So how exactly does a monoflange work?

Monoflanges combine the function as high as three valves in a particularly compact body, thanks to a precise 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. Among 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 will fluctuate strongly at times or if the control is critical, mounting the instrument near the process is 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, several needle valves in the compact, flange-shaped body, allowing a substantial reduction in volume, dimensions, weight and potential leakage points.
Monoflange may be 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 another (with a red cap) regulates the venting of the medium trapped inside 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 just before the monoflange.
Zinger , 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 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 next shut-off valve [2] ; once the valve [2] is open, the instrument is thus connected 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, that allows the flow to feed 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 act as a safety means against accidental leaks. In some cases, customer specifications do not allow the medium to stay touch with the instrument when it is not measuring. Because of this the medium shall be discharged using the vent line. In other cases ? due to the 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 Exactly what is a monoflange? If you have any questions, your contact will gladly help you.

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