Monoflanges combine the event 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 some 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 procedure will fluctuate strongly sometimes or if the control is critical, 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 inside a compact, flange-shaped body, allowing a substantial decrease in volume, dimensions, weight and potential leakage points.
Monoflange may be the solution
Depending on 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. This is mostly used in applications that 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, 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.
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];
When 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 connected 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. Both shut-off valves [1, 2] come in an angled position, which allows the flow to feed them.
The two shut-off valves allow an improved isolation from the process: In case the initial shut-off valve will not isolate the medium properly, the second one will act as a safety means against accidental leaks. In some cases, customer specifications don’t allow the medium to be in touch with the instrument when it’s not measuring. Funniest is why the medium will be discharged utilizing the vent line. In other cases ? because of 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? Assuming you have any questions, your contact will gladly help you.

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