Relief contingency

Some system uses one pressure relief device for overpressure protection, but some uses more than one. Some relief contingency is non-fire, but some is caused by fire. ASME Section VIII (ASME code) has some guideline for PRD set pressure and accumulated pressure for different cases, see Table 1. 

Another possibility exists in the design of a bus bar in the same substation where half of the motors are connected through a different bus bar. In such a situation, availability of 50% power can be assumed in estimating the relief contingency. 

The impact of total power failure is very complex and needs to be established carefully. Total power failure does not always result in maximum relief contingency. 

Sometimes a control valve is installed with a mechanical travel limit to reduce the maximum possible flow through the valve. In such circumstances, the relief contingency will be estimated for the actual CV (not the reduced CV) of the control valve. 

Though Equation 4.16 can be used, it is often difficult to establish the relief contingency. However, because the contingency is small, a relief valve of 20 x 25 is used to protect a pipeline against thermal overpressure. 

If the relieving temperature exceeds 1100°F, then it can be assumed that the vessel, if carbon steel, will fail before the PRV could relieve. In such a situation, PRV protection may not be very helpful. The use of a rupture disk or an alternative for PRV can be considered. Additional protective measures such as water spray, depressuring, or fireproofing can also be considered. The relief contingency can also be calculated as [3]... 

Overheating above design temperature may also result in overpressure, due to the reduction in allowable stress. A pressure relief valve cannot protect against this type of contingency. 

In applying this rule, the capacity of the pressure relief system must also be sized to handle the quantity of fluid released at this pressure (together with other expected loads during this contingency), so that the built-up back pressure will not result in exceeding 1.5 times the design pressure. This additional load need not, however, be considered in calculations of flare and PR valve radiant heat levels. 

Other Utihties - Failure of other uOhties, such as inert gas to seals and pinge systems, or compressed air (when used by the process) may in some cases determine pressure relief requirements. These cases are evaluated on a single contingency failure basis similar to the above. 

Conventional PR valves and discharge systems should be designed such that built-up back pressure does not exceed 10% of set pressure (both measured in psig), to avoid chattering problems. In the case where a pressure relief valve system is sized for fire conditions, with 21 % overpressure, built-up back pressure up to 21 % of set pressure is permissible. However, the lower rates resulting from other contingencies still must meet the 10% limitation. 

The required relieving rate for a pressure relief valve is determined from consideration of the contingencies which can cause overpressure. Basic... 

Failure to consider appropriate challenges when determining the required relief capacity (e.g., maximum rates of liquid withdrawal or cooling of vessel contents). Credible contingencies (e.g., thunderstorm cooling a vessel during steam-out) should be considered. 

Accumulation The pressure increase over and above the MAWP (maximum allowable working pressure) during the discharge of the pressure relief device. Expressed in pressure units or as a percentage of set pressure. Maximum allowable accumulation is established by applicable codes for operating and fire contingencies (see Section 3.6). 

Individual safety relief valve specifications shall be communicated on API 526 specification sheets or equivalent substitute provided by the end user. When the relieving contingency is vapour and liquid, rates and properties for each at the allowable overpressure shall be provided. 

After all the evaluations, a decision as to which condition or conditions will dictate the emergency reiief requirements. In some instances, the calculated relief will be impractical. When this happens, a reasonable means of reducing the maximum contingency, redesigning the evaporation system, or providing a rupture section may be required. 

If the above rule is satisfied, then a relief valve on the low-pressure side of the exchanger is not needed provided the following contingencies are true ... 

Pressure relief device (PRD), such as pressure relief valve (PRV), rupture disk and rupture pm device, are used to protect equipment and lines from overpressure. It is process engineer s responsibility to show the requirement of a PRD at an equipment or line on the P ID drawings. It is also process engineer s responsibility to select the PRD type and its set pressure, and to do a contingency analysis to check out under what conditions the equipment or line will be over pressured, and what the relief rate will be. 

In this chapter, following topics will be discussed type of pressure relief devices, their characteristics, ASME code PRD set pressure, maximum operating pressure, contingency analysts, pressure reliefvalve and rupture disk sizing, pressure relief valve inlet/outlet piping sizing, Eind PRD selection. 

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. 

The flow contingency shall be the maximum contingency flow through the valve. This is the maximum capacity of the relief valve. For convenhonal and balanced-bellows PRVs, the nonrecoverable pressure drop shall not exceed 3% of the PRV set pressure. 

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