Pressure Reducing Stations for Deep Mining
Pressure reducing stations have become the norm for the deep mining industry. There are many different designs, with varying levels of safety, as required by the pressures involved, and the needs of the mining engineers. Alongside safety, a continuous water supply, without interruptions, is essential, as these stations are often on critical supply lines to the mining areas.
Ultra are able to assist the mining engineer with complete reticulation design of underground (and above ground) water supply systems. The designs consider safety, continuation of supply, and performing waterhammer analysis, if required.
Ultra Control Valves manufactures pressure reducing stations, 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 specifications are met. These packaged stations ensure Ultra is responsible for correct operation of all items on the station, and that all safety requirements are met. Ultra’s field team is available to assist with commissioning, and after sales service and training required.
Basic Pressure Reducing valve
In its most basic form, this is a single valve designed to reduce a high inlet pressure to a lower downstream pressure.
Ultra have two different products for this as follows:
- Ultra ACV Pilot operated valve
This valve is available in 25 bar, 40 bar and 100 bar ratings, and sizes from 50mm to 400mm. This valve will keep a constant downstream pressure, irrespective of varying upstream pressure or varying flow rate. The Ultra ACV valve has been around for nearly 6 years, and is well known in the mining industry. It can handle pressure drops of up to 4:1 without cavitation.
- Ultra Ratio reducing valve
This valve 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 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. This makes it ideal for use where more than one pressure reducing valve has to act in series, a condition which can cause pilot operated valves to become unstable. It is ideal for use as a first stage “energy dissipator” upstream of the Ultra ACV control valve.
The Ultra Ratio reducing valve has been used in the mining industry for over 6 years, and is available with pressure ratings of 25bar, 40bar and 100bar.
It is recommended to install a relief valve downstream of every pressure reducing valve for the following reasons:
- Pilot operated pressure reducing valves react as water is 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 setpoint. 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.
Basic Pressure Reducing Station
In order to meet the most basic safety requirements, and ensure continuous water supply, the following layout is recommended:
This 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 above configuration 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 Ultra ACV pilot operated valve. This is generally required for inlet pressures above 40bar, and outlet pressures below 10bar.
Other Safety Features
- Shutdown due to excess flow
In case of a pipe burst on the downstream side of the station, an orifice plate, with DP switch, signals an electrically actuated valve to close after a time delay of, say, 20 seconds. This function can also be provided by a flowmeter with 4-20ma output, and an electronic setpoint controller mounted in a control panel on the station.
- 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 relief valve outlet signals the station to shutdown through the electrically operated isolating valves.
- Automatic Station start-up
Start up 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 Ultra ACV valve. This will allow foolproof start up of the station through the push of a button.
Electronic Control Panel
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.
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.
Electrically or Pneumatically operated Control valves
Should water quality and/or operating philosophy dictate that self actuated control valves are not desired, Ultra also offers process control valves of a different design. These are operated by either electrical or pneumatic actuators. Two types are available i.e. globe type or segmented ball valves as follows:
Globe Type Control Valve
The globe type control valve is shown with a pneumatic actuator, but can also be supplied with an electrical actuator. This valve is available with pressure rating up to 250bar. Should the pressure drop requirement be in excess of its capability, special trims are available. Alternatively, two valves could be installed in series.
Segmented Ball Control Valve
The Segments Ball control valve is available up to a pressure rating of 64bar. It has a very high rangeability, and can control down to “whisper” flows, which has the advantage that it offer stable control during low demand periods.
There are many options available, and it is vital to ensure optimal systems. With this in mind, it is recommended that Ultra Control Valves staff is consulted in the early stages of mine reticulation design, in order to ensure correct sizing and application of pressure reducing stations.