Monoflanges combine the function 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 higher response speed is required for most control applications. Among the variables that affect the response time is the volume and the distance between process and instruments. If Overnight to be measured is gas, and the process will fluctuate strongly sometimes or if the control is crucial, mounting the instrument close to the process may be the solution.
Vibrations may also be critical, for example, if impulse lines are connected 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 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 process 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 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 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] ; once 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. The two shut-off valves [1, 2] come in an angled position, that allows the flow to feed them.
Both shut-off valves allow an improved isolation from the process: In case the first shut-off valve will not isolate the medium properly, the next one will become a safety means against accidental leaks. Occasionally, customer specifications do not allow the medium to stay touch with the instrument when it’s not measuring. For this reason the medium will be discharged using the vent line. In other cases ? because of the vent line ? Corrupting 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 What is a monoflange? Assuming you have any questions, your contact will gladly assist you to.