Double arrangement/48-inch butterfly valves
I recently saw a configuration that included two(2) 48-inch butterfly valves proposed to be installed a few feet from each other. There was nothing in between the valves--i,e. no pipes, no fittings, no special pieces, etc. The valves were proposed for a pipe that connecting two separate shaft connections--one at each side of the double-valve arrangement. I am wondering about he reason(s) for having two valves if one could "do" (at least separate the connections) the work? Extra-safety?
A double block and bleed is a different valve arangement. It is
typically used to prevent backflow in case of some type of equipment
failure. For example, assume you have a system feeding acid (through the
double block) to a process operation. A power
failure may cause the acid flow (and whatever the acid is feeding into)
to reverse. The reverse flow may cause damage to other equipment. If
you were feeding acid into water, the reverse flow of water into the
bulk acid tank may cause the acid tank to explode.
Double block
valves are typically equipped with spring to close actuators that close
in case of a failure or loss of power. The bleed off between the valves
is equipped with a spring to open actuator so that the bleed valve
drains. The pipe between the double block valves is then depressurized
and you have a fool-proof pipeline flow blocking device. Actually double
block valves are quite similar to backflow preventer devices.
Your first thought was correct--the valves are (proposed the work has not been done yet) in series. The pipe runs east-west. The valves are installed close to each other, a few feet away (I only saw a rough drawing). There are two shaft connections (coming out of the same shaft); one connection is east of the two-valve arrangament and the other other one is west of the two valve-arrangement. I have concluded as I indicated before, this arrangement seeks redundancy for the system and the availability to isolate sections of the system. The two-valve proposition? extra safety, if one valve fails or needs to be service. The size of the mains (48-inch) imply the supply of a large section of a city, so a valve failure would have a major impact.
By providing of course only the link I did it was not my intent to put a
definitive *label" on what has apparently been designed at some obvious
expense here, nor was it my intent to pre-suppose exactly why this has
been done at least in some sort of "water" line. There is however
considerable information on the link I provided, as well as others as to
opinions e.g. regarding safety and otherwise of double valve
arangements in other fields (and maybe some of the logic e.g. concerning
"safety" or otherwise in these threads might be argued by the designer
as not unimportant in whatever apparently large work is involved here?)
Very
little information has been supplied. I guess I could wildly blue-sky
many guesses as to why someone might do something like this, including
blocking redundancy as previously discussed (whether or not it meets
someone's specialty definition of what this means, it does at least
appear to be capable in a literal sense of a "double blocking"
function!)), safety, maintenance, testing of this or multiple pipeline
contracts coming together in this vicinity, or since butterfly valves
have been called out perhaps even some obscure specialty control
function, but this may be one that "only the designer knows for sure"!
Please
let us know if you ever get more information e.g. from the original
designer. [Once it is known exactly why they did this, I guess one
could then argue if one wished with regard to cost (or
other)-effectiveness to that end.
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