Vessel design corrosion allowance

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 vessel design pressure shall be based on the wall thickness of the vessel, neglecting any allowance for corrosion or erosion. 

The national pressure vessel codes and standards require that all pressure vessels be subjected to a pressure test to prove the integrity of the finished vessel. A hydraulic test is normally carried out, but a pneumatic test can be substituted under circumstances where the use of a liquid for testing is not practical. Hydraulic tests are safer because only a small amount of energy is stored in the compressed liquid. A standard pressure test is used when the required thickness of the vessel parts can be calculated in accordance with the particular code or standard. The vessel is tested at a pressure above the design pressure, typically 25 to 30 per cent. The test pressure is adjusted to allow for the difference in strength of the vessel material at the test temperature compared with the design temperature, and for any corrosion allowance. 

The thickness t is a function of the vessel diameter, allowable steel stress, pressure rating of the vessel, and a corrosion allowance. For example, a design pressure 6f 250 psi and a corrosion allowance of in. give the following formula for t in inches (in which D is expressed in feet) ... 

Corrosion rates are expressed in terms of inches per year of surface wastage and are used to provide a corrosion allowance in the design thickness of equipment such as vessels and pipes. Operators will often use data based on historical experience from plant operations to aid them in determining appropriate corrosion allowances. Alternatively, corrosion charts are widely available that give corrosion rates for many combinations of materials of construction and process fluids, and normally a range of values will be provided for various process temperatures. In some instances, particularly where there is a mixture of chemicals present, appropriate data may not exist and corrosion tests may be necessary in order to determine the suitability of equipment. Operators should be able to demonstrate the use of corrosion allowances in equipment specification and design. The sources of data used should be traceable. 

Process design of vessels establishes the pressure and temperature ratings, the length and diameter of the shell, the sizes and locations of nozzles and other openings, all internals, and possibly the material of construction and corrosion allowances. This information must be supplemented with many mechanical details before fabrication can proceed, notably wall thicknesses. 

Good design practices to ensure that the mechanical design of the equipment is suited to normal operation, startup, shutdown, and emergency conditions (for example, ensuring that vessel wall thickness, design temperature and pressure, corrosion allowance, etc., adhere to industry or national codes and standards) ... 

Pressure vessels are subject to thinning by corrosion, erosion, or mechanical abrasion. To increase the desired useful life of the vessel, the design should include a suitable increase in wall thickness over the minimum design thickness required for safe pressure containment. In most cases, there is no specific code requirement for how much corrosion allowance a vessel requires. Vessels subject to corrosion should have provisions for complete draining as well as openings to allow for the inspection of internal surfaces. 

Corrosion is generally considered to be in one of two forms uniform or localized. Uniform corrosion occurs evenly on all surface areas of a pressure vessel. Such corrosion occurring at slow rates is quite typical and can be planned for by increasing the original wall thickness design. For vessel systems with a predictable corrosion rate, allowances can be calculated for vessel life spans of 2-20 yr. ... 

Operating pressure and temperature constrain practical size of vessel. Design codes for pressure vessels vary slightly with the country. In general, for operating pressures >10 MPa, vessel volume usually <1 m Pressure decreases as temperatures exceed 250°C. For temperatures above 350°C, consider carbon/molybdenum, and for temperatures >500°C, consider austenitic steels. Corrosion allowance 1.5 mm for corrosion rates 0.08 mm/a 3 mm for rates 0.09 to 0.3 nun/a 4.5 nun for 0.31 to 0.4 mm/a 6 mm for >0.4 mm/a. If pressure <400 kPa, use L/D of 2 to 3 1. For pressures >400 kPa, use L/D of 4 to 5 1. For surge, allow 2-min liquid residence time for draw-off, use 15 min for reflux, use 5 min, provided this allows sufficient time for controllers to function. Total volume = 1.3 X holdup if the holdup volume is >3 m ... 

Designing by the rules of a code will produce a vessel that can be expected to withstand safely the combinations of pressure and temperature for which the vessel has been designed — nothing more, nothing less. Additional requirements are beyond the scope of the code and must be specified by or for the purchaser. Most vessels for refinery and chemical plant service do require something more, even if it is only corrosion allowance. 

MAWP for a vessel is the maximum permissible pressure at the top of the vessel in its normal operating position at a specific temperature, usually the design temperature. When calculated, the MAWP should be stamped on the nameplate. The MAWP is the maximum pre.ssure allowable in the hot and corroded condition. It is the least of the values calculated for the MAWP of any of the essential parts of the vessel, and adjusted for any difference in static head that may exi.st between the part considered and the top of the vessel. This pressure is based on calculations for every element of the vessel using nominal thicknesses exclusive of corrosion allowance. It is the basis for establishing the set pressures of any pressure-relieving devices protecting the vessel. The design pressure may be substituted if the MAWP is not calculated. 

The MAWP for any vessel part is the maximum internal or external pressure, including any static head, together with the effect of any combination of loadings listed in UG-22 which are likely to occur, exclusive of corrosion allowance at the designated coincident operating temperature. The MAWP for the vessel will be governed by the MAWP of the weakest part. 

Pamphlet 78 calls for the same sort of fabrication techniques recommended for pressurized storage. It also gives instructions for commissioning, inspection, and maintenance. It covers single-wall as well as double-wall tanks and horizontal as well as vertical or spherical tanks. The minimum design pressure for a single-wall tank should be about 175 kPa (25 psig). The corrosion allowance for the internal tank and connections usually is about 2 mm this should also apply to the external vessel when one of its functions is containment. 

Z. The corrosion allowance is based on the expected annual corrosion rate for the vessel material In the anticipated process environment. That corrosion rate is multiplied by the nominal design life of the vessel (usually 20 years) to determine the total lifetime corrosion allowance. Corrosion allowance is an application-specific judgment call. 

Physical models of commercial fluidized bed equipment provide an important source of design information for process development. A physical model of a commercial fluidized bed processor provides a small-scale simulation of the fluid dynamics of a commercial process. While commercial processes will typically operate at conditions making direct observation of bed fluid dynamics difficult (high temperature, high pressure, corrosive environment), a physical model is designed to allow easy observation (room temperature and pressure, nonreactive atmosphere, transparent vessel). 

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