Monoflanges combine the event as high as three valves in a particularly compact body, thanks to a precise 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 necessary for most control applications. One of the variables that affect the response time is the volume and the distance between process and instruments. If the medium to be measured is gas, and the procedure tends to fluctuate strongly sometimes 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 is 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 substantial reduction in volume, dimensions, weight and potential leakage points.
Monoflange may be the solution
Based on the requirements of the plant it really is installed in, the monoflange can incorporate one, two or three valves. In Ultimate 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 in the instrument. This is mostly used 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 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];
Once 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 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 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 next one will act as a safety means against accidental leaks. Sometimes, customer specifications don’t allow the medium to stay touch with the instrument when it’s not measuring. Because of this the medium shall 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? In case you have any questions, your contact will gladly help you.