Automatic control valves can help safely maintain tank and reservoir levels

ACVs can help safely maintain tank and reservoir levels
Maintaining levels in water storage tanks is vital to ensuring an adequate supply, especially when water is needed during fire fighting situations.
By Jody Malo

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 (ACV) to actuated butterfly and ball valves. The primary reason is that actuators can be controlled by SCADA systems, which have been almost universally implemented in modern water systems. Therefore, many water system managers and operators have stopped purchasing the mechanical “altitude valves” which were 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. It allows the valve to open or close to maintain a user-set level of the water. Altitude valves can be designed to open once the water level begins to drop, usually around 0.3 m to 0.6 m, or they can be equipped to allow for a user to adjust drawdown of the water level to allow for more tank or reservoir turn over.

Altitude valves are still used when the location of a tank or reservoir has no power, or where getting power to the location is extremely costly. They are also better 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 still a good option.

Automatic control valve (ACV)
An automatic control valve.

How do automatic control valves work?

All diaphragm automatic control valves use 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 going into the control chamber, or leaving the control chamber, the valve can be opened fully, closed drip tight, or modulated into any position in between.

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What are the advantages?

ACV valves are designed to modulate and control water at all varying flows, and butterfly valves were originally designed to be open or closed. The advent of an actuator allowed a butterfly valve to be modulated, but it was not what the butterfly valve was truly designed for. Butterfly valves and ball valves that are only opened a little bit are prone to cavitate and nothing much can be done to stop or control this.

Many ACV manufactures also have an option for an anti-cavitation trim which can occur in level control valves when a large inlet pressure is supplied by the system. Generally, anytime 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 a lot of noise and vibration. ACVs equipped with an engineered anti-cavitation trim can stop all cavitation damage, as well as quiet the valve down and reduce vibration.

Almost all ACVs can be serviced in line without having to be removed from the line they are installed on. This allows for faster service with less downtime, since keeping the system in operation as much as possible is key.

Costs can be more when using an actuated butterfly or ball valve. Three-phase power to run an electric actuator can be expensive. The cost of a compressor with air lines for a pneumatic actuator is equally expensive. ACVs do not require these additional costs. 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 to make the valve more flexible. It is very inexpensive to add a pressure sustaining feature, or rate of flow feature to a mechanical ACV.

Electronic control of ACVs

In the past, all automatic control valves have been controlled and operated using a mechanical pilot, which is a proven and reliable technology. But in today’s water systems, having control and feedback through the SCADA system is a desired feature. This is the main reason water system managers and consultants changed over to electrically or pneumatically controlled butterfly valves. However, most manufactures have now adapted their ACVs to be operated and controlled electronically.

There are a few different ways, but the most common is by installing solenoid valves into the pilot system that controls water flowing into, and out of the control chamber. There is one solenoid valve that when opened for a controlled amount of time, allows water into the control chamber, thus it begins to close the valve. The second solenoid valve when open for a controlled amount of time, allows water to come out of the control chamber, which in turn opens the ACV a certain amount.

By controlling how much water either moves into the control chamber, or out of it, the valve can be opened, closed, or modulated. These solenoid control valves can be pulsed open and closed as much as the user wants to have the valve open, closed, or adjust opening of the ACV to a very precise position. All that is required is a 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.

Additional options for increased confidence

While having electronic control allows for excellent control and feedback of an ACV, during a power failure or outage this all changes. With battery backups, the ACVs and actuated butterfly valves can be told 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 is lost.

Unlike an actuated butterfly valve, an electronically controlled automatic control valve can also have a standard mechanical level control pilot (altitude pilot) added to it as a backup in the case of a power failure. This way the valve would not need to go open or closed, but could now be controlled with the mechanical pilot and allow the tank to keep filling and then shut off when the desired level is reached. This ensures the system is working at its optimum even during power supply issues at the tank or reservoir location.

Jody Malo is with Singer Valve. This article appears in ES&E Magazine’s June 2017 issue.

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