Pressure process equipment

This paper focuses on how to model the deterioration of static pressurized process equipment to enable efficient inspection and maintenance planning. Such equipment tends to gradually deteriorate over time from erosion, corrosion, fatigue and other mechanisms, and at some point of time inspection, repair or replacement is expedient with respect to safety, production and costs. The deterioration of the equipment is influenced by many factors such as type of equipment, system design, operation and process service, material and environment. For hydrocarbon systems, the most critical deterioration mechanisms are corrosion due to CO2 and H2S, microbially influenced corrosion, sand erosion and external corrosion (DNV 2002). In general, CO2 is the most common factor causing corrosion in oil and gas system of low alloy steel (Singh et al. 2007). 

Another example is the purification of a P-lactam antibiotic, where process-scale reversed-phase separations began to be used around 1983 when suitable, high pressure process-scale equipment became available. A reversed-phase microparticulate (55—105 p.m particle size) C g siUca column, with a mobile phase of aqueous methanol having 0.1 Af ammonium phosphate at pH 5.3, was able to fractionate out impurities not readily removed by hquid—hquid extraction (37). Optimization of the separation resulted in recovery of product at 93% purity and 95% yield. This type of separation differs markedly from protein purification in feed concentration ( i 50 200 g/L for cefonicid vs 1 to 10 g/L for protein), molecular weight of impurities (<5000 compared to 10,000—100,000 for proteins), and throughputs ( i l-2 mg/(g stationary phasemin) compared to 0.01—0.1 mg/(gmin) for proteins). 

Yokes. The need to couple the end cover to the body of the vessel may be avoided if yokes, external to the vessel, are used to resist the load arising from the internal pressure acting on the closures. However the necessity to move the vessel out of the yoke and remove one of the closures to gain access to the inside of the vessel limits its use for chemical process equipment. Yokes may be pinned, welded, bolted, or wire wound. Both the vessel and yoke maybe wire wound (136). 

Fig. 3. Solvent-processing equipment using partial condenser. Level a on the water overflow line to the receiver should be about 3 cm below level b on the solvent-return line. Dimension b—c must be great enough to overcome pressure drop in the vapor piping, condenser, solvent piping, and rotameter. In a 4 m (1000-gaI) ketde, dimension b—c would be at least 1.25 m. The volume of the piping described by the dimension c—d—e should contain twice the volume of dimension b—c, thus providing an adequate Hquid seal against normal ketde operating pressures.

Tanks are used in innumerable ways in the chemical process iadustry, not only to store every conceivable Hquid, vapor, or soHd, but also ia a number of processiag appHcations. For example, as weU as reactors, tanks have served as the vessels for various unit operations such as settling, mixing, crystallisation (qv), phase separation, and heat exchange. Hereia the main focus is on the use of tanks as Hquid storage vessels. The principles outlined, however, can generally be appHed to tanks ia other appHcations as weU as to other pressure-containing equipment. 

Belt Presses Belt presses were fiiUy described in the section on filtration. The description here is intended to cover only the parts and designs that apply expression pressure by a mechanism in adchtion to the normal compression obtained from tensioning the belts and pulling them over rollers of smaller and smaller diameters. The tension on the belt produces a squeezing pressure on the filter cake proportional to the diameter of the rollers. Normally, that static pressure is calculated as P = 2T/D, where P is the pressure (psi), T is the tension on the belts (Ib/hnear in), and D is the roller diameter. This calculation results in values about one-half as great as the measured values because it ignores pressure created by drive torque and some other forces [Laros, Advances in Filtration and Separation Technology, 7 (System Approach to Separation and Filtration Process Equipment), pp. 505-510 (1993)]. 

Much of the damage and loss of life in chemical accidents results from the sudden release of material at high pressures which may or may not resiilt from fire. Chemical releases caused by fires and the failure of process equipment and pipelines can form toxic clouds that can be dangerous to people over large areas. 

Rupture Disks A rupture disk is a device designed to function by the bursting of a pressure-retaining disk (Fig. 26-15). This assembly consists of a thin, circular membrane usually made of metal, plastic, or graphite that is firmly clamped in a disk holder. When the process reaches the bursting pressure of the disk, the disk ruptures and releases the pressure. Rupture disks can be installed alone or in combination with other types of devices. Once blown, rupture disks do not reseat thus, the entire contents of the upstream process equipment will be vented. Rupture disks are commonly used in series (upstream) with a relief valve to prevent corrosive fluids from contacting the metal parts of the valve. In addition, this combination is a reclosing system. 

For any proposed suppression system design, it is necessary to ascribe with confidence an effective worst-case suppressed maximum explosion overpressure Pred.max- Provided that the suppressed explosion overpressure is less than the process equipment pressure shoclc resistance and provided further that this projected suppression is achieved with a sufficient margin of safety, explosion protection security is assured. These two criteria are mutually independent, but both must be satisfied if a suppression system is to be deployed to provide industrial explosion protection. 

Procedural Checklist Ensures Safe, Complete Pressure-Testing and Purging of Process Equipment... 

Gaseous monomers may also be trapped within the processing equipment and accidents have occurred as a consequence of the resulting pressure buildup. In the case of the polyacetals and poly(vinyl chloride) it is reported that at elevated temperatures these materials form a more or less explosive combination so that it is important to separate these materials rigorously at the processing stage. 

From this relatively simple test, therefore, it is possible to obtain complete flow data on the material as shown in Fig. 5.3. Note that shear rates similar to those experienced in processing equipment can be achieved. Variations in melt temperature and hypostatic pressure also have an effect on the shear and tensile viscosities of the melt. An increase in temperature causes a decrease in viscosity and an increase in hydrostatic pressure causes an increase in viscosity. Topically, for low density polyethlyene an increase in temperature of 40°C causes a vertical shift of the viscosity curve by a factor of about 3. Since the plastic will be subjected to a temperature rise when it is forced through the die, it is usually worthwhile to check (by means of Equation 5.64) whether or not this is signiflcant. Fig. 5.2 shows the effect of temperature on the viscosity of polypropylene. 

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