Rebruarw I952
Equipment and Design Many chemists and even some engineers are apt to forget the design and construction codes that contribute so fundamentally to progress and safety bar C k w h n Omen m w n we in this country are burdened with a system of units and measures, which is based on nothing rational and represents a life of continuous drudgery, we can be thankful for an excellent system of standards and the codes, which are highly satisfactory. In good accomplished, the codes rank second to none-even safety-first-in saving lives and preventing accidents. We complete billions of dollars of construction in the United States each year and the correctness and excellence of this construction are due largely to the codes. Lattice steel frameworks rising as high as 1200 feet in the air support thousands of people and thousands of tons of moving loads for many decades. Kerosene lamps and burners no longer explode as they used to, and boiler explosions are very rare. This happy condition is basically due to the use of our codes. There are several interpretations t o an analysis of any engineering problem and how well-trained engineers can come up with several different results is understandable. The codes have standardized and corrected such differences by thousands of hours of study by hundreds of experienced men. The codes are really authoritative because they are based on the weighted results of long experience interpreted by many minds. An example of the thorough study given the codes can be obtained from another field of effort, but it serves our point very well. The petroleum industry has r e cently estimated the cost to construct a large project and submitted a report. In the foreword of the report appears the following paragraph :
A
LTHOUGH
Contributing companies made available the services of a large numFebruary 1952
ber of qualified specialists in research, development, and engineering. The main subcommittee, in turn, found it desirable to organize in cooperation to carry out the assignment in the most effective and expeditious manner. An idea of the size and complexity of the problem and the thoroughness with which it has been studied is indicated by the following log: No. subcommittee members Additional technical personnel used Total attendance a t meetings. man-days Y o rnnotinns
NO: EG-trrps
Total miles traveled Total dollars spent Outside companies consulted
47 106 590 197 338 400,000
300,000 115
A new edition of the “Code for Industrial Piping, ASAB 31.1 1951,” has recently been issued; the changes are important and also are indicative of the careful, slow development of this work. The opening paragraph, 101, formerly read: Section 1 of this code covers the design, manufacture, test, and installation of power piping systems.. . . I t now reads: Section 1 of this code prescribes minimum requirements for design. . , . Other changes continue to close in on the subject for greater clearness, precision, and improvement, as the following examples state: Paragraph 112 now directs that all pipe threads shall conform to American Standards for Taper Pipe Threads (3 2.1) instead of pipe threads (B 2). No change has been made in the rules for pipe joints or for gaskets. Hangers, supports, and anchors have the same treatment in subparagraphs ( b ) and (c), but pipes hung from wood structures need not be separated from contact with the wood unless carrying a fluid above 250’ F.; formerly this temperature was 230” F. Hydrostatic tests before erection were formerly guided by the in-
cluded Table 2, but now must conform to @upplement No. 1 of the American Standard for Steel Pipe Flanges (ASAB 6e6). An important change in after-erection testing is a test pressure of only one and one half design pressure, where formerly twice design pressure was required. This is a big move in the right direction, amply justified by the remarkably uniform quality of all steel today. The testsfluid is now limited to a temperature of 100’ F. The wall thickness of pipe is an important dimension t o fix. There have been many principles used in fixbg the standard thickness, all of which are compromises of one or more good design points. If pipe is to be threaded, stress calculations must be based on the metal remaining from the bottom of the threads to the inner surface. This means that the entire length of pipe to the next thread contains an outer layer of metal in excess of that required for the working pressure. Through the years this has wasted a good bit of metal, as compared to thickening the ends of pipe so that threads could be cut having a root diameter equal to the maximum outer diameter of the pipe. If this extra layer is designated as an allowance for corrosion, there is no such metal under the threaded portion of the pipe, but the threads are always enclosed in some kind of a fitting which strengthens the wall for internal pressures, 50 that’ C in the formulas is an allowance for special mechanical features, threading and/or (Continued an page 78-4)
INDUSTRIAL A N D ENGINEERING CHEMISTRY
71 A
Equipment and Design corrosioza. The minimum wall thickness (2, in inches) has been reduced slightly :
2s ( t , - C) orP = D - 0.8( t , - Cj
where P = maximum internal service pressure, and D = outside diameter of the pipe in inches. The same applies to gas and air piping. Values of C have not been changed, but additions have been made in the values for allowable stress (S) in power plant piping. Values for riveted steel have been dropped and new copper specifications added. Specifications for pit rase iron pipe have been enlarged, clarified, and changed slightly. The minimum thickriess is differently defined for Class No. 1 of ASTM Spec. A44 or Class I of AGA 1944 Interim Bulletin “Standard Specifications for Cast Iron Pipe and Special Castings.” The old edition permitted a thickness governed by AGA sperifications or Class A of the AWWA sGecifications. The efficiency of the joint, E , can now be taken as 1.0 for dottble-submerged arc-welded pipe if weld tests are equal to tensile strength of pipe met,nl. Hydrostatic pressure requirements before installation are given for butt welding fittings, which lee re not covered in the old code. P,, the mill test pressure in pounds per square inch gage, is defined more definitely as NZ
