Loads on pressure vessels

Moody, Fredrick, Predicting Thermal-Hydraulic Loads on Pressure Vessels, ASME Continuing Education Course Notes PD382. American Society of Mechanical Engineers,... 

Any horizontal force imposed on the vessel by ancillary equipment, the line of thrust of which does not pass through the centre line of the vessel, will produce a torque on the vessel. Such loads can arise through wind pressure on piping and other attachments. However, the torque will normally be small and usually can be disregarded. The pipe work and the connections for any ancillary equipment will be designed so as not to impose a significant load on the vessel. 

Proper installation of membrane modules into a pressure vessel is critical. The membrane modules are guided into the pressure vessel in series. Membranes should be loaded into pressure vessel in the direction of flow. That is, the concentrate end of the module (the end without the brine seal) is inserted first into the pressure vessel. The brine seal and O-rings on the module inter-connectors can be... 

VALVE, POP - A spring loaded safety valve that opens automatically when pressure exceeds the limits for which the valve is set. It is used a safety device on pressurized vessels and other equipment to prevent damage from excessive pressure, also called relief valve or a safety valve. 

We have now established four modes of failure in lifting lugs as validated in laboratory tests. We now will consider the lugs, and later trunnions, on pressure vessels and stacks exposed to various loads. Consider the schematic in Figure 7.12. Refer to Eq. 1.2 and Eq. 1.3, where the unit of mass, Kg, is converted to the unit of force, N, by Newton s second law. 

The thermographic activity on the pressure vessel was carried out considering a part of it because of the axial symmetry. Three different partially overlapping area were inspected since it was optically impossible to scan the curved surface of the pressure vessel by a single sweep. The selected areas are shown in fig.7 and the correspondent positions of the thermographic scan unit are also illustrated. The tests were performed with a load frequency of 2, 5 and 10 Hz. 

Probably the largest compound vessels built were two triple-wall vessels, each having a bore diameter of 782 mm and a length of 3048 mm designed for a pressure of 207 MPa (30,000 psi). These vessels were used by Union Carbide Co. for isostatic compaction unfortunately the first failed at the root of the internal thread of the outer component which was required to withstand the end load (40). A disadvantage of compound shrinkage is that, unless the vessel is sealed under open-end conditions, the end load on the closures has to be resisted by one of the components, which means that the axial stress in that component is high. 

HoUow-fiber membranes, therefore, may be divided into two categories (/) open hoUow fibers (Eigs. 2a and 2b) where a gas or Hquid permeates across the fiber waU, while flow of the lumen medium gas or Hquid is not restricted, and (2) loaded fibers (Eig. 2c) where the lumen is flUed with an immobilized soHd, Hquid, or gas. The open hoUow fiber has two basic geometries the first is a loop of fiber or a closed bundle contained ia a pressurized vessel. Gas or Hquid passes through the smaU diameter fiber waU and exits via the open fiber ends. In the second type, fibers are open at both ends. The feed fluid can be circulated on the inside or outside of the relatively large diameter fibers. These so-caUed large capiUary (spaghetti) fibers are used in microfUtration, ultrafUtration (qv), pervaporation, and some low pressure (<1035 kPa = 10 atm) gas appHcations. 

In the case of a pressure vessel subjected to cyclic loading (as here) cracks can grow by fatigue and a vessel initially passed as safe may subsequently become unsafe due to this crack growth. The probable extent of crack growth can be determined by making fatigue tests on pre-cracked pieces of steel of the same type as that used in the pressure vessel, and the safe vessel lifetime can be estimated by the method illustrated in Case Studv 3. 

Pilot-operated valves have a pilot valve combined with the main valve. The spring of the main valve provides 75 percent loading on the disk and the remaining 25 percent is offered by the gas or vapor through the pilot valve. When the vessel reaches the maximum allowable working pressure, the pilot valve relieves the gas pressure, which contributes to the disk load. Thus the safety valve becomes wide open. This is illustrated in Figure 18. 

A cylindrical polypropylene pressure vessel of 150 mm outside diameter is to be pressurised to 0.5 MN/m for 6 hours each day for a projected service life of 1 year. If the material can be described by an equation of the form s(t) = At" where A and n are constants and the maximum strain in the material is not to exceed 1.5% estimate a suitable wall thickness for the vessel on the assumption that it is loaded for 6 hours and unloaded for 18 hours each day. Estimate the material saved compared with a design in which it is assumed that the pressure is constant at 0.5 MN/m throughout the service life. The creep curves in Fig. 2.5 may be used. 

Under several possible conditions of malfunction, high pressures can occur in parts of the system and mechanical relief devices are advised or mandatory. The standard form of relief valve is a spring-loaded plunger valve. No shut-off valve is permitted between the relief valve and the vessel it protects, unless two such valves are fitted, when the shut-off may isolate one at a time [13]. Two valves are required on a vessel greater than 285 litres in volume. 

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