Monoflanges combine the function of up to three valves in an especially compact body, thanks to a precise network of internal passages and valve chambers. But what really happens in the 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 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 critical, mounting the instrument near 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 in the 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, several 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. That is mostly found in applications which are relatively uncritical (e.g. low pressure) or where a first shut-off valve is provided just before the monoflange.
The safest configuration, and the main 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 inside 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;
Once 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] are in an angled position, which allows the flow to pass through them.
The two shut-off valves allow an improved isolation from the procedure: In case the initial shut-off valve will not isolate the medium properly, the second one will become a safety means against accidental leaks. In some instances, customer specifications don’t allow the medium to stay touch with the instrument when it’s not measuring. That is why 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
pressure gauge octa 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.