Monoflanges combine the event of up to three valves in an especially compact body, thanks to a precise network of internal passages and valve chambers. But what really happens in the monoflange valve, once installed?
In a chemical process a high response speed is necessary for some control applications. One of 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 process will fluctuate strongly at times or if the control is crucial, mounting the instrument close to 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 may be 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 decrease in volume, dimensions, weight and potential leakage points.
Monoflange is the solution
Depending on 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 the other (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 where 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, 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.
Scandal following 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 next 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. Both shut-off valves [1, 2] are in an angled position, which allows the flow to feed them.
Both shut-off valves allow a better isolation from the process: In case the initial shut-off valve will not isolate the medium properly, the second one will become a safety means against accidental leaks. Sometimes, customer specifications do not allow the medium to be in touch with the instrument when it’s not measuring. For this reason 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
Further information on our valves are available on the WIKA website or in the video Exactly what is a monoflange? Should you have any questions, your contact will gladly help you.