Monoflanges combine the event 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 inside a monoflange valve, once installed?
In a chemical process a higher response speed is required for some control applications. One of 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 at times or if the control is crucial, mounting the instrument near the process may be 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, several needle valves in the compact, flange-shaped body, allowing a significant decrease 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, two or three valves. In a monoflange with two valves (block & bleed), one valve (with a blue cap) isolates the process and another (with a red cap) regulates the venting of the medium trapped in the instrument. Child-like is mostly found in applications which 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 one we advise for aggressive media or critical operating conditions, is 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 process 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 next shut-off valve [2] ; when 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 can be discharged via the vent valve [3] through the vent outlet. The two shut-off valves [1, 2] come in an angled position, that allows the flow to feed them.
Both shut-off valves allow a better isolation from the process: In case the initial shut-off valve will not isolate the medium properly, the next one will act as a safety means against accidental leaks. In some instances, customer specifications do not allow the medium to stay touch with the instrument when it’s not measuring. For this reason the medium shall be discharged using the vent line. In other cases ? because of the vent line ? instruments could be easily calibrated without dismounting them from the line.
Note
Sizzlingly 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.