Guideline for selection of instrument tubing and clamps for offshore and onshore plants Introduction...2 Requirements...2 Definitions...

Transcription

1 Guideline for selection of instrument tubing and clamps for offshore and onshore plants Technical and professional description, TD0101, Final Ver. 1, Valid from Publisher/follow-up: TEK PE D&V, Hans von der Lippe Classification: Open Validity area: UPN/Norway/All value chain/on- and offshore 1 Introduction Objective Warrant Requirements Location and service Material selection Painting and insulation Inspection during operation Definitions References...4 App A Table 1 Material selection for instrument tubing and other components...5

2 1 Introduction 1.1 Objective The objective with this guideline is to give premises for optimised material selections for instrument tubing and to avoid corrosion (i.e. crevice corrosion, etc.) between the instrument tubing and clamps for offshore and onshore installations. It is recommended to use this guideline for new projects and when performing modification projects onshore and offshore Background This document is produced on the bases of experienced crevice corrosion problems on instrument tubing at several installations onshore and offshore (i.e. Sleipner, Troll A, Kollsnes gas plant, Gullfaks), see reference (1). 1.2 Warrant This document is linked to the following documents: Operation, maintenance and modifications (AR12) (Statoil group/all locations/on-and offshore) Norsok M-001 Material Selection (ST061) Norsok M-501. Surface preparation and protective coating (ST018) Norsok M-630 Material data sheets for piping (ST071) Norsok M-650 Qualification of manufacturers of special materials (ST072) Norsok I-001 Field Instrumentation (ST007) Norsok R-004 Piping and equipment insulation (ST078) L-SP-200 Piping and valve specification (TR2000) 2 Requirements 2.1 Location and service Material selection for instrument tubing and other components including instrument clamps for onshore and offshore installations are listed in table 1 in Appendix. The table consists of two main groups, i.e. material selection in saliferious marine atmosphere and in non-saliferious atmosphere including indoor and dry conditions. 2.2 Material selection The material selection shown in table 1 is based on Norsok standard M-001 and field experience at different Statoil installations and should when implemented minimise the risk for corrosion problems as crevice corrosion, CSCC, etc. The general recommendation is to use type 316 stainless steel tubing for indoor service and type 6Mo stainless steel or type 25 Cr duplex in external marine and saliferious environments. All instrument tubing for seawater services shall be in titanium materials. Classification: Open Page 2 of 6

3 The material selection using type 6Mo and type 25 Cr duplex stainless steels should be safe for normal temperature conditions for instrument tubing, but there may be a small risk for initiation of crevices under long periods with wet and chloride containing conditions at high temperatures and between very tight and critical crevices. A more extreme but partly relevant reference is the Avesta Handbook (2) with crevice corrosion test results for type 316 and 6Mo materials in natural seawater at ambient temperature as well as at 70 o C. After 6 months of submerged testing in ambient seawater there was no crevice corrosion on type 6Mo samples, but severe crevice corrosion on all type 316 samples. At 70 o C, one of fifteen samples showed initiation of crevices on type 6Mo samples and all type 316 samples had crevice corrosion. Based on these test results it is concluded that type 6Mo material should be acceptable for use in marine atmospheric conditions and will be a much more robust material selection than the standard type 316 stainless steel. Type 25 Cr duplex material is considered to have same crevice corrosion resistance in marine environments as type 6Mo material. See also Norsok standard M-001. Based on experience in Company different material grades can be used for respectively instrument tubing and for instance manifolds, valves, etc. As crevice corrosion under clamps has been the main corrosion problem the up-grading of tubing material should solve this corrosion problem without a need to up-grade material selection for manifolds, valves, etc. However, temperature restrictions for selected materials shall apply according to table 1. For seawater service titanium shall be used. Components shall be in material grade Hastelloy C-276. Service restrictions for titanium will follow guidelines in Norsok standard M-001. Temperature restriction for Hastelloy C-276 is recommended as maximum 30 o C in chlorinated seawater. Temperature and environment restrictions for materials not listed in table 1, see Norsok standard M-001. Material selection of instrument tubing in other environments not listed in table 1 and services shall be evaluated especially. Manufacturers of type 6Mo stainless steel and type 25 Cr duplex tubing shall be qualified in accordance with Norsok standard M-650. Non-metallic clamps shall be used if there is a risk for vibrations of instrument tubing. 2.3 Painting and insulation Painting shall be applied on instrument tubing to be insulated (i.e. heat traced) and in accordance with Norsok standard M-501. It is recommended to reinforce the painted surfaces under clamps with a heavy duty mastic coated tape system. Pre-insulated tubing systems may be selected pending approval by Company. Non-metallic clamps are preferred when instrument tubing is painted. 2.4 Inspection during operation Inspection should be planed for and performed on instrument tubing under clamps within three years after completed commissioning at installations. Based on experience the most critical areas should be Classification: Open Page 3 of 6

4 selected for visual inspection. If no corrosion is observed, the actual inspection area may be deleted from the inspection program. 3 Definitions PRE Pitting Resistance Equivalent. PRE = % Chromium x % Molybdenum + 16 x % Nitrogen Austenitic stainless steel alloys with 6% Mo and PRE>40. Austenitic stainless steel alloys with 17% Cr, 12% Ni, 2.5% Mo 22Cr Duplex Ferritic/austenitic stainless steel alloys with 22% Cr 25Cr Duplex Ferritic/austenitic stainless steel alloys with 25% Cr and PRE >40 Ti grade 2 and grade 5 Titanium alloys Hastelloy C-276 Nickel base alloy with PRE>40 Operating temperature The temperature in the equipment when the plant operates at steady state condition, subject to normal variation in operating parameters. Crevice corrosion Form of localised corrosion caused within crevices in a corrosive medium Pitting corrosion Form of localised corrosion causing sharply defined holes on surfaces in a corrosive medium. CSCC Chloride Stress Corrosion Cracking SSC Sulphide Stress Cracking 4 References C025-S-0000-J-RE-001, Erfaringsrapport: Anbefalinger av instrumenttubing og klammer, gyldighetsområde land og hav, datert 15/ Corrosion Handbook, Avesta Sheffield AB, 1994 Classification: Open Page 4 of 6

5 App A Table 1 Material selection for instrument tubing and other components Atmosphere/ field environment Indoor and dry atmosphere or External nonsaliferious atmosphere External and saliferious marine atmosphere Service (Note i) Material for instrument tubing Material for other components i.e. fittings/small valves. Maximum temperature to minimise risk of external CSCC. Maximum temperature to minimise risk of internal crevice corrosion (Note iv) Material for instrument clamps with no risk of vibration. (Note ii) Material for instrument clamps with risk of vibrations. Hydrocarbon & hydraulic Max 60 o C (Note iii) Instrument air Max 60 o C (Note iii) Sea water Titanium grade 2 Titanium grade 2 and 5 85 o C 30 o C Fresh water Max 60 o C (Note iii) Hydrocarbon & hydraulic Instrument air Type 6 Mo 25 Cr duplex Type 6 Mo 25 Cr duplex Sea water Titanium grade 2 Titanium grade 2 and 5 Fresh water Type 6 Mo 25 Cr duplex Max 60 o C for type 316 Max 110 o C for type 25Cr Max 120 o C for type 6Mo Max 60 o C for type 316 Max 110 o C for type 25Cr Max 120 o C for type 6Mo Max 60 o C for type 316 Max 110 o C for type 25Cr Max 120 o C for type 6Mo 85 o C 30 o C Classification: Open Page 5 of 6

6 Notes: 1. See Norsok standard M-001 for sour (H 2 S /SSC) service restrictions. 2. for use of metal or polymer type clamps. 3. Maximum operating temperature of 70 o C indoor or in sheltered areas for type 316 stainless steel. Non-insulated systems. 4. A maximum temperature for risk of initiation of crevice corrosion will apply. Maximum 30 o C in chlorinated seawater is recommended. Classification: Open Page 6 of 6