Clad Steel Pipe for Corrosive Gas Transportation

ABSTRACT This paper describes the applicability and reliability Of clad steel pipe and its welds in sour gas environments in comparison with those of 22%Cr-5.5%Ni- 3%M0-O.15%N duplex stainless steel (ASTM A240 UNS31803) solid pipe. The example of practical applications of clad steel pipe for corrosive gas transportation is also presented. INTRODUCTION Clad steel pipe has been increasingly used for naturalgas production line in sour gas environments. In 1993, American Petroleum Institute issued the first edition of the specification for CRA clad or lined steel pipe as Specification 5LD. This will stimulates the use of clad pipe for wider range of applications. Clad steel pipe is carbon or low alloy steel pipe internally clad with a corrosion resistant alloy (CRA) to provide effective and economic utilization of CRA'S. The relatively thin cladding layer which will be in contact with the corrosive fluids is made of the CRA, while the less expensive backing steel provides the strength and toughness required to maintain mechanical integrity. In this paper, the applicability and reliability of the clad steel pipe and its welds are described in comparison with those of 22%Cr duplex stainless steel (ASTM A240 UNS31803) solid pipe. In addition, the experiences of practical applications of clad steel pipe for corrosive gas transportation is briefly introduced. MECHANICAL PROPERTIES One of the advantages of clad steel pipe over high alloy solid pipe is its higher yield strength, while duplex stainless steels also exhibit relatively high strength. Fig. 1 compares the tensile properties of an API 5L-X60 grade steel at elevated temperatures up to 200ºC with those of the typical duplex stainless steel, of which chemical composition is low carbon 22%Cr-5.5%Ni-3%Mo-0 15%N conforming to ASTM A240 UNS31803. The duplex stainless steel shows higher ultimate tensile strength at all temperatures tested, and higher yield strength below approximately 50ºC. The yield strength of the duplex stainless steel, however, remarkably decreases with an increase in temperature, and at temperatures above 50ºC it is surpassed by that of API 5L-X60 grade steel. In the gathering line of gas and oil wells, the design temperature is not lower than 50ºC, and the design calculation of pipe wall thickness is based on the yield strength. Also considering the possibility to utilize the base material with higher yield strength such as X65 or X70 grade steels, clad steel pipe is more advantageous than solid pipe of duplex stainless steel. In order to clarify whether the cladding layer of clad steel pipe can be treated as a load carrying member, bursting test of type 316L stainless steel clad pipe (diameter 8 inch (203.2mm), thickness; 2.5mm +10.2mm(API 5L-X60)) was undertaken at elevated Temperature The clad pipe contained both longitudinal and circumferential welds. Hot oil at 149ºC was used as a pressurizing medium. The repetition of pressurizing and holding was done in a stepwise manner until the bursting of the clad steel pipe occurred. The bursting originated at the parent material well apart from both longitudinal and circumferential welds of the pipe.