fiber-glass tanks

MECHANICAL ENGINEERING

FIBER-GLASS TAn;x;Ic

ACKNOWLEDGMENT This article by Edwin C. Young, of Black, Sivals, & Bryson, Inc., is re- printed from the October, 1964, issue of “Mechanical Engineering.”

FaaMwr -DING Is m a art of laying down continuous filaments impregnated with a binding material to a predetermined pattern, and polymer- ized to form a highly engineered structure of known physical, mechanical and chemical properties. The physical property of the structure is essentially second to none from a strength-to-weight standpoint.

Mechanically, the structures thus produced provide an optimum configuration with maximum strength for minimum material used. From a chemical resistant standpoint, the materials used in the structure satisfy a multitude of applications vary- ing from low acidic (low pH) up through and including high alkaline (high pH) chemicals.

One of the biggest advantages that filament winding has over other materials of construction today is that, with increased production, the basic cost of materials which go into producing filament-wound structures is constantly decreasing. This is in direct contrast to the rising cost of other materials used in similar construction today.

Filament winding is not totally Merent from the operation of a machine lathe, except that it works In reverse. Whereas a lathe removes material in a programmed manner, filament winding places the material over a mandrel to a prescribed pattern and thickness. The method of wetting the glass with the

resin and the maintenance of the glass-resin ratio, coupled with the precision employed in maintaining the filament winding pattern, will dictate the quality and the physical properties of the material produced. Thus, filament winding is a highly engineered process which can be adapted to automated production techniques to produce products which will have varied applications in essentially every industry.

Many fabrication methods are covered by United States and foreign patents. Five basic methods used are discussed here. Although any filament materials may be used in these structures, glass filaments are the most widely used and the properties given are for glass reinforced structures.

SINGLE CIRCUIT CIRCULAR HOOP WINDING

Circular windings are used to produce girth or hoop strength. They are commonly referred to as Wdeg windings although they have some angulari- ty in relation to the true 9Odeg or perpendicular plane to the winding axis. This angle results from the fact that each winding operation has a given band width, i.e., the width of several rovings of continuous filaments. The glass filaments are peck- aged by the glass manufacturer on spools with 204 filaments comprising what the industry calls an

Naval Enplrmrr Journal, Octobrr, IT65 741

FIBER GLASS TANKS MECHANICAL ENGINEERING

“end,” and then so many ends (8, 12, 20, 40, 60, etc.) to a roving. As an example, a band width of approximately

two inches is generated when 18 spools of 0.00036- india glass of 20-end rovings are used. Thus the circular windings produced would have to be indexed two inches for each rotation of the mandrel.

Figure 1 shows a typical setup for single circuit hoop windings. Bands 1,2,3, and 4 are shown lying adjacent to each other. A complete circuit is shown as one layer of windings down the mandrel and back to the starting point. This creates a double layer or two band depths of windings. This structure is extremely strong in hoop or girth loading but will carry only that end loading equivalent to the strength of the resin used in binding the filaments together.

Additional hoop strength can be obtained by using multiple layers of circular windings laid one

Figure 1. For hoop strength-single circuit circular winding.

upon the other. This type of reinforcing or multiple layer circular windings is referred to as a “doubler.” They are of importance for increasing section modu- lus at a particular point in the structure to provide stiffness or allow the structure to be subjected to external loadings as for underground storage tanks.

In combination with helical windings, a doubler close to the tangent point of the knuckle and straight section of the vessel will lock the helical windings during the initial starting of the winding circuit.

SINGLE CIRCUIT HELICAL WINDING

As shown in Figure 3, the single-circuit helical winding pattern begins at one polar port of the mandrel. It progresses down the mandrel at a helix angle alpha overwinding the polar port at the o p posite end. It returns down the opposite side of the mandrel and starts the second winding after the mandrel has been indexed one band width such that the second circuit lies adjacent to the first.

Since it is necessary to have a shaft for a mandrel out either end of the mandrel for a horizontal winding machine and out of one end or both ends of a vertical winding machine, the normal filament wound structure will have a port opening or pole piece in the end of the structure. The pole piece is normally bonded into the structure on completion or made into a manway for access into the vessel after it has been put in service.

The filament structure will have an ever-increasing buildup at the tangent point to the pole piece and will be the thinnest at the tangent point to the straight section. For a sound structure it is thus

Figure 2. A dd tank car under constructioa. This tank car which has a 22,500-gal capacity weighs nine tons less than a conventional tank car.

742 Naval Enginoo JourMI. Octobor, 1965

MECHANICAL ENGINEERING FIBER GLASS TqNKS

c - 4 TANGENT LIME

-Too\ I

CYLINDER SECTION

LAST BAWD / - POLAR PORT ’A POLAR PORT “0’

d I HELIX ANGLE I Figure 3. For high axial tensile strength-single circuit helical winding. Combine with the circular winding for a balanced

structure.

necessary to add material to the dome end of the structure in the knuckle area to provide additional thickness. This is true of all helical-winding methods discussed. In the single-circuit method the filament winding continues until sufficient circuits have been laid down to completely cover the circumference of the mandrel. The number of passes required will be the circumference divided by the band width and multiplied by the cosine of the helix angle. Usually the helix angle on a closed end pressure vessel will be relatively small. This will provide for high loadings or tensile strength in the axial direction. In order to obtain a balanced structure, it is necessary to combine helical windings and circular windings. This will allow structures to have both girth and longitudinal strength for the particular structure.

MULTIPLE CIRCUIT HELICAL WINDINGS

In this system the pattern starts similar to the single circuit except that when the windings pass the second port opening the mandrel indexes an angle dependent upon the number of circuits to be used in the pattern. In Figure 4, where a lo-circuit pattern is shown, the mandrel is indexed 1/10 the circumference of the mandrel plus 1/10 the band width. In this system the eleventh band becomes adjacent to the first. One of the features of this method is the overlapping of windings or “netting” produced. This generates a structure in which there are added benefits from continual overlapping which is similar to a woven structure. Note that in Figure 4 band 5 overlaps band 1 and band 11 overlaps band 5. This progresses with each subsequent band. In the single circuit no overlap begins until after the winding has progressed approximately 180 deg. 111

I / ’

+---- TANGENT L INE -- , . I

/I-- CYLINDCR SECTION

10

I POLU PORT -A

Figure 4. For optimum utilization of the filament’s strength without the addition of circular windingtimultiple circuit h e l i d windings.

Nevel Enpimerr JourMI, October. 196 743


The multiple circuit system produces a structure that can be designed for optimum utihation of the strength of the filaments without addition of circular windings. It is possible to develop a balanced structure by choosing the helical angle to produce the girth and axial strength required. All of the items noted in this article utilize this method of filament winding.

DUAL HELICAL WINDING

In the winding methods described previously, the port diameters of each end of the vessel were the same. In the event that port diameters are different on either end, then it is necessary to use the dual helical winding system. This system will have different helix angles for each end of the vessel in order to have a stable winding pattern around each port.

The programming of the dual helical winding system (Figure 5 ) can be complex. To change from one helix angle at the tangent point of one dome to another angle at the tangent point at the opposite dome requires that the rotation of the mandrel or the carriage speed must be varied over the straight winding system for multiple-circuit or single-circuit machines with the same end ports. This can be done but, from a production standpoint, it is not economical unless there are many units to be produced with the given configuration.

In practice it is often better to select a pole di- ameter of su5cient size to accommodate all open- ings anticipated. This then resolves itself into selection of a pole dia and using a single circuit or multiple circuit system for all jobs. The end fittings can be placed in the pole piece to meet whatever requirements arise within the limits of the pole piece.

VARIABLE HELICAL WINDING This method is used in winding odd shapes such

as cones, parabolas. It is necessary from a practical

standpoint to consider winding two units at one time back to back as shown in Figure 6. In this type system the helix angle at the largest dia will be smaller than the helix angle at the smallest dia in order to have a stable band during the winding operation. This also must be programmed on the winding machine. Because of the low helix angle at the maximum dia these configurations will have low hoop strength at this point.

To strengthen this structure it is necessary to use helical windings of high angles similar to the previously described circular windings. By proper selection of the number of helical and circular windings it is possible to arrive at a structure which will have the desired configuration and strength.

PHYSICAL PROPERTIES

Examination of the physical properties of filament-wound fiber-glass structures will reveal some interesting engineering criteria. These data must be used with caution. Only the true filament wound structure described here will have the strength properties reported. Many so-called filament wound structures must be closely examined from an engineering and design standpoint before these properties can be utilized in checking structures. Since no codes+ or standards exist at this time for filament winding, much will depend on the techniques and methods used by the manufacturer in fabricating his structure.

The data reported in Table I apply only to welldesigned and precisely manufactured structures. In these structures the composition will range from 65 per cent glass, 35 per cent resin, to 70 per cent glass, 30 per cent resin. These ratios are easily obtained through proper control in the engineering and production of these filament-wound structures.

A subcommittee o! The American Society of Mechanical mgi- neem h e m r e Vcssel Committee has been formed and has held prellminary meetings on laylng the groundwork.

744 Naval Engimen Jwrwl, 0ctob.r. 19M

MECHANICAL EXVGINEERING FIBER GLASS TANKS

TABLE I Physical Properties of Filament-Wound Structures

F-t- P U Wound Cubon

Propcrtlcs structures Steel

Density-lb/in. 0.072 0.283 Tensilestrength,psi 130,OOO 40,OOO

Strength-to-weight 1,808,OOO 138,OOO

T h e 4 4. of 5.6X10d 6x10.'

(ultimate)

reti0

expansion, in/in/F

Thermal conductivity 2.3 314

Red- Treated AUo Alnml- ~teef nllm

0.283 0.091 240,000

828,000 825,000

6x10.' 13X104

314 1,416 ~~ ~~~

From an engineering standpoint, two of the most signtficant properties are the tensile strength and weight of the filament-wound structures. For a comparable structure dimensionly, the filament- wound structure will have one-fourth the weight of steel and three-fourths the weight of aluminum. From an ultimate tensile-strength standpoint they have approximately three times that for steel and one and one-half that for aluminum.

Since the filament-wound structure can be spe- cifically designed for a given job, it is necessary to base the design on a composite strength for the structure. Individual glass fibers have an ultimate tensile strength of 300,000 psi. The figure given in Table I of 130,000 psi was determined from tests on welldesigned structures using normal glass roving~. Ultimate strengths in excess of this figure have been obtained using glass of higher tensile properties and structures with higher glass resin content.

Of significant importance to the rocket and mis- sile industry is the fact that the ultimate strength- to-weight ratio for these filament-wound structures is far superior to other types of materials of construction. For commercial products this has little if any sigmficance except that the final structure will be so light that it will tend to offset the higher initial material cost and make these structures more com- petitive.

Since the thermal coefficient of expansion for filament-wound structures is comparable to steel, it is possible to use steel attachments to the structures without fear of generating problems due to differential thermal expansions.

The thermal conductivity of filament-wound glass resin structures provides an insulation property not found in other high-strength materials of constntction. Tests were conducted on a storage tank containing 37 per cent nitrogen-ammonia 50-

lution. These tests were conducted in mid-summer, and ambient temperatures ranged from 69 F in the morning to as high as 114 F in the afternoon. Maxi- mum temperature reached in the tank at the liquid 'Tapor inner face was 88 F. Average liquid temperatures in the tank were considerably below this f gum.

Figure 6. For winding odd shepes-vnriable helical winding.

The tank used in the test was painted a standard gray and, consequently, absorbed more radiant heat than a more reflective color. The solution tested had a vapor pressure equal to atmospheric at 95 F. The tank was equipped with a two-ounce vent valve and, during the period of the test, no ammonia vapors were ever noticed at the discharge of the vent. Normally, five pounds working pressure aluminum tanks are used in t h i s application.

VARIETY

The majority of applications to date have been concerned with simple, vertical, atmospheric storage tanks. There are many more applications that can be served well with properly engineered f3a- ment-wound structures. Some of these applications, where work has been done to date, are discuss4 here. Many others not discussed are still in the embryo stage and only future engineering develop ment will bring these to the market place.

In the military, the use of filament structures is of utmost importance, especially where weight is a factor. Examination of the strength-to-weight ratios for filament structures reveals why this material has become so popular. Many components of lsockets and missiles are now being fabricated with the most significant being the rocket motor case. These cases are designed as pressure vessels capable of with- standing several hundred psi working pressure, The weight saved has resulted in increased range for these rockets. Experience gained in this field has done much to assist in the design of pressure vessels for industry.

For pressure-vessel service most of the work to date has been done in the lower pressure ranges up to 50 pig. The principal reason for pressure vessels in corrosive service has been in applications where chemicals are moved by air pressure rather than by pump. There are several advantages to this type of fluid movement which are apparent.

Several deterrents are present which keep the use of filament wound pressure vessels from wider range. Of prime importance is the fact that there are no pressure codes which cover this type of structure. State regulations, insurance coverage, etc., all become involved when pressure equipment is installed. Exempt from such considerations are agricultural installations. To date, several of these tanks have been used on the more corrosive chemicals used for liquid fertilizers, insecticides, herbi- cides, etc. For the most part these are nurse and

Nwol brqlnnra Journal. October. W& 745


applicator tanks. Another typical application is for handling acids for oil-well acidizing service. These small pressure tanks are used to move acids from a central bulk station to the well head.

Considerable thought has been given to underground storage of combustible or corrosive materials. In this case, there is a question of external as well as internal corrosion. Since ground conditions as to soil alkalinity or acidity vary from one locale to another, this problem can be either non- existent or serious. Life of underground storage tanks at gasoline service stations vary from less than two years to well over twenty years. Two other problems can exist in underground storage of petroleum products. Since these tanks breathe, moisture in the form of water vapors in the air condenses and eventually settles to the bottom of these storage tanks. The water in combination with gasoline additives creates a corrosion problem along the bottom of the tank. The other problem arises when bacteria forms at the water-petroleum inter- face in the presence of iron. Both of these impurities can produce problems by the users of these products. Filament-wound structures of glass and resin are a logical solution. A resin-rich interior and ex- terior on these underground storage tanks can eliminate both of these problems.

One factor that at present is keeping these structures from wide-usage is cost. The filament-wound structures run approximately one and one-half that of steel.

From an engineering design standpoint, these underground storage tanks must withstand external ground loading. The use of stiffeners in the form of doublers or sandwich construction offers an excellent solution to this problem, but at an increase in cost. The doubler technique was used for a sam- ple tank in a development test program. After the tank was backfilled with three feet of dirt fill, a cement truck with 33,100 gross loading was driven over the tank numerous times. The deflection of the tank under these severe test conditions was negligible. The tank used was a half tank, 5 f t dia

and 12 ft long, and wooden structure on the end was used only to hold the backfill. It added no support or assistance to the tank structure, but pro- vided a means of making internal deflection meas- urements during the tests.

In the transportation field there is an excellent possibility that filament-wound structures will find many applications. In addition to their chemical resistance, the lightweight properties mean less tare weight and higher payloads for the same gross weight. Railroad tank cars and over-the-road truck transports are currently under development. Al- though weight today is not a significant factor for railroads, an experimental railroad tank car of 22,500-gal capacity saved nine tons over conventional tank cars. For over-the-road service this weight saving would be of great significance due to limitations on highways.

Smaller tanks in the 500 to 1500-gal size also hold promise in the transportation field. These can be used for delivery of materials to customers on smaller flat bed trucks rather than trailer trucks as mentioned above. The same containers could be used by the customer with the supplier delivering another full tank as the need arises and picking up the empty for reuse. The versatility of the material in handling many chemicals make it ideal for such applications.

Although these are, perhaps, the major areas of consideration, there are many other possible areas of future development. Only liquid materials are considered above. Certainly handling and storing solid materials present another wide area of potential. Anywhere that corrosion, product purity, weight, temperature stability, etc., can be trouble- some to the user, then filament-wound structures should be investigated.

REFERENCE

[l] “History and Potential of Filament Winding,” Richard E. Young, February 4, 1958, 13th Annual Technical & Management Conference, Society of the Plastics Indus- try.

POLISH SHIPBUILDING PLANS 1965- I970 Polish shipbuilding is expected to expand considerably in the current five-year period

-1965 to 1970. The Gdansk Shipyard is to build a long series of fishing vessels: 24 factory mother ships of 10,300 tdw each and 59 factory trawlers of 1,250 tdw each. The Gdynia yard is to raise its output from the present 44,OOO tdw to I80,OOO tdw and will build 16 tankers of 19-20,OOO tdw each, ships of 23,000 tdw each and many new types of fishing vessels at Szaecin the yard will increase its production by 80 per cent in terms of tonnage by 1970, compared with 1965. It is planned to build I 2 types of ships, including seven new ones, a total of 78 ships in the next five years, 34 per cent of the total in Poland over that period. During 1964, 50 sea-going ships were built in the three yards. The total output was 320,000 tdw, more than half of which, 26 ships of 196,000 tdw, were exported. More than 700 ships have been built since 1948 when the production of sea-going ships began in Poland. The total tonnage of ships built in Poland exceeds 2m. tons.

-MARINE ENGINEER AND NAVAL ARCHITECT

746 Naval Eng1no.n Journal. October I965

fiber-glass tanks

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