Assessing automatic control valves
As automation and computer control gained a foothold in the operation and control of reservoirs and elevated water tanks, the industry moved valve control from mechanical automatic control valves (ACVs) to actuated butterfly and ball valves. The primary reason for this was that actuators can be controlled by SCADA systems, which have been almost universally implemented in modern water systems. In choosing actuated butterfly and ball valves, many water system managers and operators have stopped purchasing the mechanical “altitude valve,” which was the standard for many years.
An altitude valve is a mechanical valve that uses a sensing line from the reservoir or tank to a highly sensitive pilot, allowing the valve to open or close to maintain a user-set level of water. Altitude valves can be designed to open once the water level begins to drop, usually around 1 to 2 ft, or can be equipped to allow for a user to adjust draw-down of the water level to enable more tank or reservoir turnover.
Altitude valves still are used when a location at a tank or reservoir has no power, or when getting power to the location is extremely costly. They also are suited to cold climates, where a top layer of ice in a reservoir can interfere with electronic measurement. For smaller water suppliers that do not have the budget to implement a SCADA system, a mechanical altitude valve is an appropriate option.
How Automatic Control Valves Work
All diaphragm automatic control valves work on the same principal by using a rubber diaphragm to separate the upper control chamber from the actual water moving through the valve when it is operational. By controlling the water either entering or leaving the control chamber, the valve can be opened fully, closed drip tight, or modulated into any position in-between.
Advantages of ACVs include:
- ACVs are designed to modulate and control water at all varying flows, and butterfly valves were originally designed to be an open or closed valve. The advent of an actuator allowed a butterfly valve to be modulated, but this was not what it was designed for. Butterfly valves and ball valves that are only opened a little bit are prone to cavitate, and little can be done to stop or control this.
- Many ACV manufacturers have an option for an anti-cavitation trim. Cavitation can occur in level control valves when a large inlet pressure is supplied by the system. As a general rule of thumb, any time the inlet pressure is more than three times higher than the reservoir tank’s head pressure (which creates back pressure), the valve will cavitate. This can significantly damage the valve, as well as create considerable noise and vibration. ACVs equipped with an engineered anti-cavitation trim can stop all cavitation damage, as well as quiet the valve and reduce vibration.
- Almost all ACVs can be serviced in-line without having to remove them from the line on which they are installed. This allows for faster service with less downtime.
- Using an actuated butterfly or ball valve can be costly. Three-phase power to run an electric actuator can be expensive, as can the cost of a compressor with air lines for a pneumatic actuator. ACVs do not require these. In the event of a power failure, battery backups for the actuators are an additional expense.
- ACVs have the ability to easily add additional features to the pilot system that make the valves more flexible. It is inexpensive to add a pressure-sustaining feature, or a rate of flow feature, to a mechanical ACV.
Electronic Control of ACVs
In the past, all ACVs were controlled and operated using a mechanical pilot. Though this is a reliable technology, in today’s water systems, having control and feedback through SCADA is desirable. This is the main reason why water system managers and consultants changed over to electrically or pneumatically controlled butterfly valves. Today, however, most automatic control valve manufactures have adapted their ACVs to be operated and controlled electronically.
The most common means of electronic control is installing solenoid valves into the pilot system that controls the water flowing into and out of the control chamber. One solenoid valve, when opened for a controlled amount of time, allows water into the control chamber—thus, it begins to close the valve. When open for a controlled amount of time, the second solenoid valve allows water to exit the control chamber, which, in turn, opens the ACV a certain amount.
By controlling how much water moves into or out of the control chamber, the valve can be opened, closed or modulated. These solenoid control valves can be pulsed open and closed as much as the user desires, or they can adjust the opening of the ACV to a very precise position. All that is required is a controller (stand-alone PLC or SCADA controller) with a feedback sensor. The controller then gets feedback from a level sensor and makes adjustments to the ACV by pulsing the solenoids to open or close the valve.
While having electronic control allows for control and feedback of an ACV, during a power failure or outage, this changes. With battery backups, ACVs and actuated butterfly valves can be ordered to close fully, open fully or maintain last position, but this can have significant side effects. If the valve is told to close during a power failure, the tank or reservoir will not be filling during the entire outage, so the tank could drain very low or completely if the power outage lasts too long.
If the valve is left open, the tank could overflow and a significant amount of water could be lost, which costs money and wastes resources.
Unlike the actuated butterfly valve, an ACV also can have a standard mechanical level control pilot (altitude pilot) added to the valve as a backup in the case of a power failure. This way, the valve would not need to go open or closed, but could instead be controlled with the mechanical pilot, allowing the tank to keep filling and then shut off when the desired level was reached. As the water supply industry continues to move to electronic control, using an ACV equipped with electronic solenoids is a favorable option for level control of tanks and reservoirs. Adding a mechanical backup pilot system to control during power failures or outages allows the system to maintain optimum water levels. Adding an anti-cavitation trim makes the automatic control valve a versatile option for long-lasting level control with a guaranteed backup.