Metal seated gate valves in award-winning reservoir project

Glenfield Invicta has supplied valves, actuators and gearboxes for a fascinating project on a reservoir that carries the name of the engineer who designed it.

By Greg Morris, BD Manager – Dams, Reservoirs & Hydro, Glenfield Invicta

Scotland’s terrain and preponderance

Although Scotland’s population accounts for only 8% of the total UK population, it has almost a third of the total landmass. The wet and mountainous terrain that covers much of Scotland, along with its preponderance of lochs (‘lakes’ or ‘fiords’), makes it an ideal location for dams and reservoirs. Scotland also hosts more major hydropower projects than any other area of the United Kingdom.

The largest owner of registered reservoirs in the UK

The Reservoirs Act 1975 requires all large-raised reservoirs to be registered. Registration brings with it various obligations, including regular inspections by a Reservoir Panel engineer.
Scottish Water, the public utility which provides water and wastewater services to Scottish homes and businesses, owns 270 registered reservoirs; it is the largest owner of registered reservoirs in the UK. One aspect reviewed by a Reservoir Panel engineer is the rate of reservoir drawdown. Drawdown refers to the evacuation of water from a reservoir for maintenance, safety and emergency requirements. Several high-profile incidents across the UK, and the general ageing of the nation’s stock of reservoirs, has led to a greater focus on draw down capacity, with new guidelines issued in 2017.

Drawdown and flood discharge are key focus areas

Scottish Water has embarked on a ‘Reservoir drawdown and flood discharge enhancement programme’. Drawdown can be achieved by a number of methods including a conduit (pipe/tunnel) passing through a dam embankment, a siphon that ‘siphons’ water up and over the dam wall, and spillway weirs. Glenfield Invicta specifies, supplies and installs a wide variety of AVK valves into reservoir projects including reservoir-specification gate valves, scour valves, discharge valves and penstocks.

Loch Thom: its history

The Loch Thom reservoir is located south of Greenock some 25 miles west of Glasgow. It was constructed between 1825 and 1827.
The earth embankment that holds back Loch Thom is about 20m high and contemporaneous records state that the embankment was built in layers of puddle peat alternating with gravel, ‘…beaten together with wooden dumpers until completely mixed.’
Loch Thom is unusual in that it is named after the engineer who designed it: Richard Thom. Its principal function when first built was to power water wheels for a number of local industries including a paper mill, woollen mill, sugar refinery, foundry and shipyard.

A solution to increase discharge capacity

A Section 10 inspection report for Loch Thom Reservoir in 2013 recommended that the discharge capacity of the tunnel outlet and the associated drawdown capacity be verified.
Consequently, in 2016, consulting engineers Mott MacDonald undertook a drawdown feasibility study. The study identified that the existing discharge capacity was insufficient.
The proposed solution was to install a supplementary three-pipe siphon arrangement to provide the required additional drawdown capacity. Each of the three siphon pipes were specified as 1100mm tapering down to 900mm. The required flow rate through each siphon line was 5m3/s.

Specifying the optimal valve

Glenfield Invicta worked with Mott MacDonald to optimise valve selection for the siphon lines. The valves were to be installed on the 900mm diameter pipe section where the flow velocity was to be 7.9m/s. This velocity exceeds the maximum recommended flow velocity for standard gate valves. For example, the recommended maximum flow velocity for a PN16 rated gate valve is 4m/s as per BS EN 1074.
After reviewing the system details and the proposed valve application Glenfield Invicta recommended the use of its Series 54 ‘reservoir-specification’ metal-seated gate valve.

Supply and installation

In total, Glenfield Invicta supplied 6 x DN900 Series 54 gate valves, and 1 x DN500 Series 54 gate valve on the principal Loch Thom project works. The valves were supplied with spur gearboxes and electrical actuators. Glenfield Invicta service engineers were on site to assist with the installation of the gearboxes and actuators.

Alongside the principal works, a temporary four-pipe siphon arrangement was installed. The temporary siphon was constructed using 180mm polyethylene pipe and fitted with AVK Series 01/79 innovative socketed resilient seated gate valves with integral tensile end load restraint adaptors. Once the temporary siphon was no longer needed, the valves were cut out for use on future schemes.

The crucial aspects

Glenfield Invicta believes in continuous improvement across all areas of its business. A key element in this approach is extracting key points from each project it undertakes.
In the case of Loch Thom, there were three key points. Firstly, specifying the correct valve is fundamental to the success of drawdown and discharge projects. Secondly, at Loch Thom high flow rates meant standard gate valves were unsuitable, and Series 54 reservoir-specification gate valves provided the optimal solution. Finally, the involvement of the Glenfield Invicta site engineering team contributed to the success of the project.

It was extremely important to discuss the exact requirements for the valves being used on this critical section of the project. Glenfield Invicta had detailed discussions with the consulting engineers on flow rates, and it was these discussions that allowed us to confidently identify the optimum valve specification for this application.

Award-winning innovation

The team leading the Loch Thom project has recently received the Innovation Award at the 2021 Scottish Civil Engineering Awards. The project has also been recognised by the Scottish Parliament.

Merger of Glenfield and Invicta creates an engineering powerhouse

In October 2019, two AVK UK Group companies, Invicta and Glenfield merged to form Glenfield Invicta. One of the key drivers for the merger was to take advantage of significant growth opportunities in their shared core markets: dams, reservoirs, hydropower and environmental engineering.

Series 54 reservoir-specification gate valves

Glenfield Invicta has developed a specialist reservoir-specification for gate valves to accommodate the operating parameters required on reservoirs which can be onerous. For example, the seat and body rings in the gate valve have to be screwed and pinned to be able to accommodate considerable flow velocities that are well in excess of standard water systems.
To develop the reservoir-specification, Glenfield Invicta took the standard AVK Series 54 metal seated gate valve as its starting point and developed an enhanced design.

Key features include:

  • Aluminium bronze shoes and channels result in a very tight and continuous tolerance between the body and wedge throughout the valve stroke. This reduces potential vibration and fatigue damage. It also reduces bearing stresses on the contact areas as well as improving the alignment and sealing performance of the valve. Operational torque requirements are lower due to the reduced friction coefficients
  • The addition of a jacking screw (or two on certain sizes) at the base of the valve allows a direct axial thrust to be applied to the base of the wedge. If the valve has been closed for long periods of time, it can become increasingly difficult to operate. Rotating the jacking screw ‘pushes’ the base of the wedge up a small distance, sufficient to ‘crack’ the valve out of its seated position; normal operation of the valve from the valve stem can then be carried out with ease. The jacking screw also acts as a mechanical stop which prevents over-travel of the wedge which can cause considerable operational issues with metal seated gate valves.
  • Stainless steel fasteners and stem, and an increase in coating thickness, combine to prevent corrosion and increase the valve’s operating life.

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