Erosion, Cavitation, and Abrasion Resistance of Choke Trim Materials

Abstract

ABSTRACT An experimental investigation was performed to determine the relative erosion, abrasion and cavitation resistance characteristics of selected materials. Testing was conducted under controlled laboratory conditions to simulate service conditions encountered in production and injection chokes. The testing effort is ongoing. The data accumulated allow informed material selection of conventional and novel trim for all chokes, valves, flow metering orifices, fixed beans and other devices used during drilling, completion and production of offshore and onshore oil and gas wells. Sintered silicon carbide and tungsten carbide with minimum binder content were the most erosion and abrasion resistant of the materials tested. Cobalt base alloys bar and nickel chrome alloy 625 bar proved to be most cavitation resistant. INTRODUCTION Wear of wellhead chokes and other pressure reducing devices is a serious and common problem. Erosion, abrasion, and' cavitation are three types of wear, which is defined 1 as the volume of material removed. As the choke is adjusted to throttle fluid flow, a series of localized, time dependent and very complex flow fields are established in the region of the throttling point, such as the plug/seat gap in a globe valve. Each flow field is a function of the valve position, fluid phase and properties, pressure drop across the choke and ratio of pressure drop to upstream pressures. Choke trim design parameters, such as trim contours and clearances also greatly affect the flow field. The nature of the flow field in the gas choke is of particular concern when erosion of the trim material is considered. When the ratio of the pressure drop to the upstream pressure becomes critical, the velocity at the choke gap is sonic. Small fluid borne particles move with a velocity approaching the mean velocity of the stream, and are influenced, depending on size, shape and density, by the large scale turbulence of the stream. The high velocity conditions at the choke throat and' along the downstream trim are therefore particularly conducive to high rates of erosion. Tests were performed on several materials to determine the wear (volume loss) rates in accordance with special erosion procedure. Abrasion wear is associated with surfaces sliding along one another, and is greatly accelerated by fluid borne particulates. Those particulates with sharp corners, such as sand particles so often found in natural gas and crude oil are especially effective in promoting material removal when the choke is being adjusted frequently. Other factors influencing abrasion wear include fluid lubricity, clearances between sliding parts, particle hardness, forces on component parts, and hardness and surface finish of the component parts. Low viscosity fluids, such as gas, provide poor lubricating qualities and negligible protection against abrasive wear.