A new maintenance strategy for pH instruments

01 September 2007

Process analytical measurement technology is a significant component of measurement and control technology in chemical plants because it supplies data which are important to product quality and productivity.
by Dr. Thomas Steckenreiter (Endress+Hauser), and Dr. Armin Weinig (Lanxess)

Process analytical parameters, including pH measurements, require regular calibration and maintenance more often than other controlled variables. In the chemical production plants of Lanxess, approximately 25 per cent of the analytical systems currently in use are for pH measurement. For example, about 50 pH measurement systems are installed in the production of sulphuric oxide and about 100 in the production of iron oxide.

Calibrations or adjustments must often be carried out several times a week. The classical, analogue, high-ohm systems in use are very susceptible to moisture, salt bridges, EMC interference, and potential
overloads. This means that the sensor, cable, and transmitter must be calibrated together. For service technicians, this requires significant time and effort on site.

Often the cable and/or sensor must be replaced to enable a fault diagnosis. In addition, buffer solutions, cleaning solutions and water must be brought along when calibrating or adjusting a system. Spare electrodes must be available in case an electrode needs to be replaced. In this case, a preliminary
calibration is also required on site. All this requires careful preparation, and the necessary equipment must be packed cautiously and brought along to the plant. Worst of all, measuring points are often at locations which are difficult to access.

A new strategy
Endress+Hauser's Memosens technology makes it possible to move all maintenance operations such as the cleaning, conditioning, regeneration, calibration, and adjustment of the pH sensors to a central location. With this objective in mind, Lanxess in Leverkusen is setting up a central pH service with a computeraided
pH calibration bench ('pH laboratory'), which maintains and manages pH sensors for the different operations.

Reproducible laboratory conditions enable precise calibrations and therefore more accurate process
management. In addition, regular regeneration and cleaning prolongs the operating life of pH sensors
in particular. This approach naturally increases the availability of pH measuring points in operation. It also
reduces costs further due to the fact that no specialised knowledge of pH measurement technology is
required in order to replace a sensor, and so sensors can be replaced by operating staff.

On site, all that needs to be done is to replace the sensors with calibrated sensors. This reduces maintenance time in plants by more than 50%. An additional, essential component of this maintenance strategy is effective data and sensor management. Using the newly designed database Memobase, all data specific to a measuring point, calibration and adjustment in addition to the process data are recorded and
processed while the sensors are calibrated. This means that the complete life cycle of a sensor - from its storage through to its disposal - is tracked and analysed. These data can be used to analyse and, if needed, to optimise measuring systems and maintenance concepts.

Memobase processes all tag numbers and assigns new sensors to particular measuring points, measuring point circuits, or system components. This means that the tag number, the serial number of the sensor and the sensor type, along with the data from the preliminary calibration, are all unalterably linked with each other in the database. Successive calibrations are automatically added to the sensor history, recorded in the database and linked to the application.

Evaluations for quality assurance purposes can therefore be obtained at any time. These data can be formatted for the operational manager or the quality manager or can be made available as raw data.

For the first time, systematic evaluations of the performance of measuring points are possible. It is no longer necessary to keep a log book for a measuring point. The easy-to-manage tag number system prevents sensors from being mixed up. Each sensor is always correctly assigned to the place where it is
to carry out its measurement task.

For additional safety, a tag number test and a calibration time alarm can be activated in the case of transmitters of the Liquiline generation. This means that incorrect sensors or sensors which are too old based on the most recent calibration date cannot be used. Sensor management ensures that the operational staff always have enough sensors at their disposal. The staff are therefore in a position to clear a fault relating to pH measurement within minutes at any time of the day or night including weekends and without having to call on an expert.

This concept can be adopted across all locations. Today, the pH service at Lanxess is in a position to act as a service provider for other companies for the management of pH measuring points.

Efficiency of digital sensors
The question arises as to the cost effectiveness of a change in strategy with regard to the maintenance of pH measurements through the use of digital sensor technology.

Two cases are presented here:

....Reinstallation of pH measuring points using analogue or digital technology with the setting up of a 'pH
laboratory' in parallel; and

....Conversion of existing pH measuring points to Memosens sensor technology.

When using Memosens technology, electrodes can be replaced by operational staff without any additional
cost to the operation.

In the first case, the reinstallation of ten pH measuring points is assumed. Later on, when in operation, each
measuring point is subjected to a monthly calibration and to two electrode replacements per year. Based on established times for maintenance, both on site and in a pH laboratory, the total cost of the alternative analogue technology and Memosens technology was calculated over a period of five years. As shown in Figure 2, the additional cost of the installation of digital technology (primarily the cost of installing the pH laboratory) is paid for after little more than a year due to the lower annual maintenance costs compared to analogue technology.

The effectiveness of the conversion from installed analogue technology to digital technology is examined using the example of a production system at Lanxess. The observation is based on the conversion of 48 pH measuring points in this production operation. For the majority of measuring points, calibration/adjustment is performed on a weekly basis, and in individual cases as many as three calibrations/ adjustments are performed per week. The electrodes are cleaned in accordance with maintenance plans specific to each
measuring point. Existing fault statistics collated over many years provide information on the required number of electrode replacements.

The installed analogue measurement technology generates annual maintenance costs of (all figures are
approximate) €100,000. By converting to digital technology, annual maintenance costs are reduced to €45,000. This conversion requires investment of €70,000, which corresponds to a return on investment of 1.5 years. It is obvious that the cost efficiency of a conversion is essentially determined by calibration and
cleaning intervals. This means that specific cost efficiency observations must be made for individual
operations/operational units.

The evaluation of the pH measuring points with Memosens sensor technology installed at Lanxess shows that maintenance costs can be reduced, the quality of pH measurement increased and the operating life of pH sensors prolonged.

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