ABSTRACT

Previous tests showed type 316 stainless compression fittings can crack when exposed to NACE TMO177 solution at elevated temperatures for 30 days. Following up this finding, one-month, three-month and twelve month tests have been conducted on type 316 compression fittings and instrumentation tubing in order to assess their suitability for sour service. Loops were constructed, containing a total of over three hundred fittings from four manufacturers and filled with sour fluids comprising high chloride content solutions. They were pressurised to 50 bara (5MPa) with CO2 and H2S partial pressures in the range 0-0.8 bara (8kPa) and maintained at temperatures in the range 60- 120ºC . The tests showed that even in mildly sour conditions, at elevated service temperatures cracking can occur and that the threshold H2S partial pressure decreases with increasing temperature.

INTRODUCTION

Compression fittings are widely used in pressure and vacuum systems in a number of industries including chemical processing, nuclear power and oil and gas production. Of the fitting designs available, those based on twin ferrule arrangement are most popular in the oil and gas industry. Fittings typically are double ended, when used to join tubes (of the same or different size) or have a tube end and a specialised fitting for connecting to pressure gauges, manifolds etc.

In making up a joint the nut is tightened, which causes the front ferrule and then the rear ferrule to bite, skive or swage into the tube this is how the pressure rating is achieved. The tube end deforms and barrels out ahead of the front ferrule, to an extent governed by its wall thickness. Sealing can occur at a number of locations, depending on the design (the various manufacturers claim unique design aspects) and the quality of workmanship. If the tube is cut neatly end the joint made up carefully, sealing can be achieved between the cut end of the tube and the internal shoulder in the fitting. However, at least two leading manufacturers consider that the internal process fluid will contact the tip and front face of the front ferrule. In order to function, certain components including the front ferrule must be harder than the tubing. In the 316 austenitic stainless steel system it is recommended that the tubing be fully annealed (80 HRB max). The ferrules are much harder, typically in the range 30-47 HRC.

Prior to 1990 NACE MRO175 did not address the issue of compression fittings. Type 316 stainless steel was listed with a hardness limitation of 22 HRC for sour service. In the North Sea, as reservoir souring occurred, a number of operators with facilities designed for sweet service recognised that upgrading was required. Among these facilities was the Ninian field. Since 316 compression Wings had components with hardness well in excess of the 22 HRC, an attempt was made to clarify the meaning of MRO175, resulting in the addition of paragraph 8.4.1.1. Simultaneously, work undertaken by Cortest demonstrated that such fittings are susceptible to cracking in severe.

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