Velocity Limits for Erosion-Corrosion

ABSTRACT Erosion-corrosion is a form of metal degradation which can rapidly destroy down hole tubing and other equipment. API Recommended Practice 14E contains recommended fluid velocity limits for process piping on offshore platforms. In recent years the 1imiting velocity recommendations of API RP 14E have been questioned by a number of operators. This report summarizes the experience of several operating companies including those which use RP 14E to set velocity 1imits and those which question the advisability of the RP 14E velocity limits. INTRODUCT ION Erosion-corrosion can be defined as accelerated corrosion due to relative motion between a metal surface and an aggressive environment. For systems where mechanical damage occurs at a fast rate, the importance of corrosion is relatively diminished.1,2 Conversely, chemical reactions predominate in situations where failure is less rapid. Erosion test is frequently conducted in minutes or hours3, while corrosion exposures often take weeks or longer. While the idea of erosion-corrosion has been generally recognized in the corrosion control community since the 1940's, important questions still exist as to the chemical versus mechanical 4 aspects of this phenomenon. Ellison and Wen4–5 proposed the following classifications of the effects of velocity on corrosion:Convective Mass Transfer Controlled Corrosion: A slightly soluble species near the metal surface is rate controlling. Arriva1 or removal of the species from the metal surface determines the corrosion rate.Phase Transport Controlled Corrosion: In multiphase 1iquid flow, the presence or absence of the more corrosive liquid controls the corrosion rate. Corrosion-Erosion: Corrosion rates are determined by mechanical erosion of a protective corrosion product film.Erosion: r4echanical damage occurs so fast that no chemical component of the material damage rate can be measured. All of the above mechanisms are possible in oil and gas wells. API Recommended Practice 14E Briefly stated, API RP 14E6 suggests the following formula for limiting erosion:(Mathematical equation available in full paper) The standard provides a formula for calculating the average density of a fluid and provides a chart which shows 1imiting velocities for typical Gulf Coast conditions. It also cautions that a minimum velocity of 10 ft/sec is recommended to prevent slugging. The guidelines contained in RP 14E were adapted from petroleum refinery practice. Velocities much above these defined by RP 14E are seldom encountered in refinery practice, because pumping requirements become prohibitively expensive at high flow velocities. The recent development of deep, hot gas wells, where no pumping is necessary to obtain high pressures and fluid velocities, has caused a number of operators to question the advisability of following RP 14E recommended Limiting fluid velocities to those recommended by RP 14E means reduced production rates and could affect the economic viability of some wells. These considerations are especially important offshore, where the capital costs of field development are higher and the incentives for quick return of investment are higher.