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The isolating, safety and control valves, utilized for the mining
hydropower system can be subjected to severe working conditions such
as corrosion, scaling, cavitation, high content of solid in the
water, low or high pH values, etc and they must be designed to: -
- Reduce breakdown downtime, which affects the mine production and
can cause cost losses with no relation to the equipment cost.
- Comply with local and/or international design, safety and testing
standards.
- Be of a standard type in purpose to reduce spares holding and to
make operation and maintenance easier.
- Have identical parts irrespective of the application or the
pressure rating.
- Be designed for mounting in straight pipeline.
- Be insensitive to its orientation in the pipeline.
1) The water velocity inside the valve should not exceed: -
- 12 metres/sec for isolating valves in the fully opened
position.
- 12 metres/sec for safety isolating valves in the fully opened
position.
- 200 metres/sec for pressure reducing valves in any opened
position.
2) Allowable pressure drop through valves should not exceed the
following values at the maximum flow rate.
- Shaft isolating valves : 0.5-1.0% of valve static inlet
pressure
- Safety isolating valves : 0.5-1.0% of valve static inlet
pressure
- Haulage & Stope isolating valves : 0.5-1.0% of valve
static inlet pressure
- Pump discharge valves : 0.3-0.5% of valve static outlet
pressure
- Turbine feed control valves : 0.5-1.0% of valve static inlet
pressure
- Pressure reducing valves : 80% of incipient cavitation level.
3) Complete leak tightness is required under all working
conditions.
4) Static seals should preferably be O-Ring types.
5) Dynamic seals should preferably be pressure assisted cap seal
types. Packing and O-Rings can be used in application not exceeding
60 bars.
6) Clearances between moving and static parts of the valve should
be sufficient to prevent jamming of the valve as a result of scaling
or corrosion products.
7) The inner valve parts, which are in contact with dynamic
seals, should be constructed of corrosion resistance material.
8) Coating can be used for all valve parts that are not in
contact with dynamic seals.
9) Material of construction should be: -
- Corrosion resistant material for valve body's bore.
- Inner parts must comply with 7) & 8) above.
- Seals should be preferably made of natural rubber, Nitril or
polyurethane.
10) The minimum valve body wall thickness must comply with ASME/ANSI
B16.34
11) The valve has to be designed in such a way that will prevent
cavitation damage to the seat area and minimize cavitation damage to
other parts of the valve.
12) Isolating valves should preferably be statically balanced at
all positions and in particular in the closed position, in purpose
to enable the easy opening and closing of the valve. |
