by H. Carl Bauman American Cyanamid Co.
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Costing Piping, the Bugaboo of Chemical Plant Estimating Estimating piping from flowsheets can save time and expense in preparing study and preliminary estimates I HIS series has dealt largely with methods for quick estimation of chemical plant capital costs. I n the main, techniques for estimating complete plant costs were discussed. Reference was made now and then to ways of costing piping, electricals, insulation, and instrumentation. Of all ' components of fixed capital cost, these are the most difficult to evaluate with less than complete plans and specifications from which to work. Estimation of process plant piping presents the greatest challenge to the cost engineer. Estimating from completely detailed drawings is an arduous, time-consuming task. Evaluating on any other basis can produce results varying widely in value. Piping costs vary from 30 to 80% of process equipment value or from 4 to 1 5 % of installed plant cost. The accuracy of the entire estimate can be seriously affected by improper application of quick esti mation techniques to this single component of total plant cost. All known piping estimation methods and variations can be re duced to the following.
take-off, wherein costs of all sizes and kinds of pipe are related graph ically to reference strings for each kind of pipe which have been completely priced for a base size. The cost of a similar pipe string of another size is obtained from a chart upon which the Ν factor is plotted against size. Multiplying the cost of the original string com ponent (pipe, valve, and fittings) by the Ν factor yields the estimated cost of the desired pipe size. The successful use of this method re quires that the base charts be kept up to date by periodically repricing the base strings. Estimating by weight of specific kinds of pipe requires substantially complete take-offs, including weight calculations. Its only advantage lies in the time saved in detailed pricing of piping components. It
mate—particularly in the case of large national companies, where it may be made for any location in the country or, for that matter, abroad. Many fine detailed piping cost handbooks are available to aid the detail estimator. In all, how ever, many arbitrary factors and assumptions are suggested to correct for such intangibles as hauling, handling, warehousing, scaffolding, labor travel allowances, and other fringe benefits. Accurate estimates of piping from the most complete detailed drawings are still a function of the estimator's art. Familiarity with local labor productivity, ma terial availability, weather patterns, and economic conditions is of prime importance in arriving at competi tive pricing. The R. A. Dickson Ν method is an interesting variation of the detailed
1. Pricing of detailed take-offs 2. Dickson Ν method 3. Pricing by cost per joint 4. Pricing by weight of specific types of pipe 5. Pricing by piped equipment units 6. Pricing by per cent of equipment value 7. Pricing by per cent of total plant installed cost The first four methods require some degree of piping take-off. Even with the completed plans and specifications upon which the de tailed take-off of Method 1 depends, there is always a serious doubt as to the accuracy of the resulting esti
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Figure 1. Man-hours per joint for field erection of shop-fabricated carbon steel and low-alloy piping
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is implied, however, if the method is to be accurate, that there is available a carefully kept record from past experience of the cost of installed piping systems on a weight basis for all kinds of piping materials. The cost per joint method, 4, is similarly based on the accumula tion of previous experience data properly analyzed and plotted for convenient use. Its advantage lies in the fact that good engineering flowsheets can be used for the take-off. Figure 1 shows man-hours of field erection time per joint vs. nominal pipe size in inches from 2-inch to 16-inch shop-fabricated carbon steel and low-alloy pipe. In this method the unit of work measurement is the joint (two for couplings and valves, three for tees, etc.), because it is here where the
bulk of piping labor is expended. The cost of handling, hanging, and placing lengths of pipe only is included in the chart as a fraction per 100 feet of the cost of making up joints. Figure 1 shows the total installed cost of labor in man-hours, exclusive of the time for operating engineers and teamsters, so this component cost would not be reflected when the chart is used for jobs where such crafts are not required. The plot shows labor man-hours only. Ma terial costs are obtained by pricing a take-off from the flowsheets on which all valves and instrument connections are generally shown. Pipe lengths and fittings are taken off by a skilled cost engineer by cross reference with an equipment arrangement plan. When prepared from data fed back from a specific
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