Raising a pennant for penstocks

Penstocks have been used to control flows for hundreds of years. Here, Ham Baker Hartley's Andy Such reports on the importance of maintaining and refurbishing penstocks

There are approximately 7,000 WwTWs and 2,000 WTWs in the UK. The companies and authorities responsible for these works need to understand the condition, behaviour and fitness for purpose of individual items of equipment and have in place appropriate maintenance programmes and refurbishment strategies because these assets have to be managed effectively to maintain service levels to current and future customers.

This article seeks to illustrate this by looking at one particularly important item of flow control equipment – the penstock. The word ‘penstock’ originated in the 17th century as a description for the device used to control the flow of water to a water mill. They were as critical then to the safe and efficient operation of the mill as they are now, 400 years later, to the safe and efficient operation of treatment works.

However, due to their robust construction, general trouble-free operation and longevity, many penstocks are still working satisfactorily after more than 100 years of operation, they tend to be overlooked in capital maintenance and refurbishment programmes. The knock-on effects of a penstock failure as a result of poor maintenance can be catastrophic, not only from an operational point of view, but also environmentally. Penstocks supplied by Ham Baker Hartley, many of which have been in operation for more than 100 years, will give years of trouble-free operation, providing that a few simple maintenance procedures are adopted. The frequency of required maintenance activity is dependent upon the frequency of operation of the unit and on the operating conditions present.

The following gives minimum maintenance requirements, except for modulating (flow control) penstocks, where special and more frequent maintenance is required.
It is recommended that every three months, you should:

    hose down with clean water, operate through a full cycle and check for leakage,
    while fully open and accessible, check seals for excessive wear or damage, adjust wedges or adjusters if necessary and grease metal sealing faces,
    check all components for corrosion and erosion damage,
    check any operating gear for damage, wear and freedom of movement, and grease as necessary,
    check any guarding originally supplied with the unit,
    lightly oil or grease all other moving parts. Every 12 months, you should:
    remove the threaded door nut (non-rising spindles) or threaded yoke sleeve (rising spindles) and thoroughly check for signs of wear.

Ham Baker Hartley penstocks are designed and
manufactured for long life and trouble-free operation, and experience has shown if maintenance programmes are adhered to, then there is usually little need for spare parts.

However, should spare parts be required at some time in the future, then these can be ordered from Ham Baker Hartley irrespective of the age of the penstock. A service programme from Ham Baker Hartley, tailored to the specific types and operational duties of the penstocks on a works, will maintain the performance of the penstocks, safeguard the capital investment, reduce overall operating costs and improve planning and financial control. As with any piece of capital equipment, problems can occur during operation and Table 1 gives likely causes of problems and suggested remedial action.

Where a penstock is found to be in need of refurbishment it is generally following a risk assessment that has identified issues, which require action to meet current health and safety regulations. An example of this is the recent refurbishment of four penstocks at Thames Water’s Wick Lane Penstock Chamber, which is adjacent to the Abbey Mills pumping station.

The penstocks, two 4m x 3m and two 3m2, were installed about 100 years ago and operated via an electric motor and gearbox arrangement assisted by counterbalance weights (Figure 1). A risk assessment undertaken by Thames Water indicated several areas of concern.

The existing steelwork supporting the operating gear and counterbalance weights was proven to be unsafe, the electric motors in the enclosed building, deemed to be a confined space, were not suitably rated. The counterbalance chains had to be inspected regularly and changed at regular intervals, which required entry into a confined space.

In addition, safety is doubly important at this penstock chamber because it is opened to the public every May to allow people to see the culverts of the great northern outfall sewer constructed by Sir Joseph Bazalgette. Prior to undertaking the project, Ham Baker Hartley was involved in a site survey to establish the condition and suitability of the penstocks for continued use and to provide a budget price for target costing.

First of all the existing control building was removed, followed by removal of the operating gear, the York stone flooring and the support steelwork. The existing 16 and ten tonne counterweights were then removed. A new low-level walkway was installed to allow for inspection of the penstocks. The chamber top was then modified to accept the new steelwork and the new pillars and actuator gearbox combinations (Figure 2).

The electric actuators are rated Zone 1, explosion-proof and can be operated locally or from a remote operations room. The penstocks themselves, including the metal seals, did not require any refurbishment and the new installation is good for the rest of this century.

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