Arc Machines 307 - Manual (Page 157)

Arc Machines, Inc. Model 307 Orbital Tube Welder Training
Document No. 740096 Chapter 16. Page 4.
Rev. A
Purity requirements
Orbital welding equipment does not require gas of higher purity than manual
welding equipment, but typically, if the application requires the quality provid-
ed by orbital welding, the purging requirements are more stringent because of
the nature of the application.
In order to meet the stringent specification for discoloration Semi Standard
F79-0703 requires I.D. purge gas to be certified to less than 3 ppm total mois-
ture, oxygen and other contaminants. The ASME BPE Standard, widely used in
the biopharmaceutical industry, requires that the I.D. of welds be free of color
while a slight amount of blue or straw color in the HAZ may be acceptable. The
BPE does not specify gas type or purity levels, but bioprocess piping contrac-
tors generally use argon dewars with impurity levels of less than 20 ppm or a
purity of 99.998% to achieve the specified color-free weld bead. Purge gas pu-
rity specifications are usually more relaxed for pipe welding applications for
which argon gas of 99.99% purity is generally considered adequate.
Purging techniques
O.D. Purge
The 307 Power Supply supplies a constant gas shield to the weld head for the
entire weld sequence. Prepurge and Postpurge times are programmable. Table
1. on page 8 lists minimum prepurge and postpurge times for the various
weld heads, but the purge should continue for as long as necessary to avoid
unacceptable weld discoloration. Recommended flow rates for Model 8 and 9
weld heads are also shown in Table 1. Precise control and consistency of the
flow rate to the weld head (O.D.), is important for weld repeatability. Insuffi-
cient flow can result in a poor purge and discoloration of the weld and the HAZ.
Excessive flow to the weld O.D. can disturb the arc, causing arc-out or arc-
wander.