Relief loads

The relief load can be calculated direcdy, in pounds per hour [33a] ... 

Valdes, E. C. and Svoboda, K. J., Estimating Relief Loads for Thermally Blocked-in Liquids, Cheni. Eng., Sept. 1985, p. 77. 

A vapor depressuring valve is usually installed in a bypass line around the vessel pressure relief valve that handles the largest relief load in the process train. 

This is clearly a double jeopardy failure two unrelated events occurring at exactly the same time. One has nothing to do with the other. Therefore, you need to calculate the relief capacity for one scenario at a time. For the loss of power to a pump scenario, the relief load would be based on the amount of vapour generated at the normal rate of steam. For the steam control valve failure scenario, the relief capacity would be based on the amount of vapour generated by the heat provided by a wide-open steam valve even accounting for the amount of vapour condensed in this failure, the condenser would still be in operation. So the SRV should be sized for the worst condition. 

The first step in designing a pressure-relief system is to evaluate the possible causes of overpressure so as to determine the rate of pressure accumulation associated with each and hence estimate the relief load (the flow rate that must be discharged through the relief device). The API Recommended Practice (RP) 521 suggests the following causes ... 

In evaluating relief scenarios, the design engineer should consider sequential events that result from the same root cause event, particularly when these can increase the relief load. For example, the loss of electric power in a plant that carries out a liquid phase exothermic reaction could have the following impacts ... 

Since these have a common cause, they should be considered as simultaneous events for that cause. If two events do not share a common cause, then the probability that they will occur simultaneously is remote and is not usually considered (API RP 521,3.2). Root cause events such as power loss, utility loss, and external fire will often cause multiple other events and hence large relief loads. 

Design codes and standards such as API RP 521 and the DIERS Project Manual (Fisher et al., 1992) should be consulted for other correlations and recommended methods for calculating relief loads. The DIERS Project Manual also discusses calculation of relief loads for underpressure scenarios (Section 13.17.6). 

A toluene surge drum has capacity 500 gal and is normally operated 60% full at 100°E under 300psig of hydrogen in the head space using a level-controlled outflow The normal flow rate into the vessel is 30,000 Ib/h. Determine the vessel dimensions if the vessel is vertically mounted. Evaluate the relief loads for the blocked outflow and external fire cases and hence determine the relief valve size. 

The flare header, which collects the vapors from the safety valves for safe discharge to the knockout drum and the flare stack, is sized for the largest vapor load caused by a single failure. This vapor load is obtained from a tabulation of relief loads from safety valves connected to the flare system. The loads which may occur simultaneously as a result of fire, cooling water failure, etc., are summed up. From these summations the largest load is determined. 

Once a set of premises is available for each failure, the list of possible overpressure causes can be narrowed down. A possible cause can be eliminated from the list if it is certain that its relief requirement is lower than or identical to the relief requirement of another source. For instance, when coliunn pressure is controlled by manipulating cooling water to the condenser, failure of the pressure controller may have identical consequences to coolant failure. In this case, failure of the pressure controller can be eliminated from the list. Another example is a column whose heat-input control valve and all feed control valves fail shut, while cooling is likely to continue normally during an instrument air failure in this case, the relief load is likely to be small (if any) upon instrument air failure, and this cause can be eliminated from the list. 

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. 

Manual bypasses. A manual bypass around a control valve may affect the column relief load. For instance, an open b3rpass around the reboiler control valve can increase the heat input into the column dvm-ing reboiler controller failure (Fig. 9.2a). 

The main limitation of the secondary relief techniques is that some probability of experiencing the full relief load still remains. 

Pressure relief systems -minimize relief load... 

Implement SIS system in the units. Perform dynamic simulation to reduce the relief load from the units (Ha et a/., 2014), if the original relief load is obtained from steady-state simulation. 20-30% load reduction is possible. 

New process equipment installed within 7.6 meters from the grade/ground level will require pressure relief devices due to fire scenario. For revamp cases, it may be advantageous to place vessels on platforms at >7.6 meters if fire case relief load is most credible and bottlenecks the existing flare system. Fire circle or zone is defined as the maximum affected area during any equipment fire in the facility. API 521 (2014) defines its area as 230 to 460 m. Addition of process equipment inside an existing fire circle may increase the fire circle size. Hence, care shall be taken to review the fire circle size with each equipment addition. It will impact the peak relief load during the fire scenario. 

Buildup backpressure is PRV s additional backpressure due to the relief load from the PRV. During relieving, the total backpressure of Che PRV equals its superimposed backpressure plus its buildup backpressure. 

A typical this type PRV is shown in Figure 1. This type of PRV is not designed to avoid backpressure influence. From Figure 1, it shows that disk holder area exposed to PRV backpressure is greater than its nozzle (orifice) area. Therefore, PRV s backpressure tends to hold the disk down. If PRV s backpressure is constant, PRV s set pressure should be set at the target relieving pressure minus a correction pressure due to its backpressure. If PRV s backpressure is variable, there is possibility fiiat PRV doesn t open, when it should. Therefore, conventional PRV should be used when relief load is relieving to atmosphere or a systnn with constant pressure. 

Contingency analysis studies the causes of overpressure in equipment Or piping, and how much the relief load will he for each cause. Only one cause will be examined, unless this cause will trip another cause. The reason is that two different causes happened at the Same time is unlikely. This analysis is process engineer s responsibility. 

W is vapor relief load, in Ihdir is latent heat, in btu/lb. Latent heat can be calculated using a process simulator. But if relief conditions are close to critical point, process simulator may have difficult to estimate the latent heat Most time, 50 btu/lb will be used as a conservative latent heat for this situation. 

Liquid relief load from thermal expansion of liquid is calculated by following equation ... 

A is required effective PRV oriflce area or rupture disk area, in inch W is vapor relief load, in Ib/hr T is relief temperature, in R Z is vapor compressibility factor at relief conditions. Let Z=1.0 for ideal gas. (see note 5 at end of this section) M is relief vapor molecular weight C is a coefficient calculated by q. (7b) or let it be 315 Kd is effective discharge coefficient Kb is backpressure correction factor Kc is a correction factor for install a rupture disk at PRV inlet, set kc to 1.0 for rupture disk sizing only Pi is PRD inlet pressure, in psia,... 

For each relief case, list following information relief load (vapor in Ib/hr, liquid in gpm), relief conditions (pressure and temperature), relief fluid physical properties (for vapor, provide molecular weight (M), compressibility factor (2), ideal gas heat capacity ratio (k) for liquid, provide specific gravity, viscosity (in centipoise))... 

When equipment in a fire area may contribute a significant relief load in the event of a fire, which the relief valves discharge into another vessel outside the fire area, then the relief load from the equipment shall be considered as a contingency for the vessel to which the relief valves discharge. 

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