Reliable level switching is essential for protecting pumps, preventing overflows and automating filling or draining operations in process plants. Displacer type level switches are a proven mechanical solution for point level detection in tanks, sumps and vessels. This blog explains the working principle, design, benefits and common applications of displacer level switches, along with practical selection and maintenance tips.
What Is a Displacer Type Level Switch?
A displacer type level switch is a buoyancy-based device used to detect one or more liquid levels (e.g., high, low, intermediate) and provide an electrical switching output. It is typically installed in or on the side of a tank or vessel.
Key uses include:
- High-level alarms to prevent overflow
- Low-level alarms for pump protection
- Control of pump start/stop or valve operation
- Interface detection between two immiscible liquids, in some designs
Displacer switches are particularly suitable where stable, mechanical point level detection is required in challenging process conditions.
Working Principle: Buoyancy and Force Balance
The operation of a displacer type level switch is based on Archimedes’ principle of buoyancy.
Basic Operation
- A cylindrical or elongated displacer (solid or hollow) is suspended by a spring or torque arm inside the tank or in an external cage.
- When the liquid level is below the displacer, it experiences less buoyant force and exerts a higher downward force on the spring or torque mechanism.
- As the liquid level rises and submerges the displacer, the buoyant force increases, effectively reducing the net downward force.
- At a pre-set level, this change in force activates a mechanism (such as a magnet and switch assembly) to change the state of an electrical contact.
- The switch output can be used for alarms, control or interlocks.
Because the operation is based on force changes rather than float travel distance, displacer switches can be compact while still providing stable switching.
Construction and Key Components
Typical components of a displacer type level switch include:
- Displacer element
- Cylindrical or elongated shape
- Material selected based on fluid properties and operating conditions
- Suspension mechanism
- Spring, torque tube or lever arm
- Transfers force changes from the displacer to the switch mechanism
- Switch assembly
- Often uses permanent magnets and reed switches or micro-switches
- Provides SPDT or DPDT contacts for alarms or control
- Enclosure and mounting
- Flanged or threaded process connections
- Weatherproof or flameproof switch housings depending on area classification
Displacer switches can be installed directly in the tank or in external cages / chambers for better accessibility and maintenance.
Advantages of Displacer Type Level Switches
Displacer level switches offer several practical benefits for plant engineers and operators.
Key Advantages
- Robust and reliable
- Mechanical principle with proven field performance
- Less sensitive to foam or vapour compared to some other technologies
- Suitable for a wide range of liquids
- Can work with varying densities within specified limits
- Applicable for clean as well as some dirty services
- Multi-level detection
- Designs available to detect more than one switching point with multiple displacers
- High temperature and pressure capability
- With proper materials and construction, can be used in demanding services
- Simple integration
- Provides dry contact outputs that can be wired directly to control panels, PLCs or alarms
These advantages make displacer switches a practical choice for many tank and vessel level applications.
Typical Industrial Applications
Displacer type level switches are widely used across industries for point level control and protection.
Common applications include:
- Pump protection
- Low level alarm or trip to prevent dry running
- Sump and pit level monitoring
- Overfill prevention
- High and high-high level alarms in storage tanks
- Safeguards against spills and process upsets
- Process vessels
- Level control in separators, knock-out drums and condensate pots
- Interface level detection in two-phase systems (depending on design)
- Boiler and power applications
- Feedwater tank and condensate tank level alarms
- Auxiliary service tank monitoring
- Chemical and petrochemical plants
- Level switching in day tanks, blending tanks and recovery tanks
In many cases, displacer switches work alongside continuous level transmitters to provide independent safety interlocks.
Comparison: Displacer Level Switch vs Float Level Switch
| Parameter | Displacer Type Level Switch | Float Type Level Switch |
|---|---|---|
| Operating principle | Buoyancy and force change | Direct float movement |
| Size of moving part | Compact displacer | Larger float volume |
| Sensitivity to turbulence | Generally stable | May be affected in some designs |
| Multi-level capability | Yes, with multiple displacers | Limited, depends on design |
| Temperature/pressure | High with correct construction | More limited in some float designs |
Both technologies are useful; selection depends on process conditions and mechanical constraints.
Selection Guidelines for Engineers
When specifying a displacer type level switch, consider:
- Liquid properties
- Density range and possible variations
- Cleanliness and potential for build-up
- Process conditions
- Operating and design pressure and temperature
- Tank or vessel geometry and nozzle locations
- Required switching points
- Single point (e.g., high level only) or multiple points
- Alarm, control or trip functions
- Area classification
- Safe or hazardous area
- Need for flameproof or explosion-proof enclosures
- Wiring and integration
- Required contact configuration (SPDT, DPDT)
- Required interface with PLCs, DCS or relay panels
Providing this information to the instrument supplier helps ensure correct sizing and configuration.
Installation and Maintenance Tips
Installation
- Install the displacer switch according to the specified orientation and depth
- Ensure the displacer has free movement without fouling on tank internals
- Use proper gaskets and tightening torque on flanged or threaded connections
- Verify wiring and contact operation during commissioning
Maintenance
- Periodically check for:
- Smooth operation of the displacer and suspension
- Build-up or deposits on the displacer
- Proper functioning of switch contacts
- In aggressive or fouling services, plan periodic inspection and cleaning
- Verify alarm and trip functions as part of routine safety checks
With appropriate installation and routine checks, displacer switches typically provide long-term, dependable service.
Conclusion: Reliable Point Level Control for Critical Duties
Displacer type level switches provide a robust, mechanical solution for point level detection in a wide variety of tanks and vessels. Their ability to operate in demanding pressure and temperature conditions, combined with simple electrical integration, makes them a trusted choice for protecting equipment and ensuring safe plant operation.
Flowtech Instruments offers level switches and other level measurement solutions engineered for Indian industrial conditions, helping plants achieve reliable level control with the right combination of devices. Flowtech supports customers with application guidance and calibrated instrumentation for flow, level and related measurements.
