Design pressure rule

While generally only a single contingency is considered for design purposes, there may be situations where two or more simultaneous contingencies should be taken into account e.g., if there is some remote interrelationship between them, and pressures or temperatures developed could result in catastrophic failure. Such contingencies are also considered, and the 1.5 Times Design Pressure rule may be applied in this situation. 

In some cases where the ASME Code woidd not require pressure relief protection, the 1.5 Times Design Pressure Rule is apphcable. This rule is stated as follows Equipment may be considered to be adequately protected against overpressure from certain low-probability situations if the pressure does not exceed 1.5 times design pressure. This criterion has been selected since it generally does not exceed yield stress, and most Ukety would not occur more frequently than a hydrostatic test. Thus, it will protect against the possibility of a catastrophic failure. This rule is applied in special situations which have a low probability of occurrence but which cannot be completely ruled out. 

Examples where the 1.5 Times Design Pressure Rule apphes are ... 

If a situation occurs which involves more than one fire risk area simultaneously (such as an entire Refinery or Chemical complex), it would be classed as a remote contingency event, and the 1.5 Time Design Pressure Rule may be applied. 

When the utility can realistically be classed as "Normally Reliable" (fewer than one failure in 2 years) then its failure may be classed as a Remote Contingency event and the "1.5 Times Design Pressure Rule" may be applied. 

Internal Equipment Blockage bv Collapsed Internals - Contingencies such as collapsed reactor bed vessel internals (e.g., fixed-bed reactor grids, coked catalyst beds, accumulation of catalyst fines, plugging of screens and strainers, lines blocked with sediments, etc.) should be considered to identify any overpressure situations that could result. The use of the "1.5 Times Design Pressure Rule" is applicable in such cases, if this is a remote contingency. 

If there is a bypass around the control valve, downstream equipment must be protected so that its pressure would not exceed the 1.5 Times Design Pressure Rule, considering that the control valve is in the wide open position, and the bypass 50% open. 

Vapor space velocities normally should not exceed 100% of critical experience demonstrates that this keeps liquid entrainment into the flare line within acceptable limits. However, a velocity of 175% of critical is permitted when one is applyingthe 1.5 times Design Pressure Rule to remote contingencies,... 

Division 2. With the advent of higher design pressures the ASME recognized the need for alternative rules permitting thinner walls with adequate safety factors. Division 2 provides for these alternative rules it is more restrictive in both materials and methods of analysis, but it makes use of higher allowable stresses than does Division 1. The maximum allowable stresses were increased from one-fourth to one-third of the ultimate tensile stress or two-thkds of the yield stress, whichever is least for materials at any temperature. Division 2 requkes an analysis of combined stress, stress concentration factors, fatigue stresses, and thermal stress. The same type of materials are covered as in Division 1. 

Unstayed flat heads and covers can be designed by very specific rules and formulas given in this subsection. The stresses caused by pressure on these members are bending stresses, and the formulas include an allowance for additional edge moments induced when the head, cover, or blind flange is attached By bolts. Rules are provided for quick-opening closures because of the risk of incomplete attachment or opening while the vessel is pressurized. Rules for braced and stayed surfaces are also provided. 

Unfired steam boilers with design pressures exceeding 345 kPa (50 Ibftin") have restrictive rules on weided-joint design, and all butt joints require full radiography. 

The first guideline is often referred to as the "two-thirds rule." The basis of this rule is that if the low pressure side is designed for two-thirds of the high pressure side design pressure, the exchanger hydrotest pressure will not be exceeded due to a tube rupture. 

API RP-521 reeommends transient analysis for exchangers with wide difference in design pressure (such as cases where the two-thirds rule was not applied) because the pressure in the low pressure side of the exehanger ean spike to a level that exceeds the pressure predicted by a steady state analysis when it is liquid-filled. This pressure spike is due to pressure buildup before the liquid is accelerated out of the low pressure side and/or before the relief device opens fully. API RP-521 recommends that the basis for the tube rupture be a sharp... 

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. 

It is a general rule that as boiler design pressure and heat flux increases, so the requirement for a progressively higher quality (higher purity) MU water also increases. 

For any specific BW application, the boiler design, pressure-temperature, operation, and heat-flux rate are all contributing factors these chemistries generally function at substoichiometric levels (the coordinating and complexing polycarboxylic component of polymers aside), so that the use of reliable, directly measurable relationships is not always possible. Nevertheless, some rules and recommendations do exist, a few of which are discussed later. 

When the internal design pressure of a container exceeds 15 psig (101.3 kPa), it is called a pressure vessel. The ASME Boiler and Pressure Vessel Code is one of the primary standards used throughout the world to ensure safe storage vessels. Various substances, such as ammonia (qv) and many hydrocarbons (qv), are frequently stored in spherically shaped vessels that are often referred to as tanks. Most often the design pressure is 15 psig (101.3 kPa) or above. These are really spherical pressure vessels and fall under the rules of the ASME Boiler and Pressure Vessel Code. Discussion of pressure vessels are available (5,6) these are not covered in detail herein. 

API has, however, long used the two-thirds rule to identify tube rupture scenarios. This rule states that tube rupture protection is not required when the ratio of the low pressure to high pressure side design pressure is greater than two-thirds. 

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