Vessel design

Ship-shaped FPSOs must be designed to weather vane i.e. must have the ability to rotate in the direction of wind or current. This requires complex mooring systems and the connections with the well heads must be able to accommodate the movement. The mooring systems can be via a single buoy or, in newer vessels designed for the harsh environments of the North Sea, via an internal or external turret. Figure 10.33 shows a schematic of the Shell-BP Foinaven FPSO. 

Fig. 9. Yield pressure of multicomponent vessels designed for optimum conditions (34).

One aspect of pressure vessel design which has received considerable attention in recent years is the design of threaded closures where, due to the high stress concentration at the root of the first active thread, a fatigue crack may quickly initiate and propagate in the radial—circumferential plane. Stress intensity factors for this type of crack are difficult to compute (112,113), and more geometries need to be examined before the factors can be used with confidence. 

H. H. Bednar, Pressure Vessel Design Handbook 2nd ed.. Van Nostrand Reinhold Co. Inc., New York, 1986. 

The vessel design features a Chinese hat-like conical core stopper above the underflow sump, which is there to prevent the vortex from reaching the latter and reentraining the settled soHds. The core stopper is also beheved to stabilize and locate the vortex flow in the vessel. Overflow from the vessel is through a wide cylindrical insert through the Hd, similar to a vortex finder in a hydrocyclone (16), and an optional provision can be made for collecting any floatables in a float trap. 

Throughout the year the Welding Research Council publishes bulletins which are final reports from projects sponsored by the council, important papers presented before engineering societies, and other reports of current interest which are not published in Welding Research. A large number of the published bulletins are pertinent for vessel designers. 

Fluidization Vessel The most common shape is a vertical cylinder. Just as for a vessel designed for boiling a liquid, space must be provided for vertical expansion of the solids and for disengaging... 

United States Filter Corp. Maxi-Flo Filter. The Maxi-Flo Filter is an example of the upflow closed-vessel design. Filtration rates to 0.0081 mV(m--s) [12 gaJ/(ft- min)] and filter cross-section areas up to 10.5 m" (113 ft") are possible. Deep-bed filtration has been reviewed... 

Dyna Whirlpool A unique vessel design for capacities up to 100 t/h has been developed by the American Zinc Co. The separation occurs in a cyhndrical-shaped separatory vessel maintained in an in-chned position from horizontal. This system, known as the Dyna Whirlpool (DWT) process, provides for separate entiy of the medium and the feed sohds, as illustrated in Fig. 19-37. A distinct feature of this separator is that the feed enters the separator via gravity flow. Feed size may range from 0.05 to 0.0002 m (2 in to 65 mesh). Magnetite or ferrosilicon is generally used. 

Until Jenike developed the rationale for storage-vessel design, a common criterion was to measure the angle of repose, use this value as the hopper angle, and then fit the bin to whatever space was available. Too often, bins were designed from an architectural or structural-engineering viewpoint rather than from the role they were to play in a process. Economy of space is certainly one vahd criterion in bin design, but others must be considered equally as well. Table 21-14 compares the principal characteristics of mass-flow and funnel-flow bins. 

For vaporAiquid separators there is often a liquid residence (holdup) time required for process surge. Tables 1, 2, and 3 give various rules of thumb for approximate work. The vessel design method in this chapter under the Vapor/Liquid Calculation Method heading blends the required liquid surge with the required vapor space to obtain the total separator volume. Finally, a check is made to see if the provided liquid surge allow s time for any entrained water to settle. 

The process engineer gets involved in many mechanical aspects of vessel design such as thickness, corrosion allowance, and internals. Here are some pitfalls to watch for along the way ... 

The liquid fuel handling components of the fuel system include the fuel filler pipe, fuel tank, fuel pirmp, and the fuel supply and return lines. The fuel tank is a low pressure, low hydrocarbon emission vessel designed to contain both the liquid and vapor phases of the fuel. An electric pump located inside the fuel tank is used to transfer liquid fuel from the tank to the engine. The fuel in the tank is suctioned from a small reservoir in the tank which minimizes liquid level transients caused by vehicle motion. 

Proper vessel designs should avoid discharges from high-positioned coolers directed down a pipe. This situation reduces pressure in coolers via siphon action. 

As the operating pressure rises, the resulting force on the valve disc increases, opposing the spring force, until at the set pressure (normally adjusted to equal the vessel design pressure) the forces on the disc are balanced and the disc starts to lift. 

Fig ure 10-14. Images generated during CFD help assess the desirability of proposed vessel-design modifioations. 

Although many engineers provide only the minimum adequate vessel design to minimize costs, it is inherently safer to minimize the use of safety interlocks and administrative controls by designing robust equipment. Passive hardware devices can be substituted for active control systems. For example, if the design pressure of the vessel system is higher than the maximum expected pressure, an interlock to trip the system on high pressure or temperatures may be unnecessary. 

Process flow diagrams are more complex and show all main flow streams including valves to enhance the understanding of the process as well as pressures and temperatures on all feed and product lines within all major vessels and in and out of headers and heat exchangers, and points of pressure and temperature control. Also, information on construction materials, pump capacities and pressure heads, compressor horsepower, and vessel design pressures and temperatures are shown when necessary for clarity. In addition, process flow diagrams usually show major components of control loops along with key utilities. 

A pressure vessel designer, on the basis of test, inspection, and experience, says that a vessel has a failure rate less than 1E-6A Y, Experience has already demonstrated 9.3E5VY without failure. What is the estimated failure rate for 95% confidence ... 

As the result of a heater failure, the temperature of the vessel, designed for -29°C, fell to -60°C by evaporative cooling (see Section 10.5.2) at this temperature carbon steel becomes brittle, and cracking may have started. 

Many suppliers of sulfuric acid recommend that it is stored in pressure vessels designed to withstand a gauge pressure of 30 psi (2 bar). The acid is usually discharged from tank trucks by compressed air, and if the vent is ehoked the vessel could be subjected to the full pressure of the compressed air. 

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