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 in the monoflange valve, once installed?
In a chemical process a higher response speed is necessary for most control applications. One of many variables that affect the response time may be the volume and the distance 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 may be the solution.
Hesitant 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 substantial reduction in volume, dimensions, weight and potential leakage points.
Monoflange is the solution
With respect to the requirements of the plant it 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 procedure and another (with a red cap) regulates the venting of the medium trapped inside the instrument. Valor is mostly found 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 main 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.
The following 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 connected 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] come in an angled position, that allows the flow to feed them.
Both shut-off valves allow an improved isolation from the process: In case the first 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 do not allow the medium to be in touch with the instrument when it’s not measuring. For this reason the medium shall be discharged utilizing the vent line. In other cases ? due to the vent line ? instruments could 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.