China North West Electric Power (CNWEP) operates ten hydro power plants on the Yellow River. For all of them, the period of highest demand comes during the four-month rainy season from June to September. The plants also carry another heavy load at that time: tonnes of sediment carried down the river that causes abrasion wear in the turbine components, reducing the efficiency of the turbines and shortening the life of the plant.

Many hydropower plants suffer this kind of erosion to a a greater or lesser extent, but the problem at the Yellow river is particularly acute. This is partly due to the sheer size of the load, which can reach 500kg/m3, and partly due to the nature of the material – it has a high proportion of minerals – and partly to the particle size and irregular shape. Just how much of a problem this load represents can be seen when it is compared to the Rhône in Switzerland, which carries a load fifty times smaller than the Yellow river. Turbines on the Rhone have a life three times as long as those on the Yellow River.

The solution to the problem lies in a new joint venture company set up recently by CNWEP and Sulzer Hydro. The company will offer a technological fix, in the form of a hard coating that can be applied to components to protect them from abrasion.

The joint venture, named Xi’an Yellow River Coating Centre, arose from a meeting between CNWEP and Sulzer Hydro personnel at the International Association for Hydro Research meeting in Beijing, in 1994. Now that it is about to go into operation it has a board of directors from the Sulzer Hydro and CNWEP, while the person responsible for the technical implementation of the agreement is deputy general manager Robert Clarke, originally from Sulzer.

Two years were needed to research and prove the efficiency of coating materials that would stand up to the rigours of the Yellow River. For the severe Chinese conditions Sulzer’s SXH™70 was been chosen. This material is a cermet – a blend of a ceramic and a metallic alloy – designed to provide protection in conditions of severe abrasion. The material combines hardness and toughness, so that abrasion sistance is combined with flexibility.

Clarke says the SXH showed up well in the on-site testing programme, which was carried out in the 1995 flood season. Specimens were mounted at several points on both Kaplan and Francis turbines, and left during the entire season – a total of 3570hr. At the end of the season a comparison showed that wear on the coated region had reduced the thickness by between 5µm and 43µm, whereas the surrounding metal had been worn down by between 1mm and 10mm.

The venture in operation

According to Clarke, the cermet is applied by a high-velocity oxy-fuel process, which results in very dense and compact coating. The applcation, including the thermal spray process and application to the component is all controlled and carried out by robot. This ensures consistent quality; hand spraying can introduce irregularities that become the site of local deterioration and wear. The material has good adhesion to the metal substrate because of its extremely high impact velocity. The equipment is dsigned to be compact, Clarke says: the entire process can be moved from one power plant to another. The self-contained equipment needs only a power supply, and a supply of bottled process gases. It includes all necessary environmental protection measures, such as a soundproof cabin and a filter system for the exhaust air.

In addition a base factory will provide backup, as well as a complete coating procedure for smaller components.

According to Clarke, for each turbine some 30-40 components will be tested and coated — the most important being the runner, the wicket gates, the labyrinth rings and the turbine covers.

Before the coating process can begin, Clarke says, some preparation must be done at the damaged turbines. It is CNWEP’s responsibility to carry out general maintenance and refurbishment, as it has in the past — the joint venture’s role focuses entirely on the coating. But, as Clarke points out, Sulzer Hydro has enormous experience in maintaining and refurbishing hydro plants and CNWEP will also benefit from this experience.

The coating result depends on many parameters apart from the coating quality itself. The influence of the geometry, the hydraulic profiles and the operating conditions are very important. A careful inspection of the components and verification of these parametersare also part of the role of the joint venture, so the CNWEP turbines will have the best possible protection.

This year will see the mobile coating equipment manufactured and staff recruited locally and trained, according to Clarke. He says the joint venture is due to go into practical operation and begin coating at the end of 1998 or early in 1999.

The coating work will be carried out during the regular maintenance shutdown when the turbines are diassembled. As the schedule for maintenance is generally very tight the availability of the coatng equipment is very important.

The joint venture will initially offer its activities to the power plants on the Yellow River, but Clarke sees more opportunities ahead. CNWEP is expected to pursue new projects for the joint venture to complete. First in line are parent company CNWEP’s other plants on the Yellow River, and more on its tributaries. Further down the line, however, Clarke expects to offer the joint venture’s services in providing longer-lived turbines and maintaining high efficiency to hydro plant operators throughout China.