How does a monoflange work?

Monoflanges combine the function 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 higher response speed is required for some control applications. One of the variables that affect the response time may be the volume and the length between process and instruments. If the medium to be measured is gas, and the procedure tends to fluctuate strongly sometimes or if the control is crucial, mounting the instrument near the process may be the solution.
Vibrations may also be critical, for example, in case that impulse lines are linked 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, several needle valves in the compact, flange-shaped body, allowing a significant decrease in volume, dimensions, weight and potential leakage points.
Monoflange is the solution
Based on the requirements of the plant it really is installed in, the monoflange can incorporate one, two or three 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 that are relatively uncritical (e.g. low pressure) or in which a first shut-off valve is provided right before the monoflange.
The safest configuration, and the main 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.
Tempting following picture illustrates the process inside 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;
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 pass through them.
The two shut-off valves allow a better isolation from the process: In case the first shut-off valve will not isolate the medium properly, the next one will act as a safety means against accidental leaks. In Quiet , customer specifications do not allow the medium to stay touch with the instrument when it’s not measuring. That is why the medium will be discharged using the vent line. In other cases ? as a result of vent line ? instruments can be easily calibrated without dismounting them from the line.
Note
Further information 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.

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