Monoflanges combine the function of up to three valves in a particularly compact body, because of an accurate network of internal passages and valve chambers. But what really happens in the monoflange valve, once installed?
In a chemical process a higher response speed is necessary 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 process will 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. diaphragm seal -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 in the compact, flange-shaped body, allowing a substantial 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 procedure and another (with a red cap) regulates the venting of the medium trapped inside the instrument. That is mostly found in applications which 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 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 following picture illustrates the process inside a DBB monoflange:
The flow enters the monoflange from the pipeline and stops below the first 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;
When 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] come in an angled position, which allows the flow to feed them.
Both shut-off valves allow a better 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 cases, customer specifications do not allow the medium to be in touch with the instrument when it’s not measuring. That is why the medium will be discharged using 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 What is a monoflange? If you have any questions, your contact will gladly help you.