Page 85 - Pipeline Rules of Thumb Handbook

72 Pipeline Rules of Thumb Handbook

Table 2 procedures for making these welds, with a minimum risk of
Limitations on remaining wall-thickness for repairing cracking, are available. 1,4,5,8
without a burn-through In the case of deposited weld metal repairs, heat input must
be kept low to avoid burn-through during the first or second
Values are the minimum recommended thicknesses in inches
passes. During later passes, however, higher heat inputs can
with natural gas as the pressurizing medium at the
be used to soften the resulting repair metal microstructure
pressures and flows shown
and the heat-affected-zone of base metal. A high heat input
Maximum welding voltage, 20 volts
final pass can be made with a nonfusing tungsten electrode as
Maximum welding current, 100 amps
8
suggested by the British Gas Corp. An alternative procedure
is to make an extra pass with high heat input using a conven-
Gas flow rate, feet/second tional electrode. This pass can be ground off since its purpose
is merely to soften the heat-affected microstructure of previ-
Pressure, psia 0 5 10 20 ous passes.
(Based on a paper, “Criteria for Repairing Pipelines in
15 0.320 — — — Service Using Sleeves and Deposited Weld Metal” presented
500 0.300 0.270 0.240 0.205
by the author at the AGA Transmission Conference at Mon-
900 0.280 0.235 0.190 0.150
treal, Que., May 8–10, 1978.)

Source
horizontal or overhead positions provided flow or pressure is Pipe Line Industry, January 1980.
controlled. Analytic results predict that the I.D. wall temper-
ature will not exceed 2,000°F and a burn-through is highly
unlikely when repairs are made in this manner.
Values shown in Table 2 are believed to be quite conser- References
vative. Repairs were made without burn-throughs on 0.180-
inch remaining wall with air at ambient pressure and no flow 1. Kiefner, J. F., and Duffy, A. R., A Study of Two Methods
inside the pipe. This conservatism provides an extra safety for Repairing Defects in Line Pipe, Pipeline Research
margin and allows linear extrapolations for values between Committee of the American Gas Association, 1974,
those shown in the table. The minimum wall-thickness of Catalog No. L22275.
0.150-inch is established on the basis of the experimental 2. Kiefner, J. F., Duffy, A. R., Bunn, J. S., and Hanna, L. E.,
results. This thickness is also recommended by the British Gas “Feasibility and Methods of Repairing Corroded Line
Corp. Within these limits, repairs may be made by deposited Pipe,” Materials Protection and Performance, Vol. III,
weld metal (Table 1). No. 10, Oct., 1972.
3. Kiefner, J. F., “Corroded Pipe: Strength and Repair
Methods,” Fifth Symposium on Line Pipe Research,
Underbead cracking Houston, Texas, 1974, American Gas Association, Catalog
No. L30174.
Underbead cracking can be minimized by making repairs 4. Kiefner, J. F., “Repair of Line Pipe Defects by Full-
with low hydrogen electrodes in a manner that avoids hard Encirclement Sleeves,” Welding Journal, Vol. 56, No. 6,
weld heat-affected zones. June, 1977.
One way to help assure that extremely hard weld zones are 5. Kiefner, J. F., Whitacre, G. R., and Eiber, R. J., “Further
not formed during repair welding is to limit repairs to well Studies of Two Methods for Repairing Defects in
known carbon equivalent ranges. Unfortunately, chemistries Line Pipe,” to Pipeline Research Committee of the
of specific samples needing repairs will seldom be known. American Gas Association, NG-18 Report No. 112,
Low hydrogen electrodes prevent hydrogen from being March 2, 1978.
present in the welding atmosphere but they do not prevent 6. Cassie, B. A., “The Welding of Hot-Tap Connections
formation of hard weld zones when adverse chemistries are to High Pressure Gas Pipelines,” J. W. Jones Memorial
present. Making of crack-free weldments requires careful Lecture, Pipeline Industries Guild, British Gas Corp.,
control of heat input to avoid rapid quenching or post-repair October, 1974.
treatments to assure that extreme hardness does not remain. 7. Morel, R. D., Welded Repairs on API 5LX-52 Pipe, 13th
In the case of sleeves, the critical area in which creaking Annual Petroleum Mechanical Engineering Conference,
most often occurs is the fillet-weld at the ends. At least two Denver, Colorado, September 24, 1958.