On a large process pipe or the side of a tank, a pH sensor cannot simply be unscrewed for cleaning. Removing it from a fixed fitting means isolating and draining the line, which on a continuous process in a Malaysian chemical plant, power station or water treatment works can mean lost production. Hot-tap sensors solve this by letting the sensor be inserted and withdrawn through a valve while the process keeps running.
That convenience changes what matters in the sensor itself. A hot-tap sensor is usually removed far less often than a sensor in a small flow cell, so the length of time it holds its calibration becomes the deciding factor. This article explains how hot-tap pH installations work, what determines how long the sensor stays accurate, and how different reference technologies compare in this service.
The short answer is this. A hot-tap assembly has three parts: a full-port ball valve on the process, a compression fitting that seals around a long sleeve, and the sensor at the end of the sleeve. Because it is serviced infrequently, the sensor’s reference must resist poisoning and fouling for long periods. Conventional liquid or gel references fail fastest, solid-state references last considerably longer, and multi-stage barrier references such as LECOL’s ION BARRIER, used in the LECOL B75, are designed to extend that interval further.
How a hot-tap pH installation works
A hot-tap installation starts with a full-port ball valve fitted to a branch on the pipe or tank. The sensor sits at the tip of a long rigid sleeve, which passes through a compression fitting mounted on the back of the valve.
To insert the sensor, the operator opens the valve, pushes the sleeve through until the sensor reaches the correct depth in the flow, and tightens the compression fitting to seal against process pressure.
To remove it, the operator loosens the compression fitting, withdraws the sleeve until the sensor tip is behind the valve, closes the valve, and only then removes the sensor completely. The process never stops.
Two design details matter for safety. The compression fitting must seal reliably every time it is tightened, and the sensor must not be able to shoot out of the fitting under process pressure as it is withdrawn. Well-designed hot-tap sensors include a flared or stepped tip that prevents accidental ejection.
Getting the geometry right
Insertion depth. The glass should sit in the moving flow, away from the pipe wall where flow is slow and deposits collect, but not so far that it risks hitting the opposite wall. On large pipes this usually means positioning the sensor between the wall and the centreline, which is why hot-tap sleeves come in several lengths.
Orientation. pH electrodes need their internal fill to cover the glass, so they are typically installed with the tip pointing downward at an angle rather than upward, and away from the top of the pipe where air collects.
Valve size and port. The valve must be full-port so the sensor and sleeve pass through without catching, and the sleeve diameter must suit the compression fitting.
Why the reference matters more in hot-tap service
A pH sensor in a bench flow cell may be pulled, cleaned and recalibrated every week. A hot-tap sensor on a large pipe may stay in place for months, because retracting it, while possible, still takes planning and care. The longer a sensor stays in the process between visits, the more its reference electrode is exposed to the three mechanisms that cause most pH failures: poisoning of the silver/silver chloride element by aggressive ions, clogging of the junction by solids and oils, and depletion or dilution of the reference electrolyte.
Different reference technologies handle this exposure very differently.
Liquid or gel references connect to the process through a small ceramic junction. They are inexpensive but give poisoning ions a short path to the silver wire, clog easily, and lose electrolyte under pressure changes. In hot-tap service, their short life defeats the purpose of the installation.
Solid-state references replace the liquid with a solid matrix charged with KCl, as in ABB’s TB5 series hot-tap retractable sensors such as the TB557. This removes the electrolyte refill and greatly reduces pumping and contamination. Contaminants still diffuse slowly through the single solid medium, however, and fouling at the junction surface is not filtered by the solid itself.
Multi-stage barrier references place several different defences in series. In LECOL’s patented ION BARRIER design, process liquid passes first through a large-area PTFE or wood junction, then a wood-cell filter that traps particles and organics, and finally a matrix of randomised ionic pathways that forces aggressive ions along a long, restricted path. Each stage handles a different threat, which is why a layered design keeps the reference stable longer than either a liquid reference or a single solid medium. We explain the three reference generations in more depth in our article on how pH sensor reference technology decides life in harsh processes.
Materials for the wetted hardware
In a hot-tap assembly, the sleeve and compression fitting are wetted by the process just as the sensor is, so their materials matter. 316L stainless steel suits most water, wastewater and cooling water service. Hastelloy C is worth specifying for chloride-rich or strongly acidic streams where stainless steel would pit. Titanium suits some oxidising and chloride environments. O-rings are usually Viton, with FFKM for aggressive solvents and high temperatures.
How the LECOL B75 brings these together
The LECOL B75 hot-tap insertion sensor combines the hot-tap mechanics above with the ION BARRIER reference. It installs through a 1-1/4 inch full-port ball valve, with reducing bushings available for 1 inch and 1-1/2 inch connections, and has a flared safety tip to prevent ejection during removal. The inner sensor body is PVDF, the reusable outer sleeve is 316L stainless steel or Hastelloy C in 16, 24, 32 or 36 inch lengths or custom, the compression fitting is available in 316L, Hastelloy C or titanium, and O-rings are Viton or FFKM. The measuring glass can be HF resistant, coating resistant hemispherical or flat, low temperature, high temperature (15 to 135°C, tolerating 140°C temporarily) or platinum ORP, with PT100 or PT1000 temperature compensation, and the sensor is rated to 10 kg/cm2 at 70°C and 2.8 kg/cm2 at 135°C.
| Criteria | Fixed sensor, liquid or gel reference | Hot-tap sensor, solid-state reference (e.g. ABB TB557) | LECOL B75, ION BARRIER reference |
|---|---|---|---|
| Removal | Line isolation and draining | Retract through ball valve | Retract through ball valve, flared safety tip |
| Reference protection | Short direct path, small junction | Single solid medium | Large junction, wood-cell filter and ION BARRIER in series |
| Electrolyte refill | Often required | Not required | Not required |
| Sleeve and fitting materials | Not applicable | Stainless steel hardware | 316L SS or Hastelloy C sleeve; 316L, Hastelloy C or titanium fitting |
| Insertion length | Fixed and short | Model dependent | 16 to 36 inch or custom |
Where hot-tap pH sensors are used in Malaysia
Hot-tap sensors suit recirculation piping, large pipes without sample lines, tank side mounting and cooling water circulation, as well as chemical reactors, scrubbers and high-pressure pipelines. In Malaysia, this includes petrochemical and chemical plants, power stations, large district and industrial cooling water systems, and central water and wastewater treatment works, anywhere a line shutdown for sensor maintenance carries a real cost.
A safe retraction routine
The value of a hot-tap installation is lost if operators avoid retracting the sensor because the procedure feels risky. A short written procedure at each sensor, covering process pressure checks, the order of loosening the fitting and closing the valve, and where the operator should stand, turns retraction into a routine task. Combined with a reference that holds its calibration for long periods, it allows the hot-tap installation to deliver what it was designed for: accurate pH with minimal disruption to the process.
Autoflo Technology is the authorised distributor of LECOL pH and ORP sensors in Malaysia. For help specifying a hot-tap pH installation, contact us at info@autoflotechnology.com.
ABB, TB557 and TB5 are trademarks or product designations of ABB, referred to here for technical comparison only. Autoflo Technology and LECOL are not affiliated with ABB.