article on double isolation and bleed valve
DESCRIPTION
A summarized research article on the operation mechanism and application of Double Isolation & Bleed (DIB) ValvesTRANSCRIPT
Article on Double Isolation & Bleed Valve (DIB)
Nasir Muzaffar Kotriwala Department of Chemical and Environmental Engineering, Faculty of Engineering,
University of Nottingham Malaysia, Broga Road, 43500 Semenyih, Selangor, Malaysia Email: [email protected] , [email protected]
Ball valves for isolation applications often require a second pressure barrier
operating independently of the primary pressure barrier. The need is often due
to operational safety requirements and service (gas service, zero leakage,
cleanliness of the produced fluid, etc.). A valve with this independent second
barrier is commonly referred to as “Double Isolation and Bleed (DIB)”.
DIB differs significantly from “Double Block and Bleed valves (DBB)”.
According to Garza, Mechanical Lead—Static Equipment Engineering Group at
ExxonMobil Development Company, many users have taken DBB as a generic
term, and tend to use it when they really imply (and the applicable
specification—API 6D, Specification for Pipeline Valves, requires) the use of
DIB. The key to understanding, according to Garza says, can be found in API 6D
(2008 edition); it clarifies the distinction between a DBB and a DIB.
API 6D defines a DBB as a "single valve with two seating surfaces that, in the
closed position, provides a seal against pressure from both ends of the
valve with a means of venting/bleeding the cavity between the seating
surfaces." The 2008 note points out that this valve does not provide positive
double isolation when only one side is under pressure.
By contrast, API 6D defines a DIB as a "single valve with two seating surfaces,
each of which, in the closed position, provides a seal against pressure from a
single source, with a means of venting/bleeding the cavity between the seating
surfaces." The note adds that this feature can be provided in one direction or in
both directions.
A DIB prevents a fluid from entering an area where work is being done. Both in-
line valves would be closed, then the bleeder would be opened. This facilitates
the DIB to verify the primary seal by bleeding the cavity (the space between the
upstream and downstream valves) between; if any fluid leaked past the first
valve the bleeder would drain it off before it pressurized the cavity.
Furthermore, this serves to monitor that cavity for leakage. If the bleeder
(which is smaller than the in-line valves and may, in fact, be a needle valve)
were to be plugged the downstream valve would keep process fluid from
getting past it. Hence, the second barrier is in place should the first barrier leak
and flood the cavity between.
Besides being used as a block valve where line maintenance is expected or
common a DIB also has application in metering, particularly gas. The DIB design
for metering service differs from the design feature for the former since the
design for metering service application is driven mainly by the application’s
zero leakage requirement. A closed valve that is leaking slightly can create
errors in metering.
Bibliography: 1. Eii Valve Company. DBB or DIB. Available at:
www.enkayindustries.com/downloads/DBBvDIB%20slideshow.ppt 2. Valve Magazine. True Meaning of Double Block and Bleed. Valve Manufacturers Association
of America. Available at: http://www.valvemagazine.com/index.php/web-only/web-exclusive/2154-the-true-meaning-of-double-block-and-bleed