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MAINTENANCE/TROUBLESHOOTING
“Because our process coats everything in contact with it, we installed our pH probes in a sample loop so we could easily maintain them. Now we find that the sample loop is getting blocked, causing even more problems.”
pH (and other) probes need a lot of cleaning when they are installed in a process that tends to foul them up. Installing them in a side stream is not untypical. However, side streams create a time delay between change in process line and measurement point. The longer they are, the greater that time delay. Plus, if the sample lines become blocked, the measurement stops. To protect against that, a flow switch is needed and so the sample loop becomes more complicated and more expensive!
It turns out that installing the probe in an automated online retractor that will withdraw the probe, clean it, and reinsert it in line is a very cost-effective way to keep the measurement online. Standalone automated cleaning systems, like the EXmatic 470 from Exner, can be programmed to periodically carry out probe cleaning. Automatic cleaning can be carried out far more often than when it is done manually. Such systems consist of a pneumatic probe assembly, electronic unit and valves for water and cleaning solutions (where needed—often a water rinse is all that is needed). A cleaning sequence is programmed into the electronic unit that goes through whatever steps are necessary to clean any deposits off the probe tip, keeping it in tip-top condition and measuring reliably.
Of course, taking the probe out of line and bathing it in water cleaning solutions will change the value of the pH measurement—not very desirable if it’s being used for control or alarming purposes. Many pH analyzers have a hold function where a contact closure or applied signal voltage to its associated input will force a “hold” of the current measurement value until the cleaning cycle is finished. The automated system can be connected and work in conjunction with the analyzer to make cleaning seamless and “bump-free”. For those analyzers that do not have such a feature, the hold signal can be connected to a local PLC or supervisory control system and programmed to carry out the same function.
Implementing automated cleaning is a simple upgrade that will dramatically increase measurement reliability, extend probe life and reduce costs.
“We’ve used retractors in the past but find they start to leak after a while. Our process is quite ‘dirty’ and has a lot of fine solids in it. When retracting the probe, the shaft pulls these solids in and they cause the seals to fail.”
Some applications are quite hard on retraction mechanisms. The key to success in this type of application is to have a retractor design with a robust scraper around the moving shaft within the mechanism that pares any solids buildup from its surface as it is pulled out of the process stream. The EXtract design has a durable PTFE scraper installed in front of the O-seal around the retraction shaft to limit the amount of damaging solids material being drawn in with the probe.
A retractor design with a scraper will fare significantly better in a process with heavy, hard solids than an assembly without. However, it will not last forever and maintenance will be required. In many cases, you’ll be notified of the need for maintenance by a leak—your retractor should be designed with a weep hole that serves as a leak indicator. This helpfully removes the need for a PM program to be put in place.
The most damage a leak can do is to the retraction mechanism itself. Good designs will prevent any leakage from the process getting up into the mechanism that is turned to retract and withdraw the probe.
SAFETY
“Don’t probe retractors represent a safety hazard? What happens if someone takes the probe out and operates the retractor back into the pipe?”
Properly designed probe retractors, like the Exner EXtract series, have multiple safety features that cannot be defeated without a great deal of effort! Whatever the probe retractor you fit, make sure it has the following safeguards built in:
- The probe cannot be removed from the retraction assembly when it is inserted into the process.
- The retractor senses when a probe is installed and will not allow insertion without one in place.
- On manual retractors, there are visual indicators to show the position it is in. Lock buttons should engage at each end of the retractor’s travel to ensure it stays firmly in-process or out-of-process.
- On automatic retractors, there must be a feedback signal to the control unit to indicate whether the probe is in-holder or in-process.
- For retractors that have a ball valve entry to the process, it should not be possible to remove the probe without first closing the isolating ball valve.
These simple but effective mechanical safety features are a must for any retractable housing and will prevent any unsafe conditions from occurring.
“We operate at fairly high pressure and inserting a probe when the line is live would be almost impossible!”
We absolutely understand how pushing an object into a pressurized pipe could be very hard, dangerous even. It’s not a real problem for an automatic housing as the pneumatic pressure used for operation can be high enough to overcome any resistance during insertion, but when manually inserting, it would be certainly quite difficult. Exner manual EXtract mechanisms operate with a rotary action and allow a probe to be inserted into a process at 16 bar (230psi) without any problem.
INSTALLATION
“Why do most pH probes have to be installed at 15° from horizontal in order to work properly?”
The vast majority of pH probes contain a liquid electrolyte, which must be kept in contact with the active surfaces of the probe and the electrode within it. If you invert a standard glass pH probe so its tip is sticking upwards, you’ll see an air bubble fill the glass bulb at the end. This essentially breaks the circuit between the glass bulb (the active surface) and an electrode installed within the glass bulb assembly. The same is often true for the reference cell in a combination probe too. You need gravity to ensure that the probe can work correctly and 15° is the minimum angle needed from horizontal to ensure everything is connected properly.
It’s a common problem when probes are installed through tank walls as most tank/vessel side wall entries are horizontal. To solve this problem, Exner makes a selection of 15° angled probe adapters that ensure probes are installed at the right angle for operation. Using angled probe holders is a great solution for installing electrolyte filled probes into vessel side walls. Whether fixed or retractable, a 15° holder maintains flexibility on tank connections while keeping fabrication costs down.
CONNECTOR ISSUES
“We have ¾” NPT pH probes in our WWT plant with fixed cables because it’s very wet where they are and connectors have always been a real problem. The trouble is that they have really long cables on them and this maintenance difficult. Our probe vendor suggested cable extension boxes, but we’ve had leakage issues with these too. We would really like to change over to a probe with a digital connector, but the selection of probe brands is very limited in a ¾” NPT body style.”
pH probes with Memosens or similar digital communications are great for really wet areas because they use inductive connection—a bit like a toothbrush charger connection. However, you’re right in saying there is a limited range available in a ¾” NPT footprint. The easiest way to get to where you want to be is to use an adapter to convert from a ¾” NPT style to a 12mm style probe. These are readily available from Exner in several materials, so whatever your current setup, you can certainly find something that will be compatible.
Standardization of pH probes using adapters cuts costs.
As manufacturing plants develop and modernize over the years, newer equipment and technology is inevitably installed alongside older equipment. This can often lead to an increase in inventory of spare parts for equipment that has the same function. pH probes are a prime example—there are many different manufacturers of probes using a variety of different mounting styles. In plants with a lot of pH probe installations, one significant cost saving can be to use adapters and probe holders to enable the use of a common pH probe size and type across all installations. Adapters are available to convert from a wide variety of different connections (Triclamp, threads, flanges, etc.) to a 12mm diameter probe of standard length. Conversion means a reduction in inventory holding and increases turns on that inventory, making spare probes “always available” and within shelf-life limits.
Industrial Applications
Flue Gas Scrubbing
Many companies are required to scrub flue gases before releasing them into the environment. Plants include power stations, garbage incinerators, chemical and petrochemical plants, even sea going vessels. The most common flue gas cleanup is desulfurization.
Wet scrubbers are often employed for desulfurization. Simply put, exhaust gas is passed through a scrubbing liquor spray in the flue system. The liquid spray both reacts with the gas to form solids and forms liquid droplets around particulate matter to enable removal from the stream. The loaded liquor is captured and cleaned/recycled as needed. Maintaining the liquor at the right pH is a vital part of the desulfurization process; too low a pH promotes scaling while too high of a pH causes precipitation that creates large clumps of soft material that coats and plugs the system.
Scrubbers are very dirty, and pH probes are quickly coated because the liquor can contain 15% solids or more. Once coated, pH probe response becomes unreliable. In general, scrubber pH probes are considered a high maintenance cost item. Installing an automatic probe cleaning system that operates every few hours to clean the probe can reduce maintenance costs tenfold and dramatically improve pH measurement performance. Regular cleaning can transform pH probe life from a few weeks to several months.
Cell Culture pH and Oxygen Measurement
In biotech processes, the growth of cells can be quite slow and take many days, even weeks to achieve the required density for efficient further processing. During the growth phase, cell media must be maintained (amongst other things) at specific levels of pH and with adequate dissolved gases such as oxygen. Probes and sensors are used in contact with the culture medium to measure these parameters and ensure they are controlled within rigid limits.
Once installed and sterilized, there is no way to confirm that these probes are working correctly as the culture vessel is sealed, so should a failure occur, it can result in the loss of an (expensive) batch of material, and create a setback in terms of time lost.
Installing pH, DO2 and other probes in manual retraction assembly allows them to be removed in process without risk to the batch. Probes can be withdrawn from the vessel and tested/calibrated or replaced as necessary. Once reinstalled in the retraction housing, probes can be sterilized before reinsertion into the process to prevent undesirable infection. Once sterilized, they can be reintroduced to the batch where they can once again be used for reliable batch monitoring.
Deinking Control
Paper recycling is a challenging process carried out in several stages. Depending upon the process used, pH is an important measurement to ensure chemical balance of the recycle pulp is maintained. The process fluid is a mixture of pulp, paper, water and various deinking chemicals designed to remove ink and “stickies” from glue and labels. This process fluid coats any sensor within a few hours and in the case of pH, can render it useless unless cleaning is carried out. So severe is the coating that forms on the active portion of a pH sensor, cleaning is needed at least every two hours, and the only way to ensure this happens is to automate. Many probes are installed on dip tubes and the simplest way to achieve cleaning is to install a water jet system positioned so that when activated, it “blasts” any coating from the probe. A simple timing system operating a solenoid valve can achieve this. As the pH is often held at or very near 7pH in deinking systems, signal hold of the current measured value is not necessarily needed. For probes installed in pipes or vessels/tanks, a pneumatically operated retraction device is needed. The probe is withdrawn from the process and cleaned in isolation within the body of the retractor housing by passing water, a cleaning solution, or both across the probe tip. Once the cleaning cycle is complete, the probe is returned to the process where it can begin measurement again.
No matter the application or industry, South Fork Instruments can develop superior solutions for your mreasurement needs.
We have developed sensor housing solutions for clients in industries including food and beverage, chemical processing, pharmaceutical manufacturing, and biotechnology, and we can assist you in developing and effective and efficient means of fulfilling your measurement needs.
SOUTH FORK INSTRUMENTS
3845 Buffalo Road
Auburn, CA 95602
Tel: (+1) 925-461-5059
