What should the owner
From Vision to Supervision
furnish the engineer-contractor for process and equipment design and purch use?
Equipment Design and Purchasing
Design specifications Plant capacity and yields Simplified process flow sheet Design criteria Wind and temperature data Earthquake factor Allowable soil pressures Water analysis Waste water purity Characteristics of available utilities
L A S T MONTH’S ARTICLE showed that chemical plant design may be handled in various ways between the extremes of an owner performing all design work, including supervision of construction, and of an engineer-contractor performing all plant design and construction, often with full responsiblity for process as well as for mechanical performance. Since the majority of construction jobs are performed under arrangements somewhere between these extremes, equipment design and purchasing will also lie between. Most of the design considdrations outlined here will apply in some degree to process and equipment design groups and purchasing procedures in any owner-contractor relationship.
THE PLOT PLAN, whether suggested by the owner or developed by the contractor, is basic to both process and equipment design and layout. It is a bird’s-eye view of plant layout, but should be supplemented by preliminary elevation sketches of the major structures, and perhaps even by models. Incidentally, few plant designers pay more than the minimum attention to beauty and even fewer owners find it easy to justify extra money for the sake of architectural beauty. As shown vividly in the introductory article to this series [IwD.ENG.CHEM.,47, No. 2, 37 A (1955)], it is often possible to obtain a well balanced, esthetic layout without significant additional expenditure or loss of functional design. Piping and conduit requirements can seriously affect the plot plan arrangement. The process diagram will not reveal this. A preliminary piping and instrument diagram (or engineering flow diagram) and a single-line electrical diagram must be drawn up, so that pipeways and conduit racks, or underground envelopes, can be located and tentatively sized. It is only when a n acceptable plot plan and process flow diagram are developed, supported by a utility summary, that contractor and owner can reach agreement and fix firmly the scope of the job. ORGANIZING THE DESIGN. I n the contractor’s orgnnization a central process design group of chemical engineers 35 A
Mechanical specifications Equipment Materials Work Plot plan (suggested) The owner may also need later to point up various policies peculiar to his companyinstallation of spare equipment; preferred type of drives where not completely dictated by economy or utility balance; safety requirements; and arrangements for spare parts purchase.
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The contractor, on the other hand, should furnish the owner, after the initial discussions, with a
Project manual
This outlines the scope of the work, the functioning of both parties, and the responsibilities, procedures, and paper work for administration, engineering, procurement, accounting, and construction
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Design specification book Prepared when the design i s from 10 to 25% along. Composed of one or more volumes, containing diagrams, plot plans, and major layouts and specifications.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
JUNE 1955
PLOT PLAN LAYOUT MUST CONSIDER.. 0 Length and location of piping and electrical
..
0 Entry of utilities
conduit 0 Location of blowdown and flare headers 0 Access for construction, maintenance, and
fire fighting 0 Centralization of control stations 0 Direction of
prevailing wind (for fired heater and cooling tower locations)
e Size of foundations
if included
e Possible future expansion
prepares either full process data and specifications or data additional to those furnished by the owner, so that the project engineer may proceed with detailed design and procurement of equipment. The process group also establishes principal control methods and on completion of construction may take charge of or aid in startup. There are two ways of handling the responsibility for various detailed phases of design a,s the project proceeds. One is to parcel out much of the engineering and drafting to specialized design sections (civil, structural, architectural, electrical, piping, and instrument) that carry on work for several different projects a t the same time. In a different scheme, generally on large projects, group chiefs in the contractors’ organixation assign engineers to a project organization where they work together as a team. Each group reports to its group head-for example, the structural group will report to the chief structural engineer for tcchnical guidance and direction to be sure that the structure conforms with various state and local laws and codesbut also reports directly t o and works for the project engineer, so that its work may be integrated with that of other design groups. DESIGN AND PURCHASING ARE INSEPARABLE. Purchasing generally goes on concurrently with des+ and often precedes completion of design drawings. If the job
EDWARD
0 Location of railroad sidings,
E. DORRESTEN
Edward E. Dorresten, consultant to The Bechtel Corp., retired from operational responsibilities last December as vice president of Bechtel’s Refinery Division. He i s well known in petroleum and chemical circles through more than 25 years of engineering and management activity in these industries. After 1 1 years with E. B. Badger & Sons Co. as job engineer and district manager, Mr. Dorresten ioined the Bechtel interests in 1939. In 1948 he was named manager of engineering for The Bechtel Corp., and in 1951 elected vice president, Refinery Division. He is a graduate of Stanford University.
award is a result of competitive bidding, the contractor probably specified certain types and kinds of equipment and machinery used for pricing the bid. Purchasing is then straightforward and can proceed at any pace the contractor desires. If the job is on a cost-plus basis, the procedure is more complicated. This latter is the one discussed here. Two main factors set the pace of any construction jot>equipment deliveries and drawings completion. Sometimes one governs, sometimes the other, but equipment deliveries generally govern when a plant contains one or more pieces of large, special type equipment which require extensive engineering and shopwork. This makes it imperative that bid requests to manufacturers be forwarded as soon as practically possible. Since 3 to 4 weeks’ delay is a normal occurrence when obtaining competitive bids, direct negotiation with one vendor on supercritical occasions may he desirable. General purchasing can begin as soon as the process Bow diagram has been translated into a detailed piping and instrument diagram, showing in standard symbols every major piece of mechanical equipment, all instrumentation, piping, and valves. A separate utility diagram, and an electrical one-line diagram are also needed. The “request to purchase,” submitted to the purchasing department, should include instructions for inspection requirements for factory tests, if to be made, and deadlines for contractor’s receipt of supplier’s certified dimensional drawings of specific pieces of equipment being purchased. Control of receipt of these drawings, usually called foreign prints, may be in either contractor’s purchasing or engineering departments, handled by a materials requisitioning group which also processes requisitions and maintains suppliers’ catalog files. This control is necessary because hundreds of prints must go through the routine of being sent by the supplier to the purchasing department, then to the project engineer who makes changes or selects alternative arrangements, back to the manufacturer, and then back t o the contractor with the necessary changes made and the prints “certified for construction.” IF EQUIPMENT IS TO BE CONTRACTOR DESIGNED, usually the case with fractionators and pressure vessels, a different procedure must be followed. Process engineers determine size and shape; operating temperature and
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Xn extreme cases it may be justified to split the plant into two or more completely independent parallel “trains” or sections, so that production will never be less than 50y0of plant capacity. One moderate variation of this design is to install duplicate units for a particularly vulnerable section of a single plant. It would be folly to prescribe a pat rule for equipment duplication when so many variables affect the decision. We can only generalize by concluding that although each case should be evaluated largely in terms of dollars and cents, many intangible factors such as basic company policy and personal experience will often affect the decision. In all cases some selected individual spare parts are warehoused. The quantities will vary from a very minimum when complete equipment is warehoused to much larger quantities when single, unspared equipment is installed.
pressure; number, size, and approximate location of nozzles and manholes; corrosion allowance, and basic design of internal parts. These data are given to special designers, who prepare outline drawings and specify materials, design pressure and temperature, metal thicknesses, welding techniques, and code requirements. T h e n fabricators are able to bid, and the successful bidder preparee the actual shop drawings from which the equipment is built. Purchasing should also include erpediting, which means visits to manufacturers t o determine if delivery dates can be met, and to help if there is any doubt. A number of companies also put shop inspection responsibility under purchasing. The engineer-contractor should have an inspection manual covering procedure for various types of inspection, approved by the owner. The inspector must then enforce these procedures and see that equipment marked for inspection and shop testing on the purchase order conforms t o requirements. Take welding as an example. Usually all fabricated materials which are welded together require close inspection to be sure that the proper type of welding rod is used and the welding technique is according to specifications and the general provisions of the manual. Detailed reports must be made by the inspector to contractor and owner when the piece of welded equipment is to be used under elevated temperature or pressure or both, and must conform to some code such as API or ASRIE.
STANDARDIZATION. Many owners go on year after year adding units to their existing facilities and paying little or no attention to possible standardization until one day a serious review of spare parts inventory shows that it is far in excess of what should be expected. The prevention of this situation lies mainly in providing the engineercontractor a t the start of a project with listings of existing plant equipment supplemented with notes on the owner’s experience with the equipment. In many cases the contractor’s engineer will find it possible to justify buying other than the low bidder’s equipment, not only because spare parts can be reduced hut also because with fewer brands of equipment the operators and maintenance crews are more easily trained. One of the most striking standardization opportunities is in heat exchangers. By limiting tube lengths to say, 8 and 16 feet and diameters to 3/4 and 1inch, the warehousing of tubes can be minimized. Of course, there is still the problem of different requirements for tube materials and this is more difficult to get around, but some opportunities for standardization exist even here. Where heavy fouling of exchangers and frequent cleaning are anticipated, oversize units can in some cases be of value, but more often a payout can be shown by installing multiple parallel units. Heavy corrosion, which some-
DUPLICATION. Many chemical plants are built on prbcesses in which corrosion and wear are so severe and catalyst life is so short that if there were no spare equipment the down time for repairs and replacement would be excessive. Kot only would this result in idleness for the plant operators and loss of production but feed and product storage capacities would have to be increased. Kormally the problem is overcome by providing installed spare equipment in the most vulnerable spots such as reactors, pumps, and compressors. In other cases complete warehouse spares may be provided. This latter method is well suited to cases where space is not readily available, auxiliary but reliable equipment is costly, and change-over can be made relatively quickly.
BID REQUESTS, with specifications and flow sheets, are sent to..
..... .. . . . ..
BIDS RECEIVED ARE TABULATED on a
BIDS ARE ANALYZED b y . .
. . . .. .....
1 1 1
ACCEPTABLE SUPPLIERS, generally stipulated by owner (and perhaps added to by purchasing, with owner’s a pprova I)
BID TABULATION SHEET for each individual piece of equipment, listing salient features of each bid
PROJECT ENGINEER, on basis meeting all specifications, including desired delivery time
LOWEST ACCEPTABLE BID i s recommended to..
.... . . . . ....... ... . .. .
REQUEST TO PURCHASE is submitted to
OWNER, who will approve or perhaps suggest more expensive or slightly different equipment for standa rdization
PURCHASING DEPARTMENT
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times results in retubing after every run, requires a differcnt treatment. Spare tube bundles are then more economical, especially if exchangers of uniform design are located in different plant units so that one bundle is always out of service for repairs between runs. Standardization is often practiced on an intercompany basis. A chemical company which for marketing purposes has similar plants located in several sections of the country may find it profitable t o stock very expensive spare equipment in one plant only. Besides standardization of materials and design from a particular equipment manufacturer, the engineer-contractor is also interested in furthering more uniformity between manufacturers. All valve manufacturers, for instance, now design to AXA flange dimensions so that replacement is no problem. The same applies to electric motors with their KE-MA standards and to some extent t o TEAT-4 heat exchanger design, but among pump, turbine, and compressor manufacturers standardization is virtually nonexistant. It is not suggested here that manufacturers should standardize internal design features, but certainly it would be a great advantage to the designers and users if base plate mounting dimensions, nozzle locations, and shaft positions were more uniform for similar types of eyuipment.
manufacturers, plant owners, and engineer-contractors with an eye toward lower over-all installation costs. It is especially well suited to mild, dry climates but has been used to a lesser extent even in cold, damp, and windy areas. Chemical plants handling liquids in continuous process have made use of outdoor construction to the point that in some plants only the central control instruments are housed in buildings. Reciprocating compressors are generally installed under roofs, but pumps, centrifugal compressors, and electrical gear are placed outdoors with fractionators, reactors, pressure vessels, and heat exchangers. Modern petrochemical plants are outstanding examples of this tendency. Processing plants for solids and semisolids such as sulfur, carbon blacks, and plastic materials are generally not suitable for outdoor construction because of dangers of contamination. Likewise, batch-operated equipment which is regularly opened and closed is normally placed indoors. The move toward outdoor construction has had setbacks in some isolated cases. Lack of thorough consideration of climate, equipment design, and operating procedures was probably to blame in the majority of these. Nevertheless, continuing pressure for more economy in construction will undoubtedly spell further development of outdoor type equipment in the future.
ECONOMY IN DESIGN. One of the most important subjects in design of equipment for chemical plants is material selection. Material strength, especially a t clevated temperatures, is a problem but more frequently resistance to corrosion is of greater concern. What should be the life span of the materials-6 months, 2 years, or 20 years? The answer is seldom obvious. The most important considerations are:
OWNER DESIGNED EQUIPMENT. Plant owners seldom design equipment. When they do it is generally because their own research and development groups have produced patentable equipment designs for particular links in the process, not obtainable in the market. Often under these circumstanccs the owner will deal directly with a manufacturer and the engineer-contractor will probably receive only an outline drawing for installation purposes. Except for this one circumstance there is no advantage for the owner to do his own equipment design.
1. Chances of process obsolescence. 2. Adaptability for re-use in case of obsolescence. 3. Cost of replacement in case of failure. 4. Loss of production during replaccment. When thc owners and contractor’s engineers have determined items 1, 2, and 4,the designers are ready to tackle item 3 for whatever alternative design schemes are being considered. Again, it is a case of simple economics. If a process is considered short lived and the equipment is specialized and therefore not very adaptable for re-use in the owner’s other operations, the most inexpensive kind of safe and reliable equipment is probably the proper choice. We often hear criticism that American equipment is overdesigned. This may be true, but there are a number of benefits to be derived from this practice. It stems from several conditions which are peculiar t o the American economy, notably that raw materials are relatively abundant and cheap by European standards and also that the history of American chemical industry growth leads the designer to allow for some future expansion, a trend which is not normally practiced by European designers. Also, because the tempo of our industrial activity is so much higher than in other countries we tend to use safe approximations rather than exact decimal design dimensions. Thus we design and erect a plant in perhaps half the time required by others. I n that way earlier sales profits more than offset savings in material costs for more accurate design which, even then, is partly lost by the additional design costs. OUTDOOR CONSTRUCTION. More and more manu-
facturers are featuring “outdoor” type equipment. In most cases this move has been a cooperative effort among 38 A
OWNER’S PURCHASING. In general, it is not desirable or advantageous for an owner’s purchasing group t o buy the equipment on a contract job. The contractor gets just as good or better discounts, so that there are no savings in cost. Also, purchasing by owner and design by contractor bring divided responsibility and consequent confusion and delay. Inspection and expediting become more difficult. There is one situation where the owner can hurry the job along by initiating purchasing. This is the case when a project has been approved for construction, the major process and mechanical design has been roughed out, but a contractor has not been selected. It is then advantageous for the owner t o place orders for long delivery items. However, as soon as the engineer-contractor begins work, the procurement of these items should be passed on to him. DESIGN EVALUATION OF MANUFACTURERS’ EQUIPMENT. The engineers’ analyses of bids are very simple
when fabricators bid on engineer-contractor’s design ; price and delivery then govern. They become more difficult for manufacturers’ designed items. Price and delivery are still important, but design features must now be closely evaluated. The reason for this situation is that, in order to obtain satisfactory price and delivery, the contractor’s specifications must stipulate only the major features required, otherwise the equipment becomes “tailored.” Careful attention should therefore be given to design features which mav give more flexibility, better control, longer on-stream periods, and simpler maintenance. Their value may be sufficient to justify buying other than the low bidder’s equipment.
