Stress Analysis of a Cast Iron Split Tapping Sleeve
Authors: Sears E. C. , Miner J. W. , Mccauley E. D. 
Published: August 2009

The SR-4 strain gage was used to determine the magnitude of the stresses produced in a cast iron sleeve during assembly and while subjected to internal pressure. The object of this study was to reduce these stresses by proper changes in design and thereby obtain a greater factor of safety in the sleeve under service conditions. This sand-cast gray iron fitting, known as a 6 by 4-in. split tapping sleeve, is used to cut into a 6-in. gas main for a 4-in. branch connection without interrupting service on the main. Figure 1 shows one of the sleeves used in this study assembled on a section of 6-in. pipe. The sleeve is composed of a branch half and a bottom half bolted together around the pipe. Pressure tightness is maintained by lengths of round rubber gaskets compressed between the side flanges and by rubber end gaskets encircling the pipe compressed by glands bolted to the sleeve. The sleeve is rated at a working pressure of 150 psi. Four sleeves were tested, two of the original design, a third incorporating two design changes and a fourth having an additional design change. The steps given below were followed in testing all four sleeves: 1. Strain gages of type A-11 (1 1/16-in. gage length) and type A-8 (1/8-in. gage length) were cemented to the sleeve at selected points. After the gages were properly dried and wired, the sleeve was assembled loosely on a length of pipe and zero gage readings were taken with an SR-4 strain indicator. 2. Round rubber side gaskets were inserted into grooves in the sleeve side flanges and the side bolts were tightened to 90 ft-lb. by means of a torque wrench. Gages were read. 3. The end gaskets and glands were assembled on the sleeve and the end bolts were tightened to 90 ft-lb. Gages were read. 4. The sleeve was filled with water and the pressure raised to 200 psi., 50 psi. more than the rated maximum working pressure. 5. The sleeve was subjected to hydrostatic pressure until failure and a tensile bar was machined from the broken sleeve. The tensile bar was pulled in direct tension while readings were made on a strain gage mounted on the bar. The resulting stress-strain relation was used to convert strain readings obtained in the fitting test to stress values. Stress-strain curves for the sleeves in these tests are shown in Fig. 5. A description of the sleeves used in the four tests and the design changes made on the basis of strain-gage tests are given below: Test No. 1.—The branch half of the sleeve failed on the burst test at a corner as shown in Fig. 2. The sleeve was of the original design with side gasket grooves 1/4-in. from the side bolt holes (Fig. 3). Since no gages had been mounted at these corners, the test was mainly useful in showing the probable location of maximum stress. Test No. 2.—The sleeve was also of the original design. On the basis of test No. 1, type A-8 gages were mounted on the corners at what shall be called the critical area (Fig. 3). These gages showed that the stress produced at the critical area by tightening the side bolts accounted for 15,000 psi. of the 22,000 psi. total stress at 200 psi. pressure. Test No. 3.—On the basis of the assembly stresses measured in test No. 2, the sleeve design was changed to move the side gasket groove to 1/8-in. from the side bolt holes (Fig. 3). It was reasoned that this should reduce the stress produced in tightening the side bolts by decreasing the length of the lever arm between the load (side bolts) and fulcrum (side gasket). Also, reinforcing ribs were extended completely around the sleeve body from side flange to side flange. These changes resulted in decreasing the stress at the critical area to 13,600 psi. at 200 psi. pressure. Test No. 4.—The sleeve design was further altered to obtain a circular end flange instead of the hexagonal one on the original design. The stress obtained at the critical area on this design was only 8150 psi. at 200 psi. pressure.