It is the objective of this revised technical manual to assemble in one publication data and procedures that have been successfully used in the design of buried steel penstocks, incorporating the modern advances in steelmaking practices and pipe manufacturing methods to provide the most economical design for the wide range of conditions encountered in the industry.
Additional information is available from manual No. 79, Steel Penstocks from the American Society of Civil Engineers published in 1993.
Traditionally, steel penstocks have been considered as very high pressure conduits, usually of large diameter, and operating with frequent surges during the normal condition. Penstocks may also be subject to pulsations of varying frequency and amplitudes transmitted from the turbine or pump. When penstocks are installed above ground, this can sometimes cause excessive vibration. These are the perceived differences between a penstock and an ordinary pipeline. Based on these conditions, penstocks have been designed to standards established in 1949 with minor revisions based on an allowable design stress at normal conditions of 2/3 of yield or 1/3 of tensile strength.
In the last 25 years there have been many changes in steel making practice, pipe manufacturing methods and welding procedures. During this time, thousands of miles of steel pipe manufactured to American Water Works Association Standard C-200 have been put into service for water transmission lines including: flow lines, inverted siphons and pump discharge lines. These lines have usually been buried lines operating at working pressure of 150 to 350 p.s.i. plus transient pressures. Many lines, however, have been installed since 1960 with working pressure as high as 640 p.s.i. using high strength low alloy steels with an allowable design stress of 50% of yield at working pressure and up to 75% of yield at transient pressures in accordance with AWWA M-11 “Manual of Water Supply Practices” for steel pipe. These lines have utilized O-ring joints to working pressures of 250 p.s.i. or more and bell and spigot welded joints to working pressures of 400 p.s.i. with butt welded joints used at working pressures over 400 p.s.i.
Today there are many penstocks installed utilizing thousands of feet of pipe with operating pressures varying from no pressure at the inlet structure, to low or moderate pressures, or to very high pressures at the power plants. Most of these penstocks are buried and many parallel the stream from which the water was diverted. They are usually in remote locations. With certain types of turbines and an adequate control valve system, sometimes involving a synchronous bypass system, transient pressures can be limited. A buried penstock will not be subject to the problem of harmonic vibrations sometimes associated with the traditional penstock.
For many of these installations, when carefully evaluated by the engineer, the normal quality of AWWA C-200 pipe and the design standards of AWWA M-11 should be considered adequate and will provide the most economical material for this service. As working pressure and pipe diameter increase, the use of high strength low alloy steel will become economical. At design stresses over 21,000 p.s.i. at normal conditions, additional testing, including 100% ultrasonic or radiographic inspection of welds, is appropriate.
On many long line penstocks, the AWWA Standard and the traditional standard can be combined for the appropriate portions of the line. This manual, therefore, will address the design, materials and fabrication for both types of installation.
May 1, 1998
- Steel Water Pipe