Relief systems

Once the process route has been chosen, it may be possible to synthesize flowsheets that do not require large inventories of materials in the process. The design of the reaction and separation system is particularly important in this respect, but heat transfer, storage, and pressure relief systems are also important. 

Relief systems. Emergency discharge from relief valves can be dealt with in a number of ways ... 

Relief systems are expensive and introduce considerable environmental problems. Sometimes it is possibly to dispense with relief valves and all that comes after them by using stronger vessels, strong enough to withstand the highest pressures that can be reached. For example, if the vessel can withstand the pump delivery pressure, then a relief valve for overpressurization by the pump may not be needed. However, there may still be a need for a small relief device to guard against overpressurization in the event of a fire. It may be possible to avoid the need for a relief valve on a distillation column... 

Relief systems. Relief systems are expensive and may bring significant environmental problems with them. Strengthening vessels may be a cheaper option. 

Relief System Terminology Specific terminology has been developed for the various components which compose an emergency relief system. The American National Standards Institute (ANSI) def-... 

Designers of emergency pressure relief systems should be famihar with the fohowing hst of regulations, codes of practice, and industiy standards and guidehnes. 

DIERS. 1992. Emergency Relief System Design Using DIERS Technology, DIERS Project Manual. American Institute of Chemical Engineers, New York. 

Water addition to vessels that have been steam-purged If vacuum conditions can develop, then either the equipment must be designed for vacuum conditions or a vacuum relief system must be instiled. 

FIG. 26 13 Typical pressure relief system configurations (a) rupture disk system (h) pressure relief valve system. 

W = required mass flow rate, Ib/min F = inlet mass flow rate, Ib/min = stream density entering the relief system, Ib/ft ... 

The time-dependent nature of the emergency pressure relieving event is obtained by the simultaneous solution of Eqs. (26-27) and (26-28). Generally, the only unknown parameters in these two equations are the venting rate W and the vent stream quahty (Xo). The vent rate W at any instant is a func tion of the upstream conditions and the relief system geometry. 

The couphng equation is a vapor mass balance written at the vent system entrance and provides a relationship between the vent rate W and the vent system inlet quahty Xq. The relief system flow models described in the following section provide a second relationship between W and Xo to be solved simultaneously with the coupling equation. Once W andXo are known, the simultaneous solution of the material and energy balances can be accomplished. For all the preceding vessel flow models and the coupling equations, the reader is referred to the DIERS Project Manual for a more complete and detailed review. 

Cyclone Separator with Separate Catch Tank (See Figs. 26-17 and 26-18.) The sizing of a cyclone knockout drum for emergency relief systems is somewhat different from sizing a cyclone separator for normal process sei vice for the following reasons ... 

Since discharges of vapors from highly hazardous toxic materials cannot simply be released to the atmosphere, the use of a weak seam roof is not normally acceptable. It is best that tanks be designed and stamped for 15 psig to provide maximum safety, and pressure relief systems must be provided to vent to equipment that can collect, contain, and treat the effluent. 

Vent Sizing Package (VSP) The VSP is an extension of ARC technology. The VSP is a bench-scale apparatus for characterizing runaway chemical reactions. It makes possible the sizing of pressure relief systems with less engineering expertise than is required with the ARC or other methods. 

Unwanted reaction Clean and inspect equipment after each use Design with compatible materials contaminants. Maintain integrity of the system Design emergency relief system (ERS) for runaway scenario CCPS G-13 CCPS G-22 CCPS G-23 CCPS G-29... 

A polymerization process involving a monomer, an organic peroxide initiator and an organic solvent underwent an energetic runaway reaction. All the contents in the polymerization reactor were lost. The emergency relief system prevented major damage to the equipment. 

Wilday, A.J. 1991. The Safe Design of Chemical Plants with No Need for Pressure Relief Systems. Elazards IX—New Directions in Process Safety. IChemE Symposium Series. No. 124, pp. 243-253. Institute of Chemical Engineers, IChemE, Rugby, U.K. 

Design Institute for Emergeney Relief Systems (DIERS) Institute under the auspices of the American Institute of Chemical Engineers founded to study relief requirements for reactive chemical systems and two-phase flow systems. 

DIERS Design Institute for Emergency Relief Systems... 

The maximum pressure from an explosion of a hydrocarbon and air is 7 x initial pressure, unless it occurs in a long pipe where a standing wave can be set up. It may be cheaper to design some small vessels to withstand an explosion than to provide a safety relief system. It is typical to specify %" as minimum plate thickness (for carbon steel only). 

---