By Alejandro Duque
There are many ways to store water in a distribution system and the control system is the key component to ensuring it is done right. Common practice is to use different forms of hydraulic altitude water control valves which use different control pilots, depending on application, to measure the tank levels and maintain or adjust water storage levels.
The two key factors for water storage are to ensure the tank maintains the correct level and that there is water turnover to protect water quality. In colder climates, low tank turnover can lead to water freezing within it. Water in motion due to tank turnover is far less likely to freeze.
Freezing can cause the altitude water control valve to lose the capability to sense a full tank and therefore can cause overflows. Freezing can also cause structural concerns internally, as well as externally, with ice pressure expansion on rivets, and bolted or welded seams. The additional pressure of freezing can also lead to pipe breaks and cause leaks where corrosion has occurred.
In an ideal case, the altitude water control valve would control tank water level to the set maximum of a hydraulic control pilot. Following this, the altitude water control valve closes and then the distribution line is opened. This allows the tank to drain down to a satisfactory level, where enough water turnover has been accomplished, before the altitude water control valve opens again to fill the tank to the set maximum.
In a two-way altitude water control valve situation, the same is true, except distribution is through the same valve as the filling cycle. In this case, the altitude water control valve would still close when the maximum of hydraulic control pilot is met. However, it would then re-open when the differential pressure in the line is reversed, allowing water to distribute in the opposite direction.
Ideal operation is not always easily achievable, as system characteristics can change continuously and affect the system. As system demand changes, so does the pressure in the system. It is not uncommon for the incoming pressure to an altitude water control valve to drop off.
Altitude water control valves operate off the differential pressure of the inlet of the valve to the tank level head pressure. Consideration needs to be taken to ensure that the inlet pressure of the altitude water control valve is always greater than the tank level head pressure.
If the system fails to have a greater inlet pressure, it essentially equalizes the inlet pressure of the altitude water control valve and tank level head pressure. With zero differential pressure, the altitude water control valve will essentially sit floating open, trying to fill the tank but never reaching the maximum level setpoint. With zero differential pressure, the system is essentially sitting at a static flow and pressure. Therefore, no water will flow in or out of the tank which means there would be no tank turnover.
To fix this situation, the set maximum of the hydraulic control pilot needs to be adjusted and set to the correct tank level. Or, the system inlet pressure needs to be increased above the desired tank level maximum setpoint. Adjusting the hydraulic control pilot entails a site visit to adjust the set screw of the altitude water control valve. This can be quite cumbersome in remote sites when many adjustments are needed to facilitate system changes.
Increasing system inlet pressure entails either ramping up pumps in the system, or adjusting the feeder zone pressures higher. Both situations add time and cost, and can also increase water loss and add strain on the system due to extra pressure. This could result in additional pipe breakages.
Many systems are moving towards full remote autonomy and control. Hydraulic altitude water control valves have controlled tank levels for many years, but now the same control can be done with control panels and instrumentation. By switching out the hydraulic control pilots traditionally used to sense level, and instead using electronic tank level/pressure switches, or sensors paired with a control panel, the level control process can be achieved electronically.
The hydraulic water control valve still remains. However, there is no longer the need for a complex hydraulic pilot system as mentioned above. Just a simple single solenoid is needed.
Once automation control is installed, one of two operational sequences can be used to achieve the level control process. The first sequence entails using one level/pressure switch located at the maximum level setpoint and a secondary at the tank drawdown setpoint. If the water level is located below the tank drawdown switch, the panel will open the control valve via the solenoid and allow the tank to fill until the maximum level switch is contacted. The panel will then close the control valve using the solenoid.
This control valve will remain closed until the tank level is drawn again below the drawdown switch. Although this is an effective technique and provides more flexibility than the basic hydraulic altitude water control valve, the maximum level and drawdown switches are still sitting at a constant level. They would need to be adjusted accordingly on site to allow for variations of the setpoint, which is sometimes needed to adjust to system changing pressures.
A more versatile option is to use a level/pressure transmitter. These are available in all shapes and forms to fit specific tanks and offer a 4-20mA feedback signal to the control panel that gives exact tank level. Having a live feedback signal allows the panel to offer up a lot more options in the way the tank level setpoints are set. The level control panel can then offer a variable setpoint for both the maximum level and drawdown that can easily be changed via a user interface on the panel. This eliminates the need for site visits to change any setpoints and allows the system’s setpoints to be changed as needed to ensure optimum operations.
The level control panel with touch panel (LCP-TP) interface designed by Singer Valve was created to accomplish optimum level control feasibility and flexibility. It is designed to complement a single solenoid operated/override control valve and 4-20mA level sensor or high/low level switches. This combination package works well for filling any kind of tank that requires filling to a level setpoint and then drawing down the level of the tank to a secondary setpoint. It then activates the fill cycle again, ensuring water turnover.
The LCP-TP is quick and easy to configure to read and compare the level 4-20 mA signal to the desired setpoint. Setpoints can be set locally via an interactive button display screen or remotely via either SCADA Modbus or hardwired 4-20 mA remote setpoint signals. If a high/low level switch system is preferred, the LCP-TP can switch configuration to allow for level switch inputs and regulate the control valve accordingly. Data logging is also a useful feature to log sensor feedback and setpoint data with a time stamp, allowing for system analysis.
For the user who wants to set up a full communication network that has access to all storage tanks and controls them remotely, it is easy to do with Modbus and remote 4-20mA communication options. For the user that has a remote site, but wants to be able to data log and analyze the tank turnover, the LCP-TP offers the data logging feature.
Either option offers lots of feedback and traceability of the system operation. Based on this information, the tank level setpoints can be adjusted to match the needs/demands of the system with simple interaction to ensure it can function optimally. The ability to take all the information and then easily adjust the tank setpoints is a huge benefit.
The LCP-TP itself does not eliminate tank turnover issues. Systems still need to be operated correctly. But having access to all the feedback information should allow insight into doing so. Alarm/notification means operators instantly know if something goes wrong with the system. Procedures can then be set in motion right away to rectify the problem.
Alejandro Duque is with Mueller Water Products. This article appears in ES&E Magazine’s February 2020 issue.