The Dry-Fire Risk in Variable-Level Tanks
Process tanks with the variable liquid levels-the batch reactors, the rinse tanks, the storage vessels-risk the dry-fire of the heat exchanger. If the liquid level drops below the top of the tube bundle while the steam is flowing, the exposed tubes overheat in the air. The PTFE material itself tolerates the temperature, but the support components may not. The dry-fire also wastes the steam and creates a personnel burn hazard at the exposed tubes.
The standard protection is an external level switch that shuts off the steam when the level drops. The external switch requires a separate tank penetration, the separate wiring, and the separate maintenance. An integral level switch-built into the PTFE heat exchanger structure-eliminates the separate installation.
The Integral Level Switch Design
The level switch is a conductivity sensor mounted on the heat exchanger support frame at the safe level threshold-typically 100-150mm above the top of the tube bundle. The sensor consists of two small electrodes-titanium or stainless steel pins-encapsulated in a PTFE housing. The housing is bolted to the support frame.
When the electrodes are submerged in the conductive process fluid, the current flows between them. The conductivity is detected by the control circuit. When the liquid level drops below the electrodes, the current stops. The control circuit opens the steam supply valve-shutting off the steam and preventing the dry-fire.
The sensor cable is PTFE-insulated, running along the support frame to the tank rim, then to the control panel. No separate tank penetration is required-the cable exits through the same penetration as the steam and the condensate connections.
| Level Switch Parameter | Specification |
|---|---|
| Sensor type | Conductivity (two-electrode) |
| Electrode material | Titanium Grade 2 (or stainless steel 316L) |
| Housing material | PTFE, integral with support frame |
| Mounting position | 125mm above the top tube row |
| Response time | < 2 seconds |
| Output signal | Dry contact or 4-20 mA |
| Steam shutoff | Automatic (via solenoid valve) |
| Fail-safe mode | Steam off on the signal loss |
| Cable | PTFE-insulated, 2-wire |
The Fail-Safe Configuration
The level switch control circuit is configured as the fail-safe: the steam valve is held open only while the level signal is present. If the sensor fails, the cable breaks, or the control circuit loses the power, the steam valve closes. The fail-safe configuration prevents the dry-fire even in the event of the sensor failure.
The conductivity sensor works only with the conductive fluids. For the non-conductive fluids-the deionized water, the organic solvents-the alternative is a float-operated switch or a capacitance sensor. The integral design accommodates the different sensor types depending on the process fluid.
The Multiple Sensor Levels
For the tanks with the wide level fluctuations, the multiple sensors can be installed at the different elevations: a low-level sensor for the dry-fire protection, a mid-level sensor for the process monitoring, and a high-level sensor for the overfill alarm. The multiple sensors share the common cable routing and the common control panel.
The integral design places the sensors at the exact elevations relative to the tube bundle. The sensor positions are set at the factory, eliminating the field alignment that the external sensors require.
Summary
PTFE heat exchangers with the integral level switches provide the automatic dry-fire protection without the separate tank penetration. The conductivity sensors at the safe level threshold detect the liquid presence. The fail-safe configuration shuts off the steam if the level drops or if the sensor fails. The integral design simplifies the installation, reduces the wiring, and ensures the correct sensor positioning relative to the tube bundle. The level protection is integrated with the heating function.
Engineering support for the integral level switch specification is available upon submission of the tank dimensions, the level fluctuation range, the fluid conductivity, and the control system interface requirements.
