Problem of the flow direction of gases at different pressures in pipelines
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I’m seeking help from fluid dynamics experts! Our company has 3 environmental protection units that share one vent pipeline at a height of 15 meters. These three units handle three different types of gaseous waste, which are then combined into a single pipeline for venting at that 15-meter height. The pressures in these three treatment units range from 0 to 15 kPa, and these pressures are unstable. We are concerned that if all three units are operated simultaneously, there could be a risk of cross-contamination of the waste streams. Will this problem arise as long as there are no obstacles to venting at that high altitude?
The best solution to this problem is to set up separate vent pipes for each one, which is the optimal approach. The principle behind it is the same as that used in the exhaust pipes of primary dust removal fans in steel mills. Besides, 15m is not considered a high height. If a check valve is used, since opening the check valve requires a certain pressure, it is not a viable option given the specified conditions (pressure between 0-15 kPa).
There is a risk of air leakage, which will at least cause pressure buildup on the low-pressure side; it is recommended to implement pressure balancing at the collection point or use multiple discharge outlets
If it is connected directly to an exhaust stack, there is a risk of cross-ventilation. It depends on whether there are liquid seals or check valves of some kind to prevent backflow before these three streams of gas enter or exit. It is also related to the diameter of the exhaust pipe, that is, the flow rate of the exhaust gas. In fact, no matter what the pressure is when air enters or exits the exhaust pipe, the pressure essentially results only from the resistance loss in the exhaust pipe, with static pressure being converted into dynamic pressure. However, due to the 0KPa condition at the source, air leakage is inevitable. Suggestion 1) Install a liquid seal or check valve in front of each exhaust stack ； 2) Add a fan to the exhaust system for forced discharge ； 3) Install an exhaust stack separately. Actually, if the minimum exhaust pressure isn’t 0, but is around 100–200 PaG, things become much simpler; this can be resolved by adding partitions internally or through piping.