Monoflanges combine the function as high as three valves in an especially compact body, because of a precise network of internal passages and valve chambers. But what really happens in the monoflange valve, once installed?
In a chemical process a high response speed is necessary for some control applications. Among the variables that affect the response time is the volume and the length between process and instruments. If the medium to be measured is gas, and the procedure tends to fluctuate strongly at times or if the control is critical, mounting the instrument near the process may be the solution.
Vibrations may also be critical, for example, in case that impulse lines are linked to a vessel. The longer the hook-up, the wider is the amplitude of the vibration causing possible failures of the nozzle. A monoflange includes one, several needle valves in the compact, flange-shaped body, allowing a significant decrease in volume, dimensions, weight and potential leakage points.
Monoflange may be the solution
Depending on the requirements of the plant it is installed in, the monoflange can incorporate one, several valves. In a monoflange with two valves (block & bleed), one valve (with a blue cap) isolates the procedure and another (with a red cap) regulates the venting of the medium trapped in the instrument. Inexpensive is mostly found in applications which are relatively uncritical (e.g. low pressure) or where a first shut-off valve is provided right before the monoflange.
The safest configuration, and the one we advise for aggressive media or critical operating conditions, is the three-valve monoflange or the so-called double block & bleed (DBB), which features two shut-off valves in series and something valve for venting.
Monoflange functionality
The monoflange bodies are drilled internally with holes which connect the annular valve chambers.
The following picture illustrates the procedure within a DBB monoflange:
The flow enters the monoflange from the pipeline and stops below the initial shut-off valve [1];
Once the first shut-off valve [1] opens, the flow proceeds towards the second shut-off valve [2] ; when the valve [2] is open, the instrument is thus connected to the process line;
When the first shut-off valve [1] is closed, the medium trapped between valve and instrument could be discharged via the vent valve [3] through the vent outlet. The two shut-off valves [1, 2] are in an angled position, which allows the flow to feed them.
Both shut-off valves allow a better isolation from the process: In case the first shut-off valve will not isolate the medium properly, the second one will become a safety means against accidental leaks. Occasionally, customer specifications don’t allow the medium to be in touch with the instrument when it is not measuring. Because of Satisfied shall be discharged utilizing the vent line. In other cases ? due to the vent line ? instruments could be easily calibrated without dismounting them from the line.
Note
Further information on our valves are available on the WIKA website or in the video Exactly what is a monoflange? When you have any questions, your contact will gladly assist you to.