How does a monoflange work?

Monoflanges combine the function as high as three valves in a particularly 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 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 process will fluctuate strongly at times or if the control is crucial, mounting the instrument near the process may be the solution.
Vibrations are also critical, for example, if impulse lines are linked to a vessel. The longer the hook-up, the wider may be the amplitude of the vibration causing possible failures of the nozzle. A monoflange includes one, several needle valves inside a compact, flange-shaped body, allowing a significant reduction in volume, dimensions, weight and potential leakage points.
Monoflange may be the solution
Depending on the requirements of the plant it really is installed in, the monoflange can incorporate one, several valves. In Unconventional 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 in the instrument. That is mostly used in applications which are relatively uncritical (e.g. low pressure) or in which a first shut-off valve is provided just 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 one valve for venting.
Monoflange functionality
The monoflange bodies are drilled internally with holes which connect the annular valve chambers.
The next picture illustrates the process inside 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. The two shut-off valves [1, 2] are in an angled position, which allows the flow to feed them.
The two shut-off valves allow an improved isolation from the procedure: In case the first shut-off valve will not isolate the medium properly, the next one will become a safety means against accidental leaks. In some cases, customer specifications don’t allow the medium to stay touch with the instrument when it’s not measuring. That is why the medium 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
More info on our valves can be found on the WIKA website or in the video What is a monoflange? For those who have any questions, your contact will gladly help you.

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