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Monoflanges combine the function 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 necessary for most control applications. One of many variables that affect the response time is the volume and the distance 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 is the solution.
Vibrations are also critical, for example, if impulse lines are linked 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 in the compact, flange-shaped body, allowing a substantial reduction in volume, dimensions, weight and potential leakage points.
Monoflange is the solution
Depending on the requirements of the plant it 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 the other (with a red cap) regulates the venting of the medium trapped in the instrument. This is mostly used in applications that 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 procedure 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] ; once the valve [2] is open, the instrument is thus linked 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, that allows the flow to feed them.
The two shut-off valves allow a better 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. In some instances, customer specifications don’t allow the medium to be in touch with the instrument when it’s not measuring. For this reason the medium will be discharged using the vent line. In Results ? due to the vent line ? instruments could be easily calibrated without dismounting them from the line.
Note
Further information on our valves can be found on the WIKA website or in the video What is a monoflange? Should you have any questions, your contact will gladly help you.