Service pressure

Processes in the petroleum and chemical fields change frequently and a tower initially designed for vacuum operation may at a later date be used at pressures greater than atmospheric. Therefore, it is common to rate thin-walled vessels for both pressure and vacuum. If the vessel is thick-walled and designed for high-pressure service, it obviously will withstand complete vacuum and in this instance there is no need for vacuum rating. 

The ellipsoidal shape is usually preferred for closures of vessels 6 feet in diameter or greater. The hemispherical shape is preferred for vessels of lesser diameter. For high-pressure service, tall vertical vessels 80 feet high or more are always designed to operate at pressures under 3000 psi. 

Their design does not fall into the category of thick-walled, high-pressure vessels but rather into the field of thin-walled pressure vessels. The basic equations for thin-walled vessels under internal pressure show that the radial stress in a shell may be neglected because it is small, with a maximum 

seismic and vibrational stresses and accumulated dead weight compression loadings primarily affect the axial stress and produce only a small effect as a result of Poisson s relationship on the circumferential stress. Therefore, the shell thickness of the upper portion of a tall vertical vessel designed to operate under either internal pressure or vacuum is determined by the circumferential stress. 

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For the larger sizes in high-pressure service, the fixed-ball type with O-ring seat seals requires less operating effort. However, these require two different plastic materi s with resistance to the fluid and its temperature. Like plug cocks, ball valves may be either restricted-port or full-port, but the ports are always round and pressure drop is low. 

Tube e] ansion after welding has been found useful for low and moderate pressures. In high-pressure service tube rolling has not been able to prevent leakage after weld failure. 

FRP code vessels for pressure service over 0.1 MPA (15 IbFin") may be designed and built under ASME Sec. X. Eqmpment for seivdce from full vacuum through 0.1-MPA (15-lbf/iu ) pressure, while not... 

If the system differential pressure requirements are low, sometimes a fan can be used. A fan is limited to a maximum of about 65 inches of water (just over 2psi) in vacuum service or 77 inches of water in pressure service (just under 3psi). To estimate fan horsepower use the equations found in Chapter 6 in the section entitled Horsepower Calculation. In lieu of manufacturer s data use an adiabatic efficiency of 50% for initial work. 

Flexible tubing for high pressure service, equipped with stainless steel overbraid plus tube adapter end connections, is commonly available with a carbon black-loaded PTFE core tube to dissipate static. Numerous other designs of conductive and antistatic tubing are available for low pressure applications. The utility of conductive tubing in preventing fires during transfer of aromatic hydrocarbon liquids is described in [165]. 

The reciprocating compressor is generally in the lower flow end of the compressor spectrum. Inlet flows range from less than 100 to approximately 10,000 cfm per cylinder. It is particularly well-suited for high-pressure service. One of the highest pressure applications is at a discharge pressure of 40,000 psi. Above approximately a 1.5-to- i pressure ratio, the reciprocating compressor is one of the most efficient of all the compressors. 

Vane compressors tend to be limited to low pressure service, generally less than 100 to 200 psi discharge. They are used extensively as vapor recovery compressors and vacuum pumps. Single-stage vane compressors can develop 27 in. Hg vacuums, two-stage compressors can develop 29.0 in. Hg, and three-stage compressors can develop even higher vacuums. 

These valves are relatively inexpensive, quarter-turn valves. The seal is made by a rotating disc that remains in the flowstream subject to erosion while in the open position (Figure 15-4). Except in low-pressure service, they should not be used to provide a leak-tight seaL but they can be u.sed to throttle where a tight shut-off is not required. 

Compressors are available for vacuum as well as positive pressure service. Vacuums are obtained to 29 in. Hg abs and pressures up to 125 psig, with volumes of 2-25,000 cfm. 

Manning, W. R. D. (1947) Engineering 163 (May 2nd) 349. The design of compound cylinders for high pressure service. 

A 672 Electric-Fusion-Welded Steel Pipe for High-Pressure Service at Moderate Temperatures... 

A 691 Carbon and Alloy Steel Pipe, Electric-Fusion-Welded for High-Pressure Service at High Temperatures [Note (2)]... 

The operating company shall make certain that the service line valves installed on high-pressure service lines are suitable for this use, either by making their own tests or by reviewing the tests made by the manufacturers. 

ASTM A487 Steel castings suitable for pressure service 17245 1504 NFA 32055 G5121 G5151... 

Cast Iron and Ductile Iron Cast iron and ductile iron provide more metal for less cost than steel in piping systems and are widely used in low-pressure services in which internal and external corrosion... 

ASTM A 487, Standard specification for steel castings suitable for pressure service. 

JSA G 5151, Steel castings for high temperature and high pressure service. 

The last kind of devices used in high-pressure service are level indicators. The knowledge of the level of fluids in different apparatus is important for safe and continuous operation of chemical plants. Levels must be measured under high pressure, methods using various physical properties have been developed for this purpose. A selection of methods used in high-pressure technology is presented in Table 4.4-3. 

Traditionally, packings have not been used in distillation duties, except for small columns of less than 1 m diameter, and even less for high pressure service. The situation has changed over the last 10-15 years, partly through a better understanding of how packed towers work, particularly with respect to liquid distribution, but also through the development of new proprietary packings. 

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