perth seawater desalination plant – material … · ida world congress – perth convention and...

Wor World Congress/Perth Convention and Exhibition Centre (PCEC), Perth, Western Australia September 4-9, 2011 REF: IDAWC/PER11-292

PERTH SEAWATER DESALINATION PLANT – MATERIAL SELECTION Authors: Tako Heiner, David Parravicini Presenter: Tako Heiner Engineering Maintenance Manager – PSDP Degremont – Australia Abstract November 2006 marked the occasion of the start up of the Perth Seawater Desalination Plant (PSDP) as the first large-scale desalination plant commissioned in Australia for public water consumption. PSDP was built by the Multiplex-Degremont Joint venture, in alliance with the Water Corporation and the plant is currently being operated by Degremont in alliance with the Water Corporation. Constructed on budget and on time, the plant has been heralded as a model example of a well built and sustainable desalination plant. It has exceeded all production targets set for it, and has average availability of 99.7%. Since the start up 5 years ago, the plant has overseen several significant improvement projects, a good proportion of them stemming from the ongoing battle against corrosion. The marine environment is a very aggressive one, both on the external structures, as well as the internals of piping, valves and equipment. It creates severe demands on civil, electrical and mechanical assets. The material selection of plant items during the design phase is of paramount importance. Correct material selection has tremendous benefits for the desalination industry that is very focused on sustainable development. The correct material selection of plant items can save in terms of ongoing monetary expenses as well as labour costs. However it is a well known fact that during the design phase trade-offs have to be made between upfront capital cost and ongoing maintenance costs and lessons can be learnt from the material selection of certain plant items at PSDP. This paper seeks to highlight some of the mechanisms of failure, with a view towards raising the awareness for designers and operators alike to the impacts and of these very important decisions.

IDA World Congress – Perth Convention and Exhibition Centre (PCEC), Perth, Western Australia September 4-9, 2011

REF: IDAWC/PER11-enter log number here -2-

Introduction This paper is a summary of selected corrosion issues which have manifested themselves at PSDP in the four years since commencing production, and the manner in which these issues have been dealt with. They will be presented as a series of case studies for the different areas affected by corrosion. This paper must be seen as a return of experience rather than a definitive paper on problems and solutions – the very particular conditions which exist (nutrients, microbial activity, temperatures, water constituents) may not be applicable at other sites, and therefore the results cannot be extrapolated without a robust scientific follow-through. Materials which may be problematic under our specific conditions may perform faultlessly on another site. Seawater Environment The Perth Seawater Desalination Plant (PSDP) is located approximately 50km south of the city of Perth in Western Australia. It was built in response to a drying climate by the Water Corporation, starting production in November 2006. It draws its water from Cockburn Sound, a body of seawater bounded by the mainland on one side, and Garden Island on the other. The Intake is at a depth of approximately 10m, approximately 150m offshore of the mainland. Cockburn Sound is adjacent to a large industrial area which includes industries such as Petroleum, Fertilizer, Chemical, Nickel refinery, Power generation, Bulk Handling and Cement. Mussel farming is carried out on the southern side of the sound, and it is also the spawning grounds for Snapper, a fish species. Seawater analysis indicates significantly seasonal variation. Water temperatures vary between approximately 17 °C in Winter and 27 °C degrees in Summer. Conductivity of seawater typically ranges between 51 and 56 mS/cm. In addition, organic loading is particularly high in summer months, as witnessed by seawater Silt Density Index, loading of the Dual Media Pressure Filters and fouling rates for cartridge filters. Case Study – Seawater Intake Duplex Bolt Corrosion

Figure 1: Seawater Intake Tower Figure 2: Intake Tower Grating

Frame Securing Bolt/Insert

Retaining Plate Bolt


REF: IDAWC/PER11-292 -3-

The Seawater Intake Tower is an 8m diameter concrete structure with at a depth of approximately 10m, with 6 “windows” each with vertical GRP rods spaced 150mm apart, acting as gratings. The GRP rods fit into a Super Duplex frame on the top and bottom of each “window” and are removable by means of a retaining plate. The fixings in question are used to secure the top and bottom frames to the concrete, and constitute a threaded insert (cast into the concrete) and a Allen Head Bolt (to secure the frames. Both the threaded insert as well as the bolts were constructed of Duplex stainless steel (SAF 2205).

By June 2009, after approximately 2.5 years of operation, it was becoming clear through dive inspections that the bolts securing the frames to the concrete structure were corroding, and that an alternate solution had to be found. The corrosion was concentrated within the threaded areas of both the bolts as well as the threaded inserts. All areas open to general seawater were still in good condition.

Figure 3: Frame Securing Bolts Figure 4: Retaining Plate Bolt Figure 3 shows the Frame securing bolts, which secured the grating frames onto the concrete structure via the threaded inserts. The bolts, as well as the threaded inserts were corroded, and the head of the cap head bolt was completely intact from the outside, but almost completely hollowed out from the inside.



Figure 4 shows the bolts which secured the GRP rod retaining plate to the frame. In this case only the area in which the bolt passed through the threaded frame was corroded. The thread above and below was still intact. The case was sent to a consultant for analysis, and the report received is available from the author on request. In short, the mechanism was identified as crevice corrosion, with a potential of early microbial attack (although storage conditions prevented meaningful testing in this area). Due to the destruction of the threaded areas, and the fact that the threaded inserts were cast into the concrete, replacement of the corroded components with alternative components in different materials was not a possibility. An innovative approach was proposed, whereby the entire grating framework would be constructed in welded HDPE, and affixed to the Intake tower by use of the window sills, roof, and removable tie-bolts.

Each window has a separate lower frame, hung from the “window” sill, as shown in Figure 5. The frames are interconnected with two tie-rods to tension the ring. These tie-rods are removable to allow for replacement in case of future corrosion issues, one at a time to retain tension.

Figure 5: Lower Grating Frame Figure 6: Upper Grating Frame For the upper frame the roof upper surface and lower surface is used to locate the frame as in Figure 6, and the same principle of tie-rods was implemented to tension the ring. This solution has dealt with the



corrosion issue at the intake by removal of most of the metallic components. The remaining metallic components can be replaced easily should that become necessary. Case study - Use of Coatings over non-corrosion resistant materials Exotic stainless steels and other corrosion resistant materials can have higher up-front cost, and there is a temptation to reduce this cost by the use of coatings over a cheaper and less resistant material. This is a compromise which requires very careful consideration, since the lower up-front cost can be severely compromised by a much higher lifetime cost. At PSDP we have experienced this first-hand, and two examples will be discussed: one in which the long term cost was clearly compromised, and one on which the jury is still out. The first of these cases is the installation of a coated ductile iron Non-Return Valve in the discharge of our seawater lift pumps. The second is the use of Halar coatings on butterfly valves on the pre-treatment side of the RO. Seawater Lift Pump NRV The seawater lift pump discharge Non-Return Valves were procured with super duplex internals, but with a coated ductile cast iron body. Our first sign of problems occurred in December 2009 after 3yrs of constant production, when one of the non-return valves perforated from the inside (Figure 7). The valve was immediately taken offline and inspected, and results were alarming. Whilst most of the super duplex internal components were in reasonable condition (albeit with considerable marine growth), the coated ductile components were badly corroded. The perforation had been initiated at the edge of a coating, where there was a transition to the super duplex internals as shown in Figure 8 below.



Figure 7: Perforated Ductile Iron housing Figure 8: Internals of corroded valve In addition the pedestals on which the super duplex leaf springs are mounted were in some cases almost completely corroded away. There were many secondary failures, primarily at the edges of coatings, and in high stress areas such as near flange faces where coating could have cracked or been damaged.



Figure 9: Corrosion at coating interface In Figure 9 above the potential secondary failures can be seen at the coating boundary. The seawater had breached the coating and started to corrode under the coating. Whilst the corrosion looks limited, breaking the coating away revealed larger cavities. In Figure 10 below, corrosion initiates where the coating is compromised at the flange due to stress or direct damage. This had been noticed as a weep from the flange during operation.

Figure 10: Corrosion initiating at stressed/damaged areas In Figure 11 below the mounting pedestals to which the leaf springs are secured are corroded away, and in some cases the threaded mounting hole is compromised. The coating has either been damaged under the leaf spring, or the coating interface at the bolt (where the threaded hole enters the pedestal) has been compromised.



Figure 11: Corrosion at leaf spring pedestals The solution for this situation was to order replacement valves with full super duplex specification, and they have recently been installed. The full life cost of these tags now includes the unnecessary costs incurred in the original coated ductile iron valves, the repair costs incurred to keep the plant running in the interim period, as well as the costs of installing the new valves.



Halar Coating on Butterfly valves In much the same way that epoxy coatings can be compromised, especially at boundaries, interfaces or in stressed or damaged areas, the Halar coatings on butterfly valve discs can be compromised, and if the underlying material is not corrosion resistant this can lead to rapid failure. Whilst we have seen several failures of this type, most have so far been explained by a clear physical damage to the disc, either from misalignment of the disc with the flange (where the disc impacts the bore of the flange), debris which has been caught between the disc and the flange bore, or incorrectly specified duty such as highly abrasive sludge.

Figure 12: Halar coating in abrasive slurry application The valve in Figure 12 above is used as a control valve for transferring Backwash Effluent to a thickener. The sludge can contain abrasive particulates (sand, anthracite, mussel shells) and especially when running in a semi closed position (for flow control) and with frequent closing (multiple times a day), the Halar coating can be damaged. This valve was replaced in May 2010 after 3.5 yrs of service, although failure occurred well before that (we were able to run since this valve is not required to seal – only control flow).



Figure 13: Halar coating damaged by alignment/debris Figure 13 above shows a Halar coating damaged by a piece of debris which had become trapped in the valve. A similar failure can be expected if the disc does not have the correct alignment or spacers at a flange, and impacts the bore of the flange. This particular valve was changed in June 2009 after 2.5 yrs of service. As stated before, we are still unsure of the soundness of the decision to use Halar coatings in pre-treatment applications. Our approach so far has been to replace like for like where the failure has a clear explanation, although we have started pricing up super duplex discs with a view to stocking some of these long lead-time items should circumstances dictate. Following the manufacturer’s recommendations regarding installation is also critical, especially on larger diameter valves where disc weights become considerable. In most cases the recommendations include horizontal or angled shafts. Vertical shafts will compress liners under the discs due to weight, damaging both the liners and potentially the coatings. In addition any solids present in the fluid being pumped will settle to the bottom, and create abrasive damage to liners and coatings in the vertical position. Another recommendation generally made is to have the bottom of the horizontal disc opening in the direction of the fluid flow, as this flushes any settled debris upon closing (as velocities increase) thus minimising abrasive damage to liners and discs. case study - Inconel X750 springs in HP Discharge Non Return Valve The high pressure pump discharge non return valve is a swing check valve of super duplex construction, with an Inconel X750 return spring. After 2 years duty the characteristic of the valve changed when stopping the pump, with significant slamming and pressure shock. This was due to the fact that the return spring had failed, and the NRV required reverse flow to close. Upon opening the valve the return spring was found to have corroded and failed.



Figure 14: Inconel X750 return spring on NRV

As can be seen from Figure 14 above, the spring displayed heavy pitting across a large portion of its length, and the failure eventually occurred where the spring cross-section had been severely compromised. This spring resides in flowing water on the downstream side of the discs so stagnant conditions and microbial corrosion are unlikely to be an issue. The rest of the NRV showed no sign of corrosion, including the hinge pin, so mechanical wear from vortices shed by the upstream structure are equally unlikely. The most likely mechanism in this case would be galvanic corrosion, with the Nickel Chrome Inconel X750 positioned lower on the galvanic series than the high alloy super duplex body and connected piping. A contributing factor would be the relatively large cathode in comparison to the Inconel X750 anode, as well as the local flow disturbances and eddies which may contribute to changing the Inconel X750 from a Passive to Active state (assisting in removing the corrosion resistant coating formed on the metal surface). Our approach was to upgrade the material spec in consultation with the supplier of the springs to Inconel 625, a Nickel Chrome Molybdenum alloy, which should give us better corrosion resistance in this application. The springs were installed on all NRVs by the end of 2009, and the first follow-up inspection was done in February 2011. Figure 15 shows the result after a year of operation.

Figure 15: Inconel 625 return spring on NRV



Case Study – Rack Inlet Valve Gaskets and Monel K500 Valve Stem In PSDP each high pressure pump is fixed speed, and feeds two RO Racks via a Rack Inlet Control Valve. The valve bodies are welded into the piping (no flanging, and no ability to remove/replace without hot work). The valve is a guided plug type, and is constructed of super duplex. The gaskets used for the plug guide and bonnet were graphite spiral wound. The valve stem was a Monel K500.

Figure 16: Rack Inlet Plug Valve cross section

After approximately 1.5 years of production it was noticed that the joints between the bonnet and body on all valves were weeping, and a valve was opened for inspection. Two areas of concern were noted: the corrosion of gasket faces and the corrosion of the Monel K500 valve stem immediately above the valve plug. The symptoms in these areas will be dealt with separately, although they occupy the same area in the valve, so the solutions implemented do address both areas. A corrosion consultant was commissioned to analyse both areas, and the report is available from the author on request. Gasket Face Corrosion The gaskets are placed between the valve body and plug guide, as well as between plug guide and bonnet (the plug guide flange is sandwiched between bonnet and body as per Figure 16). Originally a graphite gasket was specified. Upon opening the valve the immediate observation was the presence of corrosion residue in the form of a red/brown paste covering the area on top of the plug guide. This seems to have originated from the top gasket face corrosion. Since the water above the plug is virtually stagnant (it is vented through the plug to allow movement, but operates for most of the time in a small range) this residue had collected on top of the plug guide flange instead of being washed away.



Figure 17: Corrosion residue at plug valve bonnet gasket faces Both the top and bottom surfaces at each gasket were corroded (the plug guide faces as well as the bonnet/body faces). The corrosion was limited to the area immediately under the gaskets. All uncovered areas on the plug guide showed no sign of corrosion. In Figure 16 above the position of the gasket can be clearly seen as the outer 15mm of the plug guide flange. The position where the residue is located towards the inner circumference of the plug guide flange was in pristine condition once cleaned.

Figure 18: Corrosion under gaskets on plug guide and valve body Figure 18 shows the gasket faces on the plug guide flange as well as the valve body in the cleaned state. Again, the corrosion is clearly only in the area covered by the gaskets. Valve Stem Monel K500 corrosion The valve stem immediately above the valve plug was very badly corroded, and in some cases less than 50% of the diameter remained. The corrosion residue was a black paste which completely covered the corroded area. There were elements of a green and yellow colour, and possible signs (once the shaft had been cleaned) of de-alloying. Together with the fact that the water in this area is largely stagnant, and the sulphurous smell reported, one suspicion was SRB.



Figure 19: Valve Stem corrosion What was also clear was that the valve stem was corroded only towards the bottom of its length, where it joined the plug. A good proportion of the shaft length which is housed in the same cavity showed little or no sign of material loss. Also noteworthy is the fact that the threads securing the stem to the plug were in relatively good condition.

Figure 20: Valve Stem lock pin in Plug Once the valve stem is screwed into the valve plug, it was secured to prevent it from rotating via a Monel K500 lock pin. This pin also showed sign of severe corrosion damage, although primarily to the Monel pin, with the super duplex valve plug remaining untouched. From the corrosion analysts report, the conclusions drawn were not clear cut. The elements primarily focused on were:

Galvanic effects of the use of graphite gaskets Affect of graphite gasket as a food source for Iron Reducing Bacteria Crevice corrosion under gaskets Possible microbial action on valve stem Chemical reactions in valve stem



The recommendations included the removal of the graphite gaskets for gaskets of a similar galvanic potential, or non-metallic gaskets. It also supported the decision taken to change the valve stem material from Monel K500 to super duplex, although it was pointed out that this would not protect against microbial corrosion. To give us the best chance of fighting the microbial corrosion our strategy was to remove the conditions in which these microbes flourish, namely the stagnant water found in the chamber between the valve plug and bonnet. It was decided to create an internal flushing path through the bonnet area, since our application does not require the valve to seal 100%. To achieve this 3 x 3mm diameter holes were drilled at 120 degree intervals vertically through the flange of the plug guide. This acts as a “bypass” past the valve plug, and constantly flushes the chamber in question. The water enters from below through the plug vents, passes into the chamber above the valve plug, and exits through the drilled flush holes.

Figure 21: Flush holes through Plug guide The super duplex shafts were implemented immediately, as were the gasket replacements with a Teflon based Novus Uniflon 51. There was a reluctance to do weld repairs on the valve bodies primarily because of the potential for heat affected zones, and because the valve body was welded in situ (not flanged) making it impossible to remove and carry out repairs under more controlled conditions in a workshop. The decision was therefore taken to test a few material options to better understand the mechanisms at work, using the plug guide (which is removable) as the test piece. The gasket faces on the bonnet and body were therefore repaired with a two component epoxy (Belzona Supermetal 1111), whilst the plug guide gasket faces were rebuilt with a variety of materials. These included Zeron 100, Inconel 625 and Ultimet as well as the Belzona 1111. The results so far have been very promising. With a combination of the flush holes in the plug guide and the new super duplex valve stems and pins, we have seen no sign of corrosion whatsoever in the valve stems. The tested variants for the plug guide gasket faces have shown that Ultimet and Inconel still display some signs of corrosion after 12 months installation under the Teflon type gaskets, with flushing



holes. This may be partially due to the fact that our Belzona gasket faces were not of the best quality, allowing ingress of seawater.

Figure 22: Ultimet plug guide and Super Duplex shaft after 12 months With testing in the various areas producing some good results, it was decided to start implementing an integrated solution including the rebuild of all gasket faces with Belzona (including the bonnet, body and plug guide), installing the plug guide with flush holes, and fitting the super duplex valve stem. We have decided to rebuild the rack inlet valves to this standard, and started with our first pair of valves in February 2011. To allow for serial rebuilds with repeatable quality we have prepared tooling and methodology to achieve good results in particularly the Belzona coatings. A machining jig was designed and built to allow for in-situ machining of the valve body gasket faces as shown below in Figure 23.

Figure 23: Gasket face machining jig The jig is driven by a variable speed drive, and mounts directly to the valve body. It allows the machining of the gasket face to remove approximately 2mm of metal necessary to mould a Belzona face.



Once the machining is completed, the relevant areas of the body are sandblasted while protecting other areas with shields. During both machining and sandblasting the valve bore is plugged to prevent ingress of debris into the valve body or piping.

Figure 24: Sandblasting of valve body Once sandblasting is complete and the area is vacuumed, the moulding with Belzona takes place. A small quantity of Belzona is first mixed and painted on with a paint brush to wet all the surfaces thoroughly. A bead of Belzona is then applied all the way around the gasket faces, concentrating on filling back of the gasket area at the step (largest diameter) as per Figure 25.

Figure 25: Applying Belzona Once the Belzona is fully applied the moulding flange is covered in release agent, and pressed into place. The air and excess Belzona is pushed toward the inner diameter. The moulding flange is dimensioned in such a way as to achieve the final gasket face with the same dimension as the original super duplex face. The moulding flange is drawn down into its final position using the studs of the valve body.



Figure 26: Securing the moulding flange After cleaning the excess Belzona from the inside diameter with a spatula, the Belzona is left to cure for 24 hours after which the moulding plate can be removed, and the valve completely cleaned and re-assembled for duty.

Figure 26: Belzona face in valve body The resultant face is very smooth, and due to the good preparation of the surface, well bonded. It is hoped that with this methodology we can limit recurrence of the gasket corrosion, as the most critical part of the joint is now the bond between the coating and the super duplex body, and if we are able to prevent crevices in this area we should see long life. The entire rebuild was performed on two valves over a 2 day period, with 24 hours of curing included, and due to the fact that no heat is created it will be able to be repeated if we do see deterioration. Conclusions Whilst the case studies show very different cause and effects, some common themes emerge from our direct experiences, which may be of considerable interest to designers in particular, and to some extent to operational staff. These include, amongst others:



Galvanic effects Seawater forms a good coupling between dissimilar metals. One needs to look at an integrated strategy for material choices. It cannot be said that a material is good in seawater – it depends on what other materials share its environment and are coupled to it through the seawater, and the relative sizes of those cathodes/anodes. Hence one can envisage that a “high spec” plant may use materials on the high end, but close to each other in the Galvanic series. When the majority of piping and valves are at the top of the galvanic series, a small component low on the series has little chance of survival. Stagnant areas Stagnant areas can be a breeding ground for microbial activity as populations can multiply and create a micro-environment which concentrates corrosive effects locally. By preventing dead-legs and a program of regular flushing of static legs (like instrument connections and vent/drain valves) the micro-environment can be disrupted. Ability to replace Operationally it is both cheaper and more productive to design plant in higher risk areas with the assumption that it will fail and need replacement. Hence a threaded insert cast into concrete in a seawater environment should possibly become a PVC sleeved hole, right through the concrete, allowing replacement of the bolt AND nut. Crevices Whilst many corrosion triggers may exist to gain an initial foothold, the existence of a crevice or a discontinuity in a coating can create a micro environment which traps and concentrates the corrosion products, leading to crevice corrosion. The challenge is to minimize these opportunities for micro environments, and these would typically include threads in wetted areas, coating boundaries and seams in construction (including flanges). Aknowledgements The Authors wish to thank the persons and companies who were contracted to perform the analysis of the failure methods, including metallurgical work, microbial analysis and interpretation of the results. These formed the basis of conclusions drawn. These include: Colin Boothroyd (Metallurgical Engineer) and Dr. Peter Farinha (Principal Engineer)

from Extrin Consultants for their work on the SAF2205 Bolt Corrosion Oliver Gasior (Project Co-ordinator) and Dr. Peter Farinha (Principal Engineer) from Extrin Consultants for their work on the and Rack Inlet Valves

In addition, thanks is extended to companies and individuals too numerous to mention, who helped to develop, implement and test the various solutions shown in this paper.

perth seawater desalination plant – material … · ida world congress – perth convention and...

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