|Steel, Stainless Steel, Monel, Hastelloy, Duplex, Super Duplex, Other|
|Flanged, Welded Mouth|
|Manual, Electric, Pneumatic, Hydraulic|
|(-200 °C) – (+550 °C)|
|3/4″ – 24″|
|Class 150 – 2500|
DB&B ( DOUBLE BALL ) VALVES
With the need for verifiable and maintainable shut off in safety critical applications, double block and bleed valves have commonly been used for primary and secondary tight shut off, in an attempt to provide and ensure system integrity. Historically, system integrity would be achieved through the use of two individual isolation valves, a spool piece and a bleed valve that had the disadvantages of extensive cost and weight and also increased the risk of emissions due to multiple flanged faces. Consequently, combined integral DB&B valves are now widely used. These valves have a single or side entry body design with dual Ball or Plug valves with a bleed valve installed between the two main line valves. These valves are available bi-directional in metal seated and soft seated style, in bubble tight shut off. Sometimes called twin valves, DB&B valves are available in Plug and Trunnion Ball type up to 600mm NB in ANSI 150 to 2500 class. These DB&B design valves replace existing traditional techniques employed by pipeline engineers to generate a double block and bleed configuration in the pipeline. There are several designs and configurations of Double Block & Bleed valves as follows:-
SINGLE BALL TRUNNION DB&B
Conventional single ball trunnion mounted ball valves have two energised generally bidirectional seats. The body cavity can then be fitted with a drain valve at the body cavity drain point to allow verification of seat integrity and this type of valve is then sold as a “Double Block and Bleed Valve” (DBB). Australian Pipeline Valve individually tests each valve to verify zero leakage at the drain point. Conventional single ball trunnion valves typically feature self relieving ‘single sealing’ seats which automatically relieve body cavity pressure downstream. Uni-directional valves can be specified which only relieve upstream. Conventional single ball design still allows a closed valve to leak up or downstream should the body cavity over pressurise.
Double piston effect (DPE) seats can also be specified on single ball DB&B trunnion valves. This style of valve is classified under API6D as a “Double Isolation and Bleed valve”(DIB). This design ensures that both seats are always sealing, even if one seat does fail, the other seat will continue to maintain a seal as the two way seats maintain a bi-directional seal. With DPE seats, a body cavity external pressure relief valve is fitted (which can be optionally by-passed back to the upstream flange of the valve or to flare). This is a cheaper option than twin ball DB&B valves whilst still providing a high level of integrity.
Double block and bleed valves
What is double block and bleed?
Double block and bleed valve is a single valve which comprises of two seating surfaces. It can provide a seal against pressure from both valve ends when the valve is closed. The basic operating feature of this valve is that it bleeds or vents the cavity between the seating surfaces. The double block and bleed valve was designed so that the need of bolting down individual valves can be avoided. A single valve can give the dual isolation that reduces the cost by over 60%.
There is a bleed valve along with two block valves that act as a manifold or a uni. This assembly is installed to provide a positive isolation. The double block and bleed valves are used for hazardous or toxic fluid processes or for the high- pressure systems and many more critical processes. These valves can bear a pressure range of usually 10,000 psig and a temperature range of generally -70.6°F to +1,002°F.
The double block and bleed valve finds applications in chemical injection connection, gauge isolation, instrument drain and chemical seal isolation. The double block and bleed valves need much lesser weight and space but can do the job of three different valves i.e. two isolation valves and one drain valve.
Some readers might be confused about when are the double block and bleed valves required. So, basically, this valve is used when a vent valve is required in between two isolation methods.
The double block and bleed valves are used to attain a positive isolation while you the maintenance activities are being performed in live process plant. The positive isolation can be performed during the shutdown, maintenance or repairing activities. At first, both block valves are closed. Afterwards, the shutdown side is vented to prepare for maintenance. The process fluid trapped in the space between two block valves is relieved by the bleed valve assembly. In this way, the shutdown side is separated from process fluids through the isolation valves.
Double Block and Bleed valve- Get additional advantage
The Double block and bleed valve allows you to get additional advantage over the traditional hook up. The cavity present in between the two balls is very small which means that an operator can evacuate the space quite efficiently and hence, establishing an environment which is absolutely safe to work.
DOUBLE BLOCK & BLEED – FLOATING TYPE
The design of thisvalves is based on the “floating ball” principle which allows the ball to turn freely between the ball seals.
A positive seal is attained by fluid pressure act- ing on the upstream surface of the ball and
producing a constant uniform contact between the downstream ball seal and the ball.
The ball is operated by a sealed spindle with a projecting square end to which the control han- dle or optional actuator is attached.
Ball valves are intended to be used as on/off flow control devices and are not to be used to throttle fluid flow. The valves should always be either fully open or fully closed.
DOUBLE BLOCK & BLEED – TRUNNION TYPE
Trunnion ball valve have a mechanical means of anchoring the ball at the top and the bot- tom, this design is the standard design applied on larger and higher pressure valves.
Sealing is achieved by spring loaded piston type seats which shut off flow when line pres- sure acts on the upstream seat. Automatic relief of cavity overpressure is assured due to the trunnion design in case of self relieving seats The ball is oper-
ated by a sealed spindle to which the operator is attached. Ball valves are intended to be used as on/off flow control devices and are not to be used to throttle fluid flow. The valves should always be either fully open or closed
BENEFITS OF THE DESIGN FOR DOUBLE BLOCK AND BLEED VALVES
- IMPROVED SAFETY IN OPERATIONS
• SPACE, WEIGHT, AND COST SAVINGS
• REDUCED LEAKAGE POINTS
• REDUCED INSTALLATION AND MAINTENANCE REQUIREMENTS AND COSTS
- COMPACT DESIGN
• UNINTERRUPTED FLOW FOR NEGLIGIBLE PRESSURE DROP • REDUCED STRESSES FROM LOADING AND VIBRATION
• DESIGNED AND ENGINEERED ON A CASE-BY-CASE BASIS