Non Suitability of using Double offset Butterfly Valve design for metal seating in #300 Pressure Rating

To understand the  Non feasibility , firstly we have to understand the types  of Rating  as per ANSI B 16.34 and then Types of  Butterfly valves as per API 609 are as follows:-

Types of rating	Bar
#150	20
#300	52
#600	115
#900	154
#1500	254
#2500	426

Type of valve	Rating	Bar	seal	Application	Remarks
Concentric Butterfly valve	PN10,PN16,#150	10,16,20 bar	Soft -EPDM,BUNA,VITON	Types of water upto 80 Degree C	Normal water application with small to medium sizes
Double Offset Butterfly valve	PN10,PN16,#150	10,16,20 bar	Soft -EPDM,BUNA,VITON	Types of water upto 80 Degree C	Mainly preferred for bigger sizes in power, water sector
Triple Offset butterfly valve	#150,#300,#600,#900,#1500,#2500	20 ,52 ,115,154,254,426 bar	Metal seating – SS 316 ,SS 316L,DUPLEX,MONEL,INCONEL	Water,steam, hydrocarbon	Used in critical application due to high pressure rating requirement

If we refer above the table of pressure rating  were ratings are defined with their operation of maximum pressure . All the valves are designed, define, distinguish & distributed in different classes # rating as per their required operation. In the table which clearly says #300 = 52 bar visa vise states that  #300 rating valve to be operated at maximum pressure of 52 bar .

Also to note and refer to API 598  says for testing of valve . On shell seat  of valve pressure to be used be 1.5 times of design pressure and on disc seat 1.1 of design pressure , which means if we take eg. For #150 class rating valve :- i.e pressure is 20 bar on Shell seat testing to be conducted with pressure of 30 bars and on disc seat = 22 bar and valve is only passed if there is no leakage as per API 598. Which means as per your query for #300 rating valve = Body shell seat pressure is = 78 bar and on disc seat = 57.2 bar .

Also we need to understand sealing of valve take place when seal is compressed on coming in mating with other part of the body i.e. seat. On tight compression of seal with metal seat only sealing of valve take place.

Now if we refer to the above table of butterfly valves it clearly indicates double offset is used for upto #150 as double offset is a soft seating valve it is having two offsets from X and Y axis and is design for higher sizes starting mainly from 40 “ and above as their was design issue in concentric design due to which double offset is made.

Your requirement is of #300 were rubber will not last in testing at 78 bars as due to higher pressure it will get torn and peeled off in testing itself , SO to sustain such higher pressure we have to go  with meat seal bounded either with PTFE or graphite Lamination . Since if we use metal seal in double offset butterfly valve design  when meal seal will come in contact with metal seat , metal seal will not compressed hence sealing will not take place an leakage will be their resultant valve will fail so to over come this design issue API 609 came with Triple offset Design where the  Third offset is in disc seat only it is angled in such a way ( or we can say each seal and seat are paired and designed in such a way) that it exerts pressure on certain area of metal bounded seal ( as we all know metal stripes are bounded together by PTFE or graphite to for a metal seal).

That metalstrips of metal seal in that area where pressure is exerted will go back from their original position and form a wedge were metal seat will sit and hence there will be no space left for leakage  .We all know Triple Offset design is used in critical application so they are expected to perform at zero leakage.( Please refer to the catalogue attached for 3 offsets design description on page-2).

So , in the end we can say please do n't look type of design as option but look at kind of sealing (soft or metal) because rating define design and design are made to accommodate rating. Firstly basic Concentric came in the market as valves are used for basic water application with upto 16 to 20 bar but when bigger sizes required then Double offset came and after that when requirement is shifted from water to other critical applications where pressure was more then Triple offset design came. So , please select the design as per the rating only.