IOM3 Underground Seminar. Health and Safety Laboratory Buxton. 27 May 2010

Transcription

1 IOM3 Underground Seminar Health and Safety Laboratory Buxton 27 May 2010

2 Applications and Limitations of Cathodic Protection within Plants Dave Harvey CEng, FICorr Engineering Manager Pipeline Maintenance Ltd

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4 The design and installation of CP systems for buried structures for plant facilities presents a unique challenge to the cathodic protection engineer. There are a multitude of different materials, processes, operating temperatures, coatings, buried and immersed structures that can suffer from both internal corrosion caused by the products contained within the system and external corrosion caused by the environment in which it is situated.

5 The following structures would normally be considered to require cathodic protection. Pressurized carbon steel hydrocarbon pipelines Bottom or soil side of above ground storage tanks Buried storage tanks and sumps Carbon steel/ductile iron drain lines Buried portions of process, gas, water, firewater or liquids pipelines Buried or the buried portions of steel valves, hydrants, monitors and fittings

6 These structures are constructed from a number of different materials, e.g. carbon steel, ductile iron, high tensile steels, stainless steels, steel in concrete, galvanized steel, copper etc.

7 Plant facilities usually require a considerable amount of cathodic protection current because of the large surface areas of buried, often bare, metalwork and are therefore normally protected with an impressed current cathodic protection system unless soil resistivities are very low.

8 STRUCTURE SURFACECURRENT DENSITY (ma/m2) Uncoated copper 50.00* Uncoated steel Poorly coated pipe 5.00 Tape or PE Wrap 1.25 Coal tar enamel 2.00 Epoxy wet applied 0.75 Fusion bonded epoxy (FBE) 0.10 Extruded Polyethylene 0.10

9 The ICCP systems utilize a variety of anode configurations, e.g. distributed anodes, local or remote surface groundbeds installed horizontally or vertically, local or remote deep anodes or a combination of the above. When choosing and/or combining anode bed types, the conceptual and the final designs must be coordinated with other plant operators to ensure all existing facilities are known, together with any planned extensions or new facilities.

10 Remote vs. close Groundbed Flow of current from an anode to a structure will be accompanied by a potential difference between the earth and the pipeline. The potential difference is used in certain criteria for determining the degree of cathodic protection being afforded to the pipeline. This potential difference can be accomplished in either of two ways: by making the pipeline negative with respect to remote earth, i.e. a remote groundbed, or by making the earth positive with respect to the pipe in local areas, i.e. close anodes.

11 For Plant Facilities, two basic cathodic protection principles may be employed: 1. Isolated systems 2. Non-isolated or total system

12 Isolated systems where every single piece of buried pipework or structure that requires to be cathodically protected is isolated by means of insulating flanged joints where it comes above ground. This can give rise to a very large number of insulated flanges which in turn leads to maintenance and reliability problems.

13 A non-isolated or total system where no isolation is provided except between the plant and off site facilities. This means that all buried metal work on the site is protected including the earthing systems. This leads to considerably higher current requirements than an isolated system.

14 Isolated System

15 Problems associated with isolated systems: Multiple insulating flanges Spark hazard Earthing system Interference with other structures Shielding Locating shorting bolts or short circuited isolation Disconnection/separation of pipework Integrity of the earthing system must be maintained despite the need for isolation Many other foreign structures exist in the vicinity of the protected pipework that need to be tested for interference In congested areas, the cathodic protection current may not reach all the surfaces that it is trying to protect.

16 NON-ISOLATED SYSTEMS With the issue of BS EN 14505:2005 Cathodic protection of complex structures, a whole new approach was introduced. The main obstacles with the plant operators for a non isolated system are : 1. They do not understand it and consider it is dangerous to have cathodic protection current flowing through all the pipework 2. That there were no authoritative standards recommending this principle. BS EN 14505:2005 changed all that.

17 A complex structure is defined as a structure to be protected and one or more foreign electrodes, which, for safety or technical reasons, cannot be electrically separated from it. The term complex structure does not refer to the complexity of the structure or to the complexity of the cathodic protection system but more the fact that there are different types of structure and metals used.

18 In a complex structure, all metallic parts of the structure included in the cathodic protection scheme must be electrically continuous. Isolating joints in incoming or outgoing pipelines are the only isolation required and these should be located outside the zone of influence of the cathodically protected complex structure so that unacceptable interference by the cathodically protected complex structure is avoided.

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20 Problems associated with non-isolated systems: Multiple insulating flanges Spark hazard Not required, only isolation between on/ff site pipes Minimised. Any separation is unlikely to create a spark as there is a multitude and parallel electrical paths through the connected pipework and earthing. However, the same safety precautions should still be taken, i.e. switch off cathodic protection and bond around pipe break until rejoined of declared gas free.

21 Earthing system Interference with other structures Shielding Integrity of the earthing system is not affected as everything is continuous Interference does not exist as all metalwork is connected into the cathodic protection system In congested areas, the cathodic protection current may not reach all the surfaces that it is trying to protect. This still exists and can only be overcome with appropriate placement of anodes to ensure that the cathodic protection current reaches all the necessary surfaces

22 Summary It can therefore be seen that the implementation of cathodic protection to plant areas is not a simple process as there are many different parameters to be considered. The cathodic protection philosophy for the areas must be determined at the outset during the feed designs. Where there are numerous different plant areas and systems that need to be brought together, a cathodic protection co-ordinator ordinator will be required to bring all the necessary disciplines and EPC Contractors cathodic protection designers together. It is therefore essential that this work is entrusted to competent experienced and Certified Level 3 Cathodic Protection Engineers.

23 This is not one of the usual hazards experienced on a construction site and I mm not sure about the quality of the wrapping!

24 Finally : Thanks for your attention. Dave Harvey CEng, FICorr. Engineering Manager Pipeline Maintenance Ltd