Refineries and petrochemical plants contain complex networks of pipelines, process equipment, storage systems, and utility lines. Reliable isolation is important during normal operation, maintenance, inspection, commissioning, and shutdown activities. A Double Block and Bleed Valve can provide two isolation barriers and an intermediate bleed function for selected critical process applications.
Importance of Process Isolation
Refinery and petrochemical processes may involve hydrocarbons, chemicals, gases, and high-pressure fluids. Equipment often needs to be isolated without shutting down an entire process unit.
An isolation valve separates the equipment from the active process. Where a single isolation point is not sufficient for the application, a double block and bleed arrangement can provide two blocking barriers with a cavity between them.
DBB Valves in Refinery Applications
DBB valves can be used in various areas of a refinery, including process units, hydrocarbon pipelines, metering systems, storage facilities, pump stations, and equipment isolation points.
They may be installed around pumps and vessels to facilitate maintenance while reducing the need for extensive valve and piping arrangements.
The bleed connection provides a means of releasing or monitoring pressure trapped between the two blocking elements. The discharge from the bleed system must be managed according to the facility's process design and safety procedures.
Petrochemical Process Applications
Petrochemical plants process different feedstocks and intermediate products under varying pressures and temperatures. Valve selection therefore depends heavily on fluid properties and operating conditions.
DBB ball valves can be considered for process lines where tight shut-off and compact isolation arrangements are required. Depending on the size and pressure rating, floating or trunnion-mounted ball valve designs may be used.
Material selection is particularly important for corrosive or chemically aggressive services.
Hydrocarbon Service
Hydrocarbon applications require careful attention to valve sealing, material compatibility, pressure containment, and leakage control.
Carbon steel, stainless steel, and alloy steel may be selected for valve construction depending on the process conditions. Seat and seal materials must also be compatible with the operating temperature and process medium.
For specialized services, project specifications may include additional requirements related to fire-safe construction, fugitive emissions, sour service, or low-temperature operation.
Advantages of DBB Valves
One advantage of an integrated DBB valve is that it can combine multiple isolation functions within one valve body. This can potentially reduce the physical space required compared with certain conventional multi-valve arrangements.
The intermediate bleed connection can also provide an additional method of managing cavity pressure and checking the isolation condition.
These characteristics can be particularly useful where equipment layout is compact or where multiple isolation points are required within a process unit.
Testing and Inspection
Before a DBB valve is placed into service, appropriate inspection and testing should be completed according to the project specification and applicable standards.
Testing may include body or shell pressure testing, seat leakage testing, functional operation, and inspection of the bleed system.
For critical refinery applications, documentation such as material certificates, inspection reports, test certificates, and traceability records may also be required.
Selection Considerations
Engineers should consider valve size, pressure class, design temperature, process fluid, material compatibility, seat configuration, end connection, actuation, installation conditions, and maintenance requirements.
The valve's isolation and bleed configuration should be compatible with the refinery's process safety philosophy and operating procedures.
Conclusion
Double Block and Bleed Valves can support controlled process isolation in refineries and petrochemical plants where dependable separation between process sections is required. Their combination of two blocking barriers and an intermediate bleed function makes them suitable for selected hydrocarbon and chemical services. Correct engineering, material selection, testing, and maintenance are essential for reliable performance.
