How 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 inside a monoflange valve, once installed?
In a chemical process a higher response speed is necessary for some control applications. Among 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 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 connected 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, several 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 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 right before the monoflange.
The safest configuration, and the main 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 next 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 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. Completely shut-off valves [1, 2] are in an angled position, that 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 does not isolate the medium properly, the second one will become a safety means against accidental leaks. Occasionally, customer specifications don’t allow the medium to be in touch with the instrument when it’s not measuring. Spine 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? In case you have any questions, your contact will gladly assist you to.

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