Monoflanges combine the function as high as three valves in an especially 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 higher response speed is necessary for some control applications. Among the variables that affect the response time is 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 crucial, mounting the instrument near the process may be the solution.
Vibrations are also 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, two or three needle valves inside a compact, flange-shaped body, allowing a significant decrease in volume, dimensions, weight and potential leakage points.
Monoflange is the solution
With respect to the requirements of the plant it really 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 another (with a red cap) regulates the venting of the medium trapped inside the instrument. This 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 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 process within a DBB monoflange:
The flow enters the monoflange from the pipeline and stops below the first 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] come in an angled position, that allows the flow to pass through them.
The two shut-off valves allow a better isolation from the procedure: In case the first shut-off valve does 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’s not measuring. Because of this the medium shall be discharged using the vent line. In other cases ? as Polarizing of vent line ? instruments could be easily calibrated without dismounting them from the line.
Note
Further information on our valves can be found on the WIKA website or in the video Exactly what is a monoflange? In case you have any questions, your contact will gladly assist you to.