Relief Capacity Pitfalls Credits and Debits

Credit can be taken for a circumstance that is certain to occur during a relief situation and that will act to lower the column relief load. Credits are applied for reducing the calculated relief vapor rate, and thus the size of the relief device. Extreme caution is required in deciding whether credit should be taken for a given circumstance. If a credit is taken for a circumstance which cannot be relied on during a relief situation, the column may be overpressured. 

The API RP521 standard (10) and Bradford and Diurett (60) present an excellent evaluation of various factors often credited, and also present pitfalls of these practices. Other guidelines and recommendations have also been presented by several other authors (9, 10, 293, 351, 414). Many of these considerations, supplemented by this author s experience, are summarized below. As in previous sections of this chapter, it is emphasized that these considerations must not be blindly adhered to they best serve as an initial checklist or starting point. Codes, regulations, and company policy guidelines must be followed. 

The following credits are commonly considered when sizing distilla- 

Instrumentation. Standard API RP521 (10) recommends that any automatic control valves, which are not under consideration as causing a relieving requirement and which would tend to relieve the system, (are assumed to) remain in the position required for the normal processing flow. Although this policy may appear conservative (it usually isX it is justified. Controllers are often operated on manual at other times, they are tuned extremely slowly. Taking credit for controller action may lead to overpressure if the controller does not respond fast enough. 

Similarly, credit is seldom taken for cutoff switches or trips that may correct a relieving situation (60, 414), as these may fail to operate. One possible exception is when a well-designed system comprising a family of (redundant) cutoff switches is installed (60, 414). This is discussed in Sec. 9.7. 

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