Condensible blowdown

To Atmosphere Via Condensible Blowdown Drum - Releases which can be totally condensed may be routed to a condensible blowdown drum, which may be vented to the atmosphere. 

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. 

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 following are the principle advantages of condensible blowdown drums ... 

The disadvantages of condensible blowdown drums can be summarized as follows ... 

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.

Because of the continuous water flow through a condensible blowdown drum, it can safely handle cold or autorerrigerating releases only to the extent that effluent liquid and vapor temperatures remain above 0°C. 

Another example of an unsteady state condensible blowdown system is the design for a phenol condensible blowdown tank. A blowdown tank is used in phenol treating plants to handle streams containing phenol and heavy hydrocarbons (lubricating oil stocks). The blowdown tank is illustrated in Figure 4. The design basis is as rollows ... 

Figure 4. Condensible blowdown tank - phenol service.

A condensible blowdown tank, designed on a similar basis to that described above for phenol, may be provided in other services where a conventional condensible blowdown drum would not be acceptable (e.g., due to effluent water pollution considerations). Examples of such cases are methyl ethyl ketone (MEK) and dimethyl formamide (DMF). A suitable absorbing material is specified (e.g., a lube oil stock for MEK water for DMF), and the design must include consideration of maximum permissible operating temperatures to prevent excessive vapor evolution or the boiling of water. 

It is important to note that in some installations where local pollution regulations would not permit venting a condensible blowdown tank in toxic service to the atmosphere, a pressure drum or sphere, vented to a flare, may be necessary. 

Reliability of water supply, if condensible blowdown and water disengaging services are combined. 

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. 

Excessive use of treating chemicals not only carries the obviously higher material cost but also increases the amount of condensate blowdown that must be taken from the system to control the dissolved solids concentration. 

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