Common issues in the design criteria for chemical industry static equipment

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As equipment used for mixing, storage, separation, reaction, and heat transfer in chemical production processes, stationary equipment such as vessels (including reactors, towers, etc.) and heat exchangers accounts for a large proportion of the investment. The reliability and safety of these individual pieces of equipment are crucial for the safe and stable operation of the entire chemical plant. Design and manufacturing are the two key aspects that determine the quality of chemical equipment, with design serving as the foundation. During the detailed design phase, the equipment team primarily designs based on the design requirements specified by teams responsible for processes, pipelines, etc. The accuracy and completeness of these requirements directly determine the quality of the final design and affect the project timeline. Based on past project experience, this article takes containers and shell-and-tube heat exchangers as examples to illustrate common omissions in the design criteria for static equipment, as well as certain inaccuracies and irrationalities, providing a reference for relevant designers in order to improve the quality of these design criteria. I. Issues related to design parameters 1.1 Controversies regarding the design pressure when it is set at “atmospheric pressure”. Design pressure refers to the maximum pressure at the top of the equipment (usually expressed as gauge pressure), and it, together with the design temperature, serves as the design load conditions. It is not appropriate to set the design pressure at “atmospheric pressure” or ATM in some container conditions, as this can lead to misunderstandings. Since the definitions of \"atmospheric pressure vessels\" in the commonly used standards for the fields of process engineering and equipment engineering are not consistent, the understanding of such terms among designers in these two fields usually also differs. In HG/T 20570-95 \"Technical Specifications for the Design of Process System Engineering\", a \"atmospheric pressure vessel\" refers to equipment with a design pressure within the range of -2 kPa (inclusive) to 0.1 MPa. In HG/T 20580-2020 \"Basic Code for the Design of Steel Chemical Vessels\" (draft for approval), a \"atmospheric pressure vessel\" refers to a vessel that is in direct connection with the ambient atmosphere or has a working pressure (gauge pressure) of zero. 1.2 The conditions should specify the components and concentrations of the medium. The main materials for the equipment are determined primarily based on the information provided in the process package or engineering experience; the equipment specialist is responsible for confirming this, and if necessary, additional technical requirements regarding materials, manufacturing, and inspection should be specified in the design documents. The corrosivity of the medium is one of the main factors affecting the selection of equipment; raw materials, intermediate products, impurities, etc., in the production process must all be taken into consideration. Therefore, the conditions should specify all components of the medium along with their concentrations. For example, the medium in a certain ammonia tank is liquid anhydrous ammonia, but even for top-quality products, relevant standards permit no more than 0.1% moisture content. When liquid ammonia at a temperature higher than -5°C is used, in conditions where the water content is no more than 0.2% and air pollution is a possibility, there is a tendency for stress corrosion in carbon steel or low-alloy steel; the equipment may suffer from sudden, stress-induced brittle fracture without any prior signs, posing a significant risk. Therefore, a series of special requirements must be taken into account during design. A lack of understanding in this regard, assuming that 99.9% of ammonia is a pure medium and that trace amounts of water at ≤0.1% do not need to be considered, and failing to specify the components and concentrations of the medium in the design parameters, can lead to improper design. 1.3 Determination of nominal diameter and height-to-diameter ratio: On the basis of meeting the requirements of chemical processes and equipment layout, the diameters of custom-made containers and heat exchangers should, as much as possible, be selected in accordance with GB/T 9019-2015 \"Nominal Diameter of Pressure Vessels\". This facilitates the use of standard-sized components such as end caps, container flanges (and gaskets), supports (excluding skirt supports), and other accessories during design, eliminating the need for additional design and calculation efforts. This not only reduces the workload but also ensures design quality. 1.4 Determination of the maximum liquid level in storage tanks: In the design criteria for petrochemical storage tanks, the process engineering team typically determines the maximum liquid level of the tanks in accordance with SH/T3007-2014, the \"Design Code for Tank Areas in Petrochemical Storage and Transportation Systems\", and uses this value to determine the required height of the tank walls. Regarding the difference between the wall height of commonly used fixed-roof storage tanks and the height of the highest liquid level, SH/T 3007 takes into account the distance between the fire foam generator and the tank roof, the thickness of the foam mixture layer, the height of liquid expansion, and the height equivalent to the maximum liquid inflow rate of the tank over 10–15 minutes; however, it does not consider the amplitude of seismic shaking waves. In fact, seismic design requirements for oil tanks stipulate that the distance from the design liquid level to the top edge of the tank wall should be greater than the wave height of liquid surface fluctuations, in order to prevent the tank roof from being subjected to impacts. Of course, the amplitude of the oscillating wave is related to factors such as the location where the tank is built and the diameter of the storage tank; the exact value needs to be determined through calculation. II. Issues related to equipment pipe connections 2.1 Pipe flange standards The pipe connections on equipment are mainly divided into process and instrumentation types; in design, there is often a need to match pipes using different standard flanges. The equipment engineering field typically uses the HG series, while the piping engineering field prefers the SH series; the instrumentation engineering field, on the other hand, mainly uses ASME B16.5 flanges. In fact, when the nominal diameter DN and pressure class PN of the flanges are identical, whether flanges from different standards can be paired depends mainly on whether factors such as the diameter of the bolt center circle, bolt specifications and quantity, bolt hole diameter, type and size of the sealing surface, and gasket size are consistent. Therefore, the design specifications should specify the standard number of the flanges for equipment nozzles, DN, PN and their type, as well as the type of sealing surface. 2.2 Circular arrangement of flange bolts at the top outlet: For the valve and instrument mounting ports at the top of the equipment, the “centering” or “mid-span” arrangement of the flange bolt circle (with respect to the north-south or east-west axes) should take into account the requirements related to piping and operation, such as the direction of the pipes connected to the safety valves and the convenience of operating the valve stems; this aspect is often overlooked. 2.3 Break-siphon opening in the liquid inlet extension tube: If it is necessary to provide a small hole at an appropriate position in the liquid feed inlet extension tube for breaking siphoning, the process engineering team should specify this in the equipment specifications; if no such requirement is given in those specifications, the equipment engineering team should not create such a hole. Generally, the protruding tubes at the inlet and outlet do not require such a siphon break. The process engineering department should pay attention to checking this issue during the equipment drawing review. 2.4 The connections between the device nozzles and pipes are made without the use of flanges. To ensure a good seal, the nozzles of high-temperature and high-pressure devices, as well as those of large-diameter pipes on ordinary devices, are generally connected to pipes by welding rather than using flanges. This is because flange joints used for high temperatures have high manufacturing costs and their sealing performance is not entirely satisfactory. For such pipe ends, the specifications of the pipes to be connected shall be specified in the design requirements, with alignment of the inner or outer diameter required, and the pipe ends of the equipment pipe ends to have welding grooves. III. Problems with shell-and-tube heat exchangers 3.1 Heat transfer area The heat transfer area of a heat exchanger refers to the external surface area of the heat transfer tubes involved in the heat exchange process; lengths that extend into the tube sheet and are not involved in heat exchange should be deducted when calculating this value, a point that is often overlooked in the design specifications. The thickness of a single tube sheet in common medium and low pressure shell-and-tube heat exchangers is usually between 50 and 200 mm. 3.2 Metal wall temperature: The metal wall temperature on the shell side and tube side of a shell-and-tube heat exchanger refers to the average temperature of the metal along the axial length of the shell tube and the heat exchange tubes, respectively. It has a significant impact on the axial forces acting on the shell tube and heat exchange tubes, as well as on the pulling forces between the heat exchange tubes and the tube sheet. It is an important parameter in the strength calculation of heat exchangers, directly affecting the reliability and cost-effectiveness of the design; therefore, its exact value must be specified in the design parameters and cannot be estimated arbitrarily. 3.3 Tube bank anti-erosion plates: GB/T151-2014 \"Heat Exchangers\" specifies several situations in which anti-erosion plates must be installed at the inlet of the shell side of heat exchangers. This is done primarily to prevent the fluid from causing erosion of the tube bank or inducing vibrations. The area of the anti-impact plate must not be smaller than the projected area of the inlet channel, and in order to ensure adequate flow area, the anti-impact plate needs to be at a certain distance from the inner wall of the shell. If the diameter of the inlet pipe is large, the use of anti-impact plates will severely limit the number of pipes that can be installed. This should be taken into account during the heat exchanger design calculations, and if necessary, the nominal diameter of the heat exchanger should be increased. 3.4 Arrangement of heat exchange tubes: When determining the arrangement of heat exchange tubes, the flow direction of the medium should be that in which the fluid flows around the first baffle, that is, as specified in GB/T 151 where the flow direction is perpendicular to the gaps in the baffles; it is inaccurate to determine it solely based on the direction of the shell-side inlet. Taking a horizontal heat exchanger as an example, if only the relationship with the inlet direction is considered, then whether the baffle notches are arranged horizontally or vertically has no impact on determining the arrangement pattern, which is clearly not in line with reality. 3.5 Vent ports: Vent and drain ports should be provided at the highest and lowest points of the heat exchanger’s shell side to prevent the formation of dead zones that could lead to residue of the fluid; this aspect is sometimes overlooked in the design specifications as well. For vertical fixed-tube-sheet heat exchangers, if the tube sheet is thick, the drain port is usually provided on the tube sheet; if the tube sheet is thin, the drain port should be located close to the tube sheet. For horizontal fixed-tube-sheet heat exchangers, there should be drain ports at the highest and lowest points on both the left and right sides; the inlet and outlet of the shell side can also serve as drain ports.