Cleaning turbine blades is an essential part of power plant maintenance but requires downtime. Methods where the turbine can be left in situ and where waste can be easily managed are favoured, especially by operators in competitive markets.
Mike Watson, Tube Tech International, UK
One of the last outstanding traditional cleaning methods in the power industry has finally found its match in a new solution that threatens to eradicate the more traditional method.
Tube Tech International has, quite literally, put shot blasting on ice when it comes to turbine blade cleaning, one of just a number of new methods the tube, pipeline and vessel cleaning specialist has devised.
Based in Essex, UK, Tube Tech has already proved the advantages of its new system to several clients. The method that the company sees as revolutionizing turbine blade cleaning utilizes the strong cleansing power of polymerized water and dry ice. The company rolled out its polymerized water and ice application for Siemens Power Generation at a power station at Bradwell-on-Sea in Essex where shot blasting had been the norm for the regular cleaning of the steam turbine blades for decades.
Turbine blades were just one such case of a traditional approach needing rethinking and the Tube Tech International answer was in super high-pressure water and CO2 (dry ice).
Figure 1. Tube Tech was faced with a difficult task at Dam Head Creek
Both high and low pressure steam turbine blades need regular cleaning because they collect magnetite scale as they rotate at high speed. A build-up of this scale cannot only damage the blades but also affect the overall balance of the turbine if not removed.
Traditional shot blasting with aluminium oxide is so messy that the turbine has to be removed from the turbine hall to be worked upon in an airtight tent. A complete shot blasting operation can take on average four days and this decades-old method has a number of drawbacks in equipment costs, time, health and safety and post-operation repairs.
Complete removal means craneage time and cost, plus the expense of scaffolding and the construction of a dust tight containment area. The dust created within the containment tent not only has safety implications for personnel but also the blasting medium can get trapped in other parts of the turbine causing further problems later.
The grit, while leaving a shiny, aesthetic finish, actually roughens the turbine blade surface, which can be a key for future scale build up. Reprofiling the blades to smooth the surfaces is both time-consuming and expensive. All of these operational disadvantages are banished with the new in-situ cleaning system that uses an infusion of ‘dense’ water and CO2 with super-high pressure water jets reaching up to 4000 bar. At Bradwell the Tube Tech International team completed the work in just 12 hours and overall cost and downtime were reduced for the client.
Because the turbine remained in place and the Tube Tech International system is so safe, power station staff could remain engaged on other duties at the plant, working in close proximity to the cleaning operation.
Tube Tech International’s system generated no secondary waste to contain and the removed magnetite scale was collected at source. With the blade metal left in a pristine condition without reprofiling, blade life was also increased and the period over which future scale would build up was minimized.
Tube Tech International’s technical sales manager Scott Donson said: “The old methods of turbine blade cleaning were very time consuming, messy and required moving the whole turbine.
“Our system removed all those three elements as we were able to do the work faster, cleaner and in situ. Because they were cleaned with our ice method the blades we worked on will require longer periods between maintenance.”
Figure 2. Stubborn molluscs met their match at Teeside
Keith Jones, operational services manager for Siemens Power Generation, said: “We believe that the Tube Tech International process offers advantages over more traditional cleaning procedures. In particular we felt that the procedure offers a time saving as against more traditional methods, with no deterioration of the blade aerofoil surface, which is particularly important for high-pressure turbine blading.
“Tube Tech’s process does not require the removal of large quantities of debris or sludge, as compared with air or water-borne media, and is completed with no damage to the components being cleaned.”
The speed with which the dense water and dry ice application can be carried out was a major factor when Tube Tech International was asked to take advantage of a brief shutdown period at the Keadby power station near Scunthorpe in Lincolnshire, UK to clean turbine blades.
This time the solution recommended by Tube Tech International was a variation on its ice theme – its FastIce system. This operation meant raining bespoke CO2 pellets propelled using compressed air at extremely high pressure to remove hydrocarbon deposits on the turbine blade surfaces. With the help of various accessories the system was able to penetrate even the most awkward of positions or corners. It is also more efficient and time saving, proving to be around ten times faster.
Again the Tube Tech International method created minimal surface abrasion thereby not upsetting the delicate balance of the blades and saving the cost of reprofiling.
Tube Tech’s Donson said: “The FastIce surface preparation and blasting system was developed for this and other applications where a controlled, waste-free cleaning environment is required. It has proved successful in many other similar surface preparation contracts. While the Tube Tech system removes the scale deposit build up, customers do not have the added expense of secondary waste disposal or mess. There were also production downtime savings for the client.”
Tube Tech International has created around 50 new systems and applications that have replaced old cleaning methods in a number of industries. Elsewhere in the power generation industry the company has devised new solutions to cleaning more than turbine blades.
In other instances, Tube Tech International solutions have been adopted not for just their time and money saving properties but also safety considerations. Safety was paramount when one of Britain’s more-recent power stations chose Tube Tech International’s descaling and inspection expertise to clean a massive air cooled condenser (ACC).
The Apex finned ACC was one of the largest of its kind in Europe when Damhead Creek, on the northern banks of the River Medway in Kent, UK was commissioned in February 2001. With a 105 hectare surface area for the heat exchanger, the ACC takes up nearly one third of the site of the combined cycle gas turbine station operation. It was suffering reduced performance on the steam driven turbine and required an additional clean of the finned tube bundles from inside to deal with debris sucked up by the 36 enormous cooling fans. The total bare surface area that needed to be cleaned measured more than 34 000 m2.
Tube Tech International was the only company in the competitive tender that could satisfy the desired criteria of using the existing walkways to clean the tubes more than 10 m above. Competitor companies had advised the use of rope access methods or secondary staging to reach the full extent of the tube surfaces.
Figure 3. A Tube Tech technician at work on power station turbine blades
Tube Tech International won the contract for its proposal of the safest option and, faced with the largest and highest ACC it had tackled, used several systems. It also developed an additional technique that involved a high-tech lightweight alloy system and a handling process specific to the site. The rig provided safe and easy access to areas that were being cleaned without putting personnel at risk and in a reduced time of only 16 days.
Donson said: “We created a mock-up prototype and tested its effectiveness on a life-size replica at our premises. The final result exceeded expectations.”
Since then Tube Tech International teams have returned to Damhead Creek to carry out further maintenance cleaning.
Few cleaning specialists such as Tube Tech International have found their adversaries to be the common or garden mussel but it was another power industry job that required some original thinking.
Mussels find the condenser at the Sita Tees Valley waste to energy site a perfect spot for breeding but traditional water jetting is not enough to remove them. The tenacious molluscs would not loosen their grip from the tube walls at the plant on Teeside, which supplies electricity to the national grid in the UK.
To get rid of the bivalve squatters, Tube Tech International used an advanced form of darting in the 8000 tubes of the turbine condenser. The mussels still return from time to time and so do the Tube Tech International teams who have reduced subsequent cleaning times from an initial two days to less than 14 hours.
Donson said: “A major way in which our system scores over traditional methods is that we guarantee that the tubes will be cleaned all the way through to the end which involves multiple systems, namely Conco cleaners to remove thin tenacious scale, Rotaflex to remove very thick scale and Wysperflex to unblock tubes.”
A plant spokesperson said: “We had tried conventional jetting some time ago but the quality was just not up to what we wanted. Tube Tech has been continually managing to reduce the time the project takes on each occasion.” He added: “The last time Tube Tech was on site, their team was out in record time, which is the most important aspect for us.”