SINGER BLOG

This post originally appeared on WaterActive

Hydraulically-actuated automatic control valves or regulators have been a mainstay in the waterworks industry for over 100 years. The primary benefit of using a hydraulically actuated valve is the use of line pressure as motive power to operate. Whether these valves are used to control pressure, flow, or level, these valves are operating without the need for external pressures and/or electricity.

These automatic control valves are closed-loop systems. The main valve is controlled by a pilot that compares its spring setting (set-point) to the process variable (ie, pressure, flow, and/or level). As the process variable changes, the pilot senses the change and compensates to keep the process variable as close to the desired set-point as possible.  As such these valves are single set-point control valves, and a set-point change will require operators to change the spring setting of the pilot.

As the waterworks industry grows and systems become more complicated, the need to have multiple set-points and the ability to remotely change it is becoming necessary. For pressure management applications, downstream pressures need to be lowered during periods of low flows to reduce leakage. Others require the need to shutdown the valve on emergency situations. Furthermore, operators require the need to change the process variable as conditions change in the system. For example, on a tank fill application, operators may want to switch the valve to pressure sustaining to maintain a minimum line pressure. Whatever, the application/case may be, if versatility is required and electricity is not an issue, using electrical components with the valve is a good option.

There are two ways to effectively control a diaphragm valve: solenoid control and motorized actuation of the pilot. 

1. Solenoid Control

The principle operation of a hydraulically actuated, diaphragm actuated control valve is to control the pressure on the control chamber.  Increasing the pressure on the actuator closes the valve and decreasing the pressure opens the valve. 

A solenoid uses a similar principle but operates on establishing a hydraulic lock. Water is an incompressible fluid and the volume of the control chamber is proportional to the valve position. If the control chamber is full of water, the valve is fully closed. If the control chamber is empty, then the valve is fully open. Of course, the statement can be said assuming the valve has sufficient available differential pressure to operate.

Solenoids are used because they are the most efficient devices in terms of cost and simplicity in the market today in actuating a valve. There are numerous manufacturers and they are readily available.

Control Valves with Solenoid Overrides 

106-PR-SC

If control valves with hydraulic control systems require shut-off, a solenoid to isolate the pilot from the process can be employed to act as an override. This option is used if the valves are required to remove the valve from service. Selecting the solenoid to be used will depend on the de-energized state of the valve. For example, a pressure reducing valve that fails closed on power failure uses a Normally Open solenoid.

Control Valves with Two Pilots (Solenoid Selectable)

2PR-SC-BT

If a control valve is required to have two set-points only, a solenoid can be employed to select between two pilots.

On/Off Non-Modulating Solenoid Control Valves

106-SC

These valves employ a three-way solenoid and will either pressurize the control chamber or relieve it. This type of valve has two positions only: fully open or fully closed. Applications for this valve type includes: on/off level control, sprinkler systems, and can be used to flush stagnant water in a pipe.

Modulating Solenoid Control Valves

106-2SC-PCO

These valves employ two two-way solenoids that in turn establish a hydraulic lock on the control chamber. The volume of the control chamber is directly proportional to valve position. By adding or relieving the existing volume of water in the control chamber, the valve can be positioned from 0% to 100% open. Using this type of control valve as the final control element, a process control system can be used to pulse the appropriate solenoid so that the process variable is regulated as close to the set-point as possible.

For this type of valve, the process control system and feedback transmitters are the key. The valve is now the final control element that manipulates the valve position, pressure, flow, and/or level. Depending on the feedback transmitters and the process control system, multiple process variables can be controlled with only one valve. 

For example: operators require a control valve that will control flow into a reservoir and shutdown on high level. In addition, the valve shall monitor the upstream pressure of the line and modulate to make sure that there is sufficient line pressure and will prevent it from dropping further. Furthermore, when the reservoir is being by-passed due to maintenance, the valve controls downstream pressure and limits flow into the system.

With the example, a single dual solenoid control valve can be used to manipulate multiple processes. The benefit of having an electronic control system is that the operators can remotely change the process based on the conditions and allow them to monitor the process variables in real time. 

Flow Metering and Control

Control valves can be used as variable area flowmeters. The concept arises from the theory that valves are variable restrictions or orifices. Using a position transmitter and differential pressure transmitter to calculate the flow rate, the valve can be used to control flow without the need for an inline flow metering device.

2. Motorized Pilot Actuation

420-DC (shown installed onto a 106-PR)

With dual solenoid control, the valve’s function is reduced to being a final control element. Without a control system to actuate the solenoids or transmitters, the valve cannot do any useful function. In case of power failure and depending on the orientation of the solenoids, the valve will merely fail open, fail close, or fail in the last position. To operate the valve, manual actuation is required.

For many years the control valves were controlled by pilots. The valve together with the pilots makes a closed loop system. The pilot acts as the control system and feedback to control the valve, and this allows the valve system to be completely automatic.

When changing a set-point, an operator is required to turn the pilot’s adjusting screw. If there is a need to remotely change the set-point of the pilot a multi-turn motorized actuator can be used.

With each pilot there is a fixed change in set-point per turn of the screw. This linear relation to change in set-point per turn is surprisingly accurate and the linear relationship can be programmed in a multi-turn actuator. Using an analog signal, operators can now remotely change the set-point of the valve. In case of power failure, the valve will control its last set-point and will not fail open, close, or last position as is the case with a dual solenoid valve.

In conclusion, hydraulically controlled, diaphragm-actuated control valves are the mainstay of the waterworks industry to control pressure, flow, level, pump control, and at times to meter. These automatic control valves are controlled by pilots to do specific tasks and are mainly single-set-point systems. With the need to remotely actuate and control the valves and the applications becoming more complex, electronic controls offer versatile and feasible solutions.