Field-erected vessels

Since this failure, most designers have used only ellipsoidal or hemispherical heads that require a minimum temperature, typically 80° F (27° C), for hydrostatic tests of field-erected vessels. 

Large field-erected vessels for storage and barge shipment of liquid cryogens (e.g., liquid hydrogen and methane) can now be readily insulated with quilted superinsulations. Moreover, this particular application technique makes possible further use of superinsulations to a number of unusual applications. 

Atmospheric Tanks The term atmospheric tank as used here applies to any tank that is designed to operate at pressures from atmospheric through 3.45 kPag (0.5 psig). It may be either open to the atmosphere or enclosed. Atmospheric tanks may be either shop-fabricated or field-erected. The most common apphcation is storage of fuel for transportation and power generation. See the subsections Pressure Tanks and Pressure Vessels later in this section. 

Horizontal pressure vessels Vertical pressure vessels Storage tanks, shop fabricated Storage tanks, field erected... 

Large vessels for the storage of cryogenic fluids are field-erected, rather than assembled in a factory, because it is difficult to ship parts much larger than 12 ft in diameter. It is impractical to insulate these field-erected tanks with high-efficiency multilayer insulations by conventional methods, so a new technique has been devised to apply superinsulations to vessels which cannot be moved or rotated. 

Since it is necessary to construct tanks of this type by field-erected techniques rather than to shop-fabricate and ship to the field, it was obvious that standard methods of applying multilayer insulation could not be used. The inner vessel could not be rotated around its axis to spirally wind these materials. In addition, the construction schedule required installation of the superinsulation within a seven-day time period. To meet these stringent requirements, quilted superinsulation, a new technique of applying multilayer insulations, covered by U.S. patents 3,007,596 3,009,600 3,009,601 and patents pending was developed. 

Field erection of stirred tanks is much more expensive than shop erection, and the maximum size of a stirred-tank reactor may be limited by the size that can be shipped. Thus plants immediately adjacent to water may be able to have larger reactors than plants accessed only by rail or road. Rail and road shipments limit reactor sizes to about 100 m. Much larger vessels are field-erected, but these designs are usually more suited as low-pressure storage tanks or fermentation reactors than as polymerization reactors that are typically rated for 10-20 atm pressure. 

Although it is significantly more expensive to field fabricate a vessel, the total installed cost may be cheaper than a shop fab that is erected in a single piece due to the cost of transportation and erection. 

The above discussion supposes that the vendor assembles all fabricated process equipment before shipping it to the plant for installation. In some cases, equipment cannot be shipped to the plant site in one piece and pre-installation field assembly will be required. Examples are furnaces and very large distillation colmnns and other vessels, which cannot be trucked, barged, or sent by rail in one piece to the plant site. Large columns may be fabricated in sections in the shops of the vendor and transported to the plant site where the sections are welded in a horizontal orientation before the column is erected to a vertical position. In this chapter, the purchase cost of field-assembled equipment includes the cost of pre-installation assembly at the plant site. Field-assembly costs are usually included in the purchase-cost quote from a vendor. 

Process vessels erected too close to walls (not enough clearance from walls or permanent stmctures). In this case either the valves cannot be fitted, or the ducts cannot be welded properly very less space becomes available for fitting field instruments, and it becomes difficult to operate, clean, and maintain the equipment. 

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