Circulating Water Tank Level Control: 80 GHz Radar Level Transmitter Case Study for Industry
2026/10/27
The plant operates a circulating cooling water system with an open concrete tank of 6 m height and a working level of 4.5–5.5 m. Water temperature is 30–45 °C, the surface is agitated by the returning water from the tower, and in summer a stable foam layer forms on part of the surface. The tank supplies the cooling water pumps; a low level means pump cavitation and a lost production run, so the level signal is used both for indication and for make-up water valve control.
| Field parameter | Typical value | Engineering consequence |
|---|---|---|
| Tank height / working level | 6 m / 4.5–5.5 m | Level control band is narrow, so repeatability matters more than absolute accuracy |
| Water temperature | 30–45 °C | Warm, humid air above the surface condenses on cold surfaces |
| Surface condition | Agitation, foam in summer | Contact devices swing and foul; foam weakens echo returns |
| Control use | Make-up valve modulating control | Signal must be stable; noisy output causes valve hunting |
| Existing system | DCS with 4–20 mA input cards | Analog output is the cheapest integration; Modbus available as an option |
- A float switch arrangement gave only start/stop of the make-up pump, so the tank cycled between two levels and the pumps saw varying suction head.
- A pressure-type sensor installed at the tank bottom clogged with algae and scale and had to be lifted out for cleaning, which required a shutdown permit.
- The summer foam caused a radar on trial to lose echo and drive the make-up valve fully open, overfilling the tank.
- Nobody could explain to management why make-up water consumption varied from month to month, because there was no level trend.
The control loop needs a stable, repeatable level signal with no moving parts and no shutdown for cleaning. The selected instrument is an 80 GHz radar level transmitter with a PTFE lens, 4–20 mA plus Modbus output, mounted on an existing handrail bracket and aimed away from the return water inlet.
| Selection item | Specified value | Reason |
|---|---|---|
| Principle | 80 GHz FMCW radar | Narrow beam avoids the tank wall and the return inlet structure |
| Range / accuracy | 0–10 m / ±3 mm | Repeatability well inside a 1 m control band |
| Antenna | PTFE lens, self-cleaning | Reduces condensation and algae adhesion; wipe with water, no acid |
| Output | 4–20 mA + Modbus RTU | 4–20 mA for the existing DCS card, Modbus for the plant network |
| Damping | Adjustable 1–20 s | Filters surface ripple so the make-up valve stops hunting |
| Protection | IP67 | Humid tank area, washdown nearby |
- The bracket is positioned on the handrail on the side opposite the return inlet, at least 1 m from the wall, so the beam sees calm water.
- The antenna is set 400 mm above the maximum level, and the dead zone is confirmed against the tank drawing before drilling.
- Damping is set to 10 s and the output is linearised in the DCS; controller output is then tuned to the 1 m band.
- The echo map is taught under two conditions — calm water and full return flow — so the inlet structure is masked in both cases.
- Foam handling is configured in the device: a moderate filter with a hold time of 10 s, plus a DCS sanity check that rejects a jump of more than 0.3 m in 30 s.
- Because the transmitter is non-contact, the tank is never opened and no shutdown permit is needed for maintenance.
- Continuous level replaced two-point pump control, so the make-up valve now modulates and the suction head stays stable.
- Signal damping and the jump-reject logic removed the overfill event caused by foam.
- Cleaning is now a wipe of the lens from the walkway, with no shutdown and no lifting of a heavy probe out of the tank.
- Level trend data is available for the monthly water balance, so make-up consumption differences can finally be explained.
Will the radar handle the foam? Light foam is tolerable with a filter and hold time. Thick stable foam needs a guided-wave radar with a stilling tube.
Can I keep my existing DCS card? Yes — order the 4–20 mA version and use the Modbus port for diagnostics or a second system.
- Provide the tank drawing and the position of the return inlet, not only the tank dimensions.
- Ask for adjustable damping and a jump-reject function in the DCS specification.
- Specify a PTFE lens where algae or scale is expected, and confirm it can be cleaned without removing the housing.
- Keep one spare antenna seal in stock; it is the lowest-cost part with the highest failure impact.
