Kayden CLASSIC 812 Thermal Flow, Level & Interface Switch

• Ammonia Storage
• Biogas Flow
• Catalysis Vessels
• Chemical Injection & Additive Flow Monitoring
• Chemical Reactors
• Chlor-alkali Processes
• Compressors
• Condensers
• Deionization Tanks
• Digester Gas Flow
• Distillation Columns
• Drain Line Flow
• Emergency Eye Wash & Shower Stations
• Fermentation Vessels
• Flare Gas Monitoring
• Flare Knock-out Drums
• Flow Monitoring & Verification
• Heat Exchangers
• Heat Recovery Steam Generator (HRSG) - Power & Utilities
• High Pressure Flows
• Leak Detection
• Lube Oil Systems
• Monitoring Purge Air Flow
• Natural Gas to Boilers
• Pumps
• Reboilers
• Refinery Flow Applications
• Relief Valve & Rupture Disk Flow Monitoring
• Remote Indication of Flow via Analog Output & Digital Communications
• Pump Protection - Dry Alarm
• Tank Blanketing
• Tank Overflow Protection
• Tanker Loading & Unloading
• Water & Wastewater
• Vaporizers
• Vent Monitoring

• Biogas Dehydration
• Blending Operations
• Carbon Slurry Level
• Compressor Scrubber
• Compressor Waste Liquid
• Condensate Receiver Tanks
• Cooling Tower Basins
• Crude Desalting
• Crude Dehydration
• Deionization Tanks
• Digester Blow Tanks
• Free Water Knock-out Tanks - High, Low and Interface Level
• Geothermal - Degasser Tanks
• Geothermal - Steam & Brine Separators
• Leak Detection (Accumulation Reservoir)
• Lime Slurry Level
• Mixing & Blending Systems
• Oil Separators & Treaters - High, Low & Interface Level
• Production Fluid Storage
• Pulp Bleaching Towers
• Pulp Digesters
• Pulp Storage
• Pulp Washing Systems
• Sampler Systems
• Sand Separators
• Scrubber Vessels
• Storage Tanks
• Sumps - All Types of fluids: Water, Oil, etc.
• Surge Tanks
• Tank Overflow
• Turpentine Recovery
• Vapor & Liquid Separation
• Vapor Recovery

• Crude Desalting
• Interface Control in Separation Vessels
• Interface Detection
• Interface Control & Level Detection in Settling Vessels
• Liquid-Liquid Extraction - LLX

• Liquids, Air & Gases
• Slurries
• Corrosive Liquids

• Flameproof, Hazardous & General-Purpose Areas

• Consistent Process Composition & Temperature
• Sufficient Straight Run Flow Profile (minimizes turbulence)
• Recommended Minimum of 5 Pipe Diameters from any Disturbance

• Consistent Process Composition & Temperature
• Sufficient Straight Run Flow Profile (minimizes turbulence)
• Clean and Dry

• Consistent Process Composition & Temperature
• Sufficient Straight Run Flow Profile (minimizes turbulence)

• Consistent Process Composition & Temperature
• Sufficient Straight Run Flow Profile (minimizes turbulence)

• Inconsistent Process Composition or Temperature
• Insufficient Straight Run
• Aerated Fluids
• Turbulence

• Inconsistent Process Composition or Temperature
• Wet or Saturated Air & Gas

• Inconsistent Process Composition or Temperature
• Insufficient Straight Run
• Aerated Fluids
• Turbulence

• Inconsistent Process Composition & Temperature
• Insufficient Straight Run
• Aerated Fluids
• Turbulence

• Dry granulated processes are NOT good candidates for thermal switches

The adoption of thermal dispersion flow, level, and interface switches confers tangible operational, economic, and safety advantages. From their high durability and minimal maintenance needs to their seamless integration and adaptability, these instruments support more reliable, efficient, and compliant industrial processes. Thermal dispersion, or calorimetric, flow, level, and interface switches offer a multitude of advantages that contribute to improved process reliability, safety, and cost-effectiveness in industrial settings. These benefits span operational performance, lifecycle economics, and integration flexibility.

HIGH RELIABILITY & LONG-TERM STABILITY
Robust sensor elements and protective housings are highly durable in harsh conditions, allowing them to withstand corrosive fluids, extreme temperatures, high pressures, and abrasive or viscous media.

The fundamental thermal measurement principle is stable over time, ensuring stable, repeatable performance over years of operation.

BROAD APPLICABILITY & VERSATILE SENSING CAPABILITY
A single thermal dispersion instrument can often serve multiple functions—detecting flow/no-flow conditions, monitoring liquid levels, and identifying interface boundaries in multi-layer fluid systems, as it's operating principle and technology are inherently versatile. The same fundamental principle applies to both gases, liquids, and slurries accommodating a wide range of process fluids, including water, oils, chemicals, solvents, and slurries.

Thermal dispersion switches can accurately detect very low flow rates and respond reliably in varying fluid densities, viscosities, and temperatures.

COST-EFFECTIVE OWNERSHIP & LOW MAINTENANCE
With no moving parts to recalibrate, repair, or replace, scheduled maintenance requirements and associated labor costs are drastically reduced, keeping maintenance demands to an extreme minimum.

The inherent ruggedness and simplicity in design lead to an extended service life with intervals between service and fewer unscheduled shutdowns, lowering the total cost of ownership. Kayden thermal dispersion devices require no periodic recalibration and hold their calibration well, further cutting maintenance expenditures.

SAFETY & REGULATORY COMPLIANCE
Kayden's models, standard with fail-safe operation, can be configured to go into safe states under defined fault conditions, supporting plant safety initiatives and regulatory compliance. All models are available with explosion-proof certifications, enabling use in flammable or otherwise hazardous environments while meeting regulatory standards.

EASY INTEGRATION & REDUCED PROCESS IMPACT
Thermal dispersion switches are designed as insertion-type instruments that can be installed through a simple nozzle or port, minimizing piping modifications and associated costs. Because the sensor elements are small and streamlined, their presence in the flow path introduces negligible pressure loss, helping maintain efficiency in fluid transport.

Available in various geometries and installation options, these switches readily fit existing systems and can be located in difficult-to-access or space-restricted areas.

ENHANCED PROCESS CONTROL & OPTIMIZATION
The continuous, real-time monitoring of a steady, non-interrupted data stream enables immediate response to changing conditions, improving process control and product quality. Reliable flow/no-flow and level detection help identify early detection of anomalies and problems such as pump failures, line blockages, leaks, or interface layer shifts before they escalate.

THERMAL FLOW, LEVEL & INTERFACE SWITCHES
Kayden's thermal dispersion flow, level, and interface switches and transmitters offer many distinct advantages that make them highly valuable for monitoring and controlling the flow or level of liquids, gases, and slurries in various industrial applications, making them highly effective and dependable for critical applications in industries such as chemical processing, oil and gas, water and wastewater management, and pharmaceuticals, where precise monitoring of flow or level is essential for operational safety and efficiency.

NO MOVING PARTS
Thermal dispersion devices rely on temperature differential rather than mechanical motion, eliminating wear-prone components such as paddles, floats, or turbines. This inherently reduces the likelihood of mechanical failure.

Thermal dispersion switches do not have any moving parts, which significantly reduces the potential for mechanical wear and tear. This absence of moving parts leads to increased durability and reliability, minimizing maintenance requirements and prolonging the lifespan of the device, while providing simpler design and operation, reducing the risk of operational failures, and simplifying maintenance procedures. Choose thermal dispersion switches for their high reliability and simplicity.

VIRTUALLY NO PRESSURE LOSS
The design of thermal dispersion flow sensors ensures that they introduce very little resistance to the flow. This characteristic is crucial in applications where maintaining the existing pressure level is critical.

Thermal dispersion flow sensors, having no moving parts and providing minimal intrusion into the flow, ensure that they do not significantly disrupt the fluid's path or create turbulence, thereby causing minimal instinterference obstruction to the flow of the medium and reducing the potential for pressure drops. By minimizing pressure loss, these sensors help maintain energy efficiency in the system, reducing the need for additional energy to compensate for pressure drops. This feature makes them ideal for use in systems where pressure stability is crucial, such as in certain chemical processing or HVAC applications.

Kayden's Classic 800 Series of thermal dispersion flow switches and transmitters are particularly advantageous in applications where maintaining the original pressure of the system is crucial. Their design minimizes pressure drop, ensures high accuracy and reliability, and offers a versatile solution for a wide range of industrial applications. These benefits make them an excellent choice when pressure loss is a critical consideration.

EXAMPLE APPLICATIONS

Chemical Processing
Accurate flow measurement without pressure drop helps in precise mixing and reaction control.

Water and Wastewater Treatment
Ensures efficient flow measurement without adding to the system’s pressure load.

HIGH ACCURACY AND SENSITIVITY
Thermal dispersion switches are known for their high accuracy in measuring flow or detecting fluid levels. They can detect low flows and small changes in the flow rate or level, making them suitable for applications requiring precise control, and providing consistent and repeatable measurements, which is critical for maintaining the efficiency and quality of industrial processes.

Kayden thermal flow sensors measure the flow based on the cooling effect of the fluid or gas on a heated sensor, providing accurate and direct flow rate measurements without significantly affecting the fluid dynamics.

WIDE RANGE OF MEDIA COMPATIBILITIES
Thermal dispersion switches can be used with a wide range of fluids, including corrosive and dirty media, further broadening their range of suitable applications. They are effective in both liquid and gas applications, which makes them versatile tools for diverse industrial environments. Thermal dispersion switches can be customized with various materials and coatings to handle different types of fluids and operating conditions, enhancing their utility in specialized applications.

EASY INSTALLATION AND INTEGRATION
These switches are fairly compact and easy to install. They can be mounted in various orientations and configurations to suit different system designs. Kayden's thermal dispersion flow sensors are designed for easy insertion into the flow stream, allowing for straightforward installation without major modifications to the system. They can easily be integrated into most process control systems, offering outputs that are compatible with digital control systems, which facilitates easier monitoring and automation.

LOW MAINTENANCE & COST-EFFECTIVE
The minimal maintenance requirements reduce ongoing operational costs. Since they are durable and very reliable, Kayden's thermal dispersion switches and transmitters decrease the need for frequent replacements or extensive and expensive system downtime for repairs. The no-moving parts' construction results in low maintenance requirements and reduced operational downtime. Their robust design means fewer failures, which enhances the safety of the operations, especially in critical processes involving hazardous materials.

OPERATIONAL IN CHALLENGING CONDITIONS
Thermal dispersion technology performs well in extreme environments, including high temperatures and high pressure, as well as in hazardous areas where explosive gases or liquids are present.

Insensitive to Pressure and Temperature Changes
Unlike some other flow or level sensing technologies, thermal dispersion switches are less sensitive to variations in pressure and temperature, which can affect the accuracy and reliability of flow measurements in other sensor types.

NO CALIBRATION REQUIRED
Once installed and configured, thermal dispersion switches generally do not require recalibration. They maintain their accuracy over time, offering stable and precise readings, which is essential for maintaining process control without introducing pressure variability.

Thermal Dispersion Flow Switches in Pump Protection Applications

Pumps play a critical role in the transfer and circulation of fluids across a range of industrial sectors—ranging from petrochemical refineries and wastewater treatment plants to power generation facilities and food & beverage operations. Ensuring their reliable and efficient operation is paramount to maintaining productivity, product quality, and operational safety. In pump systems, conditions such as dry running, blocked suction or discharge lines, and cavitation can cause severe damage to equipment, increase maintenance costs, and threaten process integrity.

Thermal dispersion flow switches have emerged as a proven solution to detect these undesirable conditions early and provide protective interlocks that can prevent costly failures. This paper presents an in-depth examination of how thermal dispersion flow switches work, their benefits, best practices for selection and installation, and integration methods for comprehensive pump protection strategies.

1.  Introduction
Pump protection is one of the most critical aspects of fluid handling in industrial processes. Pumps often operate in challenging environments—high pressures, corrosive or abrasive fluids, fluctuating temperatures, and variable flow conditions—that can stress mechanical seals, bearings, and other components. If a pump attempts to operate under no-flow or low-flow conditions, or if a pipeline obstruction or valve closure upstream or downstream restricts movement, the pump may overheat, cavitate, and eventually fail.

Traditional mechanical flow sensors, such as paddle switches, floats, or turbine meters, can provide detection of no-flow conditions, but their moving parts add complexity, increase maintenance, and reduce long-term reliability. Thermal dispersion technology, utilizing heat transfer principles rather than mechanical motion, provides a robust, low-maintenance solution. Thermal dispersion flow switches can reliably detect very low flows, respond quickly to changing conditions, and withstand harsh media—making them ideal for comprehensive pump protection.

2.  Operating Principle of Thermal Dispersion Flow Switches
Thermal dispersion devices typically consist of two temperature sensors embedded within a single probe that inserts directly into the fluid. One sensor is self-heated by a small, controlled current, while the other remains at the ambient fluid temperature. The flowing fluid cools the heated sensor at a rate proportional to the mass flow or velocity of the medium. Under no-flow or reduced-flow conditions, the heated sensor remains at a higher temperature since there is insufficient fluid movement to remove heat. Conversely, when fluid flow is present, heat is carried away, reducing the temperature differential between the two sensors. By continuously measuring this temperature differential, the switch can accurately discern whether flow is present, below a critical threshold, or absent.

Key Attributes of Thermal Dispersion Technology

No Moving Parts
Eliminates mechanical wear and reduces maintenance.

High Sensitivity to Low Flow
Accurately detects small changes in flow rate, enabling early warning of conditions such as dry running.

Robust Construction
Capable of withstanding corrosive, abrasive, and high-temperature or high-pressure environments.

Minimal Pressure Drop
A small sensor probe results in negligible disruption to fluid flow, maintaining process efficiency.

3.  Pump Protection Scenarios and the Role of Flow Switches

3.1  Dry-Run Protection
A common cause of pump failure occurs when the pump’s suction line is starved of liquid—often due to a closed valve, blocked filter, vapor lock, or depletion of the feed tank. Running the pump dry can cause excessive heat buildup, damage seals, and degrade bearings. Thermal dispersion flow switches mounted in the pump’s suction line can detect a sudden or gradual loss of flow and send a signal to shut down the pump before it overheats.

3.2  Blocked Discharge Monitoring
Should the discharge side of the pump’s line become blocked or the downstream valve remain closed, pressure builds and flow ceases or drops to a negligible level. Without adequate flow, the pump suffers from elevated internal temperatures and mechanical stress. By installing a thermal dispersion switch downstream from the pump, operations personnel can detect reduced or no flow conditions and initiate corrective actions—such as opening bypass lines, adjusting valves, or shutting down the pump.

3.3  Pump Cavitation and Low-Flow Conditions
Inadequate suction head or changes in fluid characteristics (e.g., vapor pressure shifts due to temperature changes) can lead to cavitation. While thermal dispersion switches do not directly measure cavitation, early detection of a drop in flow, especially when feed conditions deviate from normal, can preempt cavitation events. By alerting operators to unusual low-flow conditions, these devices enable timely intervention—correcting fluid conditions, adjusting valve positions, or triggering protective interlocks.

4.  Selection Criteria and Considerations for Thermal Dispersion Flow Switches in Pump Protection

4.1  Fluid Compatibility
The sensor’s wetted materials must be chosen to withstand the process fluid’s corrosiveness, temperature extremes, and particulate content. Common materials include stainless steel, titanium, and hastelloy C-276, or other exotic alloys. Proper material selection ensures long-term reliability and minimal contamination risk.

4.2  Operating Temperature and Pressure Ranges
The switch must be rated for the pump’s operating envelope. Ensure that the switches specified maximum temperature and pressure ratings are not exceeded. Thermal dispersion switches commonly handle a wide range of temperatures (from cryogenic to several hundred degrees Celsius) and pressures (often exceeding 100 bar, depending on the model).

4.3  Flow Sensitivity and Setpoint Range
Different pumps and fluids require distinct low-flow thresholds. The thermal dispersion switch should be adjustable or selectable to trigger at the desired setpoint. Fine-tuning the alarm point is crucial to distinguishing normal operational flow variations from actual hazardous conditions.

4.4  Response Time
The switch’s response time should be aligned with the pump’s operational characteristics. Faster response times are preferable when dealing with rapidly changing conditions—such as sudden valve closures or pump priming phases. Most thermal dispersion switches offer adjustable time delays to avoid nuisance trips due to brief flow interruptions.

4.5  Insertion Length and Mounting Configuration
Insertion length and mounting arrangements are critical for achieving representative flow measurement. The sensor must be fully immersed in the process fluid and positioned in a region of stable, representative flow. Avoid turbulence-generating bends, valves, or other fittings immediately upstream of the sensor. Follow manufacturer guidelines regarding recommended straight run lengths.

5.  Installation and Commissioning Best Practices

5.1  Installation Location
Place the thermal dispersion flow switch in a straight run of pipe, downstream of the pump or upstream in the suction line, depending on the target condition (no-flow at suction or discharge). Ensure minimal hydraulic disturbances—generally, a minimum of 10 pipe diameters of straight run upstream and 5 diameters downstream is recommended (specific guidelines vary by manufacturer and application).

5.2  Wiring and Signal Integration
Most thermal dispersion flow switches provide a relay contact or 4-20 mA output. Integrate these signals into the pump’s control loop, interlocking logic, or Distributed Control System (DCS). When the switch detects no-flow or a low-flow threshold, it can trigger a pump shutdown, activate alarms, or adjust control valves. Ensure proper grounding, cable shielding, and adherence to hazardous area classifications if applicable.

5.3  Calibration and Setpoint Adjustment
Follow the manufacturer’s instructions to set the trip point. Commissioning typically involves operating the pump under known flow conditions to ensure the switch activates at the correct threshold. Adjusting the setpoint can be done via onboard controls, software tools, or external instrumentation, depending on the device.

5.4  Periodic Functional Tests
While thermal dispersion switches require minimal maintenance, periodic functional checks are advisable—especially in critical applications. These checks may involve verifying that alarms and shutdown commands occur at the correct flow thresholds. Over time, if process conditions change (fluid viscosity, temperature, or composition), re-verification of setpoints may be warranted.

6.  Integration into Overall Pump Protection Strategies

6.1  Redundancy and Layered Protection
For critical pumps, a single flow switch may not be sufficient. Consider employing redundancy—installing two sensors in different pipelines or at different points in the same line—to enhance reliability. Combine flow switches with pressure sensors, vibration monitors, temperature sensors on bearings, and motor current measurements to form a comprehensive pump health monitoring and protection system.

6.2  Data Logging and Diagnostic Features
Modern thermal dispersion switches with digital communication capabilities (e.g., MODBUS®) can provide diagnostic information such as sensor health, drift detection, and predicted failure conditions. Integrating this data into a plant asset management system can enable predictive maintenance, reducing unplanned downtime.

6.3  Regulatory and Safety Compliance
In hazardous process environments, ensure that the selected thermal dispersion flow switch is certified for the relevant regulatory standards (e.g., CSA, UL, NRTL, or CRN approvals). Intrinsically safe or explosion-proof models are commonly available.

7.  Case Study Example

Background
A large chemical processing facility experienced frequent pump failures due to intermittent dry running conditions during batch startups. The pumps fed a reactor vessel, and upstream filters occasionally became clogged, restricting flow.

Solution
The engineering team installed thermal dispersion flow switches in the suction lines of each pump. The switches were calibrated to trip when flow fell below a critical minimum—just above the pump’s specified minimum flow requirement.

Results

Reduced Maintenance Costs
Early shutdowns prevented overheating and mechanical seal damage.

Improved Operational Efficiency
Operators used the signals to quickly identify and clear blockages in filters, ensuring smoother startups.

Increased Equipment Life
By preventing frequent dry runs, the pumps’ mean time between failures (MTBF) increased, lowering overall lifecycle costs.

8.  Conclusion
Thermal dispersion flow switches provide a reliable, low-maintenance, and highly adaptable means to protect pumps from costly damage associated with no-flow or low-flow conditions. By leveraging the robust operating principle of thermal dispersion technology, these devices excel in harsh industrial environments and offer exceptional sensitivity to minimal flow changes. With careful selection, proper installation, and integration into broader pump protection systems, thermal dispersion flow switches can significantly enhance equipment reliability, reduce maintenance overhead, and safeguard the continuity and safety of critical fluid-handling operations.

References

ISA (International Society of Automation). “Pump Protection and Reliability Standards,” ISA Publications, 2020.
API (American Petroleum Institute) Standard 610, “Centrifugal Pumps for Petroleum, Petrochemical and Natural Gas Industries.”
Galvan, J., “Flow Switch Technology Review and Selection Guidelines,” Instrumentation & Control Magazine, Vol. 46, No. 3, 2021.
Cruz, P. and L. Martin, “Integrating Flow Measurement Devices in Pump Protection Schemes,” Chemical Engineering Progress, Vol. 117, No. 2, 2022.

Acknowledgements
The development of advanced thermal dispersion technology and its ongoing improvement are indebted to the research efforts of instrument manufacturers, industrial consortia, and standardization bodies dedicated to improving safety, efficiency, and reliability in industrial process operations.