Measuring pH at a pump discharge, on a pressurised reactor or on the wall of a high-pressure tank is a different problem from measuring it in an open channel. Pressure acts directly on the most vulnerable part of a pH sensor, its reference electrode, and the sensor must also be removable without shutting down a line that is expensive to depressurise. In Malaysian petrochemical, oleochemical and power plants, these measurement points are often where pH sensors give the most trouble.
This article explains what high pressure does to a pH sensor, how the main reference technologies respond to it, how retractable high-pressure assemblies work, and how LECOL’s B86 is designed for this service.
The short answer is this. In a conventional liquid or gel reference, process pressure pushes sample through the junction into the electrolyte, contaminating it and making the reading move with pressure. Solid-state references remove the liquid that pressure acts on, which is a large improvement. A multi-stage barrier reference such as LECOL’s ION BARRIER keeps that pressure tolerance and adds separate stages to resist fouling and poisoning, while a retractable housing allows the sensor to be serviced without stopping the process.
What pressure does to a pH reference
A pH sensor’s reference electrode is a silver/silver chloride element in potassium chloride (KCl), connected to the process through a junction. In a conventional sensor, that KCl is a liquid or gel, and the junction is a small porous ceramic pin. Three things happen when this design meets high pressure.
Sample is forced into the reference. When process pressure is higher than the pressure inside the reference chamber, process liquid is driven through the junction into the electrolyte. This dilutes the KCl, carries contaminants straight to the silver/silver chloride element and shifts the reference voltage. Some designs pressurise the reference to counter this, which adds complexity and maintenance.
The reading follows pressure changes. As pressure rises and falls with pump starts, valve movements and process cycles, the direction and rate of flow through the junction change. The junction potential changes with them, so the pH reading moves even when the true pH has not.
Seals and bodies are stressed. Plastic bodies and single O-ring seals that are adequate at low pressure can creep or leak at high pressure, especially combined with high temperature.
How the three reference technologies respond to pressure
Liquid or gel references are the most pressure sensitive, for the reasons above. They are rarely a good choice for pressurised lines unless the reference is actively pressurised and maintained.
Solid-state references replace the liquid with a solid matrix charged with KCl. With no liquid volume for pressure to push sample into, the main pressure problem largely disappears. This is the approach used in established high-pressure retractable sensors such as ABB’s TB564, part of the TB5 series, which pairs a solid-state reference with a Kynar (PVDF) electrode body and 316 stainless steel retraction hardware. The remaining limitation is chemical rather than mechanical: contaminants still diffuse slowly through the single solid medium, and the junction surface is not protected from fouling by the solid itself.
Multi-stage barrier references keep the pressure tolerance of a non-liquid design and add separate defences against fouling and poisoning. In LECOL’s patented ION BARRIER design, process liquid passes 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 to the silver/silver chloride element. Because each threat is handled by a dedicated stage, the reference stays stable for longer than with either a liquid reference or a single solid medium. Our article on how pH sensor reference technology decides life in harsh processes explains the three generations in more detail.
How a retractable high-pressure assembly works
A retractable high-pressure sensor sits inside a stainless steel extraction housing mounted on a full-port ball valve. During operation, the sensor is pushed through the open valve into the process. To service it, the sensor is drawn back into the housing, the valve is closed to isolate it from the process, and the housing is relieved of pressure before the sensor is removed.
Three features make this safe and practical. The housing must retain the sensor against process pressure throughout withdrawal, so it cannot be ejected. Flushing or vent ports on the housing allow trapped process liquid to be flushed out and pressure to be relieved before the housing is opened. And the valve must be full-port so the sensor passes through cleanly.
How the LECOL B86 is built for this service
The LECOL B86 combines a retractable hot-tap design with the ION BARRIER reference. The sensor body is PPS (Ryton), carried in a 316 stainless steel extraction housing with a 1-1/4 inch full-port ball valve and two 1/4 inch NPT ports for flushing and pressure relief. The housing is available with or without an isolation valve, to suit what is already installed. The assembly is rated to 20 bar maximum and 135°C maximum. The junction is notched PTFE or notched wood, and the measuring glass can be hydrofluoric acid resistant, coating resistant hemispherical, high temperature (15 to 135°C, tolerating 140°C temporarily) or platinum ORP, with PT100 or PT1000 temperature compensation.
| Criteria | Fixed sensor, liquid or gel reference | Retractable sensor, solid-state reference (e.g. ABB TB564) | LECOL B86, ION BARRIER reference |
|---|---|---|---|
| Effect of process pressure on reference | Sample forced into electrolyte | Largely eliminated | Largely eliminated |
| Protection against poisoning and fouling | Short direct path, small junction | Single solid medium | Large junction, wood-cell filter and ION BARRIER in series |
| Removal | Depressurise and isolate line | Retract into housing | Retract into housing, flushing ports |
| Body and housing | Plastic body | PVDF body, 316 SS hardware | PPS body, 316 SS extraction housing |
| Published rating | Typically a few bar | 300 psi (about 20.7 bar), 140°C | 20 bar, 135°C |
For almost all high-pressure pH points, both solid-state and ION BARRIER designs comfortably cover the process conditions. Where a process genuinely runs at the very top of the pressure and temperature envelope, the actual maximum conditions should be confirmed when selecting a sensor.
Where high-pressure pH sensors are used in Malaysia
Retractable high-pressure sensors are used for high-pressure tank side mounting, pump outlets and other pressurised process points. In Malaysia, these are found in petrochemical and oleochemical plants, chemical reactors, power generation and pressurised process water systems, wherever pH must be measured reliably under conditions a normal sensor would not survive.
Safety comes first
Withdrawing a sensor from a line at 20 bar is a hazardous task if the procedure is not followed. Before any retraction, operators should confirm the housing is correctly assembled, use the flushing and vent ports to relieve pressure as the procedure requires, and never stand in line with the sensor during withdrawal. A clear written procedure posted at each high-pressure sensor is the simplest way to make maintenance safe and consistent.
Autoflo Technology is the authorised distributor of LECOL pH and ORP sensors in Malaysia. For help specifying pH measurement for high-pressure processes, contact us at info@autoflotechnology.com.
ABB, TB564 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.