Pressure mechanical design

Figure 13 shows two pipe distributors, one in a branched and one in a ring configuration. These distributors minimize weeping, have good turndown, may requite the lowest pressure drop, and avoid the need for a plenum chamber. They are also well suited to multiple-level fluid injection. The disadvantages of these distributors are that there are defluidized soHds beneath the distributor and the mechanical design is more complex. 

Single-stage or two-stage cooling is used to achieve the desired degree of cooling. These alternatives achieve the same thermal efficiency, but offer different approaches to low pressure drop operation and mechanical design. 

The plate thickness of bubble-cap and sieve plates is generally estabhshed by mechanical design factors and has little effect on pressure drop. For a sieve plate, however, the plate is an integral component of the vapor-dispersion system, and its thickness is important. 

Balanced bellows PR valves need not be restricted to the same built-up back pressure limit (10% of set pressure) as are conventional valves, since they are not subject to chattering from this cause. However, maximum back pressure is limited by capacity and in some cases by the mechanical design strength limitations of parts such as the outlet flange, bellows, or valve bonnet. 

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. 

Consider a weak roof seam for API tanks if the tank is going to split under internal pressure, the roof seam should fail, not the bottom seam. The weak roof seam must be specifically included in the specifications and the mechanical design must address the issue. This emphasis is made because smaller tanks (less than 50 feet in diameter) manufactured under API 650 will not automatically have a weak roof seam. 

To determine the column (with trays) diameter, an approach [130] is to (1) assume 0 hours (2) solve for V, Ib/hr vapor up the column at selected, calculated, or assumed temperature and pressure (3) calculate column diameter using an assumed reasonable vapor velocity for the type of column internals (see section in this volume on Mechanical Designs for Tray Performance ). 

The mechanical design of thermosiphon reboiler piping must be carefully examined for (a) system pressures and (b) elevadon reladonship between the liquid level in the disdl-ladon column and the verdcal or horizontal reboiler. Kem provides an excellent presentadon on this topic, including the important hydraulics. AbboT also presents a computer program for this topic. 

System efficiency is influenced by the air plenum chamber and fan housing. Industrial axial flow fans in proper system design will have efficiencies of approximately 75% based on total pressure. Poor designs can run 40%. Speed reducers are about 75% mechanically efficient. 

The standpipe provides the necessary head pressure required to achieve proper catalyst circulation. Standpipes are sized to operate in the fluidized region for a wide variation of catalyst flow. Maximum catalyst circulation rates are realized at higher head pressures. The higher head pressures can only be achieved when the catalyst is fluidized. Table 7-5 contains typical process and mechanical design criteria for standpipes. 

Designing an optimum set of cyclones requires a balance between the desired collection efficiency, pressure drop, space limitations, and installation cost. Cyclone process and mechanical design recommendations are shown in Table 7-8. 

The pressure drop in the Y or J-bend section could be from improper fluidization or a flaw in the mechanical design. There are often fluffing gas distributors in the bottom of the Y or along the J-bend that are designed to promote uniform delivery of the cataly.st into the feed nozzles. Mechanical damage to these distributors or too little or too much fluffing gas affect the catalyst density, causing pressure head downstream of the slide valve. 

Reactor temperature is usually directly controlled by adjusting the slide valve openings or changing the pressure differential between the regenerator and reactor. Mechanical design conditions of the reactor systems can limit operating at more severe conditions. To debottleneck these limitations ... 

Four elements of microchannel scale-up models will be described pressure-drop design, heat-transfer design, reactor design, and mechanical and manufacturing designs. 

Singh, K. P. and Soler, A. I. (1992) Mechanical Design of Heat Exchanger and Pressure Vessel Components (Springer-Verlag). 

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

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