Mechanical pressure relief

For most processes, the optimum operating point is determined by a constraint. The constraint might be a product specification (a product stream can contain no more than 2 percent ethane) violation of this constraint causes off-specification product. The constraint might be an equipment hmit (vessel pressure rating is 300 psig) violation of this constraint causes the equipment protection mechanism (pressure relief device) to activate. As the penalties are serious, violation of such constraints must be very infrequent. 

To avoid this situation, some regulators are designed with a built-in over-pressure relief mechanism. Over-pressure relief circuits usually are composed of a spring-opposed diaphragm and valve assembly that vents the downstream piping when the control pressure rises above the set point pressure. 

To prevent chattering from this mechanism, conventional PR valve discharge systems should be designed for a maximum built-up back pressure of 10% of set pressure, when relieving with accumulation of 10%. In cases where pressure relief design is controlled by fire conditions, with 21 % overpressure, built-up back pressures up to 21 % of set pressure are permissible. 

In the case of a balanced bellows pressure relief valve, to the maximum pressure permitted by considerations of bellows and bellows bonnet flange mechanical strength. This maximum pressure may be obtained by applying the following correction factor to the maximum back pressure listed for 38 °C. 

A distinction must be made regarding the length of service of the pressure reducing systems. Fatigue failure of any mechanical system depends on time, i.e., the number of cycles to failure. Therefore, the treatment required for a continuous service may not be justified for a short term service. A System in short term service is defined as one which operates a total of 12 hours or less during the life of the plant. Pressure relief valves typically meet this limit. Systems in short term service exceeding the screening criteria indicated above should be evaluated. 

Many aspects of quench pool design (vessel, spargers, mechanical design loads, etc.) and operation are covered in Guidelines for Pressure Relief and Effluent Handling Systems (AIChE-CCPS, 1998), and it should be consulted when a quench pool has to be designed. 

The term pressure relief refers to the automatic release of fluids or gases from a system or component to a predetermined level. Pressure relief systems are designed to prevent pressures in equipment or processes reaching levels where rupture or mechanical failure may occur by automatically releasing the material contained within. 

The autoclave was equipped with a separate pressure relief system consisting of a rupture disk and a safety valve connected to it in series (set pressure 50 bar in each case). It was clear from the debris that these mechanisms had been activated. 

The pressure relief system was not designed for two-phase flow. Thus, the resulting mass flow in the relief line was too high and could not resist such a high mechanical load. As a consequence, the relief line ruptured, causing the spillage. 

The conditions of temperature, pressure, tank metallurgy, pressure-relief device, and the level of liquid in the vessel interact to determine which of the two mechanisms will occur when a vessel is exposed to fire loading. 

While preparing for the test, two mechanics blinded a six-inch (15 cm) pressure relief flanged top-mounted nozzle. The craftsman also capped two smaller nozzles on the tank roof with flanges. These smaller nozzles normally were equipped with a pressure gauge and... 

Where water is involved in a PC, as in mold/die cooling controllers, with improper construction the most common problem (due to expansion/contraction not properly incorporated) is that water leaks occur. With their external pressure relief valves, they ensure discharge outside the cabinet. With inside discharge, severe damage can occur to mechanical and electrical components. 

Fig. 11 Differential pressure relief of mechanical seal on a semi-hermetic high-pressure canned-motor centrifugal pump...

This chapter covers those aspects of the mechanical design of chemical plants that are of particular interest to chemical engineers. The main topic considered is the design of pressure vessels. The designs of storage tanks, heat exchanger tube sheets, and pressure-relief devices are also discussed briefly. 

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