Gulf Construction Online - Increasing the life expectancy of plant

Corrosion

Welding a copper nickel elbow.

Increasing the life expectancy of plant

Alan Brown, sales manager at the UK-based Arc Energy Resources, discusses the benefits of weld overlay cladding to protect the internal surfaces of oil and gas pipe systems and equipment from corrosion and reviews the options available using corrosion-resistant alloys.

01 June 2003

Corrosion is the ultimate dirty word, leading to a waste of precious natural resources and creating problems for those entrusted with the containment and handling of aggressive liquids and ecologically-damaging toxins.

Paints and coatings are readily available to protect the external surfaces of piping systems and equipment from the atmosphere; but what can oil and gas engineers do to ensure the integrity of the internal surfaces and maintain an efficient and reliable service, when the transported media can degrade pipes, valves and other equipment?

In such applications, what techniques are available to enhance the life of new plant or refurbish worn or corroded equipment? And what are the cost benefits compared to expensive base materials and/or replacement parts?

Engineers have a number of options when considering corrosion protection for the wetted surfaces of a pipeline system. However, the final solution will depend in part on an evaluation of factors such as the presence of chemically-aggressive compounds; the level of suspended solids, if any, in the product; the installation deadline; and budget restraints.

Where budget is not a constraint, engineers can simply specify components in corrosion or wear-resistant alloys known to withstand the specific service conditions.

However, this is rarely the case and other, more cost-effective options must be sought.

Where standard, off-the-shelf carbon steel components such as valves, pumps, pipe, flanges or fittings are used, weld overlay cladding is certainly the most versatile option, providing the assurance of a heavy-duty metallurgically-bonded protective layer that will not be undermined or dislodged in service.

After first identifying the areas within the system that need to be protected, and the properties of the medium from which protection is required, engineers can choose from a number of welding processes and a wide range of cladding alloys. The final choice will depend on the size and geometry of the components to be clad and the alloy best suited to combat the surface degradation.

Of the main welding processes, automated GTAW (TIG) is ideally suited to smaller components, particularly where the deposit needs to be finely controlled to ensure that the finished surface will not disrupt the flow characteristics of the transported medium. It is also most appropriate where subsequent machining will be required, for example, for the seal areas of valves and flanges.

GMAW (MIG), submerged arc and electroslag welding processes are used where larger areas and thicker deposits are required. Faster deposition rates mean these methods also offer cost savings. A wider selection of consumable materials, which may not be produced in the standard solid wire form, is also available.

Even in systems that do not suffer anything worse than normal aqueous attack and where pipes are internally protected by other methods, it is well worth considering the application of weld overlay cladding to the system's seal areas. For example, on flange seal faces where mechanical damage may be caused during construction and subsequent maintenance activities, a conventional and inexpensive stainless steel deposit would be adequate.

Where the medium is more corrosive or can be variable in content, the use of either a higher grade stainless steel or one of the more complex nickel chromium alloys is normally recommended. Whilst these tend to be difficult to source and are prohibitively expensive for use in solid form, a 3 mm-thick layer on the affected surface will offer the same working performance and could lead to significant savings from the extended life of the equipment.

Copper nickel and aluminium bronze deposits have been used traditionally for marine environments and are still a popular and reliable choice.

Mechanical wear is normally limited to areas where a directional change of flow takes place - for example at a bend or diversion point, or where the medium is physically propelled by pumping or other means.

In these areas of wear, which are generally well recognised and documented, weld overlay cladding offers clear advantages because protection can be applied specifically to the areas under attack, eliminating the need to produce the whole component from an expensive corrosion-resistant material. Often the wear resistance of some available cladding consumables can be far greater than that of solid cast or forged options.

The overwhelming advantage of weld overlay cladding is its versatility. Whatever the shape or size of the piping component, there is a process that can be applied and whatever the medium, there is an alloy to counteract its corrosive or abrasive properties.

This is also true when weld overlay cladding is used to protect oilfield valves from corrosion. As oil and gas wells are sunk deeper and produce hotter and more aggressive 'cocktails' of corrosive media, equipment manufacturers are forced to specify highly alloyed materials for valves and associated components to overcome the inevitable corrosion problems.

The alloys used to manufacture oilfield valves and associated components can range from carbon and low alloy steels through to stainless steels. All are capable of being overlay clad to provide corrosion resistance in specified areas or indeed over all surfaces in the case of the carbon and low alloy varieties, to provide total resistance.

For some applications, limited corrosion may be tolerated on the body of a component manufactured from low alloy or stainless steel, provided complete integrity is assured in sealing areas. It is here that a more corrosion-resistant material can be applied using weld overlay cladding.

Similarly, where total corrosion protection is required, it makes economic sense to manufacture the component in a cheaper alloy and overlay clad with a corrosion-resistant alloy, rather than manufacture the whole component in a more expensive, highly alloyed material.

To summarise, significant developments by equipment suppliers to improve productivity and quality have helped to ensure that weld overlay cladding is now widely accepted as a versatile and economic process with which to protect oilfield pipe systems and equipment against corrosion.