Always assess the consequences of system additions. While sometimes this may involve detailed calculations, often just paying attention to the laws of physics can head off problems.
One plant learned this lesson on a tank vent line for volatile organic vapor capture. Originally, the tank vented to atmosphere. The organics in the tank had approximately 12 psia vapor pressure. During normal operation, tank breathing losses amounted to a significant environmental and safety issue.
So, carbon canisters were put on the tank to reduce the breathing loss and a nitrogen purge was added. Upstream process upsets could occasionally get air into the tank but the continuous nitrogen purge prevented an explosive mixture from forming.
Initially the system seemed to work well. Sniff testing of the vent downstream of the carbon canisters indicated good performance. The canisters had to be changed once or twice every year. All project objectives were met.
However, a follow-up review aimed at reducing nitrogen consumption identified an inconsistency. For the nitrogen consumption rate with the entrained organic rate expected, the carbon canister life would be less than one day. (Evidently the likely life of the canisters wasn’t checked during initial size selection.) So, the plant started to delve into why the actual canister life was so much longer than expected. It found that the reason was that most of the organic vapors didn’t reach the canisters.
Figure 1 shows the field-confirmed tank and vent line configuration. The tank was protected from over-pressure by a vent relief valve set at 2 in. of water column. The calculated pressure drop in the line from the tank to the carbon canisters was less than 0.5 in. of water column. The line followed existing pipe racks. Because of this, the line from the tank to the carbon canisters included two large low spots.