Gas-Trap Manhole

All laterals leaving the unit enter the upstream compartment the branch line to the trunk sewer flows from the downstream compartment. Incoming laterals are elbowed downward to below the water level as in junction boxes. 

It is of great importance to maintain seals when units are shut down, whether permanently or for repairs. Most methods of sealing require a small flow to offset evaporation losses. Although it is 

It is usually advisable to blind or block an unused branch or lateral where it enters an active sewer. When welding is in progress near open catch basins, the usual procedure is to seal the catch-basin lid with sandbags or place a tarpaulin with a cover of sand over the lid. Similar precautions should be taken near floor drains or other open drains that may be discharging flammable vapors. 

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Gas-Trap Manhole. Branch lines from refinery and chemical plant units and processing areas generally flow through a gas-trap manhole before entering the trunk sewer system. Such manholes are often located at the unit limits. A typical arrangement is shown in Figure 8-8. 

When gas-trap manholes are used to sectionalize tnmk sewers, it is usually impractical to elbow the incoming line because of the larger size piping. Such is the case in Figure 8-7. The gas-trap manhole is an effective device for preventing backup of vapors from the tnmk sewer into branch systems. 

When there is a possibility that the sewer may flow only partially full, it is probably wise to provide some positive means of sealing the system. Gas-trap manholes can be installed in the existing system. Weirs installed in manholes to back up the flow and keep the sewer full provide another means of sealing the system. 

The vent should drain to the manhole or box and be free of pockets, which might be subject to clogging or other obstructions. On crude oH units the blow-down stacks will frequently drain to the gas-trap manhole at the battery limit. Under these conditions the blowdown stack is usually equipped with an internal spray nozzle to prevent the emission of condensibles through the stack. The vent from the manhole is sized to handle the larger quantities of vapor and is connected to the blowdown stack. 

The effectiveness of dry well flooding could be improved if the reactor vessel support skirt were vented in order to reduce the trapped gas volume and increase the fraction of bottom head surface area contacted by water. Partial venting could be achieved by loosening the cover on the support skirt manhole access hole. This would increase the wetted portion of the bottom head from 55% to 73% of the total outer surface area, which delays the predicted time of bottom head creep rupture by about one hour. The predicted failure times for the basic case without skirt venting and for Ae case of partial venting at the manhole access are indicated in the first two entries of Table 2. 

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