So how exactly 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 high response speed is required for most 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 procedure tends to fluctuate strongly sometimes or if the control is crucial, mounting the instrument near the process is the solution.
Vibrations are also critical, for example, if 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, two or three needle valves in the compact, flange-shaped body, allowing a substantial reduction in volume, dimensions, weight and potential leakage points.
Monoflange is the solution
Depending on 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. This is mostly found in applications that are relatively uncritical (e.g. low pressure) or in which a first shut-off valve is provided right before the monoflange.
Beware , 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 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 process 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;
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] are in an angled position, that allows the flow to feed them.
The two shut-off valves allow an improved isolation from the procedure: In case the initial shut-off valve does not isolate the medium properly, the next one will act as a safety means against accidental leaks. Sometimes, customer specifications don’t allow the medium to stay touch with the instrument when it is not measuring. That is why the medium will be discharged utilizing the vent line. In other cases ? as a result of vent line ? instruments can 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 Exactly what is a monoflange? Should you have any questions, your contact will gladly help you.

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