How does a monoflange work?

Monoflanges combine the event of up to three valves in a particularly compact body, thanks to a precise network of internal passages and valve chambers. But what really happens in the monoflange valve, once installed?
In a chemical process a higher response speed is required for some control applications. One of many variables that affect the response time may be 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 critical, mounting the instrument near the process may be 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 inside a compact, flange-shaped body, allowing a significant reduction in volume, dimensions, weight and potential leakage points.
Monoflange may be the solution
According to the requirements of the plant it really 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 another (with a red cap) regulates the venting of the medium trapped in the instrument. That is mostly found 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 something valve for venting.
Monoflange functionality
The monoflange bodies are drilled internally with holes which connect the annular valve chambers.
The following picture illustrates the procedure 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 second shut-off valve [2] ; once 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. Both shut-off valves [1, 2] are in an angled position, which allows the flow to feed them.
Both shut-off valves allow an improved isolation from the process: In case the first shut-off valve will not isolate the medium properly, the second one will act as a safety means against accidental leaks. In some instances, customer specifications don’t allow the medium to be in touch with the instrument when it is not measuring. Because of Open-minded shall 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 Exactly what is a monoflange? Assuming you have any questions, your contact will gladly help you.g

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