How to Reduce the Variability of Piping Tie-Ins

Tie-ins, or connections to existing piping systems, are typically required to replace corroded systems or to install lines for new equipment or facilities.

Branch connection tie-in for new line

The API 570 standard applies to in-service piping systems, or those that have been in operation, while the B31.3 code applies to new construction piping systems. In-service piping systems can pose challenges during welding due to the process contaminating the steel. For instance, hydrogen can penetrate the base metal, leading to delayed cracking after welding. The location where you plan on making your tie-in can have general or localized metal loss, leaving you scrambling to find sound metal to weld to.

Asset owners can choose to perform 100% ultrasonic (UT) or radiographic (RT) examination instead of pressure testing, provided this is permitted by code. Non-destructive Examination (NDE) in lieu of pressure testing is advantageous when pressure testing in-service systems is impractical. However, this volumetric exam can reveal pre-existing defects in the base material that quality control may not have detected during the construction process. It is good practice to perform magnetic particle (MT) or liquid penetrant (PT) testing of the weld root and cap where volumetric examination may not be as effective, in addition to RT or UT of the entire tie-in weld.

Liquid Penetrant test reveals multiple indications on the Tie-In root weld.

Tie-ins to existing piping systems should be made at flanges, whenever possible, even if this means adding more new pipe than what is required. Field welding may still be necessary; however, eliminating a new pipe-to-old pipe weld reduces the variability and risk of your installation. Reducing the variability inherent in welding existing piping systems increases the likelihood of your project completing on time and within budget.

"Tie-ins to existing piping systems should be made at flanges, whenever possible, even if this means adding more new pipe than what is required."

Piping Flange at Tie-in with Backing Gas Dam for Welding

If a flange is not readily available on the existing piping system to connect to, and a new-to-old pipe weld is required, consider adding a new flange at this tie-in location. A flange at the tie-in allows for the backing gas to be applied adequately and for the inside root weld to be visually inspected, with slop in the bolt holes that can provide some leeway in alignment during fit-up. The new flanged spool can be pressure tested off-site and bolted into place. The benefits of installing flanges at tie-ins should be studied against the risk, as bolted flanged joints are a potential source of release. The exact location for a welded tie-in on an existing system should be examined early in the project during design to ensure adequate wall thickness, thereby eliminating this variable during installation.

Utilizing laser scans and point clouds to locate tie-ins can reduce dimensional errors and field welding when done correctly. Always account for the operational state of the piping system when laser scanning, as the dimensions of a system at operating temperature and under load can differ from its dimensions in its cold state during installation.

Point cloud for a like-in-kind piping replacement

In conclusion, tie-ins to existing piping systems should be made at flanges, when possible. When welding to existing systems is required, examine the wall thickness of the tie-in location during the design phase. Consider adding flanges to ease the installation process and utilize point clouds to eliminate dimensional discrepancies.