Double arrangement/48-inch butterfly valves

[LCid]

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?

 

[rconner]

[Was just curious are there any taps or outlets located between the two valves, and if so what is their ("clock" etc.) orientation?]

 

[stanier]

Double isolation for safety purposes? Although a butterfly valve in itself is not considered adequate for single isolation!

Some ill informed engineer who thinks he is doing the right thing?

 

[bimr]

Maybe the construction is not finished so the intent was unclear.

Or maybe there is provision for future expansion of some sort.

Perhaps you can put a picture online.

 

[greggy]

I am familiar with type of valve arrangement to provide "either way" feed to critical customers (e.g., hospitals) during water main emergencies. It allows the customer to draw their water supply from either direction when one side of the main is out of service for a repair.

An alternate that is less expensive materially is to have two individually valved customer taps a few feet apart on a main with ONE large mainline valve in between them. The two customer taps are joined to become one service on the customer side. This arrangement allows the customer to pull water from either side of the closed mainline valve if there is a water emergency (break), using a smaller valve to isolate them from the out of service portion instead, saving some expense of a second larger main line valve. During nonemergency periods all valves are kept open.

 

[rconner]

fyi see also

http://www.eng-tips.com/viewthread.cfm?qid=48267

 

[LCid]

Sorry no pictures available yet; still looking.
The arrangement of the two adjacent butterfly valves I am referring to is for trunk water mains (48-inch in diameter). So the concept of "either way" feeding is still applicable on a large scale (i.e. feeding a large section of a city). My question is more specifically related to the use of two (2) valves instead of only one (1) valve. Operationally, one (1) valve can actually do the work and separate a run in two branches. Then, in the "shut" position, any side (on either side of the one (1) valve), may be shutdown and dewatered. The other side still feeds the system (whatever system may be).
If two (2)adjacent valves (in the drawing the 48-inch valves were a few feet apart) are then proposed, what's the reason for that arrangement?

 

[LCid]

I think that the reason for the 2-valve arrangement is operational safety, redundancy for the system. If only one (1) valve is installed and that valve needs to be serviced/repaired a large section of the system is affected. The "bigger picture" here is that the arrangement is large (48-inch), it feeds a large section of a city; just considered a shaft connection in the vicinity and that the two-valve arrangement actually split in half the water supply coming out of it. Thank you all for your thoughts.

 

[bimr]

rconner,

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.

LCid,

My first thought was that you were enquiring about valves in series. However, it seems that you are referring to valves in parallel.

Valving off a water distribution into segments is good practice because it will allow you to valve off and isolate areas that required maintenance.

 

[LCid]

bimr,

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.

 

[bimr]

If you have two valves in series, than I would concur with Stanier's comments, it does not make any sense.

There is little value to installing redundant valves. One would be better off economically to buy a better quality valve than buy two valves that they do not trust.

 

[rconner]

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.