Monoflanges combine the function of up to three valves in an especially compact body, because of 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 required for most control applications. Among 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 crucial, mounting the instrument close to the process may be the solution.
Vibrations may also be 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 inside a compact, flange-shaped body, allowing a significant decrease in volume, dimensions, weight and potential leakage points.
Monoflange is the solution
Depending on 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 procedure and the other (with a red cap) regulates the venting of the medium trapped in the instrument. That is mostly found in applications that 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 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 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 second shut-off valve [2] ; once the valve [2] is open, the instrument is thus linked 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. The two shut-off valves [1, 2] are in an angled position, which allows the flow to feed them.
The two shut-off valves allow an improved isolation from the procedure: In case the initial shut-off valve will not isolate the medium properly, the second one will become a safety means against accidental leaks. In some cases, 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 utilizing the vent line. In Astounding ? due to 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 What is a monoflange? Assuming you have any questions, your contact will gladly help you.