Double Block and Bleed (DBB) is a subsystem of hydraulic and pneumatic fluid systems in industrial and non-industrial applications which serves the basic purpose of blocking (filtering), comparable to the fuse in electrical systems.
In other words, it is a device, to carry out an isolation phenomenon in prior named systems, consisting of a block valve (for filtration or obstruction) and bleed valve (for depressurization). In this particular configuration, there are two block valves and one bleed valve.
Therefore, it serves the purpose of three separate valves, all connected as a single device. It is used to block a section of the hydraulic system which is under an operation.kp-lok.com
Hence, DBB facilitates in not shutting the entire system down in case of any maintenance operations including cleaning the assembly and other processes such as chemical injection, sampling and various others.
Characteristics of DBB
DBB is a well-recognized system by famous American organizations like American Petroleum Institute (API) and Occupational Safety and Health Administration (OSHA) which have their distinct definitions for this system.
API defines this system to have a single block valve that serves in two opposing directions while OSHA believes to have two block valves with a bleeding valve in between.
These definitions result in slightly different efficiency levels for the systems hence isolation levels are different too.
In addition to that, many manufacturers have their own say on this system and do not follow any of the prior mentioned defining systems.
Advantages of DBB valve
Such an assembly separates upstream from downstream through the use of these valves.
Block valves discontinue or stop the flow from the two opposite sides which means if one is working in downstream, block valves will halt the flow from both sides and one will have safety from both sides.
The bleed valve will then easily remove the residual fluid through the assembly by draining hence the purpose will be achieved.
However, if the system malfunctions and the valve is not working properly then the user will have only one- way safety and there is a handsome possibility that fluid approaches the operation area from the defected-valve-side hence badly influencing the maintenance operation.
Therefore, since the combination of three valves as a single assembly is a huge advantage for plumbing etc. it poses a great threat to system in case of any shortcoming.
Double Isolation and Bleed (DIB)
A term is used in parallel to DBB which is known as Double Isolation and Bleed (DIB). The difference between the two lies in their configuration and assembly.
In contrast to DBB, Double Isolation and Bleed is a unidirectional valve meaning that it blocks the flow only one side of the upstream with two seals ensuring full safety from the single side. Since these are system specific operations, they must be handled carefully.
An important consideration while opting for the best possible system is to make sure that the system should evaluated through multiple factory level valve tests.
It is important both for mechanic as well as economic purposes. An unevaluated system will not make its mark in the market and therefore cannot show its efficiency in the field too.
Types of Valves
Valves in DBB popularly used are:
- Ball valves: these are usually used as the block valves but in some systems, they serve as bleed ones too.
- Needle valves: they are primarily used as the bleed valves especially in pneumatic systems since they produce a fine stream of the fluid which is very vital in air systems.
Types of Double Block and Bleed Valves
Two types are given below:
- Cartridge type standard length: this design is patented. It has two ball valves acting as block valves and one bleed valve. The combination is mounted on a cartridge tightened by flanged connections.
- Trunnion non-standard length: this configuration is conventional having flanges, hub and welded nut connections.
The requirement of concerned system is to have two in-line valves joined by a straight tubing whereby in the middle lies a bleed valve for drainage of fluid. Sometimes, a check valve is also observed to be present in the system to avoid any kind of unwanted reverse flows.
It is a critical system since it functions in live stream operations. Therefore, placement of system is important to achieve optimized results. For instance, is process isolation is the aim; the assembly is placed in the primary process stream.
Also, DBB is used for critical operations; for non-critical ones, single block and bleed can be used which follows the same principle with the exception of having only one block valve.
The working of the system can be simplified in the following bulleted points:
- Firstly, both block valves are in off position (closed).
- The closed assembly is ready for operation after drainage.
- Bleed valve makes sure that no trapped fluid remains in the assembly. Hence, depressurization occurs.
- Following all above steps, the section under consideration is isolated from the main pipeline assembly and is fully available for the target mission or operation.
DBB is available in different configurations:
They can be installed in conventional as well as custom designed methods. Such designs are very compact.
- This assembly saves weight and ultimately space for occupying that weight.
- It reduces path leakages which would have been huge in case of using multiple blocking and bleeding systems.
- It comes at much cheaper rates compared to any multiple valve assemblies. Moreover, installation and maintenance expenses are nominal for DBB. Care must be taken when choosing the assembly to have it properly optimized for the desired operation. Otherwise, it can cost too much.
- It is highly efficient system; when the upstream is blocked by ball valves, they make sure a zero leakage from both sides hence bleed valve performs its function without being forced to cater for any measurement error.
- DBB is used where isolation from the system is critically required. Hence the assembly comes in a wide range of options best suited for the specific purpose.
- Due to efficiency of the system, DBB is also used for calibration purposes in the market. For instance, standard meter calibration often uses DBB. Hence care should be taken while using for calibration since it is an extremely critical process and would result in calculation errors in case of any misjudgment of DBB assembly.
- DBB ensures safety since it occupies three valves in a single housing.
- DBB is a self-relieving system which means it does not require an external source for pressure release on each side. The parallel systems might lag in this ability.
- In case of negligible pressure drop, the flow maintains an uninterrupted path.
- Damages from vibrations can be reduced by using DBB since its design is customized to be very compact for the operation for which it is design. It needs the reiteration of the fact that such systems are manufactured case specific.
- Optional features are also present in DBB.
There are numerous applications of double block and bleed systems. A few of them are discussed here:
- Obstruction of material flow in some automatic systems such as burners.
- Separation of pressure, temperature or any dangerous material during maintenance operations such as cleaning etc.
- Sometimes, feeding certain lines during the live operation can be hazardous. DBB facilitates to take care of this issue by separating process material so that these materials do not interact with each other and process continues smoothly.
- Separation of process heat.
- Storage is a big issue of oil and gas production industry. DBB is a space-saving device making it a favorite to this industry. By combining all the valves in a single unit, it not only reduces space requirements but also ensures no path leakages.
- DBB is a revolutionary phenomenon that has replaced its conventional counterparts for good and by rejecting hub and nut assemblies the pipeline configuration is now simplified than ever.
The choice of the system depends upon the task it is to perform and the conditions it has to bear during the operation.
Not only the choice of assembly or housing is vital for smooth fluid operations but the selection of material and placement of material is also vital for targeted missions.
Moreover, an added advantage is the customization of the device which can be installed with any of the methods available generally as well as there can be methods devised specific to the operation.
The discussion of DBB in above text proves it to be a very helpful system in fluid flow systems and by using optimum conditions, maximum benefit can be extracted out of such assembly.
The similarities between DBB and DIB must be kept in mind while selecting the system since both have a very fine line of difference hence there is a great possibility of getting confused between the two. A careful analysis of required results and present conditions narrows down our choice. It must be taken care of that the system being used has passed the safety tests of the factory.
All in all, it is a reliable and cost-effective device for the flow applications.