Instrumentation: Dos And Don'ts For The Water Industry
By Oliver Grievson, Group Manager of Water Industry Process Automation & Control
To say that instrumentation, automation, and control are key to the modern wastewater industry is somewhat of an understatement. With growing populations, tighter consent parameters, and the need to do more for less, operational efficiency is more and more important. The need to give the operators in the field all the necessary information in order to actually operate the wastewater treatment works is also becoming increasingly important. Some see this as a threat to staff numbers and for the less enlightened operating companies it is. For those who actually want to achieve operation efficiency it isn’t— it is merely a change in the day-to-day job.
Instrumentation is quite literally the “eyes and the ears” for both the operator on the ground and the manager back in the office. Those in the water industry need to realize when networks or treatment works are designed, installed or reworked instrumentation needs to become a fundamental part of that design.
By not realizing that, potentially valuable instrumentation opportunities may be missed. This article will highlight the common areas surrounding the selection, installation, operation, and maintenance of instrumentation in the modern water industry.
The selection of the correct instrumentation for the job is a vital first step and there are various pressures on the engineers within the industry. These pressures are:
Cost And Procurement: In the UK we have a framework process where a water operator makes an agreement with a supplier for a certain time and often a certain volume of sales. The problem with this is that it excludes developments over time of new technologies. The driver is to go for the cheapest technology available.
In this case the thing to remember is that the seemingly ‘cheapest’ instrument is not always actually the cheapest. Some instruments have significant installation costs so the overall cost of the product is less than an instrument that seems more expensive upfront.
Figure 1: Which is cheaper – The electromagnetic flow meter on the left or the radar flow meter on the right, in terms of meter cost the flow meter on the left. In terms of total expenditure the meter on the right
What is the right instrument?: In some situations choosing the right instrument for the right job is the most important factor. Often when instruments are selected a number of mistakes are made. These include a company not asking the supplier what is an appropriate application for a particular instrument or a supplier being ‘adventurous’ as to the application of their technology in order to get a sale. At this point honesty is certainly the best policy as a single sale of an instrument can be made relatively easily, but if a relationship between company and supplier is developed significantly more sales can be made.
Care also needs to be taken where specific regulations exist around particular applications. An example of this in the UK is the Self-Monitoring of Effluent Flow under the MCERTS Regulations.
A mistake that I saw recently involved the purchase of ammonia monitors with Ion Selective Electrodes (ISEs) for the monitoring of effluent flow from the treatment works. The ISE, although it is a perfectly good and cheaper instrument, was inappropriate for this application and failed within a few weeks.
The installation of instruments is probably one of the most common reasons why an instrument goes wrong. This happens for a variety of reasons.
The first question to ask is whether or not an instrument is placed in the right location to measure the right thing.
- Is there undue vibrations that are going to cause the instrument to be damaged in the transit of normal operation?
- Is the instrument in the right place to take a representative sample or sub-sample with adequate mixing (for quality analysis) or with appropriate condition (flow as well as quality)?
- Is the instrument in a place where it can be operated and maintained or replaced as needed when the instrument breaks?
- Are all of the elements of the instrument installed correctly and in the right place?
Figure 2 below shows some poor instrument installations.
Figure 2 (L to R): An electromagnetic flow meter with inappropriate clearance between it and a control valve, a suspended solids monitor inaccessible for operations and a temperature compensation monitor installed in a cabinet
The example in Figure 2 above shows an electromagnetic flow meter which was installed within 3 ½ pipe diameters downstream of a modulating penstock. This is contrary to the manufacturers minimum requirements of 5 pipe diameters and certainly contrary to studies that have suggest it should be 15 pipe diameters. By chance there was a regulated flow meter for this site and by comparing the two flow meters the error can be compared and contrasted.
Figure 3: Flow data showing the interference of a modulating penstock on a electro-magnetic flow meter (in red) when compared to an effluent ultrasonic flow meter (in black)
Figure 3 shows 15 minute flow data over about three weeks with the black line (final effluent ultrasonic) compared to the red line (inlet electromagnetic flow meter) which shows an error of anywhere between -50% to +150% due to the modulating penstock.
This shows that if an instrument is to be used, the installation is key.
Operation And Maintenance
If the correct instrument is selected for the right task and is installed correctly, the last barrier to an instrument working correctly is the operation and maintenance. Part of this comes down to whether or not operational members of the staff were consulted or even considered in instrumentation selection. There are a number of barriers at this stage and one of them is how complicated an instrument is to operated and maintain. Sometimes operational staff find it impossible to operate an instrument because it is simply too complicated.
Putting this aside, there does need to be some robust operational and maintenance procedures put in place to cover areas of typical errors. These include: cleanliness of instrumentation structures or pipework (particularly important in the measurement of flow), drift of the instrument, and calibration and scaling of the instrument within the analogue or digital communication.
Of these, cleanliness of the installation is probably the chief cause of error with the use of instrumentation. Figure 4 shows typical problems that have been seen on treatment sites.
Figure 4: Obstructions in pipes and dirty channels are a chief cause of flow meter inaccuracies
The other causes— drift and scaling— should be addressed in routine tasks that maintain the operation of the flow meter. As long as procedures are in place to address these issues then the meter will operate as it should
As long as instrumentation is selected, installed and operated/maintained correctly it has the power to assist the operator in actually operating the wastewater treatment works they are responsible for acting quite literally as the “eyes and the ears” of both the operator and the operating company as a whole. However they still need some attention and in no way should be used as a replacement of operational staff, but merely as a tool to help the operational staff in their day-to-day tasks.