Hi Bharti,
Your question is way too general. What is the purpose your question? You've mentioned that you had perform some studies, what are the conclusion you got so far and what are the gap between your summary and your reference? Read forum policies... quote "Please conduct internet-based research (such as Google.com) and review the site FAQs, along with performing an Eng-Tips.com site search prior to posting a new question on these fora."unquote
For at least two simple reason:
-Answer in this limited forum can have multiple interpretation, even though the intention was not.
-Some of us gain this knowledge with years of experience, and it is not appreciated shall some parties took commercial advantage by claiming ‘this forum's explanation’ as their ‘invoice-able’ answer towards internal/external client. You can say you’re studying, but most of us don’t buy that reason.
I am only sharing below information so someone is not miss-used the general butterfly valve concept. And minimize reinventing the wheel stigma.
Not to be generalized, but in summary
Why one or some Plants choose butterfly valve:
-Generally have spatial constraint. By far butterfly valve have more compact face to face, thus suitable for very limited space
-Understand the risk such as: would have disc within its flow path (not full bore or PIG-able), have limited controllable capacity (in general butterfly valve Cv at 70-75% opening or bigger is not much different with 100% full open)
-Have preference for quarter turn type
-Etc.
Butterfly valve type are concentric, double-offset, triple offset and quadruple offset (don’t have much experiences with this one so I will not discuss this one)
Concentric:
-Butterfly valves utilize an elastomeric liner into which the disc is an interference fit, thus affecting the seal.
-Normally used in water service and available in ASME Class 150 only.
-In general torque seated. However, applying excessive torque may have damaging consequences
Double offset:
-Generally soft seated (polymeric).
-The seat is normally housed in the body spring energized onto the disc, giving effective bi-directional sealing.
-Available in ASME Class 150 and 300.
-In general torque seated. However, applying excessive torque may have damaging consequences
-Quite common for chemical application which have temperature limit due to polymeric seat. Some geothermal and offshore application with moderate temperature also use this kind of valves
Triple offset:
-Have a metallic seat and are suitable for higher temperatures.
-Classes up to Class 600, but some of uni-directional designed up to class 2500.
-It is arguable that manufacturer claim it is position seated, but torque (seating) do play role on its disc vs seat tightness.
-it is becoming quite common to be used for tight shut off application for hazardous product.
-since Disc and Seat only "touch" right when it is about to reach close position, thus can be an alternative for fouling medium. Not suggested to use ball valve for fouling application due to ball and seat is always in contact when valve being operated.
Concentric butterfly valves are the lowest cost, followed by double-offset and tripleoffset.
Double-offset butterfly valves have a considerably lower operating torque than tripleoffset, resulting in a lower actuated valve package.
Regards,
MR
All valves will last for years, except the ones that were poorly manufactured; are still wrongly operated and or were wrongly selected