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.
