pH and ORP sensors get installed in a variety of liquid processes. The mounting and installation of these probes in vessels and pipelines often raises questions. This article discusses some basic installation tips to consider when installing sensors.
How will you remove the sensor?
pH/ORP sensors need to be removed for cleaning, calibration, and eventual replacement. In batch processes, removal between each run for maintenance can easily be accomplished. In continuous processes the sensor must be isolated from the process prior to removal to avoid shutdown. There are several design ideas for isolating the sensor.
Retractable sensor holders
Retractable sensor hardware mounts directly to the process and allow the sensor to be isolated prior to removal. The sensor is removed from the process and o-ring seals are used to avoid any leakage. There are multiple retractable holder options for either manual or pneumatic sensor exaction. Retractable holders allow the pH sensor to be mounted directly at the reactor or pipeline for quick response to changing measurement conditions. Retractable holders such as the Hamilton Retractex also offer additional ports for flushing of the sensor and any residual liquid prior to removal from the process.
- Sensors used in retractable holders may need to be longer lengths (often 225mm or 325mm) to accommodate the hardware needed to extract the sensor.
- If an automated retractable sensor holder is desired then a pneumatic air supply. solenoid valves, and control logic needs to be considered.
Sample line installations
Sample line installations move the sensor off the main process vessel or pipeline to a smaller sample stream. Process liquid passing through the sample line for measurement may go to waste drain or downstream at lower pressure in the process. Sample lines are typically smaller diameter tubing routed to a flow cell where the sensor is installed. Isolation valves upstream and downstream of the sensor are used to close off the flow of process liquid so the sensor can be removed for service. Sample line installations are preferred if the process is at elevated temperature or pressure or whenever a retractable holder is not practical. The sample liquid can be cooled or pressure reduced prior to measurement.
- Depending on the length of the sample line there may be some delay in measurement which needs to be accounted for in the process control logic.
- The flow cell and related isolation valves are normally metallic thus are best mounted on a wall or panel for easy access.
- The flow cell should be installed so that it is self-draining after shutdown.
Submersible installations require that the sensor must be immersed vertically into the liquid. These applications usually do not allow for retractable holders or sample lines. The sensor should be mounted at the end of a rigid pipe or tube with the cable passing through the inner diameter to the process control system. Sensors should be inserted at the same depth each time so stratification of the process does not affect the measurement. The dip tube should be as short as possible and be easily removable so the sensor can receive proper maintenance. Sensors with detachable cables should be avoided as they may create more issues with leakage and shorted outputs. A sensor with fixed cable such as the Hamilton InchTrode are preferred for these types of installations.
- Sensor mounting apparatus such as a dip tube are fairly simple but unique for each installation. The length of the dip tube, coupling to the sensor, and method of securing of the dip tube within the vessel need to be considered.
- If the sensor cable must get to a pH transmitter or process control system (PCS) then cable length can be an issue.
The InchTrode has no connector on the back of the sensor, but relies on a fixed cable for submersion in processes that require it
Other Installation Considerations
Good practice for pH sensors means that they should be mounted vertically with the tip pointing downward. This ensures that internal liquid electrolyte stays within the glass membrane bulb. The probe can be mounted up to 15° above horizontal with good measurement accuracy. If mounted vertically in a horizontal pipeline then the installation point should be chosen where there is no potential for a partially full pipe. Note that polymer filled sensors such as the Polilyte Plus pH sensor offer more mounting flexibility as the sensor can be mounted with the tip points up.
pH and ORP sensors do not require flow velocity for measurement; however, some flow past the sensor helps avoid potential coating and build-up from occurring. A flow velocity of at least 1 m/s (3 ft/s) is often adequate to prevent coating from occurring. Flow velocities higher than 3 m/s (10 ft/s) should also be avoided if particulates are present. These higher velocity flowrates may abrade the glass membrane of the sensor causing shortened lifespan. Sensors with flat glass electrodes may be an option in these installations.
The glass bulb at the tip of the sensor should be fully exposed to the process by protruding out into the liquid roughly 8-10mm. This insertion depth helps avoid air pockets and ensures that the pH sensitive glass membrane and liquid junction is past any laminar flow area near the inner wall of the process vessel / pipeline. Good mixing with turbulent flow allows for quick response and helps prevent any potential process related coating that could occur. Avoid excessive insertion depth that could put mechanical stress on the sensor and increase potential for breakage of the glass shaft.