Sizing the normal vent depends on the in-breathing (vacuum) and out-breathing (pressure) limits established by the process. The flow into the sump sets the in-breathing while out-breathing is based on the pump. Without additional data, use the right end of the (heat flow) curve for two pumps as the flow into the sump. Although assuming both pumps are operating is conservative, this approach usually is acceptable for out-breathing. With the limits set, you can specify the normal and emergency (fire) vents.
Given the conditions, it might be tempting to eliminate the emergency vent and go with the normal vent alone. This usually is not acceptable but it's worth checking into. Crane's method for divided flow won't work but iteration using the flow for the normal vent with the emergency vent set point is effective.
Using a vendor's package, I sized a 3-in. normal vent with 6-in. piping and a 3-in emergency vent. I found that a 6-in. single inlet pipe would feed both vents. I estimated a set point of 11 IWC for the emergency vent, 0.9 IWC for normal vacuum and 4 IWC for pressure. Sizing the emergency vent is tricky because the tank is rectangular and the vendor program calls for a height; use an equivalent diameter based on the top area and ignore the height.
Dirk Willard, senior engineer
Ambitech Engineering, Downers Grove, Ill.
We use a vacuum knockout pot to push vapor produced from a batch polymerization reactor and a neutralization tank to a caustic scrubber (Figure 2). Caustic is recirculated over a chevron demister and through packing. The vacuum is only about 12 psia to avoid losses from the polymer reactor, which contains a solid catalyst. The process periodically suffers from pH upsets in the neutralization tank, corrosion in the vacuum pot, poor temperature control, loss of vacuum from seals, fouling of the vacuum pump rings, and fouling in the demister pad, which only survives a few weeks. We're tearing down this process every week to avoid trouble and the cost is killing us. There's a further complication — we periodically use the batch reactor and tank to make other products. Can you suggest any ways to improve reliability?
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