What are the primary types of filter vessels?
	Why did the filtration industry move to FRP vessels and tanks?
	How is the filter vessel constructed?
	What is the FRP vessel made with?
	How are the filter vessels sealed?
	Are filtration vessels resistant to chemicals?
	How long does the filter vessel exterior last?
	Are the filter vessels pressure tested?

What are the primary types of filter vessels?
There are three primary types of filter vessels; Plastic, Steel and Fiberglass Reinforced Polymer Plastic (FRP). Plastics and Steel tend to work best in situations that can accommodate a shorter life span in a non-corrosive environment. However, both materials quickly break down over time, especially in corrosive environments. In other words, steel rusts; FRP doesn’t. In certain corrosive conditions, a carbon steel vessel might last 1 year and a 316 stainless steel vessel might last 3 year. An FRP vessel, however, is likely to last 15 years or more. All steel structures will eventually deteriorate due to galvanic corrosion. The ionic exchange that occurs removes metal ions from steel structures and leads to leakage or explosive structural failure. This is due to the fact that like all metals, steel contains crystals. FRP does not suffer from galvanic corrosion. This is because FRP is a polymeric composite made from continuous filaments of glass (not crystals) that are individually coated with a thermosetting resin matrix that bonds them tightly together. In fact, glass is actually a super-cooled liquid that becomes increasingly hard and strong as its viscosity increases when its temperature is decreased from its 12,700 C. melting point.

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Why did the filtration industry move to FRP vessels and tanks?
Thirty years ago saw the beginning of FRP pressure vessels and tanks. These were made the same way as today – layering fiberglass or carbon fibers with various resins. The material was as light as magnesium and stronger than carbon steel. When used as pipe material, one pound of this filament reinforced plastic (FRP) was able to replace 8 to 12 pounds of steel. The most commonly used corrosion-resistant resins were those of the polyester family. In extremely corrosive chemical environments, a stronger, more corrosion-resistant resin was needed; one that would stand up to not only highly corrosive materials such as hydrochloric acid and sulfuric acid but also resist the high stress and cyclic loading placed on pressure vessels.

Dow Chemical hypothesized that the addition of an epoxy backbone would greatly increase the resistance dilemma. The strength and toughness of the epoxy would make the resin highly resistant to chemicals and solvents. Then, to enhance the reactivity of the resin, vinyl acrylate groups could be placed on this epoxy backbone. The result, Dow Derakane 411 epoxy vinyl ester resin, which gives broader corrosion and solvent resistance than older established polyester resins. It has been designed for 10 to 20 percent more laminate strength, 2 to 4 times better adhesion and 3 to 5 times greater elongation than other resins. Chemical attack on these types of resins occurs through hydrolysis of the ester groups or the splitting of unreacted carbon-to-carbon double bonds through actions such as oxidation or halogenation. If the laminate layer of a polyester resin cracks, chemicals can seep in through the corrosion barrier and destroy the laminate. Without going into a detailed chemical lecture, the vinyl groups of polyester resins are dispersed throughout a molecular chain. This results in cross-linking and brittleness. Derakane epoxy vinyl ester resins place the double bonds at the end of the molecular chain. These react completely on polymerization, resulting in a more chemically resistant structure. With these bonds confined to the ends of the molecules the entire length of the molecular chain is able to elongate under stress and absorb mechanical and thermal stress or shock. This inherent resistance to cracking means three things:

	When properly designed for a given load, the wall of an FRP filter vessel can be made thinner than comparable steel vessels.
	When designed with a tensile strength of 12,000 PSI and a 10:1 safety allowance, laminate made with Derakane 411 resin shows an actual safety factor of 16:1 or greater while polyester resin laminates have a safety factor of less than 7:1.
	When designed to an allowable strain of 0.1 percent, the laminate made with Derakane 411 resin has a true safety allowance of greater than 11:1 compared to 5:1 for the polyester resin.

Table 1 presents data on the room temperature properties of 1/8” thick clear castings of Derakane 411 epoxy vinyl ester resins. The combination of high clear casting strength plus high elongation is a good indicator of the toughness of Derakane resins.

TABLE 1

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How is the filter vessel constructed?
Eden Equipment Company has employed the advantages of the Dow Derakane epoxy vinyl ester resin and used its inherent strength and corrosion resistance to provide a filter vessel that is a superior choice for a wide variety of uses. Eden Equipment Company’s line of Excel filter vessels are available in five sizes ranging from 8 to 30 inches in diameter with flow rates up to 3,200 gallons per minute.

During the construction of pipe, tanks, or pressure vessels, filament winding is an automated process that yields a hollow shape. Typically, the filament winding process involves continuous glass roving passing through a tensioner and into a resin bath. From here the saturated glass strands are wound onto a rotating mandrel at a precisely predetermined angle and speed. The filaments are collimated and aligned by a proprietary process that resists both longitudinal and circumferential stresses imposed by the vessels internal pressure. This patented process assures dimensional control on the interior sealing diameters when the vessel is subjected to a wide range of both temperature and pressure loads.

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What is the FRP vessel made with?
The FRP vessel is made with a reinforcement of fiberglass fibers. The reinforcement provides the finished product with the strength and structure while the resin acts as a corrosion barrier preventing corrosive elements from penetrating the FRP material. FRP materials can be stronger than steel on an equal weight basis (termed “structural efficiency” or “strength-to-weight” ratio). Optimal strength can be realized through proper orientation of the glass reinforcement.

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How are the filter vessels sealed?
FRP filter vessels are designed with side piston seal o-ring mechanisms similar to that used in automobile engines. This method of construction actually increases the seal compression as the pressure in the vessel increases. The vessel is sealed when the top seal plate is put into place. There is no need to torque cover bolts to ensure integrity. Mere hand tightening of the cover bolts is sufficient to provide a leak-proof vessel. The lower seal plate of the vessel seals with two o-rings to prevent leakage and maintain the integrity of the clean and dirty sides of flow.

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Are filtration vessels resistant to chemicals?
Filtration vessels constructed of Dow Derakane 411 have excellent resistance to corrosion by many different chemicals, including both acids and alkalis. This resistance extends from below ambient conditions to elevated temperatures. FRP vessels require little, if any, maintenance during the course of a long service life. Excel filter vessels are able to operate in the following fluid and load conditions:

	Internal pressure – unless otherwise specified a maximum allowable working pressure of 150 PSI is recommended.
	Working fluid – solutions having a pH range from 2 to 13 with solutions of inorganic salts or chemistries deemed acceptable to Excel filter vessels.
	Temperature – ambient or fluid temperatures ranging from 20 to 150º F.

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How long does the filter vessel exterior last?
In addition to the corrosion resistance for the interior of the filter vessel, Excel filter vessels can be designed with long exterior life in mind. While there is no direct fluid contact with the externals of the filter vessel, every aspect should be well thought out. For example, anodized 6061-T651 aluminum (anodized) and 303 stainless steel can be used for standard housings. For special applications, all the supporting externals can be ordered in 316L stainless steel or other polymers. The steel is certified per ASTM and ASME Codes and has no welds or weld repairs. Even though there are no metallic wetted parts; this provides external materials free from mercury and/or other foreign contaminants. However, this is not always the case with welded steel vessels. The external fiberglass may be painted with a modified oil alkyd enamel that has excellent resistance to weather, salt and sun. In the case of exceptionally aggressive chemicals, the inside of the vessels may be lined with either polypropylene or PVDF. These liners are bonded to the fiberglass to form one integral component. One other available option is the addition of Benzoyl Peroxide. (BPO) to the Derakane resin. This high boiling point plasticizer is used as a catalyst in the polymerization process and halts runaway decomposition. This application is especially useful in sodium hypochlorite (bleach) applications.

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Are the filter vessels pressure tested?
All Excel filter vessels are designed to withstand a cyclic pressure test comprising a minimum of 100,000 cycles, each cycle ranging from atmospheric pressure to a maximum use pressure at a cyclic rate ranging from two to twenty cycles per minute. The cyclic test fluid is water at a maximum temperature of 1,500º F. The vessels are tested prior to shipment following ASME Code, Section X test procedures. Each vessel is hydrostatically tested with water at ambient temperature to a pressure equal to two times the maximum use pressure. ASME Code requires the vessel be tested for not less than thirty minutes and no longer than twenty-four hours. During test procedures no noticeable pressure drop will be permitted and no leaking shall occur throughout the tube wall or around the end closure seals.

Due to the flexibility of the design, vessels may utilize either double or single open ended cartridges. Either style will provide a no by-pass filter vessel with minimal pressure drops. Along with being highly corrosion resistant to a wide range of chemicals, Derakane 411 resin complies with the U.S. Food, Drug and Cosmetic Act as amended and applicable FDA regulations (21 CFR 177.2120). These resins may be used as articles or components of articles intended for repeated use in contact with foodstuffs.

In summary, the process listed above is designed to provide the customer a filter housing with unparalleled corrosion resistance, safety and longevity.

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