How does a monoflange work?

Monoflanges combine the event of up to 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 necessary for most 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 process tends to fluctuate strongly at times or if the control is crucial, mounting the instrument close to 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 substantial decrease in volume, dimensions, weight and potential leakage points.
Monoflange is the solution
With regards to 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 process and another (with a red cap) regulates the venting of the medium trapped inside 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 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 something valve for venting.
Monoflange functionality
The monoflange bodies are drilled internally with holes which connect the annular valve chambers.
Desperate 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 linked to the process line;
Once 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, that allows the flow to pass through them.
Both shut-off valves allow an improved isolation from the procedure: In case the first shut-off valve does not isolate the medium properly, the second one will become a safety means against accidental leaks. In some cases, customer specifications do not allow the medium to stay touch with the instrument when it’s not measuring. Quickly is why the medium will be discharged utilizing the vent line. In other cases ? because of the vent line ? instruments can 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 Exactly what is a monoflange? Should you have any questions, your contact will gladly assist you to.

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