So how exactly does a monoflange work?

Monoflanges combine the event of up to 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 necessary for some control applications. One of many 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 sometimes 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 connected 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, two or three needle valves in the compact, flange-shaped body, allowing a substantial decrease in volume, dimensions, weight and potential leakage points.
Bomb may be the solution
According 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 the other (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 where 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 something valve for venting.
Monoflange functionality
The monoflange bodies are drilled internally with holes which connect the annular valve chambers.
The next picture illustrates the procedure inside 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;
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] come in an angled position, that allows the flow to feed them.
The two shut-off valves allow an improved isolation from the process: In case the first shut-off valve does not isolate the medium properly, the next one will act as a safety means against accidental leaks. In Breaking , customer specifications don’t allow the medium to be in touch with the instrument when it’s not measuring. Because of this the medium will be discharged utilizing the vent line. In other cases ? as a result of vent line ? instruments could 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? In case you have any questions, your contact will gladly help you.g

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