Correct sizing and application
Peter Telle discusses key factors to be considered when designing pressure reducing valve stations for deep mining applications
Since the early days of Deep Mining, water has traditionally been distributed underground through Dams in a “Cascade” system whereby water flows from one dam to the next one at a lower level and usually about 100m apart. Control of the water presented many problems such as different demands at different Levels and each dam relying on the supply to and from the dam upstream of it. Also contamination and heat gain is inherently a negative side effect.
In about 1974 a Control system was pioneered by Anglo American at Elandsrand Gold Mine in Carltonville, whereby water pressure was reduced from an inlet pressure of up to 95bar (950m from the feeding surface dam), using a Self Actuated Pressure Reducing valve imported from USA. Although a lot of problems were initially experienced, after a lot of R&D and modifications to the valve, the systems were finally commissioned successfully and have since proven to be a much better way of Water Distribution in the Deep Mining Industry
Design Considerations
Pressure Reducing Stations have become the norm for the Deep mining industry. They include many different designs with different levels of safety as required by the pressures involved and the needs of the mining engineers. Together with safety, a need also exists for continuous water supply without interruptions, as these stations are mostly on critical supply lines to the mining areas. Suppliers of Pressure Reducing valves (PRV’s) should be able to assist the mining engineer with the performance parameters of their PRV’s, whereas some of these suppliers are able to assist with complete reticulation design of underground (and above ground) water supply systems, considering safety, continuation of supply and performing Waterhammer analysis if required.
Pressure Reducing stations are manufactured by a few suppliers with all valves, piping and instrumentation (if required), skid mounted and supplied as a unit with connecting flanges to the mine’s requirements. Coded welders are used and all the mine’s quality and corrosion requirements are met. Supplying packaged stations puts all responsibility for correct operation of all items on the station on the supplier, and ensures that all safety requirements are met. In considering the suitability of a company supplying such complete stations, they should prove the availability of Field staff to assist with commission and trouble shooting of these PRV Stations.
Basic Pressure Reducing valve
In its most basic form this is a single valve to reduce from a high inlet pressure to a lower downstream pressure. Ultra have two different products for this as follows:
- Pilot operated valve
This valve is available in various Pressure ratings up to 150bar and sizes from 50mm to 400mm. This valve will keep a constant downstream pressure irrespective of varying upstream pressure or varying flow rate. A few different reliable designs are available in the market and the Design Engineer should check the track record of the product and supply company. Users of these valves forget the critical function these valves have to perform 24/7 and they need to have proven reliability and ready supply of spares.
Ratio reducing valve
- This valve will keep a constant ratio between inlet and outlet pressure. It does not use a pilot and is the simplest form of Pressure Reducing valve. Any ratio can be manufactured to suit a need with the maximum ratio of 5:1 recommended for good practice and to minimise cavitation. The Ratio valve has a very quick reaction time to flow or pressure changes, which makes it ideal for use where more than one Pressure Reducing valve has to act in series, and where pilot operated valves can become unstable. It is ideal for use as a first stage “energy disipator” upstream of a Pilot Operated Pressure Reducing valve.
The ratio Reducing valve is the simplest form of Pressure Reduction and ideally suited for the rough environments experienced in the Mining Industry.
