I/EC
Equipment
&
Description of the Equipment
Design
Failure of a 16-Inch Screwed Flange Study of causal factors shows how proper design, fabrication, and maintenance can prevent a recurrence consuming 46 tons of hydrogen sulfide. Considerable damage occurred from the blast effect which shattered and scattered 300 square feet of concrete slabs on catwalks. T h e heat of the fire caused buckling of structural steel within a 30-foot radius of the heat exchanger. Elec trical gear and instrumentation were burned and weatherproofing on insulation was ignited as far as 100 feet downwind from the ex changer. T h e early use of fire hoses prevented destruction of the weatherproofing and insulation outside of the 30-foot radius. There were no injuries to personnel.
by J. L. Rector, E. I. du Pont cfa Nemours & Co., Inc.
β \ PORTION of the facilities for pro duction of heavy water at the Atomic Energy Commission's Savannah River Plant ( 7) was severely dam aged on M a r c h 18, 1960, when a 16-inch screwed flange parted from a heat exchanger slip tube. Hydrogen sulfide gas at an initial pressure of 280 p.s.i.g. was released with explosive force through the open 16-inch pipe. T h e gas ig nited and the fire raged through ad jacent equipment for 30 minutes,
Tube Inlet
Chsmil cover
Channel
Heat exchanger slip-tube joint is shown above after the flange parted from the slip tube Floating HeadSlip lube Assembly
Design of the heat exchanger
44 A
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
T h e joint that failed was on one of the nineteen tube-and-shell, single pass, vertical heat exchangers that are parts of each of 24 units in the hydrogen sulfide dual-temperature exchange process for extraction of heavy water. T h e joint design is one which is commonly used; the flange is screwed to the slip tube so that the floating head and bottom shell cover of the heat exchanger can be disassembled. T h e process seal is not m a d e with a raised-face gasket and tapered threads, but is made by means of a gasket seated between the end of the slip tube itself and a mating pipe flange; the threaded joints provides only mechanical strength and is not exposed to the corrosive process gas. American Standard straight locknut threads are specified to provide the minimum reduction of pipe wall thickness at the thread. T h e calculated load-carrying ca pacity of the thread, based upon specified tolerances and allowances for the thread form, is in excess of 3,500,000 pounds. T h e load nomi nally applied by a stress of 30,000 p.s.i. in the bolt-studs is 560,000 pounds. Thus, the joint is ample to carry the load, if the proper thread engagement is maintained. Nature and Cause of Failure
Inspection of the slip tube and flange after the incident showed no
evidence of metallurgical damage or stripping of the threads. However, the thread on the flange was ta pered 0.076 inch and the slip tube was tapered to a small degree. T h e failure resulted from inadequate engagement of threads caused by fabrication defects and by dishing (deflection) of the screwed flange from normal loads imposed by the bolt-studs. T w o errors in fabrication were evident. First, the slip tube thread was machined with a V^-inch eccentricity that produced a 45degree segment of imperfect threads. Second, the engagement of com pletely formed threads was less than the American Standard locknut thread specification of 0.067 inch. T h r e a d engagement was zero at the segment of imperfect thread and about 0.040 inch at the completely formed threads. It is estimated that the segment of imperfect thread accounted for a 2 0 % loss of strength and that poor engagement at com pletely formed threads accounted for another 4 0 % loss of strength. T h e remaining loss of strength in the joint resulted from dishing of the flange and resultant tapering of the flange thread due to the bolt-stud load. Calculations, verified by tests on a new flange, indicate that permanent distortion begins at a bolt-stud stress of 13,500 p.s.i. T h e amount of taper on the flange that failed is produced by a bolt-stud
stress of only 25,000 p.s.i., as shown on the graph. As the flange is dished, about l/3 of the total taper is caused by a decrease in diameter at the flange face where the thread engagement increases. T h e net ef fect, however, is less engagement and a weaker joint than with the untapered thread. As an expedient to prevent a recurrence of this type of failure, and at the sacrifice of easy dis assembly, all 16-inch and 12-inch screwed flanges in the heavy water extraction units were replaced with welding neck flanges. Conclusion
T h e findings from this incident have important application to all users of large screwed flanges in which the gasket is retained within the bolt circle. These lessons apply equally to the design, the fabrica tion, and the maintenance of such joints. T h e following was con cluded : • I n the design of the joint, the loss of thread engagement due to dishing of the flange should be considered, with particular note that a ring gasket on the pipe ends transmits the bolt-stud load through the flange thread. T h e bending moment, and thus the flange de flection, is larger with a gasket on the pipe ends than with a ring gasket on the raised face.
0.12 J
16 Sti.eC Pipe A m i . SM. loctaul ( 8 per in.) 300 lb. Snail Hale flange
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15.700" Η 15.010" Du. 14.688" ID
-| ι " ' "m—•• Asbestos Basket, IMS " I n k . Η """ 16 3 0 0 - U . Welding Neck r Flange with Small Female / Facing. Bore Sen. SO, 20 —-j sleds. 1-1/4" Da·.. A1S3-B) —I I
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(1) Bebbington, W. P., Thayer, V. R.. Chem. Eng. Progr. 55, 70-8 (1959). The information contained in this article was developed during the course of work under Contract AT(07-2)-l with the U. S. Atomic Energy Commission.
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Literature C i t e d
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• Thread diameters should be specified clearly for flange sizes larger than those tabulated in cur rent standards (larger than 12 inches). T h e specified diameters should be maintained during machining. • Bolt loads should be carefully controlled to avoid exceeding design limitations, thus producing excessive flange deformation. It is particularly important during overhaul and main tenance to avoid overstressing to compensate for gasket seats that have deteriorated and flange align ment that has become poor. • T h e importance of suitable inspection techniques should be stressed to detect flaws before initial service and during routine mainte nance.
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Slip tube joint design
•^ Thread on the southwest segment o f the slip tube is pictured at left. Truncated t h r e a d on the northwest segment is shown a t right. C o m p a r i son shows t h r e a d is o f f - c e n t e r from tube axis
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Tapering o f a 16-inch flange thread from a p p l i e d stud l o a d
VOL. 53, NO. 8
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AUGUST 1961
45 A
