|A diffuser pump that has had a solvent-free epoxy coating applied to protect it from erosion-corrosion damage and thereby maintain its performance|
Considering that pumping systems account for 20 per cent of the world’s electrical demand and a typical steam power plant will run more than 60 pumps sets, it is obvious that the power industry would benefit from any solutions aimed at increasing pump performance.
Pumps fulfil the critical role of continuously delivering water throughout a thermal power plant. Circulating water is an integral component of thermal power generation and the water is typically used in two ways: in the internal steam cycle to create steam via the thermal energy source and convey it to an electricity-generating turbine; and in the cooling cycle to cool and condense the after-turbine steam and then discharge surplus heat to the environment.
This fluid handling equipment can suffer from several physical and mechanical problems, including general and/or localised corrosion, cavitation or reliability linked with poor efficiency or performance.
All these parameters may affect the power consumption of the equipment, increasing considerably its lifetime running cost and reducing the energy efficiency of the power plant. Minimising performance deterioration is a major consideration for pump manufacturers and users.
One effective way, other than corrosion-resistant alloys, is to protect pumping systems using erosion-corrosion resistant coatings.
We should start by recognising that it is virtually impossible to design a pump that is totally immune to in-service deterioration. Typical problems encountered with fluid handling equipment will be similar whether we look at nuclear or fossil power generation or turbine components in the hydro power industry.
Pumping systems are designed to work at specific flow and heads, however, they are rarely running at full efficiency, which could be explained by different mechanisms, aside from purchasing an off the shelf pump where the intended service does not exactly match the Best Efficiency Point (BEP).
Three main categories can summarise the origins of pumps performance reduction. Mechanical losses due to friction in bearings, worn wear rings or seal problems. Leakage losses are explained by recirculation through wear rings, seals and balancing devices.
Energy losses, also called hydraulic losses, represent the majority of the efficiency reduction, highly relying on surface conditions. The metallic substrate is subject to erosion-corrosion, leading to its degradation. In order to offer a better understanding of the problems, we should consider different forms of corrosion and erosion.
There are several main mechanisms of corrosion, in particular those which affect metals used in pumping systems, such as stainless steel, cast iron or bronze.
Uniform corrosion is most widely known as the oxidation of the entire surface, however it also includes tarnishing, active dissolution and polishing in chemicals (especially acids), anodic oxidation and passivation. Passivation, or anodic polarisation, occurs with alloys such as stainless steels and aluminiums, where the surface oxidises, stabilising and preventing further corrosion.
Experience has shown that passivated alloys, such as aluminiums and stainless steels exhibit excellent resistance to corrosion in some immersed conditions, however, despite this passive oxide layer, when in close proximity to a more noble metal such as bronze, can still suffer bi-metallic corrosion.
With Localised corrosion, certain areas of the metals corrode faster than others and it is in localised corrosion where the difference between oxidation and corrosion is seen. The process is accelerated as clear anodic and cathodic areas are defined, often with the corroded area invariably becoming increasingly anodic to the neighbouring cathodic area. It is one of the most problematic types of corrosion, and it is these forms of localised corrosion which often lead to component failure due to their severity.
The various manifestations of this form of corrosion are as follows:
Bi-metallic corrosion occurs when two metals of differing potentials are placed in solution and electrically connected together, a current flows between the two and electrons are given up by the metal with the greater potential – the anode. This principal is true for many types of corrosion, including uniform corrosion, where different potentials are present in the different grains of the structure. In the case of two separate, different metals, the results can be quite dramatic.
If we compare the potentials of cast iron with bronze in flowing sea water, we see typical potentials of -0.61 V for the cast iron compared to -0.23 V for the bronze. Specific attack will occur because the cast iron is the cathode, compared to the anodic bronze.
Deposit corrosion occurs under or around a discontinuous deposit on a metallic surface. In sea water, gaskets, fittings, and marine growth are primarily the cause of propagation, leading to pitting. This form of corrosion is similar to crevice corrosion.
The most probable causes of pitting corrosion are defects in the surface of the alloy, which may be either in the smoothness of the surface, or the internal structure of the alloy. In both cases there is small localised corrosion, leading to oxygen depletion. Corrosion is propagated as the greater area outside of the pitted area, which has ready access to oxygen, becomes cathodic despite its being of the same material.
Selective dissolution occurs in certain alloys, where the more active element can become corroded away. A good example is the graphitisation of iron, where the iron corrodes in preference to the (non-corroding) carbon, and dealuminification in some brasses.
The high fluid velocities encountered in fluid handling equipment contribute to the rapid degradation of the components. In addressing the solution for erosion problems it is important to be able to diagnose the erosion sub category.
|An example of impingement|
Impingement is caused solely by high velocity fluid flow, and its effect on the substrate, such as in this example where high fluid velocities are occurring due to high pressure to low pressure leakage.
|An example of entrainment|
Entrainment is when silts and gravel are carried up in the fluid stream – they are said to be entrained. This type of erosion causes greater material loss than impingement alone for the same given fluid velocity.
Cavitation occurs as a result of a pressure difference in the fluid and can be found on the pump body or most commonly on the impeller, in particular the low-pressure surfaces. It is recognized by a ‘peppering’ of the surface, caused by the implosion of vapour bubbles onto the substrate.
All of these deteriorations will greatly reduce the efficiency with which the pump is running. It is important to find solutions to reduce these effects and to extend the lifetime of equipment.
Pump manufacturers have been looking at different ways to reduce corrosion and erosion damage onto metallic substrate.
In addressing the erosion-corrosion problem, the first consideration is always the material. One possible way to reduce the rate of deterioration is to select the best material suited for specific operating conditions.
Conventional materials such as cast iron are generally used whenever possible due to cost implication. However, their resistance to erosion-corrosion is relatively low, implying a quick degradation of the substrate.
Stainless steels are extensively used for their resistance to general corrosion, through the creation of a protective passivation layer. Providing this passive film stays undamaged, corrosion rate will be very low. However, if the film is damaged and the environment does not favour rapid film repair, then localised corrosion can occur.
By understanding the principle of the oxidation/reduction process, it is clear that the noblest metals are more likely to be protected against corrosion, but no metals are completely immune against erosive and corrosive attack.
As stated earlier, the usual criterion for the choice of a particular material, or combination of materials of construction will be cost, provided that the materials have sufficient physical properties to function within the environment. However, this design ethic can cause more problems than it solves, particularly if the equipment is to be immersed in an electrolytic solution. The resultant bi-metallic corrosion, which will inevitably ensue due to the different metals used, will lead to the premature failure of the equipment.
Over many years attempts have been made in areas of fluid flow to select specific materials with corrosion resistant properties and also to try to match up galvanic potentials to minimise the difference and thus the galvanic effect. However, ultimately when using only metals, there will always be a compromise either in performance or cost.
The only way to greatly reduce erosion-corrosion effects is to isolate the metal surface from its environment. For fluid flow situations, there is a wide range of factory applied coatings, including PTFE, FBE and rubber linings, but a more limited range of options available to the designer which can be field applied or repaired in situ.
Historically, glass flake coatings have been used and specified for the protection of fluid handling, processing and storage vessels. They have good corrosion protection properties and with correct selection of binder, have a good chemical resistance.
Nevertheless, glass flakes systems do have many drawbacks. The level of volatile organic compound through solvents as well as styrene may be a serious health and safety issue. The polymerisation process involved in curing process of glass flake system leads to shrinkage causing the bond line to be permanently stressed.
The adhesion, cavitation and impact resistance are relatively poor and in comparison with conventional solvent free epoxy system, their general erosion resistance is lower. Glass flake systems are notoriously brittle and easily damaged during routine maintenance of equipment.
Glass flake are also high build, typically 1.5-2 mm thick. This could cause flow restriction in critical areas affecting performance characteristics. In terms of performance, this thick glass flake coating will shift the efficiency curve to the left. This will improve the efficiency of a pump which is operating left of BEP but will reduce efficiency of pump operating at or to the right of BEP.
If we compare, for example, the 2 mm thickness of a glass flake coating on a 100 mm pump inlet, we see a reduction of the cross section area of approximately 8 per cent; and flow rate is highly influenced by the cross sectional area.
Modified solvented epoxies are very versatile in use, as they can be designed with many different properties, depending on the binders used. Generally, they offer good resistance to erosion-corrosion. Epoxies can be modified using phenol, coal tar and hydrocarbon resin to give special properties, such as better chemical resistance, better penetration and improved water resistance.
One drawback with solvented epoxy coatings is that they contain large quantities of solvent, which is associated with health and safety problems. The content of solvent also implies shrinkage, thus stress within the coating. Poor immersion resistance of modified solvented epoxies may limit their use within fluid handling equipment.
The design of thermosetting polyurethane coatings allows them to be stiff or flexible as required, offering good curing at low temperatures, cavitation and impact erosion resistance.
Their disadvantage, however, tends to be in long term immersion as some can be moisture sensitive, tending to absorb water more readily than other coatings.
Applying at a greater thickness helps to overcome this problem, and there have recently been developments in diffusion resistance to provide systems which avoid these shortfalls.
Modified solvent-free epoxies offer similar benefits as solvented epoxy coatings, such as resistance to erosion-corrosion and chemical resistance. The key benefit of solvent free material eliminates associated health and safety problems whilst also reducing the shrinkage to a negligible level. There is also significant scope to work with the properties of the coating and required service parameters, modifying for strength, flexibility, corrosion and erosion resistance, and temperature and chemical resistance. Resistance in immersion is generally excellent which offers long term protection for fluid handling equipment.
Solvent-free epoxy coatings, such as those developed by Belzona, are applied at a relatively low thickness, about 500 microns, which do not lead to interference with the fluid flow, compared to glass flake coating for the majority of pumping situations
Improving on ‘as new’ conditions
We have seen that in-service deterioration is possible through mechanical damage, leakage and hydraulic losses. Leakage and hydraulic losses can be addressed by material selection, or the use of performance coatings to provide the pump with protection against damage caused by bi-metallic or general corrosion, or erosive effects such as cavitation.
Use of the correct performance coating has been shown to not only improve the performance of the pump compared to its ‘as new’ condition, but to maintain this throughout its life with minimal maintenance of the coating necessary. Preventing bi-metallic corrosion under wear ring seats can prevent leakage from the high to low pressure side (on split casing pumps for example) and general corrosion can be halted in the main body of the pump and impeller.
Jérémie Maillard is an engineer in the Technical Services Department at Belzona Polymerics. For more information, visit www.belzona.com.
Belzona 1341 case studies
|Split casing pump coated with Belzona 1341|
Celebrating its 25th birthday, Belzona® 1341 Supermetalglide was designed to meet the key service requirements of the pump, i.e. immersion, corrosion, erosion and cavitation resistance, excellent adhesion, flexibility and ease of application and maintenance.
• In 1991, a water supply company in Australia had a rapidly deteriorating water transfer pump with falling flow output. Belzona 1341 was specified and applied. The customer reported an 8 per cent efficiency gain along with 11 per cent flow increase, which resulted in close to A$60,000 ($55,500) energy savings per year. In 2010, the pump was brush blasted and a further coat applied over the original layer of material. The pump is still in service today.
• In 2010, two cooling water circulating pumps in a power plant in Northern Ireland, each weighing over 11 tonnes without the motor, were failing to deliver sufficient flow. Belzona rebuilt the eroded and cavitated pump components using paste grade composites, pump casing were coated with an immersion grade coating and the internals of the pump protected with Belzona 1341. Both pumps were returned to service within an eight week period and are still in service.
• In 2012, the continuous flow of water through cooling water pumps in a French power station had caused the concrete construction to wear both upstream and downstream of the pump, as well as the pump volute inside. This erosive wear had led to micro cracking of the concrete and the rough concrete surface had led to a loss of efficiency as the worn concrete walls created a more turbulent flow. Severely eroded sections were restored using Belzona materials for the rebuilding of concrete and coated with Belzona 1341 to create smooth surface with optimum efficiency. This restoration work was repeated for all the colling system lines at the power station.
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