Relief scenarios

After specifying the location and type of all relief devices, the relief scenarios are developed. 

A relief scenario is a description of one specific relief event. Usually each relief has more than one relief event, and the worst-case scenario is the scenario or event that requires the largest relief vent area. Examples of relief events are  

A pump is dead-headed the pump relief is sized to handle the full pump capacity at its rated pressure. 

The same pump relief is in a line with a nitrogen regulator the relief is sized to handle the nitrogen if the regulator fails. 

The same pump is connected to a heat exchanger with live steam the relief is sized to handle steam injected into the exchanger under uncontrolled conditions, for example, a steam regulator failure. 

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A vapor poeket on the exchanger s low-pressure side can create a cushion that may greatly diminish the pressure transient s intensity. A transient analysis may not be required if sufficient low-pressure side vapor exists (although tube rupture should still be considered as a viable relief scenario). However, if the low-pressure fluid is liquid from a separator that has a small amount of vapor from flashing across a level control valve, the vapor pocket may collapse after the pressure has exceeded the fluid s bubble point. The bubble point will be at the separator pressure. Transient analysis will prediet a gradually inereasing pressure until the pressure reaches the bubble point. Then, the pressure will increase rapidly. For this ease, a transient analysis should be considered. 

Using the results of Problems 8-9 and 8-11, determine the relief scenarios for each relief device. 

The design of a relief system often involves iteration and recycle. The flow chart in Figure 2.2 shows that possible recycle in the design process may involve changing the assumptions about the worst case relief scenario or changing the sizing method used. 

If all the identified relief scenarios give rise to the same system type, then the worst case is likely to be that which gives the highest rate of reaction, at the appropriate temperatures, in the screening test, as follows ... 

Non-adiabatic screening tests can be used to narrow down the range of relief scenarios which may be the worst case, but in many cases two or three possibilities may still remain. ... 

For multi-purpose vessels there will be a much larger number of possible relief scenarios. The worst case can be found using the methods above, but the process may be more time-consuming because many more possible scenarios are likely to be identified. Sometimes different relief systems may be specified for the different reactions that are carried out in a reactor, and the bursting disc must be changed to the correct one at the beginning of a campaign. A robust procedure is clearly needed to ensure that this occurs. -... 

The relief size obtained can be reduced by optimising the design parameters, such as relief pressure (see 6.2 below), vessel design pressure, or even changing the worst case relief scenario by designing out certain possibilities (see Chapter 3 and Annex 1). 

A reactor of volume 3.5 m3 has a design pressure of 14 barg. A worst case relief scenario has been identified in which a gassy decomposition reaction occurs. The mass of reactants in the reactor would be 2500 kg. An open cel test has been performed in a DIERS bench-scale apparatus, in which the volume of the gas space in the apparatus was 3,800 ml, and the mass of the sample was 44.8 g. The peak rate of pressure rise was 2,263 N/m2s at. a temperature of 246°C, and the corresponding rate of temperature rise was 144°C/minute. (These values include corrections for thermal inertia.) The pressure in the containment vessel corresponding to the peak rate was 20.2 bara. 

Note e.g. increase design pressure, take measures to change relief scenario. Check that design changes do not alter whether or not the hybrid is tempered, e.g. increasing the maximum accumulated pressure could cause a reacting mixture to no longer temper. 

DEVICES WHICH LIMIT THE WORST CASE RELIEF SCENARIO... 

There are a lot of possible ways in which the worst case relief scenario can be limited. These include ... 

Closed system tests, using an unvented test cell (see Figure A2.5) or Dewar flask, can be used for vapour pressure systems. The runaway is initiated in the way that best simulates the worst case relief scenario at plant-scale. The closed system pressure and temperature are measured as a function of time. Most commercial calorimeters include a data analysis package which will present the data in terms of rate of temperature rise, dT/dt, versus reciprocal temperature (-1 / ), and pressure versus reciprocal temperature (see Figure A2.10). However, it is important to correct the temperature data for the effects of thermal inertia. See 2.7.2. 

Where failure of the glass could cause injury, pipes and equipment should be protected by external shielding or wrapping with plastic tape. Glass apparatus should allow adequate venting to the atmosphere to handle anticipated relief scenarios without accumulating high pressure. 

The usual approach in design is to prevent explosions from occurring, for example, by not allowing flammable mixtures to form in the process. If internal explosion is a possibility, then it must be considered as a pressure-relief scenario and the pressure-relief devices must be sized to prevent detonation. This will usually require the use of large bursting disks. Flame arrestors should also be specified on process piping to... 

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 ... 

Figure 13.41. Instrumentation response to pressure-relief scenarios, (a) Instrumentation response increases relieving load, (b) Instrumentation response would reduce relieving load, but API RP 521 recommends taking no credit for instrumentation response.

Both API RP 521 and ASME BPV Code Sec. VIII allow multiple vessels connected together to be considered as a single unit for relief scenarios, provided that there are no valves between the vessels and that the design considers the full relieving load of the system (ASME BPV Code Sec. VIII D.l UG-133). 

If the temperature is constant (which is valid for a blocked outlet relief scenario). 

For some relief scenarios, correlations have been established for the relieving load. For the external fire case, API RP 521 (Section 3.15.2) gives... 

List possible relief scenarios for the vessel designed in Problem 13.3. 

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