FEATURE TEST & MEASUREMENT
MAINTAINING CONTINUOUS WATER ANALYSERS
ABB provides some recommendations that will help in the design of an effective maintenance strategy and prolong the life of continuous water analysers
1. INITIAL WARNING SIGNS Many of the latest generation of analysers incorporate self-diagnostic features and alarm capabilities to warn when something is wrong. Although these signals are useful in determining when maintenance needs to be performed, it is nevertheless advisable to have a preventive maintenance regime in place to help extend the life of the instrument.
T
ightening environmental legislation, coupled with customer expectations
of quality and availability are seeing continuous water analysers being used to measure an extensive range of parameters, from dissolved oxygen in aeration processes through to chlorine and turbidity. Capable of taking ongoing readings
which can be automatically relayed to a central monitoring system, continuous water analysers help to eliminate the delays and added uncertainties associated with extractive testing methods. Despite their advantages, however, the performance of continuous water analysers is only as good as the condition of the analyser itself. With the introduction of ever more
stringent standards by the UK Environment Agency, it is particularly important to maintain analysers to the highest possible levels as a matter of best practice. In the event of a water quality incident occurring, operators must demonstrate that their monitoring and measurement equipment was being correctly maintained in accordance with recognised best practice. The following provides some
recommendations for measures that can be put in place to help ensure continued effectiveness.
28 MARCH 2018 | INSTRUMENTATION
Having a preventive maintenance regime in place can help extend the life of an instrument
2. CHECK REAGENT LEVELS Reagents are crucial for producing the chemical reaction needed for an accurate measurement to take place. As such, it is important to ensure that reagents are replenished as soon as they are exhausted. Checking reagent consumption regularly will help you predict the best time to carry out a changeover. As reagents have a shelf life, check the expiration date before exchanging the used reagent with the new one. In addition, regular maintenance will also help to show whether the reagent is being consumed at the rate that should be expected for the application. Under or over consumption can be indicative of a problem in the analyser’s fluid handling system, such as a blockage or restriction.
3. PERFORM REGULAR VISUAL CHECKS Analyser functions and readings should be thoroughly checked to ensure they work at optimum efficiency. Checking for leaks (pipe connections and tubing and liquid handling components), liquid
levels (reagents, calibration and cleaning solution bottles) and potential obstructions to the sample flow should be performed as regularly as possible. It is also worth remembering that erratic readings on the analyser display unit may not be indicative of a fault with the transmitter, but rather external factors such as unexpected changes in water quality. Where doubts arise, it is advisable to check the analyser’s historical log to compare the period during which erratic readings took place against the known operating conditions. In addition, always check the analyser’s high and low measured values. Any discrepancies in the measured values may be symptomatic of a number of potential issues. These could include incorrect calibration or variations in ambient temperature and humidity conditions that could be affecting the sample. If a filter is fitted prior to the analyser, there is also the possibility that some important parameters may be being filtered out.
4. CHECK CALIBRATION Ensuring that your analyser is properly calibrated will have a material impact on its ability to deliver continued measurement accuracy. Errors can arise when changing reagents, with potential failures arising if the reagents are not fitted properly, if tubing is blocked or restricted or if the reagent lines were not properly primed. Failure of a routine calibration can also be symptomatic of issues within the analyser, such as blocked tubes and valves, leaking seals or contamination of the reference standard.
5. CHECK THE SENSOR It is worth checking analyser systems that are in direct contact with the process, as sensors can become obstructed or damaged as they often bear the brunt of any adverse conditions. In waste water treatment processes in particular, sensors can quickly become fouled by the build-up of organic matter or subjected to abrasion by sediment. The accuracy of electrochemical sensors used in dissolved oxygen monitoring applications, for example, can quickly become impaired due to fouling of the sensor membrane.
Analyser functions and readings should be thoroughly checked to ensure they work at optimum efficiency
6. REPLACEMENT OF COMPONENTS Follow the manufacturer’s recommendations regarding the replacement of any key components. Depending on the type of analyser being used and its application, consider changing
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