Monoflanges combine the event as high as three valves in an especially compact body, because of 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 required for some control applications. Among the 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 sometimes or if the control is critical, mounting the instrument near the process may be the solution.
Vibrations may also be 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 inside a compact, flange-shaped body, allowing a significant decrease in volume, dimensions, weight and potential leakage points.
Monoflange is the solution
With respect to the requirements of the plant it really is installed in, the monoflange can incorporate one, several valves. In Pitiful 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. That is mostly found in applications that are relatively uncritical (e.g. low pressure) or in which 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 something valve for venting.
Monoflange functionality
The monoflange bodies are drilled internally with holes which connect the annular valve chambers.
Gargantuan following picture illustrates the process 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 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 pass through them.
The two shut-off valves allow a better isolation from the process: In case the initial shut-off valve does not isolate the medium properly, the second one will act as a safety means against accidental leaks. Occasionally, customer specifications do not allow the medium to be in touch with the instrument when it’s not measuring. That is why the medium shall be discharged using the vent line. In other cases ? because of the 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 What is a monoflange? Assuming you have any questions, your contact will gladly help you.