Whether you use industrial storage tanks for harvesting rainwater, providing municipal drinking water, irrigating fields, emergency fire suppression, or other purposes, you know how important it is to maintain accurate tank level pressure. One reliable solution is a pressure switch. This instrument is known for its accuracy, repeatability and versatility in water level management applications in water treatment systems, bubblers, scrubbers, and more. But how do they work?
Ashcroft has been manufacturing pressure switches for decades and understands the unique challenges of pressurized tank systems. In this article, you will see how a pressure switch works and learn how multiple tank levels can be monitored and controlled using one or more pressure switch connections.
When you are done reading, you will also find additional resources to answer other switch-related questions you may have.
How pressure switches monitor tank levels.
Pressure switches are known to deliver accuracy, repeatability and versatility for level control when using bubblers, scrubbers, and pressurized tank systems on water treatment systems. They are designed to be easily installed in existing tank drains or access fittings.
Multiple tank levels can be monitored and controlled from a single pressure connection. Level switches usually require costly connections at each designated control level. Once the pressure switch installation is completed, setpoint adjustments are easily done to accommodate different operating conditions. Typically, pressure switches are mounted outside of the tank and can be isolated with manifold valves for easy maintenance and replacement.
Here's how pressure switches work in tank applications:
Pressure switches sense the weight of the liquid column, which is proportional to the level. This is independent of tank diameter or piping configuration. The pressure in any container is the same at all points at the same level. For most applications, the pressure switch is connected to the tank anywhere below the level at which switching is desired. The switch is then adjusted to trip at specified levels.
For tanks where only top access is available, a bubbler system is used. In this approach, air flows through a tube to the bottom of the tank and is allowed to bubble up through the process fluid. A regulator adjusts pressure to keep flow to a minimum, and a pressure switch monitors the back pressure, which is proportional to the height of fluid above the bottom of the tube. Plants and mills have been using this method successfully for many years.
Pressure switches offer easy installation into existing tank drains or access fittings. Once installed, pressure switches allow for simple setpoint adjustments to suit various operating conditions. They can be conveniently mounted outside the tank and isolated with manifold valves for maintenance and replacement.
Depending on tank access and number of switches needed, different systems may be used:
1. Using a bubbler system and one pressure switch with two set points to control the level.
For slurry applications, where side or bottom connections are difficult, the bubbler system is ideal. The
purge meter adjusts flow to keep bubbles to a minimum. Backpressure is monitored with a pressure
switch with two independent set points.
- Backpressure equals the head or liquid level.
- Water is flowing constantly under normal operation.
- Set point one activates the switch when level pressure reaches L1 and opens the solenoid valve
to prevent spillage. The Solenoid valve closes when the pressure drops to the switch reset value. - Set point 2 activates to sound an alarm if the level pressure drops to L2.
Figure 2. Tank level control using one switch and a bubbler system.
2. Using two pressure switches to monitor and control tank level.
In sewage treatment plants, level control is crucial in the operation of wet air scrubbers to minimize the usage of chemicals and maintain a consistent treatment. For this type of application, two pressure switches are used with single set points. There are some cases, however, where one pressure switch can be used with two independent set points instead of two pressure switches.
- The suction pump operates continuously, with the make-up water solenoid typically open during normal operation to remove treated water at the same rate as untreated water enters.
- Water passing through baffles is aerated to enhance the process.
- The first pressure switch (PS1) stops the suction pump if the water pressure drops to level L1 to prevent pump cavitation.
- The second pressure switch (PS2) stops the make-up water solenoid when the water level reaches L2 to prevent high water levels from interfering with the baffles' aeration.
Figure 3. Pressure switch scrubber liquid level control in sewage treatment plan.
3. Using three pressure switches to monitor and control tank level.
Level control is also critical in improving reliable and efficient water treatment operations. In these applications, the first pressure switch (PS1) has an adjustable deadband to control the level between L1 and L2 while the second and third pressure switches (PS2 and PS3) have single set points and a fixed deadband to control alarm level L3 and L4.
- PS1 activates the switch when the pressure drops to L1 and resets when it rises to L2.
- The solenoid valve opens at L1 to allow dirty water in and closes at L2.
- PS2 triggers an alarm and stops the suction pump if the pressure drops to L3 to prevent cavitation.
- PS3 activates if the pressure reaches L4 to prevent spillage.
- A separate control system manages treatment chemical addition, reaction timing, and suction pump activation.
Figure 4. Pressure switch level control in water treatment system.
4. Using differential pressure switches to monitor and control pressurized tank level.
Differential pressure measurement is a reliable method for monitoring liquid levels in pressurized tanks. In these applications, the differential pressure switch offers excellent accuracy and flexibility for level control systems.
- In pressurized tanks, the air pressure above the liquid acts on the low differential pressure ports of the switch, directly correlating the differential pressure to the hydrostatic pressure tank level.
- For this application, a differential pressure switch with a single setpoint and adjustable deadband is used.
- If the water level reaches L2, the switch is activated and opens the output solenoid and resets if the level drops back to L1.
Figure 5. Differential pressure switch level control in pressurized tanks.
Switch selection can be a complex challenge. Let us help.
Now that you understand how pressure switches can help you monitor and control tank levels for your application, here are a few related articles that can answer additional questions you may have:
Choosing a Pressure Switch: 9 Factors to Consider
What is a Deadband on a Pressure Switch and How Does it Work?
What is the Cost of Industrial Mechanical Pressure Switches and Temperature Switches?
You can also contact us directly and speak to an industry expert to get your questions answered. In the meantime, download our guide to learn more about switches and other instrumentation used in water/wastewater applications.
