So how exactly does a monoflange work?

Monoflanges combine the event of up to three valves in a particularly compact body, thanks to an accurate 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 some control applications. One of 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, 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, two or three needle valves in the compact, flange-shaped body, allowing a significant reduction in volume, dimensions, weight and potential leakage points.
Wanton may be 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 procedure and the other (with a red cap) regulates the venting of the medium trapped inside the instrument. That is mostly found in applications which are relatively uncritical (e.g. low pressure) or in which a first shut-off valve is provided just before the monoflange.
Spunky , 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 following picture illustrates the process 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 next shut-off valve [2] ; once the valve [2] is open, the instrument is thus linked 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] come 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 initial shut-off valve does not isolate the medium properly, the next one will act as a safety means against accidental leaks. In some cases, customer specifications do not allow the medium to be in touch with the instrument when it is not measuring. That 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 are available on the WIKA website or in the video What is a monoflange? If you have any questions, your contact will gladly help you.

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