Vapor relief

2 Vapor relief This happens when vapor knock-out drum Is blocked-in and exposed to fire, or some fluid become supercritical fluid during fire, 

W is vapor relief, in Ib/hr F is environmental factor M is vapor molecular wei t PI is vapor relief pressure, in psia T1 is vapor relief temperature, in R As is vessel surface area exposed to fire, in tl2 Tw is vessel wall temperature, in °R Tn and Pn are normal opetatiog temperature and pressure, in °R and pisa. For carbon steel, Tw-1560 R. 

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The following eonsiders a situation that involves all vapor relief. The size of a vapor phase rupture disk required is determined hy assuming that all of the heat energy is absorbed by the vaporization of the liquid. At the set temperature, the heat release rate q is... 

Thus, the size of the relief deviee is signifieantly smaller than for two-phase flow. Sizing for all vapor relief will undoubtedly give an ineorreet result, and the reaetor would be severely tested during this runaway oeeurrenee. Table 12-7 gives the results of the VENT software program of Example 12-4. 

W = required vapor capacity in pounds per hour, or any flow rate in pounds per hour, vapor relief rate to flare stack, Ibs/hr W(. = charge weight of explosive, lb Wj. = effective charge weight, pounds of TNT for estimating surface burst effects in free air W, = required steam capacity flow or rate in pounds per hour, or other flow rate, Ib/hr Whe = hydrocarbon to be flared, Ibs/hr Wtnt equivalent charge weight of TNT, lb Wl = liquid flow rate, gal per min (gpm)... 

W = vapor relief rate to stack, lbs/hr Pt = pressure of the vapor just inside flare tips (at top), psia (For atmospheric release, P, =... 

Valve open too far Internal valve malfunction. Operator error. Calibration error. Operation High N2 pressure at HF cylinders, HF vaporizer - HF vaporizer vessel rupture - HF released to environment. High HF flow to HF vaporizer - high HF flow to B-l wing -potential liquid HF to B-l wing. Local pressure indication on N2 line. Local pressure indication between rupture disk and PRV-4 at vaporizer. PRV-3 at V-13 outlet. PRVs on N2 feed lines to HF cylinders. PRV-4 at HF vaporizer. ii If N2 line relief valves lift, vaporizer relief valve should not lift. Relief valve discharges piped to D-wing stack. 

If the reactor has a relief device, the pressure response depends on the relief device characteristics and the properties of the fluid discharged through the relief. This is illustrated by curve A (Figure 8-2) for vapor relief only and by curve B for a two-phase froth (vapor and liquid). The pressure will increase inside the reactor until the relief device activates at the pressure indicated. 

B ecause the relief character of two-phase vapor-liquid material is markedly different from vapor relief, the nature of the relieved material must be known in order to design a proper relief. 

This requires a relief device with a diameter of 0.176 m, a significantly smaller diameter than for two-phase flow. Thus, if the relief were sized assuming all vapor relief, the result would be physically incorrect and the reactor would be severely tested during this runaway event. 

Determine the diameter of a spring-type vapor relief for the following conditions. As- ... 

Also compute the relief vent size assuming all vapor relief. Assume 20% backpressure. 

A horizontal vessel, 10 ft long and 3 ft in diameter, contains water. What relief size is required to protect the vessel from fire exposure Assume the following vapor relief only, M AWP of 200 psig, conventional spring-operated relief. 

A laboratory test has shown that the reaction will not result in a two-phase relief. Thus a vapor relief system must be designed. Furthermore, calorimeter tests indicate that the maximum self-heat rate is 40°C/min. The physical properties of the material are also reported ... 

Blowdown and flare What may or may not be vented to the atmosphere or to ponds or to natural waters, nature of required liquid, and vapor relief systems. 

In the case of a fire, there is a pressure buildup and the required relief capacity can be determined provided that the heat absorbed can be estimated. API 520 gives recommendations for heat of absorption during vapor relief [12]. Not all the causes will happen simultaneously, but the pressure relief or safety relief should be sized for conditions that require the greatest relief. 

THIS PROGRAM PRINTS THE RESULTS OF VAPOR RELIEF SIZING ONTO A PRINTER... 

FORMAT(20X, VAPOR RELIEF VALVE SIZING ) WRITE(1, 110)... 

For v r relief when pressure of the protected system approaches PRV set pressure (about 9S% to 100% of PRV set pressure), a small portion of vapor will start escaping frem the orifice of the PRV. This is simmer. As system pressure continue rising, PRV disk will be lifted little hit, relieving vapor will expand under the disk holder and provide more force to lift the disc. This phenomena will accelerate as disk lift higher and more vapor is relieving. Finally, the disk will pop to its maximum lift position. A typical vapor relief path in terms of disk lift and system pressure is shown in Figure 6. As system pressure is reduced, PRV disk will close gradually, until fully dosed (at about 92.5% of set pressure). The pressure difference between PRV set pressure and the PRV fully dosed pressure is the blowdown of the PRV,... 

Figure 6 A typical spring-loaded PRV disk lift path versus system pressure during vapor relief. [Courtesy of Tyco Valves and Controls LP]...

For liquid relief, PRV disk H3 is gradualty to its full lift, as shown in Figure 7, Its closing is gradually as well. There is no pop lift of disk as vapor relief There is no (or very tittle) simmer or blowdown. E Iier liquid relief is not very stable. API 520 reports that at 2-4% overpressure, PRV disk move is little, but at 2-6% overpressure it will suddenly lift to 50-100%. [ 1 ] However, this suddenly lift of disk is not as steep as the v r relief. Figure 7 is drawn based on this report, using two straight lines to represent the liquid relief. However, the intersection point of these two lines is arbitrarily chosen, since there is no information available. 

Vapor relief fixim fire case is calculated by following equation ... 

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. 

For preliminary calculation, PI can be assumed to be equipment relieving pressure at 10% ovetpressure for non-fire case or 21% overpressure at fire case. After PRD inlet and outlet piping are designed, an updated PI and P2 based on hydraulic calculation should be used to recheck the vapor relief is sonic or not... 

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