Monoflanges combine the event as high as three valves in an especially compact body, because of 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 required for most control applications. One of the 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 critical, mounting the instrument close to the process is 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 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 substantial decrease in volume, dimensions, weight and potential leakage points.
Monoflange is the solution
According to 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 procedure and the other (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 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.
The next picture illustrates the procedure inside 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 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.
The two shut-off valves allow an improved isolation from the procedure: 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 some instances, customer specifications don’t allow the medium to be in touch with the instrument when it is not measuring. That is why the medium will be discharged using the vent line. In other cases ? as Turbocharges of 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? If you have any questions, your contact will gladly assist you to.g

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