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DESIGN AGAINST CORROSION

IP.com Disclosure Number: IPCOM000221418D
Publication Date: 2012-Sep-07

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Page 01 of 21

AGAINST CO RR0 S 10N

By J F LANCASTER

         On the 22nd of June, 1974, a carbon steel bend in the CO2 removal unit of an ammonia plant in Madras, India, burst, projecting a jet of hot potassium carbonate solution against the window of the control room. The window failed and~ as a result, the potassium carbonate solution inundated the control room, causing a number of fatalities.

         Investigation of this tragic accident showed that the carbon ateel bend had been placed immediately downstream of a stsinless steel control valve(1). For economy, the contr~l valve was made ~o pipe sizes smaller than the piping, so that, when it was open, a jet of carbonate solution impinged on. the outer radius of the bend. Carbon steel is normally a satisfactory material of construction for inhibited potassium carbonate solution, provided that the velocity
is kept below prescribed limits. In'this instance the velocity
was excessive, due to the constriction of the valve, and the affected area corroded to paper-thlckness in a relatively short period of
time.


Page 02 of 21

2

         This is a clear instance where a deslgu error led to a
disastrous failure. The control valve should have been positioned
in a straight piece of pipe or, alternatively, the bend should have been specified to be stainless steel(2). Either way, the organlsation of a piping design group should be such as to pick up errors of this type and rectify them at an early stage of the design. Fortunately
it is rare for a corrosion failure to have such a disastrous effect, because no organisatlon is perfect and no piping system is entirely without faults. Moreover, only ~proportlon of corrosion failures are associated with deficiencies in design. However, experience indicates that a significant number are so caused, and that proper design is important if corrosion problems are to be avoided.

          In the hydrocarbon processing industry there are two main types of corrosion problems ~here the influence of design must be taken into account. The first is the type of attack that caused the failure in the CO2 removal plant instanced above and which is associated with fluid flow. The second is stress-corrgsion, cracklng, particularly under those conditions where the design is such as to permit concentration of a stress-cracking agent at the met@l surface. In other industries other forms of corrosion may be equally important; for example, in the food processing "industry the crevice corrosion of .stainless steel is a potential source of failure and is clearly .influenced by details of design. In the present article, however,
we shall be concerned only with the first two categories.


Page 03 of 21

Corrosion associated with Fluid Flow :
Cavitation and Impingement

         In reviewing corrosion due to the flow of liquids,
we may distinguish between three flow regimes, all of which may remove metal from the surface but which act in different ways. t

The first is a region of very high fluid velocity in which impingement
or ea~itation erosio...