Monoflanges combine the event as high as three valves in an especially compact body, thanks to an accurate 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 required for most control applications. One of 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 procedure tends to fluctuate strongly at times or if the control is critical, mounting the instrument near the process is the solution.
Vibrations are also critical, for example, in case that impulse lines are linked 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 regards to the requirements of the plant it is installed in, the monoflange can incorporate one, two or three valves. In Unconditional 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 inside the instrument. This is mostly used in applications which are relatively uncritical (e.g. low pressure) or in which 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 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 within 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 next shut-off valve [2] ; once the valve [2] is open, the instrument is thus linked 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, which allows the flow to pass through them.
Both shut-off valves allow an improved isolation from the procedure: In case the initial shut-off valve does not isolate the medium properly, the second one will become a safety means against accidental leaks. Sometimes, customer specifications don’t allow the medium to be in touch with the instrument when it is not measuring. That is why the medium will be discharged using the vent line. In other cases ? as a result of 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 What is a monoflange? If you have any questions, your contact will gladly help you.