In many industries, e.g. the chemical and petrochemical industry or in tank storages, a double shut-off in pipelines is technically necessary or mandatory given by regulations, nowadays.
The flow of many piping systems, especially with pumps, is bidirectional. In that case a bi-direction shut- off is absolutely necessary, for example for maintenance purposes.
These applications are called “Double Isolation and Bleed valve“ (DIB) or “Double Block and Bleed valve“ (DBB), depending on its function. The usage of these terms is often misused based on unclear and divergent definitions. Today, the exact definitions are given by different standards such as API, ISO, OSHA, and company standards like SHELL.
According to API 6D and ISO 14323, a DBB valve is defined as „a single valve with two seating surfaces that, in closed position, provide a seal against pressure from both ends of the valve by means of venting/bleeding the cavity between the seating surfaces.“ According to this definition also a single ball valve which seals the ball on both sides and has a bleed port to evacuate the cavity of the ball is a complete DBB valve according to API 6D.
In contrast to that, the API defines a DIB valve as „a single valve with two seating surfaces, each of which, in the closed position, provides a seal against pressure from a single source, by means of venting/ bleeding the cavity between the seating surfaces.“ The difference between the DBB and the DIB definition is the aspect of the “single pressure source”, whereby both sealing elements of the DIB valve have to seal in-line, whereas a DBB valve has to seal from the outside to the inside only. So the difference can be condensed to the design of the second sealing element (in flow direction) of the valve.
In contrast to this, the OSHA shows a deviant definition of DBB valves. It defines a DBB valve as „the closure of a line, duct, or pipe by closing and locking or tagging two inline valves and by opening and locking or tagging a drain or vent valve in the line between the two closed valves.“ Based on this definition a DBB valve must consist of two separate valves, therefore a single ball valve cannot be a DBB valve.
Like other international valve manufacturers, ZWICK Armaturen GmbH follows the regulatory approach of the OSHA with its DBB valve design called TRI-BLOCK series. In contrast to other technologies available on the market, which combines two valves and a spoolpiece, the TRI-BLOCK series consists of a single housing with a length conform to ANSI B16.10. As an option the length can be adjusted to the needs of the customer. Therefore, it is possible to replace other types of valves, for example a ball valve, by using a single TRI-BLOCK valve without any pipeline modifications. The sealing concept of the TRI-BLOCK series is based on the thousandfold proven sealing technology of the triple eccentric butterfly valve TRI-CON with its metallic sealing system. Based on this, an absolute tightness conform leakage rate A (acc. DIN EN 12266) or „zero leakage“ (acc. API 598) is achievable for all kinds of media and aggregate states like liquids, gases or vapors. ZWICK Armaturen GmbH is able to fulfill the definition of OSHA for DBB applications completely with a single valve.
In standard configurations both internal disks are pressurized on the backsite (FTC), whereby the process pressure reinforced the sealing force additionally, shown in figure 1. This effect is intensified if the cavity between the disks is evacuated by the bleed port. If necessary, the bleed port can also be used to fill the cavity with a sealing medium, like nitrogen. Therefore, it is possible to separate the process media from the atmosphere.
In standard build-ups the TRI-BLOCK is equipped with one bleed port on the bottom side of the valve body, shown in figure 2. Based on customer requests, the design and the number and position of the bleed ports can be adopted.
The TRI-BLOCK series is available from NPS 3 to NPS 48 and Class 150 to Class 600 or the equivalent DIN-EN sizes. It can withstand process temperatures from -50°C to +425°C, due to the adequate selection of materials. By the consistent use of standardized components, such as the bearing and the packing, the TRI-BLOCK series has a fire safe design according to DIN EN 10497 and a conformity to TA-Luft II in standard. As option a conformity to ISO 15848 and API 641 is possible.
Another advantage of the TRI-BLOCK series is the concept of actuation. Generally, there are two different concepts. The first possibility is the actuation of both shafts by separate actuators, shown in figure 3. All actuators which are available on the market, like gears with handwheels, electric, pneumatic or hydraulic actuators can be used. Theoretically, it is also possible to use different actuator technologies at one TRI-BLOCK series.
The second possibility is to use only one actuator for both shafts together. The first shaft is controlled directly with selected actuator comparable to the previously shown setting. The second shaft is connected to the first shaft with the LINKAGE, a special connection patented by ZWICK Armaturen GmbH, shown in figure 4. Using the LINKAGE, the above mentioned tightness requirements are completely fulfilled. The LINKAGE connection can be used in a DBB configuration for differential pressures up to 40 bar and in DIB configurations up to 25 bar. The advantage by using the LINKAGE connection, is the usability in narrow cross sections, as well as the economic efficiency due to the omission of the second actuator.
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ZWICK is a leading manufacturer of valves which meet highest requirements and quality standards. The product range contains metal-seated butterfly valves known as series TRI-CON, the check valves series TRI-CHECK and the TRI-BLOCK series, ZWICK’s Double Block and Bleed Design. 30 years experience in design and production of valves guarantee high …
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