In the second of this blog series about the resistance to the effective use of instrumentation I am going to talk about the subject of instrument reliability. This is one of the first reasons given by any water or wastewater treatment operators. The typical cry is “we tried to use it but it simply doesn’t work”. On one side you have instrument manufacturers who have invested many man hours in developing a particular instrument and on the other, once the instrument is installed it is not maintained, it breaks down and then falls out of use. The first line of process automation is lost and the system breaks down and all of the systems that were meant to save money are lost. The question is, why does this happen?
There are many reasons for why a instrument is not reliable but there are two I would like to talk about in this blog
a) The instrument has not been selected for the right application or is simply in the wrong place within the treatment process or has not been installed correctly
b) The instrument has not been maintained for a variety of reasons.
If we examine the first reason then the reason for the failure of instrumentation becomes a bit clearer. In the pictures below there are a variety of instruments that I have taken photographs of in my travels around the various wastewater treatment plants. Some of you will recognise the instruments but I will stress that there is nothing wrong with the instruments themselves but the way in which they have been installed. This doesn’t give the instrument a fair chance to operate.
In this particular case there is a pair of instruments. The left two pictures show an ammonia monitor installed in a final settlement tank distribution chamber, the wet end (actually from another instrument) is partially submerged in mixed liquor. The environment that it is submerged in is so turbulent that not only does the wet end get shaken so much that the mixed liquor can’t settle within the instrument to take analyse the sample but physical damage is done to the instrument. The manufacturer makes a stilling tube that would resolve this issue. In the pair of photographs to the right hand side the instrument is blocked by a rather large piece of debris that has not only managed to bypass the screens but has gone through a feed pump!
As can be seen by these two examples that not only has care got to be taken in selecting instruments for the right application but the installation is vital to. The ammonia instrument mentioned is actually a very reliable instrument when it is installed in the correct environment and of course installed correctly. As a result of all the problems that has been experienced on the particular site in this case study the instrument has fallen into disuse. The result being that the dissolved oxygen ammonia control mode is rarely used and the company is loosing £1000’s per year in over aerating the activated sludge plant.
The second and one could say more important reason for questionable instrument reliability is lack of maintenance. In the case of the ammonia monitor above the operators of the treatment works were left with an unreliable instrument. As it was known to be “unreliable” then there was no driver for the operational staff to maintain the instrument. It wasn’t cleaned and the reagents that the instrument relied upon where not refilled. This lead to an inevitable decline in the instrument performance and with time the instrument being sent away for an overhaul leaving the site with no working ammonia monitor and the confidence in instrumentation irrevocably damaged by not only the site operators but the treatment mangers to.
There are of course other reasons why instrumentation isn’t maintained and again this falls to the way that they are installed.
The two pictures on the left hand-side show a solids monitor and how it is installed. The instrument is installed on the return RAS line to an activated sludge plant. The instrument should be removed from where it is (in between two large belmouths) approximately once a week for a clean in addition to the automated wiping system. The opening (centre photograph) is narrow and only allows the instrument to be withdrawn with considerable effort. The instrument is not heavy but the way it has been installed does not make it an easy job to complete, hence operationally it tends not to be completed. The picture on the far right shows an actuated valve with instrumentation nearby, the access issues for maintenance I think are self explanatory.
The message for this blog is that there is a general resistance to the use of instrumentation that is partially centred on instrument reliability. On one hand you have manufacturers that produce instruments that are reliable “in all environments” and on the other you have operators that have to maintain these instruments. Sometimes something happens in between the two that results in an instrument not working as reliably as it should do and this does the whole industry a great dis-service and as instrumentation is the backbone of any large wastewater treatment works and fundamental for process automation & control then before the industry takes a step forward it is an issue that must be resolved.
– Oliver Grievson, Group Manager, Water Industry Process Automation & Control