Thick walled cylinders requirements

Smaller quantities of gas at high pressure are usually stored in bottle-shaped gas cylinders. Although convenient to handle, they suffer from the disadvantage that their thick steel walls make them very heavy. Cylinders must be held in position by securing chains and they must not be banged or exposed to sources of heat. Owing to the severe risk in the event of fire, only the minimum number of cylinders required for immediate use should be stored at the plant. 

A thin walled cylinder with closed ends of radius r and wall thickness is fabricated from a polymer with a yield stress in pure shear of k. Calculate the internal pressure required to produce yielding of the cylinder walls if the yield criterion under appropriate conditions of temperature and strain rate may be written as... 

Compound temperature inside the cylinder 200-250 °C To avoid sink marks on very thick-walled components, it may be necessary to work at 170 °C. This gives a very high melt viscosity which requires a high screw torque (risk of overload). 

For the repetitive inspections the required hydrotest can only be performed for a limited number of the small cylinders, and even then the drums have to be removed from the line and the cylinders will be supported in defined distances for the weight of the water and the pressurisation. For the new and long cylinders even this is impossible, because they loose due to the additional weight of the water and the over-pressurisation their roundness and balances. Therefore the law in the most countries within and outside of the EU accept as a replacement of the hydrotest an additional application of different NDT methods, which were often done by an ultrasonic measurement of the wall thickness of the cylindrical part and a MT of the flat covers. 

The minimum wall thickness required to resist the hydrostatic pressure can be calculated from the equations for the membrane stresses in thin cylinders (Section 13.3.4) ... 

The structure of this formula can quickly be related to the thin-walled pressure vessel cylinder equation. Using the equation that calculates the stress at the center of the vessel wall, ux = P R + 0.5t)/t, and rearranging to solve for the thickness, results m. t = PR/ ux — 0.5P. The addition of the weld joint efficiency, E, and changing the coefficient before P to 0.6 results in the ASME code formula, t = PR/ SE — 0.6P), which they feel best represents the minimum wall thickness required to contain an internal pressure, P, in a cylindrical vessel having a radius, R, and made of a material with an allowable stress, S. 

Figure 3 is a detail drawing of the freeze-out coil-and-trap assembly. The coil is wound to a diameter of approximately 1.5 in., using 7 turns of 0.093 in. OD stainless steel tubingwith a wall thickness of 0.008 in. The coil terminates in a trap machined from 3/8 in. stainless steel rod. The trap consists of a "body and a "cap", as shown in the inset in Fig. 3. The cap is silver-brazed to the body. This trap serves as a reservoir for the solids formed in the coil. The volume of the coil-and-trap assembly is slightly less than 10 cm. Liquid helium requirements are kept low by making the cooled portion small and using material of low-thermal conductivity. The two legs of the coil-trap assembly are 0.120 in. OD 304 stainless steel tubing having a 0.013 in. wall thickness. Bellows valves with V-point, are employed in the trap assembly. Use of two traps permits impurity concentrations of duplicate samples from each cylinder at a faster rate than would be otherwise possible. To prepare many concentrated samples, additional coil-and-trap assemblies are helpful. In this laboratory, twelve of these assemblies are used.

Minimum Allowable Wall Thickness—The minimum wall thickness required by DOT cylinder specification. 

Any defect of appreciable depth having a sharp bottom increases the localized stress. Even though a cylinder may be acceptable from the defect depth standpoint, it is common practice to remove such defects. After any defect removal operation, the wall thickness must be measured to verify that the remaining wall thickness meets requirements, and a hydrostatic retest shall be performed. 

As previously stated, considerations for visual inspection of steel cylinders are also applicable to aluminum cylinders. However, some differences exist with acceptance criteria. For example, aluminum cylinders must be condemned when impairment to the surface (corrosion or mechanical defect) exceeds a depth so that the remaining wall is less than three-fourths of the minimum allowable wall thickness required by the specification under which the cylinder was manufactured. 

DOT and TC regulations require the hydrostatic testing of compressed gas cylinders by water jacket or other suitable methods that provide accurate data [1, 2]. The hydrostatic test determines the total expansion and permanent expansion under specified test pressure. These expansion readings determine wall thickness, average wall stress, and level of yield strength of the steel. DOT or TC must approve in writing the type and operation of the test apparatus. 

Minimum Allowable Wall Thickness— Minimum allowable wall thickness is the minimum wall thickness required by the specification under which the cylinder was manufactured. 

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