Cocentric vs eccentric butterfly valves

[Ahooram]

Hi
kindly, could someone inform me about the selection factors for Cocentric vs eccentric butterfly valves.

Thank you in advance.

 

[JLSeagull]

Eccentric butterfly valves are also known as offset butterfly valves. By moving the disc off the centerline of the butterfly and a few other mechanical tricks the offset valves can may reduce wear when seating and unseating, improve the seat seal and reduce undesirable actuation force characteristics.

Perhaps the water or HVAC industry still use traditional concentric butterfly valves.

 

[JimCasey]

Centric/Flat disc/rubber-lined butterfly valves were the first developed. Originally conceived to isolate dry granular solids, they were later found to be useful on liquid streams.

They are very simple to make and they are the most inexpensive possible valves.

THe rubber (Elastomer) liner performs multiple duties of stem seal, flange gasket, and process isoplation.

THe elastomer also limits the temperature and pressure that the valves can tolerate.

Friction is high because the disc never fully breaks away from the elastomer liner.

High-Performance, or Double-offset butterfly valves move the sealing line of the vane away from the stem. The sealing point a continuopus line, not interrupted by the stem. The stem is then sealed by a conventional packing set instead of just interference between the liner and the shaft.

Friction is much lower because the disc breaks completely away from the sealing line (seat) as soon as the valve moves a couple of degrees.

The seal can be made of higher-tempearture materials, including flexible metallic designs, and published limits of Double-offset valves can exceed 1000F.

 

[insult2injury]

Triple offset valves can also be used if Class IV shutoff is required with metal seats.

I2I

 

[bheart790]

As I know, triple or double eccentric valve always be used in high temperature condition.

 

[insult2injury]

Double offset valves are commonly used in low temperature (even cryogenic) systems where better performance is desired.

I2I

 

[NGiLuzzu]

In my opinion, some of the statements by insult2injury and bheart790 need to be amended (probably due to outdated practices and ideas about valve type selection...?).

As a matter of fact, triple-offset metal-seated valves can achieve tightness performances even better than Class VI and are also used as on/off valves.
They may work fine in many different applications, from cryogenic up to high temperatures, and are more and more required by the Oil and Gas market...: please take also a look at thread408-210294 within this Forum (and at the links included there), for example.
Of course those valves are are not piggable, but they are much lighter, easier to operate and more compact than gate or ball valves (for instance) of the same size and rating.


Coming back to the main topic, triple-offset metal-seated valves should be considered as "the top" of the range which starts from "simple" rubber lined butterflies and continues with double-eccentric ones (HPBV's), including all the possible intermediate variations in the middle.


Hope this helps, 'NGL

 

[Ahooram]

Thank you very much everyone for your very informative responses.

 

[JimCasey]

THat's why they call double-offset valves "high performance". They can handle colder temperatures, or higher temperatures. Centric valves are limited by having an elastomer liner that can't work if it gets too cold, and melts if it gets too hot.

Double-offset valves can handle higher pressures. Elastomer-lined valves are very rare at 250 psi. Elastomer-seated Double-offset valves easily handle class 300 (740 psi) and there are designs that will handle class 1500 or higher.

DOuble-offset valves have lower dynamic torque so they are more suitable for control applications. Flat-plate rubber-lined valves are probably the reason that the legend got started that rotary valves don't control well.

 

[bheart790]

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