Emergency vapor blowdowns

This chapter covers the design of facilities to handle equipment drainage and contaminated aqueous effluents that are sent for appropriate disposal blowdown drum systems to receive closed safety valve discharges, emergency vapor blowdowns, etc. and facilities for process stream diversion and slop storage. Also covered are criteria for selecting the appropriate method of disposal. Design of flares is covered in a subsequent chapter. 

Vapor load considerations must include all safety valve, emergency vapor blowdown and vapor stream diversion sources which release as a result of a single contingency. 

Types of Equipment The three most commonly used types of equipment for handling emergency relief device effluents are blowdown drums (also called knockout drums or catch tanks), cyclone vapor-liquid separators, and quench tanks (also called passive scruh-hers). These are described as follows. 

Usually, the closed liquid drain header is run as a separate line to the drum and provided with a high level cut-off valve with local manual reset. In some cases the closed drain system is segregated into a number of subheaders, as described earlier. Hydrocarbon liquids may be bypassed around the drum through a connection from the closed drain header directly to the pumpout pump suction, provided that the liquid can be routed to a safe disposal location, considering its vapor pressure and temperature. Emergency liquid pulldown connections, if provided, are routed to the blowdown drum via the closed drain header. 

In severe accidents in PWR s, the availability of boric acid vapor and aerosols for reactions with CsOH and Csl depends on the particular accident sequence (see Section 7.3.2.3.I.). These reactions are of greater signiflcance in the high-pressure accident sequence, since in this case the boric acid inventory of the primary coolant and the emergency coolant solutions becomes concentrated in the residual water volume inside the reactor pressure vessel because of the considerable volatility of boric acid at a pressure of 16 MPa, this compound will be partly volatilized with the steam. In contrast, in low-pressure accident sequences most of the primary coolant boric acid inventory will be ejected during blowdown and, thus, not be... 

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