Determine Overpressure Protection Requirements

Now let us consider utility failure as a cause of overpressure. Failure of the utility supphes (e.g., electric power, cooling water, steam, instrument air or instrument power, or fuel) to refinery plant facihties wiU in many instances result in emergency conditions with potential for overpressuring equipment. Although utility supply systems are designed for reliability by the appropriate selection of multiple generation and distribution systems, spare equipment, backup systems, etc., the possibility of failure still remains. Possible failure mechanisms of each utility must, therefore, be examined and evaluated to determine the associated requirements for overpressure protection. The basic rules for these considerations are as follows ... 

The quantity of material to be relieved should be determined at conditions corresponding to the PR valve set pressure plus overpressure, not at normal operating conditions. Frequently, there is an appreciable reduction in required PR valve capacity when this difference in conditions is considerable. The effect of friction pressure drop in the connecting line between the source of overpressure and the system being protected should also be considered in determining the capacity requirement. If the valve passes a liquid which flashes or the heat content causes vaporization of liquid, this should be considered in determining PR valve size. 

Heatup and cooldown rates from 0 F/hr ("Isothermal" pressurization) to the design limit of 100°F/hr at 10 F/hr intervals are examined in determining the allowable heatup and cooldown rates as a function of temperature to meet low-temperature overpressure protection (LTOP) requirements for the Reactor Coolant system. LTOP considerations are discussed in Section 5.2.2.10. 

Tayout also has a significant role in minimizing the probability of ignition of a flammable release. Area electrical classification provides the basis for the control of electrical ignition sources. This classification is also used to determine the areas that require protection from vehicular access, etc. Frequently, highly hazardous processes that can result in overpressure (e.g., hydrogenation) are placed behind blast resistant structures/walls. 

Determine the size and location of relief devices required to protect an exchanger from overpressure during a tube rupture. 

A 500-gpm pump is used to provide water to a reactor vessel. If the pump continues to operate, the reactor might be overfilled and overpressurized. Determine the relief diameter (in inches) required to protect the vessel. The MAWP of the vessel is 100 psig. Please state clearly any additional assumptions required for your calculation. Assume a 10% backpressure and a 10% overpressure in the relief system. 

First, we need to define generally what we are talking about A pressure relief device is any device that can purge a system from an overpressure condition. More particularly, an SRV is a pressure relief device that is self-actuated, and whose primary purpose is the protection of life and equipment. Through a controlled discharge of a required (rated) amount of fluid at a predetermined pressure, an SRV must prevent overpressure in pressurized vessels and systems, and it operates within limits which are determined by international codes. An SRV is often the final control device in the prevention of accidents or explosions caused by overpressure... 

A bellow style pressure relief valve is required to protect a vessel containing an organic liquid. The required relieving capacity is 310 gpm. Inlet temperature is 170°F. Set pressure is 100 psig. Allowable overpressure is 25%. Built-up back pressure is 25 psig. Specific gravity is 1.45 and viscosity is 3,200 cP, Determine the orifice size of the valve. The correction factors are ... 

Positive-displacement pumps have an almost vertical head—flowrate curve the decline in capacity at increased pressure results mainly from increased internal leakage, a relatively small quantity in an efficient pump. It is not, however, normal to present the performance of a positive-displacement pump on a head—flowrate basis it is essentially a constant flowrate device (for constant speed), where the discharge pressure is determined by the discharge system only. Such pumps usually require a relief valve as protection against overpressure in the event of flow restriction (typically, a closed valve), and capacity is controlled by a bypass flow rather than by a series control valve. 

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