Sloy Hydro Station

Sloy Hydro Station Glenfield Valves Needle Valve Refurbishment

Glenfield Valves Limited was awarded a contract by Dales Engineering Services Ltd for overall client Scottish and Southern Energy (SSE) to refurbish two Needle Discharge Valves – one 48” and one 12” on the Sloy Hydroelectric Power Station.

In May 1945 construction began on the Sloy Hydroelectric Power Station on the banks of Loch Lomond in Scotland. The power station was completed five years later and was opened on 18th October 1950 by the late Queen Mother - It is still the largest conventional hydroelectric power plant in the UK.

The Loch Sloy Dam, built as part of the project, is 56m high and 357m long and raised the surface level of the loch by approximately 47m. The resulting Sloy Reservoir has a 17km2 direct catchment area, although various pipes and intakes have provided a further 63km2 of the indirect catchment area. The total volume of water held in the reservoir by the dam is approaching 36million m2, and a 3km long tunnel takes water from Loch Sloy to a valve house positioned approximately 197m above the tank. From the valve house, four DN2000 steel pipes carry the water down into the powerhouse that is situated on the west coast of Loch Lomond.

The above image shows Sloy Hydro Station and the original valve.

Jim McAllister, Glenfield Valves Project Manager for the project, explained about the valves and the company’s involvement.

“The Needle Discharge valves are the original ones fitted in the 1960’s by Glenfield Valves, and it is the first major refurbishment they have had. We still have the original drawings in our extensive drawings library that enabled us to understand what was needed for the refurbishment."

The valves were originally removed by Dales Engineering Services Ltd and were delivered to the Glenfield Invicta workshop. 

From there the Glenfield Valves engineering team were able to remove and replace the bronze seat and face rings on both valves. The most challenging aspect of replacing the seat and face rings is ensuring that the ‘bedding’ or “lapping” between the seat and face rings was accurate. This is done by hand and is an highly skilled process; one which is fundamental to successful and cost effective valve refurbishment.


The images above show the valves during various stages of refurbishment and the original Glenfield drawings.

Jim explained,

“During the assembly, we have to rebuild all the gearing within the valve and replace the bronze piping for the grease lubricating pipe. We then have to undertake the ‘lapping” element to match the seat faces - this is also carried out by hand. It entails introducing a marking dye on one surface and then closing the valve until the two surfaces are engaged. This indicates the high points on the surface that have not been dyed. It is then that the most intricate and skilled aspect of the works takes place. The engineer fitter has to file or grind the surfaces by hand until they are completely engaged and ‘drop tight’ – no leakage whatsoever! 

The valves are then fully assembled, painted and hydrostatically tested. We received a visit from the Dales Engineering Services Limited and SSE Engineers who witnessed the successful testing of the valves post refurbishment. The SSE engineer commented that he looked forward to another 50 years of successful operation.”

Looking for expert advise for your Hydropower project? Then contact Greg Morris, who can advise on  the right solution for your needs, covering every stage of your project including product selection, application and installation. Greg's contact details are shown above.

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Hydropower from Glenfield Invicta

Hydropower plays a critical role in delivering clean energy. As market leaders, we have been designing and manufacturing specialist engineered valves and penstocks for the Hydropower industry for more than 170 years and we stand behind every product we install.