Pen stock are to be considered one of the most important component of any hydro power project and act as key member between the reservoir and power house. Pen stock may be of different materials and each type has its own protection technique and Support anchorage system in the following article two there different of pen-stocks are discussed along with their protection measures and Standard requirements.
Protection of Steel Penstock
1. Above-ground steel pen-stocks should be clear of the ground to avoid corrosion and to allow easy access for paint.
2. Steel pipes must be given two coats of paint on the outer side and the maintenance plan must include regular repainting. Internal painting is recommended. Standard metal oxide paint can be used, although the recommended paints are:
- Romate primer followed by polyurethane enamel top coat.
- Inside: Bitumen (easier to apply, use rag or brush); or lead oxide (more difficult to apply).
3. If steel pen-stocks are buried, they must be coated with bitumen. It is preferred that they are galvanized, and double-coated with either bitumen or high zinc content paint.
4. Tar paper or bitumen must be used between steel pipe work and supports/anchors to exclude water and air which may collect in inaccessible areas and promote corrosion. Rubbing plates or tar paper are recommended between pipe work and support piers to prevent abrasion due to thermal expansion and contraction. Rubbing plates should not be used if they pose danger of air and water entrapment which may lead to corrosion. Tar paper or bitumen may be needed on either side of a rubbing plate to avoid this.
5. Expansion joints are recommended on steel pen stocks:
• One immediately below forebay tank or integrated into forebay tank wall
• Between each anchor block one below final anchor block on sites of more than 10 kW with a high head and complex manifold which may be vulnerable to stress. When visiting verify the above and in addition:
6. Over ground steel penstocks should be inspected to ensure that there are no areas where rain water can accumulate and cause corrosion. Check that this is being done as part of a routine maintenance schedule, and that corrective action is routinely taken (i. e. clearing of plant growth touching the penstock); and that repainting is undertaken periodically.
7. Over ground penstocks should be checked to ensure that thermal expansion will not cause wear at any sliding support.
8. During commissioning check that expansion joints are not closed up tight to one end of their movement as this may cause leaks and failure. Check that operators know how to reset expansion joints after works are carried out on the pen stock.
Protection of PVC and HDPE penstocks
1. PVC and HDPE pipes often give good service without special protection from either sunlight or thermal expansion and contraction forces. Nevertheless it the following precautions can improve the cost effectiveness of the scheme:
a. PVC penstocks can be protected from sunlight by paint or some other sort of effective and durable covering; or by burying.
b. Expansion/contraction. PVC pipes with spigot and socket joints can accommodate thermal forces without special measures. If a glued PVC pipe is not buried and is straight, a socket and spigot joint is recommended between anchors and at the forebay to accommodate thermal forces; alternatively some inclusion of flexibility in the run of pipe.
2. If PVC penstocks are buried they must be laid on sand beds without danger of stress points occurring below or above the pipe surface due to objects such as stones butting against the walls of the pipe.
Penstock supports and anchors
1. Anchors and supports must be sized and designed to reliably accommodate all possible sliding, toppling, and sinking/lifting forces.
2. Supports should be designed to allow some longitudinal sliding between the pen-stock and the support. Supports must be fitted with a vertical movement retention method, to accommodate lifting forces.
3. Anchors should be constructed from concrete, preferably reinforced. Special attention should be given to ensuring that their foundations are sound and not likely to move (as this will cause severe difficulties in handling the turbine manifold, and jeopardize the forebay and penstock).
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