Valve technology veteran and managing director of Ultra Control Valves, Peter Telle, has been involved in pressure reducing valve station installations in South Africa’s deep level mines for over four decades. He was part of the team of specialists that pioneered pressure reducing valve technology at a gold mine.
The role of Pressure Reducing Valve Stations in contemporary mining operations might be taken for granted, but it is barely credible how water flow was controlled over 40 years ago.
Until the advent or popularity of pressure reducing valve stations, water was traditionally distributed underground through Dams through a “Cascade” system, where water flows from one dam to the next one at a lower level and usually about 100m apart. With this system, Control of the water flow presents many problems, as there are different demands at different levels and each dam relies on the supply to and from an upstream dam. Also, contamination and heat gain is inherently a negative side effect.
A ground-breaking development in 1974 was to revolutionise water flow control in South African underground mines forever. 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 Research and Development (R&D) and modifications to the valve, the were finally commissioned successfully and have since proven to be a much better way of Water Distribution in the Mining Industry. The writer is proud to have been involved in this pioneering project.
The industry norm
Ever since, Pressure Reducing Valve Stations (PRVS) have become the norm for the underground mining industry in South Africa and other mining countries in Africa. Typically, they come in a variety of designs, depending on pressure levels specified by mining engineers for projects.
The vital aspects in PRVS are safety and, as they are mostly located in critical supply lines to the mining areas, the ability to sustain a continuous water supply without interruption.
That’s why suppliers should assist the mining engineer with performance parameters of their product requirements. Specifically, this entails complete reticulation design of underground (and above ground) water supply systems, focusing on safety, continuation of supply, in addition to performing water-hammer analysis, if required.
Typically, PRVS are supplied as a unit, with all valves, piping and instrumentation (if required), skid mounted and with connecting flanges to the mine’s requirements. It is recommended that coded welders should be used, and the mine’s quality and corrosion requirements should be met.
Valves for PRV Stations
To address the pressure reducing valve station requirements, Ultra Control Valves (www.ultravalves.co.za) has a wide range of products under the categories below.
- Basic Pressure Reducing Valve
The basic Pressure Reducing Valve is utilised in reducing a high inlet pressure to a lower downstream pressure. Ultra Control Valves has two different products for this application, namely Pilot Operated Pressure Reducing valve (POPRV) and Ratio Pressure Reducing Valve (RPRV).
- The Pilot Operated Pressure Reducing valve (POPRV)
The Pilot Operated Pressure Reducing valve (POPRV) is available in various Pressure ratings up to 150bar and sizes from 50mm to 400mm. It keeps a constant downstream pressure irrespective of varying upstream pressures or varying flow rates. A few different reliable designs are available in the market.
- The Ratio Pressure Reducing Valve (RPRV)
The Ratio Pressure Reducing Valve (RPRV) keeps 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 particular need. 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 also ideal for use as a first stage “variable energy dissipater” 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 mining environment. As column sizes in Mines are normally big, friction losses are minimal and upstream pressures vary very little, so an adjustment to downstream pressure is seldom needed. The benefits of the RPRV far outweigh the lack of adjustability and is a real Mining-friendly Pressure Reducing valve. The maximum ratio of 5:1 is recommended for good practice and to minimise cavitation.
Should water quality and/or operating philosophy dictate that Self Actuated Control valves are not desired, other options, such as process control valves of a different design and operated by either Electrical or Pneumatic actuators, can be considered. Ultra Control Valves has two types of process control valves, Globe type or Segmented Ball valves.
- Globe Type Control Valve
The Globe Type Control valve can be supplied with options of either a Pneumatic actuator or an Electrical actuator. This valve is available with Pressure Rating up to 250bar. Should pressure drop requirement be in excess of its capability, special trims are available or two valves could be installed in series. The use of the Ratio Reducing valve upstream would be a good option in such cases.
- Segmented Ball Control Valve
The Segment Ball control valve is available up to a Pressure rating of 64bar. It has a very wide range, controlling down to “whisper” flows. It ensures stable control during low demand periods.
- Surge Relief valves
It is recommended to install a relief valve downstream of every Pressure Reducing valve for the following reasons:
- Pilot operated Pressure Reducing valves have a delayed reaction time as water is being bled in and out of a control chamber to close and open the valve. If downstream demand is reduced too quickly (by fast closing of an isolating valve), the pilot operated valve may react too slowly thereby allowing the downstream pressure to momentarily rise above the required set-point. The relief valve would react to this and open to keep the pressure at say 1bar above the setting.
- Any foreign dirt particles in the Pressure reducing valve may prevent its closure during demand shutdown. The relief valve will blow off to maintain the downstream pressure within required limits. On smaller Stations, where the actual PRV’s are up to 80mm size, direct acting Relief valves which are much simpler and without the Pilot Tubing can be considered. Specifically for this task, Ultra Valves has a totally spring operated Surge Relief valve which acts very fast and is available up to 150mm.
Ultra Control Valves has two designs tailored for the mining environment, basic pressure reducing valve station and dual pressure cuts station.
- Basic Pressure Reducing Station
In terms of Basic Pressure Reducing Station, in order to meet the most basic safety requirements and ensure continuous water supply, the layout in the figure on the preious page is recommended and is most often used. The station has an additional feature of an automatic Start-Up system, which allows automatic controlled filling of the downstream piping after shutdown of the PRV station. The configuration below is recommended for pressures up to 50bar.
- Station with Dual-Stage Pressure Cuts
This is recommended for higher inlet pressures where the required pressure drop is too high for the normal pilot operated valve. This is generally required for inlet pressures above 40bar and outlet pressures below 10bar.
- Electronic Control Panel for PRVS
The Panel can be supplied with the following features, which are also dependant on the instrumentation provided on the station.
- Mimic panel indicating which leg is operational.
- Alarm light to indicate fault condition has occurred, with indication of what the fault condition was.
- Readout of Flow
- Readout of pressure
All the readouts can be sent to a remote location (such as a surface control room). Further sophistication is possible such as remote adjustment of pressure settings, remote shutdown and start-up of the station.
- Safety Shutdown due to excess flow
In case of a pipe burst on the downstream side of the station, an orifice plate with DP switch will signal an electrically actuated valve to close after a time delay of say 20 seconds. This function can also be provided by a flow meter with 4-20ma output and an electronic set-point controller mounted in a control panel on the
- Shutdown due to extended relief valve blow off
If the relief valve blows off for an extended period (exceeding say 20 seconds), it could mean that there is a permanent component or system failure. A flow sensor on the reliefvalve outlet will signal the station to shut
down through an Electrically operated isolating valve.
- Automatic Station start-up
Startup of the station after shutdown, has to be done slowly through the manual operation of the pilot operated valve or through the slow opening of the downstream isolating valve. If this operation is done too quickly, resulting surging and unstable operation could cause
damage to downstream piping.
An electronic “soft start” system is available whereby the slow start up is achieved through a control system added to the PRV Station. This will allow foolproof start up of the station through the push of a button.
- Water Quality
If the water quality is suspect it is always a good idea to add a Strainer on each leg of the PRV station. A few different types are available and Ultra can assist with the ultimate choice. Of course what is always important with these strainers is the ease of maintenance – it has to be done while the flow is continuing through the alternative leg, and must not interrupt the supply of water to the mine.
With all the options available and in order to ensure correct sizing and application of Pressure Reducing Stations, it is recommended that specialists from the supply company should be consulted in the early stages of mine reticulation design, in order to ensure optimal performance systems at all times.
It is worth mentioning that, with respect to packaged PRV Stations, the supplier is responsible for correct operation of the components, in addition to meeting all safety requirements. On their part, the design engineer, on behalf of a mine, should check the track record of suppliers and the reliability of their products. The supplier should be able to provide spare parts as and when needed, in addition to capable field staff to assist with commissioning and trouble shooting of the PRVS.
Last but not least, the other important part is maintenance of PRV Stations. Because of high staff turnover on the mines and the fact that their primary focus is on extracting minerals, maintenance of PRV Stations is often neglected. And as a result of this, most PRV Stations are in total neglect with parts missing, a situation which increases safety risks and downtime. And so, most mines should consider outsourcing maintenance duties to well-versed specialists to ensure continuedsafe operation and minimize downtime.