Overpressures definition

Tweeddale (Tweeddale, 1995) identified two general sorts of deviations, i.e. hard and soft deviations. He identifies hard deviations as malfunctioning equipment, and soft deviations as faults in the system or procedures. In this thesis these definitions are slightly modified to cover all deviations identified in the operational process preceding and directly related with an accident. Hard deviations are defined as the actual loss of containment or demonstrable loss of control, e.g. small leakages, overpressure, override of control systems, etc. Soft deviations refer to indications of possible deviations, but cannot be demonstrated by actual facts, e.g. operator complaints, deficiencies of maintenance activities, or bad housekeeping activities, etc. 

Another area of confusion might involve the definition of capacity and how the term is used in ASME and API. Relieving rates are determined from what can go wrong scenarios and, if allowed to go unchecked, would overpressure the vessel. 

Definitions of terms used in connection with overpressure relief systems. (Reproduced courtesy of the American Petroleum Institute from API Recommended Practice 520, Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries, Part I - Sizing and Selection,... 

During a detonation, a shock wave is produced outside this shock wave the pressure is ambient, but inside the shock wave the pressure is above ambient. This shock wave continues to travel outward until, as a result of the expansion of the gas behind the shock front, the pressure inside the wave falls to ambient pressure and the temperature is below the initial temperature of the gas. If the initial pressure is sufficiently high, the shock wave will travel at, but not exceed, the speed of sound. This is the strict definition of an explosion. If the shock wave travels at a slower speed, the event is referred to as a deflagration. It is the overpressurization or pressure difference resulting from the passage of a shock wave that causes most of the damage in an explosion. 

In 1924, Waterman and Kortlandt ( ) observed that semicoke obtained from lignite was liquefied more rapidly if there was an overpressure of hydrogen and/or carbon monoxide. Fischer and Schrader ( ) observed that sodium formate in large amounts facilitated the liquefaction of various materials including peat and cellulose at 400 C. The effect of formate or carbon monoxide on the rate of biomass liquefaction was reported in 1960 by Appell, Wender, and Miller ( ) working at the Bureau of Mines in Pittsburgh. In a series of publications (36-40), these and other workers at the Bureau of Mines showed a definite effect of carbon monoxide on the alkaline liquefaction of biomass. 

The term set pressure is used to denote the pressure at which the relief device is open fully. This is not the definition used by some — for example, British Standards. For a safety valve the set pressure is often 10% above the set pressure as specified by the manufacturer or measured in the workshop, since this higher pressure is needed to open the valve fully. For a bursting disc, the set pressure is the nominal burst pressure plus any tolerance (or alternatively it is the maximum specified bursting pressure). The term overpressure is used here to mean the difference between the (redefined) set pressure and the maximum pressure reached during the venting process. 

If the ciunulated probability depends on a physical size that is by definition only positive and increases with the physical size, in our case the overpressure and the blat impulse, the lognormal density and cumulated distribution function of (9) is the right choice. In this case the lognormal density for all physical sizes smaller than z are used to compute the cumulated probability F+ (z). This can be interpreted as considering all damage cases, also for smaller z-values up to the actual z-value. 

An LTOP enable temperature is defined in Branch Technical Position RSB 5-2, "Overpressurization Protection of Pressurized Water Reactors While Operating at Low Temperatures," to Standard Review Plan Section 5.2.2, "Overpressure Protection," issued November 1988 as Revision 2. The definition is based on measuring the degree of protection provided by the low temperature overpressure protection system (LTOP System) against violations of the P-T Limits in terms of the of the reactor vessel beltline material at either the l/4t or 3/it location,... 

Determine the final overpressure and impulse from the definitions of the scaled variables. 

STEP 7 Determine the final overpressure. From the definition of the scaled pressure,... 

By EPA definition (40CFR68.3), Worst-case release means the release of the largest quantity of a regulated substance from a vessel or process line failure that results in the greatest distance to an endpoint defined in 68.22(a). This end point is a specified concentration of a toxic substance, an overpressure of 1 psi for explosions, a radiant heat flux of 5 kW/m for 40 seconds for a fire, or the lower flammability limit for a flammable substance, whichever is appropriate. Thus, the worst case is defined by the size of the area adversely affected by the release. There are also definitions of how much of the material in a tank (often 100%) should be considered over what period of time (often 10 minutes) in the scenario. 

It is certainly my experience that the most common and catastrophic accidents in process units are related to corrosion-type failures. 1 cannot bring to mind any process vessels that were definitely overpressured and failed because a relief valve did not open. 

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