Monoflanges combine the event as high as three valves in an especially 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 high response speed is necessary for most control applications. One of the variables that affect the response time may be the volume and the distance between process and instruments. If Effortless 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 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 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 is the solution
With respect to the 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 procedure and another (with a red cap) regulates the venting of the medium trapped in the instrument. This is mostly used in applications which 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 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 following picture illustrates the procedure within 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] ; once 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. Stupid-simple shut-off valves [1, 2] are in an angled position, that allows the flow to pass through them.
The two shut-off valves allow an improved isolation from the procedure: In case the first shut-off valve does 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 be in touch with the instrument when it is not measuring. Because of this the medium shall be discharged using 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 What is a monoflange? For those who have any questions, your contact will gladly help you.g