Enhance the life of pH sensors and re-calibrating existing practices

Abstract

In chemical industry, one device that can represent significant cost is pH sensor, particularly in brine section & cell house. In a study of number of plants, the replacement cost of sensors is ranging 10-12% of installation cost. The sensors are often exposed to or immersed in highly corrosive solution subjected to high operating temperature and other severe conditions. pH probe is the critical frontline sensor that gathered and delivered the key data. Proper selection, installation, maintenance, recoding history of problem, can extend the probe life thus reducing cost.

This article is presented after studying the life of pH sensor in number of Chemical Industries for getting the minimum, maximum, and average life attain by the sensor in harsh service. The same results and root causes are discussed to enhance the life of sensors and reduce re-work.

Economics of pH sensors 

In the study of 10 chemical site, the replacement cost of sensors is observed ranging from 5% to 16%. The sensors are selected such a way that use of area and fluid is common. The wide variation in similar kind of services is indicating the gaps site to site. It is possible due to handling, maintenance, and installation. The standard value of pH sensors replacement should be in range of 2% to 3%. This is as per the value’s available literature and cited reference.

Present Performance & Life of sensors 

The data is collected followed by a questioner, the present life of sensors shown below in the figure 1, It is varying from the 4 months to 18 months.

The figure -1 is representing the pH sensors life in similar services. It is used in harsh condition. The sensors life is dependent on many factors, these factors are needed to maintain from sensor selection to throughout the operation.

As it is very much visible from the figure, the life of sensors achieved from 20% to 120% of OEM recommended life.

The root cause of early failure are collected. The major cause behind the lower life of sensors is high temperature, acidic, suspended solids, and sticky fluids. In most of the cases or say 90% time the failure reason is one of them. The rest 10% are fall under compatibility, installation and type of sensors.

Maximizing the pH probe performance starts with choosing right sensors, which is process dependent. The more demanding the application, the more critical it is to consider process operating condition and sensor expectation. It is especially significant when you consider the harsh environment require frequent probe replacement. Hence, first selection is to be enough process and fluid information before choosing the sensor.

During survey, it is observed that 60% of end user are not getting adequate life, the life of sensor is less than 50%. Few areas identified, where the sample cooler was not installed, it is due to costly MOC. The pH sensors are placed at high temperature of 70°C. The life of sensors is observed 3 to 5 months. In such scenario, process has to look the requirement and importance measurement and plan the sample cooler in design stage. The ROI model can be used to justify the sample cooler cost.

Hence, it is more important to focus on installation, fluid condition, adequate sample cooler, and review the calibration and maintenance practices. As more handling of probes is also detreating the life of the sensors.

Sensors Selection 

Maximizing pH probe performance starts with choosing right sensors, which is process dependent. The more demanding the application, the more critical it is to consider process operating condition and sensor expectation. This is especially significant when you consider the harsh environment require frequent probe replacement. It is well worth the upfront effort to select the proper pH sensor, when you consider the potential saving by extended service life, reduced replacement, and maintenance costs and ultimately accurate reliable pH measurement.

Selecting the appropriate pH probe requires a basic understanding of its construction. The main components are

·      Body

·      Measuring Electrode

·      Reference Junction

·      Temperature sensing element

·      Reference Electrolyte

When selecting body housing, two key consideration are material and mounting configuration. Choose a material that compatible with the process chemistry, temperature, and installation physical parameters.

Sensors bodies made of polypropylene sulphide (PPS) are suitable for most general purpose application. PVDF is an ideal material due to its excellent pressure, material, and temperature properties, including chemical resistance.

Existing mounting arrangement dictate what mounting configuration should be specified. The historical problem has been specifying, ordering, stocking, training on and maintaining a variety of sensor accommodating the different mounting configuration throughout the plant.

Another factor to consider is disposable vs rebuild able bodies. Disposable sensors are more widely used because they are easy to use; specially, they are straight forward to install, replace, calibrate and maintain. However, convenience has its price. Ongoing cost containment has renewed interest in rebuildable sensor bodies, particularly in harsh environment that require frequent replacement. With rebuildable, new electrodes, reference junction and reference electrolytes are inserted into reusable pH bodies. Rebuild kits are less costly than disposable sensors, and rebuilding allows the body and cable to be reused and avoid opening and rewiring the pH transmitters. However, rebuildable do require inventory internal spares parts and skilled technicians that can assemble them.

The measuring electrode is the business area of the pH sensor and includes the glass that actually comes in contact with the measured media. The three most popular type glass configurations are available.

·      Doomed glass with protective guard

·      Doomed glass only

·      Flat glass

At present, the domed glass bulb with a protective guard is best suited for severe application where breakage is concern while in service or while being handled during installation and maintenance.

During the survey, we have identified 95% pH sensors are domed glass with protective guard. Hence, the rebuildable sensors and flat glass use

A domed glass bulb accommodates applications where the process stream has continuous solid material that coat or wear the electrode. The smooth sensing surface is self-cleaning due to action provided by the process flow.

A pH sensor compatibility always shown by the supplier, the table below is showing the same.

Standard Installation

Industrial pH probe can be installed in the process piping, immersed in a tank basin, or placed in a side –stream sample. In a side stream, a portion of the process liquid continuously removed from the main process and directed to the sensor side stream, a extreme care to be take when the pressure and temperature exceed or running to probe rating. The location always be flooded, if it is not possible than pay attention to how far the probe insert to the pipe for flooded conditions.

Take into consideration that pH probes need regular calibration and, in some applications, frequent cleaning. When possible, install sensor to allow easy access and removal. If the probe is installed in the process pipe or inserted through the vessel wall, the process might have to shut down before pH probe can be removed. In these situations, retractable sensor may be solution.

Always consider the liquid velocity and density flowing past the sensor. A dense liquid moving at a high flow rate can deform the insertion shaft.

Sensors Maintenance

One FAQ and SOP were collected during the study and checked present maintenance practices. The present standard operating procedure (SOP) are not much elaborated in front of the parameters and the recorded values.

It is observed that high frequency is also not good due to impact upon the life of the sensor and time provided during maintenance. The sensors regular maintenance is observed from daily to half yearly.

Maintenance involves mostly cleaning and calibrating. However, service call frequency can be determined only by experience.

For instance, an application that requires highly accurate measurement of a process liquid that coats or fouls the probe will require more frequent cleaning, possible in a day. The SOP's are observed silent on thermal shock, fluid temperature measurement, and stains observed over the glass. This information is need to recorded and it is used as input for further selection & improvement.

Cleaning methods are application dependent but there are few general guidelines for optimum results. Never scratch or aggressively scrub the sensing element. These are delicate glass electrochemical electrodes and can break easily by force. To remove oil and grease, wash the glass bulb with mild detergent and rinse thoroughly with water. For loose scale or debris, use a water stream from a wash bottle to rinse away the solid from the sensor tip. If water does not work, gently wipe out the glass bulb with soft cloth. 

Hard scale (oxides, carbonates, and other corrosion product) requires more aggressive cleaning. If wiping the sensor tip with a tissue does not remove the scale, soak the glass only in a 5% HCL solution, be sure to keep the acid away from the liquid junction and any stainless-steel parts. Some scale such as calcium sulphate, cannot be removed easily with acid.

Soaking the glass bulb in a 2% disodium ethylenediamine acetic acid (EDTA) solution for 5 minutes should help. However, be careful not to soak to long, since EDTA solution can attack glass.

The standard shelf life for most of pH probes is one year from shipping date. Sensors should be stored wet in a PH 4 buffer solution or dilute KCL, with a probe tip down.

Now a day’s intelligent sensor management will help to diagnose and predict sensor maintenance and replacement.

 The continuous monitoring process condition, sensor slop and reference system impedance value give the data to predict the life and type of maintenance required. The time-based management you can fix the adequate life of the sensor.

Replacing sensor too early or too late and may be over calibrating when it is not required.

We need to maintain four parameters record continually for better sensor management.

·      Fluid temperature

·      Operating PH

·      Sensor tip fluid Velocity 

·      Sensor slope

By ensuring the above four factors, the life of the sensor can be improved drastically from 200-400% times.

As present, the fluid temperature and velocity are not much focused and not ensured. This is also not mentioned in SOP.

It is better to include the performance of sample cooler in SOP, as this is contributing most of cases for lower life and early failure as per the feedback. The best practices to start measuring the cooler temperature and size the cooler according your requirement. Many of times, it has seen that the due to poor cooler surface area & high fouling, the failures are very high.

The critical gaps are listed down below, these inclusions will be helpful to focused maintenance and excellent sensor management.

1.    The sensor removal process is not mentioned, how to handle the bulb, it cannot be touch or hit with any surface.

2.    After removal of sensor what time required to hold the sensor to acclimatize with atmospheric temperature before rinsing with water. It is to avoid thermal shock.

3.    In SOP, it is not mention that during visual check. We need to record the type of stains observed over bulb or protective guard.

4.    Stains possible due to suspended solids, sticky material, microbes, oily, chemical stains etc. The type of stains is fixing criteria of cleaning and fluid used for the cleaning.

5. The possible cleaning fluid will be different compared to DM water and time of removal these1.    stains also changes. It is varying from second to hours.

6.    In present SOP, the fluid temperature is not mentioned. The fluid temperature should be checked and recorded during the maintenance.

7.    The temperature is directly recoded from the housing to ensure safe temperature for the sensor probes.

8.    The velocity of the fluid can be ensured by looking the pipe size, flow, and pressure. The velocity should be limited to less than 2 m/s and if it is high so use orifice in the same line to reduce the velocity in housing.

Installation Practices

Industrial pH probe can be installed in the process piping, immersed in a tank basin, or placed in a side –stream sample. In a side stream, a portion of the process liquid continuously removed from the main process and directed to the sensor side stream, an extreme care to be take when the pressure and temperature exceed or running to probe rating. The location always be flooded, if it is not possible than pay attention to how far the probe insert to the pipe for flooded conditions.

Take into consideration that pH probes need regular calibration and, in some applications, frequent cleaning. When possible, install sensor to allow easy access and removal. If the probe is installed in the process pipe or inserted through the vessel wall, the process might have to shut down before pH probe can be removed. In these situations retractable sensor may be solution.

Always consider the liquid velocity and density flowing past the sensor. A dense liquid moving at a high flow rate can deform the insertion shaft.

Conclusion and Way forward

Sensors are common and critical component used throughout cell house, ETP, DM water, Cooling water, etc. The accuracy is very important to avoid sustainable operation. The findings are shared in this study based of the fact and data collected.

Provided regular monitoring and validation, based on industry statistics, it is estimated that, by maximizing pH probe performance, a medium scale chemical plant can realize 30-40 Lacs/year saving attributable to fewer probe replacement and maintenance call. It only makes sense to be familiar with the proper selection, installation and maintenance of PH probes, and to stay current on how you can benefit from advances in sensor technology. We are hopeful, these gaps will serve our purpose to improve the pH sensor life.

Literature Cited 

1 Bloch, H. P., Pump Wisdom, John Wiley & Sons, Inc.,Hoboken, New Jersey, 2011.

2 Bloch, H. P. and F. K. Geitner, Compressors: How to Achieve High Reliability and Availability, McGraw-Hill, New York, 2012.