Outlets: The Rest of the Story

The 5th edition of AWWA M11 has been available for a little over a year and the benefits to engineers have been recognized for the value it has for steel pipe design. There were numerous changes and additions that are intended to clarify important design criteria when designing steel water pipe. 

The nature of engineering design is that the knowledge used to design large facilities is learned and remembered mostly during the work on a project. This series of articles will highlight many of the major changes in the new AWWA M11.

The first article in the series will concentrate on outlets in a pipeline. This article, along with the remaining articles, will become useful references for the design of pipeline and treatment plant projects.

Outlet Design

There are three main categories for outlets on a steel water line.

• Single outlets
• Double outlets – non-full sized
• Full Crosses

The terminology for configurations within these categories can be listed as:

• Single radial
• Single radial – tangential
• Single radial – lateral

The 5th edition of AWWA M11 has been expanded to include design examples of outlets and needed reinforcement. One of the great features of steel as a material is its ability to be designed and built to any shape and form. When a portion of the steel pipe wall is removed for purposes of creating a tee, a wye, or an outlet, the pipe is obviously weakened and needs to be reinforced with additional steel. The reinforcement can be via additional steel thickness, a reinforcement plate around the opening to further stiffen that specific area, or a crotch plate.

This paper highlights and expands upon the details given within the M11, with a short explanation of the changes.

PDV
A PDV is a representative number for Pressure-Diameter Value that was originally proposed by the Los Angeles Department of Water and Power for designing large diameter, high pressure outlets as an alternative to the ASME-based design method of reinforcing. The original work on PDV in 1981 was based on a 9000 PDV as a factor of when to use the crotch plate design. It was not clearly identified what to do with surge or test pressures.

The 5th edition clarifies the definition of design pressure and how it relates to surge and test pressures. It goes back to the original 9000 PDV value for design pressure for most outlets. The earlier M11 editions did not provide guidance of how to handle full cross design, but the procedure has been used by many engineers in the industry for many years to design and fabricate full-size crosses. The committee clarified that for full-size crosses, the original AWWA value that was in earlier editions of M11, with a 6000 PDV, should be used to determine when to go to crotch plates for a full cross. This was handled by introducing a “K” value of 1.5 for designs with a full-size cross. The PDV is used to determine the various types of outlet reinforcement as shown in the table below that can be used for the different outlet configurations.

Tangential Outlets
In previous versions of M11, the tangential outlet formula was not based on stress design but was based on historical ratios. The formula that is in the 5th edition of M11 has a basis in ASME design and is based on replacement of removed material and stresses. This gives a better design basis.

Minimum Edge Width
In previous editions of M11, the minimum edge width of a collar or wrapper was the effective outlet size divided by 3, whichever was greater. The 5th edition has changed the minimum edge width to be simply 1.5.” Basically, the closer to the outlet that the reinforcement is placed, the better. The maximum edge width remains unchanged at effective outlet size divided by 2.

Crotch Plate Design
The crotch plate design in the 5th edition of M11 was enhanced in two different areas. There was no methodology in the previous editions of M11 to explain a consistent method to design when the diameter of the pipe was outside the ones listed on the nomograph. This was clarified.

The original papers for crotch plate design were based on a low stress which was consistent with the material strengths available at the time the papers were written. The use of higher grades of material was acknowledged but how to apply them was not clarified in previous editions. This edition of M11 has clarified how to apply higher grades of steel and the resultant stresses to the nomographs and the design of crotch plates.

Welded Connections
Welded connections for the different configurations of outlets were not addressed in previous versions of M11, other than weld symbols. There is now information included for welded connections for all the outlets used in AWWA M11.

Future Articles
One of STI/SPFA’s goals with our experts’ design tips is to point out important changes in the new edition of AWWA M11 and to provide background of why such revisions were made. Steel pipe systems have been designed, fabricated, and installed with decades of experience, and reliable, and proven safety factors. The discussion above about outlets is the first in a series of design tips prepared by experts in the industry. Future articles will feature Harnessed Coupling Joints, Anchor Ring Design, Bolted Connections, Above Ground Pipelines, Bracing for Shipping, and other changes incorporated into the 5th Edition of AWWA M11 Steel Pipe – A Guide for Design and Installation.