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 some control applications. Among the 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 procedure tends to fluctuate strongly at times or if the control is crucial, mounting the instrument near the process is the solution.
Quickly are also critical, for example, in case that impulse lines are connected 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, two or three needle valves in the compact, flange-shaped body, allowing a substantial reduction in volume, dimensions, weight and potential leakage points.
Monoflange may be the solution
Depending on requirements of the plant it 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 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 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 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];
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;
Once 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. Both shut-off valves [1, 2] are in an angled position, that allows the flow to feed them.
The two shut-off valves allow a better 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’s not measuring. For this reason the medium will be discharged utilizing the vent line. In other cases ? due to the vent line ? instruments can be easily calibrated without dismounting them from the line.
Note
More info on our valves can be found on the WIKA website or in the video Exactly what is a monoflange? When Obnoxious have any questions, your contact will gladly assist you to.