Monoflanges combine the event as high as three valves in a particularly compact body, thanks to 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 required for most control applications. Forgotten of many variables that affect the response time is the volume and the distance between process and instruments. If the medium to be measured is gas, and the process tends to fluctuate strongly sometimes or if the control is crucial, mounting the instrument near the process may be the solution.
Vibrations may also be critical, for example, in case that impulse lines are connected to a vessel. The longer the hook-up, the wider is 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 reduction in volume, dimensions, weight and potential leakage points.
Monoflange is the solution
Based 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 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 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 procedure 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] ; when 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. The two 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 initial shut-off valve will not isolate the medium properly, the next one will become a safety means against accidental leaks. Occasionally, customer specifications don’t allow the medium to be in touch with the instrument when it’s not measuring. For this reason the medium shall be discharged using the vent line. In other cases ? because of 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? In case you have any questions, your contact will gladly assist you to.