By Roger Lah, Technical Products Specialist, Cla-Val
A recent visitor to our facility described the lengthy process that entails when a power failure occurs and what that means to his utility’s water treatment plant operator. The operator has to quickly close fifteen motorised valves manually in order to prevent water overflowing onto the plant floor. The valves are usually controlled by a Programmable Logic Controller (PLC), to control flow into a filtration backwash process, utilising venturi flowmeters. These types of valves are susceptible to cavitation problems and therefore a hydraulic pressure reducing valve is normally installed upstream to reduce the pressure and prevent cavitation.
This traditional arrangement requires significant piping space for two valves and a venturi flowmeter. Furthermore, power failure to the motor operated valve can cause significant problems.
Fortunately, the hydraulic pressure reducing valve can now be replaced with a hybrid multi-function electronic and hydraulic metering valve.
This valve is capable of measuring and controlling flow based on valve position and differential pressure sensors installed on the valve. It also incorporates anti-cavitation trim to prevent cavitation due to the high pressure drop into the filtration process.
The electronic metering function utilises dual solenoids and simply changing the upstream closing solenoid from normally closed to normally open will result in hydraulic valve closure when a power failure occurs.
This allows the plant operator to handle other important duties when the inevitable loss of power happens. The single metering valve replaces the previous lengthy piping arrangement, saving significant space.
And because the replacement valve discharges directly into the filtration process, a hydraulic pressure limiting function can be added to provide a ‘hydraulic guarantee’ against over pressurising the system.
Hydraulic pressure reducing with electronic ‘time of closure’
In another treatment plant application, this time utilising prefabricated treatment equipment, a hydraulic pressure reducing valve is used to limit the pressure into the process.
A relief valve provides protection for a long, ten mile pipeline feeding into the plant, when a fast closing butterfly valve, shuts down the process.
The engineer’s surge analysis determined that the pressure reducing valve was required to close in 12 minutes when this occurs and a standard closing needle valve is really incapable of controlling these long closing times.
The simple answer was to modify the pilot control system to perform a switch between the normal pressure reducing function and electronic controlled closing of metering flow to zero flow, in a programmable time period.
A third solenoid installed switches between the normal hydraulic control to electronic control when the system shuts down. Operators can easily see on the valve controller screen the status of the closing sequence.
They are also able to adjust closing times, if the required closing time requires amendment.
This valve therefore accomplishes two very important functions: a hydraulic pressure reducing function during normal operation, and an electronic “time of closure” function to prevent surging in the long upstream pipeline.
Numerous hybrid electronic and hydraulic combinations
Many other hydraulic and electronic functions can be combined onto one valve, which can reduce the number of valves in an installation, protect aging and sensitive pipes, or prevent overflow of tanks and reservoirs.
A particularly popular combination is an electronic flow control with a hydraulic pilot control to limit downstream pressure. This example of hybrid electronic and hydraulic control allows normal electronic flow control unless downstream pressure exceeds the limit of the hydraulic pilot setpoint.
Electronic flow control valve with hydraulic protection of downstream piping
At one recent project startup, an electronic flow control valve began to open, but was unable to barely crack open. A check of the downstream pressure gauge revealed that downstream pressure had already reached the setting of the pressure limiting pilot control.
It transpired that the water district had forgotten that an isolation valve had been closed some distance downstream. In effect, the hydraulic control system prevented a pipe break from occurring.
In fact, the electronic control system is capable of multiple functions such as flow control and pressure reducing. In this instance, the hydraulic pilot can be used as “backup insurance” where the pilot control adjustment is set to not interfere with normal electronic multi-function control.
Normal operation consists of electronic flow control and electronic pressure reducing control, and seamlessly transfers between functions with specialised software. In the event of power failure or solenoid malfunction, the hydraulic pilot will take over and limit downstream pressure, preventing pipe breakage.
Up to four modulating electronic functions are possible, including flow, pressure sustaining, pressure reducing and level control. Reservoir level control, for example, can include flow control and pressure sustaining control.
Conversions of existing altitude control valves often include the hydraulic altitude control pilot for backup control to prevent overflowing of a tank.
Often there is a long pipeline upstream of these valves which is susceptible to surging during valve closure. A hydraulic ‘surge relief override’ pilot control prevents this in case of faster than normal valve closure.
Motorised valves are typically used to control a single function and offer limited or no capabilities to provide multiple electronic functions with hydraulic backup functions.
Replacing these valves with hybrid electronic and hydraulic control can certainly reduce the number of valves required and can offer very useful “insurance” against power failures and costly line breaks.