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 inside a monoflange valve, once installed?
In a chemical process a high response speed is required for most control applications. Among the variables that affect the response time is the volume and the distance 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 crucial, mounting the instrument near the process is the solution.
Vibrations may also be critical, for example, if 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 reduction in volume, dimensions, weight and potential leakage points.
Monoflange may be the solution
With respect to the requirements of the plant it really 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 process and another (with a red cap) regulates the venting of the medium trapped inside the instrument. That is mostly used in applications that are relatively uncritical (e.g. low pressure) or where a first shut-off valve is provided just before the monoflange.
The safest configuration, and the main one we advise for aggressive media or critical operating conditions, may be the three-valve monoflange or the so-called double block & bleed (DBB), which features two shut-off valves in series and one valve for venting.
Monoflange functionality
The monoflange bodies are drilled internally with holes which connect the annular valve chambers.
เกจวัดแรงดันถังแก๊ส illustrates the procedure within a DBB monoflange:
The flow enters the monoflange from the pipeline and stops below the initial shut-off valve [1];
When the first shut-off valve [1] opens, the flow proceeds towards the next 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 can be discharged via the vent valve [3] through the vent outlet. Both shut-off valves [1, 2] are in an angled position, that allows the flow to pass through them.
Both shut-off valves allow an improved isolation from the process: In case the initial shut-off valve will not isolate the medium properly, the next one will act as a safety means against accidental leaks. Sometimes, customer specifications do not allow the medium to be in touch with the instrument when it is not measuring. Because of this the medium will be discharged utilizing the vent line. In other cases ? as a result of vent line ? instruments can be easily calibrated without dismounting them from the line.
Note
More info on our valves are available on the WIKA website or in the video Exactly what is a monoflange? In case you have any questions, your contact will gladly help you.