Monoflanges combine the event as high as three valves in an especially compact body, because of 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 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 procedure will fluctuate strongly at times or if the control is crucial, 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, two or three needle valves in the compact, flange-shaped body, allowing a significant decrease in volume, dimensions, weight and potential leakage points.
Monoflange is the solution
According to the 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 the other (with a red cap) regulates the venting of the medium trapped in the instrument. Tenacious 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, may be 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 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 second shut-off valve [2] ; once the valve [2] is open, the instrument is thus connected 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. Both shut-off valves [1, 2] come 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 initial 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 don’t allow the medium to stay touch with the instrument when it is not measuring. That is why the medium shall 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
More info on our valves are available on the WIKA website or in the video What is a monoflange? Should you have any questions, your contact will gladly assist you to.