ABSTRACT

In this study, the effect of turbulence on corrosion inhibitor performance was investigated with a rotating cage autoclave (RCA) in order to select a corrosion inhibitor for a Japanese field piping. The maximum rotating speed is 2,000 rpm, corresponding to 8 m/s in circumferential speed of 80 mm diameter of the cage. The capacity of the autoclave is 2.3 L. The flow pattern was observed through a transparent acrylic resin container. As the rotating speed became higher, a vortex appeared clearly on the free surface of test solution. The bottom of vortex reached the top of the rotating cage at 1,000 rpm and air bubbles were dragged into the cage. Computational fluid dynamics (CFD) modeling revealed the same phenomena of vortex and bubbles. The maximum wall shear stress (WSS) was estimated at 180 Pa in CFD modeling at 2,000 rpm. The WSS range of this RCA covered the actual field piping. Three brands of oil-soluble water-dispersible inhibitors were evaluated with the RCA. Some inhibitors showed enough efficiency at high rotating speed but the others require more dosage to maintain the same result.

INTRODUCTION

Corrosion inhibitors are commonly used to mitigate corrosion in oil and gas pipelines. The selection of inhibitor for a particular field depends on the field conditions (water cut, water chemistry, temperature, flow rate etc.). In order to evaluate inhibitors recommended by manufacturers, several laboratory methodologies are adopted in oil and gas industry companies. A rotating cage autoclave (RCA) is one of the options which are listed in ASTM G170-061. RCA seems more simple to implement than other methodologies such as flow loop, rotating cylinder electrode, jet impingement and so on. RCA is suitable to evaluate the effect of wall shear stress (WSS) rather than to reproduce the flow regimes of actual piping. The flow pattern in RCA has been studied using Computational Fluid Dynamic (CFD) calculation and the WSS range on test coupons are revealed2,3. Vortexes are observed in RCA at higher rotating speed but the influence has not been discussed sufficiently.

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