Increasing reservoir resilience using free discharge valves

Greg Morris, Business Development Manager, Dams, Reservoirs and Hydropower.

Burnhope Reservoir is operated by Northumbrian Water. It is located in the North Pennines AONB (Area of Outstanding Natural Beauty).

Glenfield Invicta was tasked with specifying and designing valves to increase draw down at Burnhope; draw down is the ability to quickly lower the level of a reservoir in an emergency situation.

Construction constraints were also a determining factor; the tailbay into which the drawdown water was to be discharged was narrow and there was no space for a stilling pond.

Specification

When water is discharged from a reservoir at high speed it possesses high kinetic energy. If unchecked, it can cause damage to surrounding structures and natural features. This kinetic energy can be dissipated safely using a free discharge valve.

The Glenfield Invicta engineering team specified a DN600 Series 857 free discharge valve fitted in conjunction with a bespoke hood. With the Series 857, draw down rates can be finely controlled. The bespoke hood was incorporated to limit the width of the discharge plume thereby safeguarding the structural integrity of the tailbay. The calculations on which the valve specification and design were based took into account the reduced Coefficient of Discharge value resulting from incorporating the hood into the design. Remote electric actuation was specified for ease of operation.

A DN800 Series 54 reservoir-specification gate valve was also proposed to provide upstream isolation. 

Design

The general operating principle of a free discharge valve is relatively straightforward. However, the design element is critical to enable the valve to achieve maximum discharge rates whilst ensuring hydraulic forces do not cause excessive vibration. Vibration is minimised through the use of multiple aerodynamic ‘ribs’ which are specially designed and connect the main body of the valve to the downstream cone section.

Sizing calculations are based on the required discharge through the valve at the corresponding available pressure head. At Burnhope, flow rates of up to 4.5m³/s had to be controlled by the discharge valve; this equates to 4.5 tonnes of water passing through the valve every second.

Further calculations were undertaken to determine the relative position of the valve and fabricated hood in order to ensure the optimal discharge flow was achieved.

Installation and commissioning

Whilst this article focusses on the specification and design of the valves, the input of the Glenfield Invicta engineering team was also integral to their installation and commissioning.

Read the full case study here.

Contact: Greg Morris, Business Development Manager, Dams, Reservoirs and Hydropower.

Email:             Greg.morris@glenfieldinvicta.co.uk

Mobile:           +447810377246

Website:          Glenfield Invicta

Enquiries:        enquiries@glenfieldinvicta.co.uk

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