Non-condensible blowdown drums

The header is normally a 80 mm diameter pipe (50 mm may be adequate for small units) and is routed via an overhead pipe rack (which is generally sloped) to a non-condensible blowdown drum. 

A typical non-condensible blowdown drum and its associated equipment and headers are illustrated in Figure 1. A single blowdown drum may be used for more than one process unit, if economically attractive. However, when this is done, all units served by it must be shut down in order to take the drum out of service, unless cross connections are made to another system of adequate capacity. Normally all closed safety valve discharges are combined into one header entering the drum, although separate headers and inlet nozzles are acceptable if economically advantageous. The following releases are also normally routed into the safety valve header ... 

Figure 1. Typical non-condensible blowdown drum arrangement.

Liquid hydrocarbons accumulated in non-condensible blowdown drums, originating from safety valves, closed drain headers, knockout drum drainage, etc. Facilities are normally provided at the drum for weathering volatile liquids and cooling hot liquids before disposal. 

As an alternative to special pressure slop storage, the necessary holdup may be provided in a non-condensible blowdown drum. 

Select a condensible blowdown drum for condensible releases, rather than the non-condensible type. If a condensible blowdown drum is not suitable for handling the total blowdown service (e.g., if cold liquids are involved), then a combination of a condensible and a non-condensible drum may be used. 

The first vessel in the blowdown system is therefore an acid-hydrocarbon separator. This drum is provided with a pump to transfer disengaged acid to the spent acid tank. Disengaged liquid hydrocarbon is preferably pumped back to the process, or to slop storage or a regular non-condensible lowdown drum. The vented vapor stream from the acid-hydrocarbon separator is bubbled through a layer of caustic soda solution in a neutralizing drum and is then routed to the flare header. To avoid corrosion in the special acid blowdown system, no releases which may contain water or alkaline solutions are routed into it. 

Lx)cate the blowdown drum (when the non-condensible type is used) at a minimum permissible spacing from the flare, to minimize condensation in the flare header. 

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