properties as composition, melt index. density. MATERIAL SELECTION
When p1ast)ics are added to the list of engineering materials, additional considerations are added t o the problem of material selection. Flammability is a serious consideration from the safety standpoint, particularly in extensive installations which could promote the rapid spread of fire. For many applications i t is becoming manadatory t h a t only the self-extinguishing plastics be used. This can be a serious limitation to the application of several
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important types of plastic, such as polyethylenes and styrenes. Research must be pointed t o developing nonflammable or selfextinguishing types or grades of material if the full engineering potential of plaatics in chemical plant8 is to be realized. Chemical resistivity is a property of plastics that has received great attention b y industry The resistivity of individual materials wi1I not be detailed here, but a plea must be made for more data concerning exposures above room temperature and for more complete evaluation of exposure effects. For instance, one manufacturer reports his material t o be satisfactory for use in 78% sulfuric acid a t 25’ C. and satisfactory for limited service a t 65’ C. What is the significance of “satisfactory”? From an engineering standpoint, it means little. The effect of chemical exposure on mechanical properties must be known for engineering applications involving equipment under stress. Likewise, the effect of mechanical stress on chemical resistance must be known. For instance, are there environments which will cause stress cracking? Visual examination, determination of weight changes, and changes in dimension are insufficient criteria for determining suitability in most applications. It is recognized t h a t laboratory resistivity data must be supplemented by plant exposure tests, but laboratory data are still valuable in screening potential materials for application T h e upper temperature limit for materials such as polyethylene, polyvinyl chloride, and high-impact styrene copolymers, which are among the more promising thermoplastic materials for chemical service, is generally considered t o be 50’ t o 6 5 O c. This limitation is qualitative from a design point of view. The temperature effects on physical properties and chemical resistivity must eventually be established by measurement. I n our own plants, however, pipeline failures have occurred when these temperatures were exceeded or when pressure surges occurred in pipelines operating near these upper limits. This experience alone indicates that improved properties will be required. Present materials must be modified or new ones developed. Just recently a test report was received from one fabricator experimenting with unplasticixed polyvinyl chloride (PVC) reinforced with a polyester-glass laminate. It appears t h a t such construction may raise the useful temperature limit of PVC above 100” c. The aging characteristics of many materials have not been measured, particularly in exposures involving ultraviolet light. These data must be established before extensive outdoor installations can be risked. As a n example, plastic flange guards molded
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
Vol. 47,No. 7
Plastics Construction Materials of clear polyethylene have failed in as little as 4 months (Figure 1). These guards are safety shields t o prevent spray of material in case of leakage at the pipe flange. Carbon-filled polyethylene is now used in this service. I n another installation a pipeline of unfilled polyethylene has failed by cracking after 3 years' outdoor exposure. This failure is attributed t o t h e action of ultraviolet light on t h e unfilled polyethylene. Fortunately, in this case a 3-year life is an economic life. EQUIPMENT DESIGN
*
*
Having selected one or more candidates for actual design use, the next step is a detailed engineering analysis of mechanical suitability. T h e design problem with plastics differs considerably from t h a t with metals. To show this difference, consider only the temperature range for which most thermoplastics are suitableup t o 60" C. I n this range metals exhibit elastic behavior. Design can be based on short-time tensile strength data. Most plastics cannot be figured on short-time tensile strength data. Under a stress t h a t is small compared with the short-time tensile strength they will continue t o deform and eventually fail. T o be confident in design, i t is necessary t o have stress rupture data and creep data for plastics over t h e entire range of allowable working temperature. T h e significance of such data is illustrated in Figure 2 by reference t o German experience with nonplasticized polyvinyl chloride. Plotted are data on applied stress versus time for rupture failure at 20' C. T h e short-time tensile strength of this material is 7500 pounds per square inch T h e long-time strength is only 2600 pounds per square inch. At stresses above 2600, failure can be expected in 100 hours or less. For design purposes, German practice is t o apply a minimum safety factor of 4 t o the long-time tensile strength. The final result is a n allowable design stress only about 8% of the short-time tensile strength. B y comparison, if the structure were designed in carbon steel, a n allowable stress 25% of the short-time tensile strength would be used. Figure 3 for the same kind of material shows the effect of temperature on tensile strength and allowable design stresses.
At 60' C. the allowable design stress is only 140 pound8 per square inch. This is about 3% of the short-time tensile strength a t 60" C. and only about one fourth t h e allowable stress a t 20" C. These stresses, based on the minimum safety factor, are allowable only if stresses in the equipment can be calculated accurately, notch effects can be avoided, there are no thermosealed joints in the design, and there will be no deterioration due to corrosion. These data demonstrate t h e risk involved in designing structures on the basis of short-time tensile tests at room temperature -point A on the curve. For most American materials we have only the one point. We know neither the shape nor t h e slope of the curve with respect t o temperature. All materials cannot be expected t o behave in t h e same manner as PVC nor are these data for German PVC necessarily the values for American materials. Suppliers of the chemical industry must include in their research and development programs the tests t o determine these data, using standard methods for uniform results. The d a t a must be developed for each grade of material and must be so identified; else they will be of little value t o designers. Of equal importance to stress rupture is creep of plastic materials. Creep data tell t h e engineer t h e elongation or change in shape of materials under stress over the useful life of t h e equipment. Initial loading of a material, as shown in Figure 4, will cause a certain deformation known as initial creep. Under the same loading, elongation will continue a t a much reduced rate during t h e so-called second stage of creep. I n the third stage deformation increases rapidly t o failure, or failure occurs suddenly with little additional deformation. If the designer is t o keep the equipment within allowable limits of deformation during its useful life, he must know the rate of creep under various loadings a t various temperatures. The stress-strain diagram of Figure 5 indicates a yield point for polyethylene of about 1600 pounds per square inch at 25" C. If we were to use this as a basis
TENSILE STRESS
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TENSILE STRESS
9
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Figure 2. Tensile stress us. rupture for unplasticized
PVC July 1955
Figure 3.
20
30 40 x) TEMPERATURE -.C.
60
TO
Mechanical properties of unplasticized PVC
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of design, however, the results might be disastrous. We must first look at the creep characteristics, Figure 6. A sustained load 5oy0 of t h e indicated yield point or 800 pounds per square inch would result in 55% strain or elongation. Whether 400 pounds per square inch might be used a t room temperature from the creep standpoint alone depends on the particular design. For applications above room temperature, we do not have the data.
suppliers. As a n example, four manufacturers of polyethylene pipe show a difference of 75 pounds per square inch, or 225y0 in rating for 1-inch pipe and 20 pounds per square inch for 6-inch pipe. Referring again t o German experience with PVC, they found stress rupture data t o be satisfactory for use in pipe design. We must determine whether such is the case for American PVC and for other types of plastics. The Society of the Plastics Industry is sponsoring such a program a t Batelle Memorial Institute, the results of which are awaited with interest. Several major manufacturers also are conducting test programs. It will be a happy d a y when these investigations have progressed t o t h e point where designers can have such data as shown in Figure 8, showing allowable working pressure for various sizes of pipe over the allowable temperature range. Having good pipe of known quality and strength will be insufficient until methods of joining are standardized, and the strength of joints is known. A piping system is no stronger than its weakest link, which may well be t h e joint or fitting used. T E N S I L E STRESS L E . / SO. IN.
U TIME
Figure 4.
v
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General creep curve
At this point in the engineering analysis one might ask, "Can plastics be used before these data are available?" They not only can be used, but they must be used on a calculated risk basis so t h a t field experience can be gained simultaneously with t h e development of basic data. Figure 7 shows a large ventilating duct which has been installed on a calculated risk basis, fabricated of a high-impact styrene copolymer material. It was designed on 20% of the short-time tensile strength a t room temperature. It is yet t o be determined whether economic life will be obtained. I n this case i t was necessary to develop a fire retardant grade of material before the installation was permissible from a safety standpoint. This illustration can be used also t o show the need for fatigue data. I n a duct of this size it is impractical t o eliminate flutter of panels. Continuous reversal of flexure stresses can cause failure if the fatigue strength of t h e material is exceeded. There are no measured data on fatigue strength, so we can only hope t h a t the fatigue limit will not be exceeded. Consider now a special item in equipment design-piping. Piping in chemical plants usually represents about %yoof total investment. For this reason alone plastic piping systems deserve major attention. Certainly, the steel industry considers t h e potential great. Republic Steel already is marketing polyethylene and butyrate pipe. Youngstown Sheet and Tube is offering glassreinforced polyester oil country tubing. U. S. Steel has just announced its entry in the field with P V C and polyethylene. Tube Turns Plastics has been organized t o produce injection molded PVC fittings. It appears there will be plenty of pipe and fittings, b u t there are still problems t o be solved before maximum use of plastic pipe can be realized. I n design of piping systems the first property beyond chemical resistivity that we think of is bursting pressure. Allied with it is t h e allowable working pressure. Another important use consideration is external loading for buried pipe or pipe under vacuum conditions. The design engineer cannot calculate these properties accurately since they are dependent on reproducible quality of raw material, the workmanship in producing the pipe, the uniformity of wall thickness, out of roundness, and the concentricity of bore and outside diameter. This is evident from the diversity of pressure ratings recommended by various
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40
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Figure 6.
Creep of polyethylene
Uniaxial loading, 30' C.
More information on supporting plastic pipe a t various temperatures must be developed. The problem of support may be in many applications the factor t h a t makes plastics uneconomical, Figure 9 shows a plastic pipe which requires continuous support. Although the pipe itself is adequate for the chemical service, the cost and maintenance of the support makes questionable the economy of this piping system. It further illustrates the need for corrosion-resistant plastics having improved mechanical properties at temperatures above 50" C.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Plastics Construction LVaterials FABRICATION AVD INSTALLATION
Figure 7.
V e n t i l a t i n g duct of high-impact s t y r e n e conolvmer
The thermal expansion coefficients of most plastics range from four t o fourteen times t h a t of carbon steel. A major problem in design is t o provide adequate compensation for expansion and contraction. I n t h e literature and in advertisements we see reference t o various means for accommodating expansion. T h e economics and practicability of using lyre-type bends, inserting flexible tubing, or using expansion joints are not well defined. The practicality of each method must be determined for use by the designer as well a s the construction and maintenance engineers.
Once material is specified, equipment must be fabricated and installed. Xethods for fabrication of equipment in a vendor’s shop or on the plant site during construction are usually determined by the designer, selection being based on the mechanical efficiency of various joining methods and the facility with which each can be accomplished. Three methods are generally available-thermosealing, solvent joining or cementing, or mechanical (flanges, bolts). An example of thermosealing is the joining of flanges t o polyethylene pipe. The technique is similar t o welding but is not welding. The hot gas gun melts only t h e surface of the filler rod and stock t o be joined. Extreme care must be exercised to obtain a strong joint. Both solvent joining and flanging are illustrated in Figure 10. The rectangular duct sections are assembled b y solvent-cementing the side panels into the corner angles. Each length of duct is flanged and connected t o the adjacent section b y bolting. Specific data are required on t h e mechanical efficiency of each method; else an adequate or economic design cannot be made even though complete mechanical property data be available on the basic materials being fabricated. T h e Germans, for instance, have found that a thermosealed joint efficiency of 70% can be obtained with well-trained technicians under careful supervision. Other WORKING PRESSURE LE.ISO.IN
SPECIFICATION
Specification defines the grade of material within a type t o meet the engineering requirements. For example, polyethylene may be selected as the type material t o use, but there are various polyethylenes having different properties. Specification must isolate and identify t h e particular polyethylene t o be used. Properties t h a t are important must be defined for each material, such as melt index, density, and others. The exact relationship of these properties t o mechanical properties needs to be measured before engineering specifications can be prepared. We must work toward the type specification t h a t has been developed for metals. Stainless steel pipe, for example, can be ordered t o ASTM Specification, which defines the method b y which the pipe is manufactured, the limits of chemical composition of the material, and the mechanical properties of the material. It defines the tests which t h e pipe must withstand and permissible variation in weight and dimensions. The SPI is currently working in this field for plastics. The Batelle program will assist in drawing up specifications. Specification concerns also availability of the specific items required. This presents difficulty a t present because of lack of standardization in pipe dimensions, lack of fittings for certain lines of pipe, and a multitude of methods of joining pipe. Work is in progress by industry t o effect standardization in dimensions. One suggestion for pipe is that standard iron pipe size dimensions for outsidediameter be used wherever possible. This will permit the use of standard tools and methods of installation, which will be a large factor in obtaining acceptanceof plastic pipe in the chemical industry. I n the field of sheet materials and structural shapes, the problem of specification is even more complicated, since this Dhase of nlastic utilization has not advanced t o the extent of piping. Figure 9.
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EASED ON STRESS -RUPTURE TESTS WITH FACTOR OF SAFETY
c
. 4
40
30 INCH INCH INCH INCH
20 10
20
30
40
TEMPERATURE
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- OC.
60
F i g u r e 8. Allowable working pressure us. temperature for Schedule 80 IPS polyethylene P’Pe
H i g h - i m p a c t s t y r e n e copolymer piping
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thermosealed joints may have an efficiency as low as 30%. We cannot design and fabricate on the basis of assumed efficiency but only on measured efficiency. One American fabricator now claims 100% efficiency with PVC at greatly increased speed of thermosealing. This can be accomplished only with the best technicians.
Figure 11.
Unplasticized PVC piping SUMMARY
I n summary, the major phases of engineering have been reviewed: Figure 10. Air supply duct of fire-retardant grade highimpact styrene copolymer MAINTENANCE
The maintenance engineer must h i v e a knowledge of fabrication, installation, and repair procedures on all materials if he is t o keep the plant operating a t maximum efficiency. H e cannot call in specialized talent for every pipeline failure and every equipment breakdown or for minor rearrangements or additionu. H e must be able to solve most of these everyday problems by having the proper materials and methods at hand. Figure 11 shows an installation of plastic pipe designed by maintenance engineers t o solve a serious corrosion problem. This installation required a knowledge of corrosion resistance, methods of cutting and joining like and unlike materials, and metliods of suppoit. It includes nonplasticized polyvinyl chloride for maximum corrosion resistance, polyethylene for flexibility, and both graphite and phenol formaldehyde for temperature resistance. This example illustrates the ingenuity required of the maintenance engineer in solving problems on a day-to-day basis.
Material selection Equipment design Material specification Fabrication and installation Maintenance The problems facing the engineer have been described and the kind of d a t a he requires has been illustrated. This all points t o the fact t h a t in the field of plastics the greatest engineering need is for additional data on mechanical properties over the entire working temperature range; particularly stress to rupture, creep, and fatigue. Secondly, standardization is necessary, both in materials and in methods. Until these requirements are met, maximum utilization of plastics will not be realized. Although most installations cannot now be adequately engineered, field experience must be gained simultaneously with the development of basic data if large scale use of plastics in the chemical industry is t o develop rapidly. During this growth period, care must be taken t o avoid major misapplications which will result in retardation in t h e acceptance of plastics as engineering materials. RECEIVED for review September 17, 1954.
ACCEPTEDApril 27. 1955.
One of the world's largest injection molding machines is at Prolon Plastics
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