We have an office argument going about the use of ball valves versus gate valves on a sewer force main.  This particular project is a 12" force main running about 2.5 miles and discharging into a manhole, but I would like your opinions on sewer force main valving in general.  The project is located in SE Georgia.

I think that gate valves are appropriate up to 20".  Over 20" I would probably use a butterfly valve, although I haven't given it much thought because I've never designed a force main over 20".

I've reviewed four utility design standards from Florida and Georgia.  Three use gate valves, and one uses plug valves.  There is no local design standard, and we are basing our design on a military technical manual, which provides no guidance on this issue.

The answers and advice you have been given are typical selections for US. In Europe (for EN standards) eccentric plug valves are not that readily commercial available in all sizes, market and competition somewhat limited. Soft sealing gate valves are common for smaller versions,  dominating up to 10 inches, but available and used for dimensions up to 20 inches and larger.

Parallell to this (good constructions of) knife-gate valves are used in treatment plants and on mains, typically up to 24 inches.

Knife-gate valves would be considerably cheaper than plug valves, and dependable with long-lifetime. Good constructions of plug valves will probably have longer lifetime and be somewhat more dependable over a long (twenty year plus) lifetime.

There exists one exception to ball-valves not recommended  for sewer. In Berlin, Germany, the Erhard double-excentric ball valve (Tyco Waterworks), was selected for a main sewer, I believe in 40 inch (1000mm) requiring low builing height. This valve has a full bore, seat sealing fastened on the ball turned away from main stream when open, and in addition fairly large open room between ball and inner wall of housing beeing flushed clean by faster flowing part of main stream. This has proven a very dependable, trouble-free solution. Price at least as high as plug-valves.

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