This guide is intended to assist prospective buyers of glass fiber Reinforced Plastic (RP) tanks. It is especially designed for infrequent buyers who may not be familiar with the intricacies of a material so different from the metallic vessels traditionally used for corrosive chemical services. Without becoming experts in reinforced plastics, buyers can follow this guide to make more informed purchasing decisions. Hopefully, the guide will help form the basis of successful communications between buyer and seller.
Historically, the RP tank fabrication industry has grown steadily since the 1950's. RP tanks have proven themselves consistently as excellent choices for containment of numerous corrosive chemicals. They represent an economical alternative to metallic vessels for a wide range of chemical applications. However, because entry into this business does not require large capital investment in sophisticated machinery and equipment, the industry today is characterized by a large number of fabricators with varying talents, specialties and capabilities.
This variance in talents and capabilities is precisely why the relationship between the tank buyer and seller is critical to the sale and the tank's subsequent performance. It is very important for the purchaser to know the fabricator he's selected. Fabricators must demonstrate their experience and capabilities in manufacturing the type of equipment under consideration, because, unlike other tank constructions, the fabricator of RP equipment actually manufactures his own materials. He does not merely purchase finished raw materials and shape them into the final product; the curing process, which takes place while he is fabricating, determines the ultimate mechanical and quality characteristics of the material. Quality control in both the manufacture of materials and the method by which they are applied should be a serious concern for a purchaser. Criteria other than price alone must govern his final choice.
"How To Purchase" Highlights
Specification Requirements - Inquiry Design Data - Inquiry Requirements - Bid Evaluation
A buyer should rely upon good purchase specifications. Fortunately, there are several good specifications covering vertical cylindrical tank construction, which is the type most commonly used today. The elements of RP tank construction are described in two basic industry standard specifications: ASTM D3299 -Standard Specification for Filament Wound Glass Fiber Reinforced Thermoset Resin Chemical Resistant Tanks. ASTM D4097-Standard Specification for Contact Molded Glass Fiber Rein-forced Thermoset Resin Chemical Resistant Tanks. (These tanks were formerly covered under PS 15-69.) These are specifications which describe the minimum design parameters and construction elements essential to glass fiber Reinforced Plastic tank fabrication. The scope of both of these industry standards should be studied carefully. The standards were written to cover the design and construction of tanks intended to operate under a specific set of conditions. If the conditions of the application under buyer consideration exceed those stated, other special designs will be required, and in some cases, specialized construction procedures wilt be required. If there are no unusual conditions involved in the tank's application, the selection of one of the two industry specifications may be governed by buyer preference or may be left to the choice of the fabricator. The difference between the two specifications involves the fabrication method used in the structural portion of the tank shell. Also, the two methods of fabrication, which use different reinforcement materials as well, produce tanks with different physical properties. When it is properly designed and fabricated, RP equipment may be produced to either industry specification and give comparable, satisfactory service-life. As we noted earlier, the final choice may be dictated by the specific physical requirements for a given application. More often, the choice is a matter of fabrication economics or the technology available from a given fabricator. Past buying practices may also play a meaningful role in the decision.
In order to expedite the purchasing process and to avoid as much confusion and misunderstanding as possible between buyer and seller, a complete purchase description of the tank should be developed, including:
1) a complete description of the chemical environment (including concentrations and specific gravitates of all chemicals to be contained) and the operating temperature of the environment;
2) special loading requirements such as pressures, vacuum conditions, loading from ladders, catwalks, or agitation;
3) any seismic design requirements and/or wind loads.
It is quite common to utilize a worksheet to help gather this information. Worksheets are often called Vessel Data Sheets or Tank Design Data Sheets or Service Environment Questionnaires. An example of one such form is included as an attachment to this guide. It is also necessary to list (but even better to provide a drawing of) the accessories and ancillary items which will be part of the tank and its installation conditions. The importance of detailing the complete tank description cannot be overemphasized. Such details will govern construction items like sizing the vent in relation to the tank's shell design: For example, will the tank be filled by air-loading, or by pumping?; Will the tank be vented into the atmosphere, or into a fume conservation system? These items and others are absolutely necessary to proper tank design, and knowledge of details is necessary to determine the proper classification of the tank under the industry specifications. Having developed a comprehensive picture of the tank application, it can be determined if the tank falls within the scope of the Industry Standard Specifications, or whether a special construction is needed. Once determined, all the buyer has to do in his purchase specification is reference and require compliance with the selected industry specification. There is no need to further describe construction requirements clearly detailed in the industry specification.
The buyer should require the fabricator to take full responsibility for designing the tank in accordance with the specifications and application conditions described. In this way, much of the burden felt by the inexpert buyer in RP construction can be placed where it properly belongs. However, this does not mean the buyers should blindly trust a fabricator's alleged expertise. Here are some suggestions the buyer may follow in order to be sure a correctly designed tank has been bid, and that a correctly fabricated tank will be produced:
require bidders to submit complete design calculations with their quotations, including safety factors used.
require bidders to submit with the quotation the material properties used as the basis of the design calculations.
require bidders to submit with the quotation a description of the quality assurance program and procedures in effect at the plant where the product will be manufactured.
require that the successful bidder be prepared to mail a final inspection report on the tank at the time of shipment.
require the successful bidder to submit detailed installation instructions.
require the successful bidder to submit drawings for approval.
By requiring this type of information with the bids, the prospective buyer will have the means to evaluate the bidders on the basis of their accuracy and the thoroughness with which they have complied.
The evaluation of bids is never an easy task. However, by requiring detailed information and production according to specifications, the buyer has given himself some immediate advantages, without struggling with the intricacies of RP fabrication. The correctness and comprehensive-ness of the design can readily be checked and evaluated/ and valuable insight can be gained concerning the care with which the fabrication has been carried out. Other questions for evaluating bids might include the following:
Does one bid stand out . . . well below the rest? If so, confirm that the bid is in compliance with specs and other details required. Find out if costly items have been omitted intentionally to get the orderonly to be picked up later at an extra cost.
Is each tank described in detail? - capacity - diameter height - top style - bottom style - specific gravity for which designed - method of fabrication - laminate composition a) resins used b) inner surface reinforcement c) thickness of corrosion liner d) total wall thickness e) type of exterior surface -specific list of all accessories - environment for which tank is recommended
Is specification compliance clearly stated? If not, are all exceptions detailed?
Are design calculations provided or available? Is there a charge for calculations if requested?
Are unit prices provided for each tank?
Are freight costs and responsibilities for freight cost clearly defined? Who's responsible for damage in transit?
Are prices firm, and for how long?
Are all drawing and engineering requirements and costs included? Are representative sample drawings of similar equipment available?
What is the fabrication and delivery schedule?
Who will be responsible for installation and final inspection of equipment before it goes on line? (See Inspection Guide)
By establishing bidding conditions and mandatory detailed quotations, the buyer has greatly simplified his comparison process for the tendered bids.
As we noted earlier, the purpose of this guide for purchasing glass fiber Reinforced Plastic tanks is to provide the infrequent buyer with information and a plan that will help him judge a vendor's bid and product by important criteria other than price. With adequate information about design data, quality control, and specifications, a buyer should feel more comfortable determining whether a tank quoted is designed in accordance with his needs, fabricated properly and consistently, and capable of performing as intended in the field.
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