Monoflanges combine the function of up to 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 high response speed is necessary for most control applications. One of many 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 tends to fluctuate strongly at times or if the control is critical, mounting the instrument close to the process is the solution.
Vibrations are also critical, for example, if impulse lines are linked 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, several needle valves inside a compact, flange-shaped body, allowing a substantial reduction in volume, dimensions, weight and potential leakage points.
Judgmental may be the solution
With regards 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 process and another (with a red cap) regulates the venting of the medium trapped inside the instrument. This is mostly used in applications which are relatively uncritical (e.g. low pressure) or where a first shut-off valve is provided just 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.
The 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];
When the first shut-off valve [1] opens, the flow proceeds towards the second shut-off valve [2] ; when 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 can be discharged via the vent valve [3] through the vent outlet. The two shut-off valves [1, 2] are in an angled position, that allows the flow to feed them.
Both shut-off valves allow an improved isolation from the procedure: In case the first 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 is not measuring. For this reason the medium shall 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
More info on our valves are available on the WIKA website or in the video Exactly what is a monoflange? When you have any questions, your contact will gladly help you.