Water and Effluent Treatment Magazine
Issue link: https://fhpublishing.uberflip.com/i/843774
10 WET NEWS JULY 2017 ProjEcT SPEcS • Understand how and whether individual asset failures escalate to system failure • Assess water and sewerage industry reliability • Predict the future NEED To KNoW 1 Reliability engineering was first used shortly after WWI in the context of aeroplane safety 2 British Standard (BS 65000:2014, Guidance on Organisational Resilience) is mainly focused on the cycle of learning from your mistakes 3 Loads might be interpreted as water demand, wastewater load, weather THE VErDIcT "Reliability is a means with which to measure our resilience both now and in the future as required by Ofwat. Using standardised or observed rates of failure and calculating the chances of the rare simultaneous failures needed to cause system failure, we can arrive at an objective measure. This helps with many decisions including those difficult calls involving safeguarded systems" Alec Erskine then we understand the reliability. This is fairly comfortable terrain for engineers and fits well with the "variability" part of the resilience definition. Loads might be interpreted as water demand, wastewater load, weather and so on. The actuarial approach is more to do with lifetimes and deterioration and captures the asset performance in terms of its "time to failure" or "failure probability". Failures happen, especially when you have huge numbers of things. Predicting which ones are going to fail is really difficult but predicting how many are going to fail is surprisingly easy – just look at a graph of the monthly pipe bursts for a water company. Calculation The last step is understanding how and whether individual asset failures escalate to system Alec Erskine failures. When does the asset failure lead to an impact on the customer and when does the standby just kick in so the customer never knows it happened? How resilient are our systems and our networks to the inevitable breakdowns? Have we got enough standby, enough cross-connection to be able to cope? Have we got too much? Techniques and soƒware such as Reliability Block Diagrams and Fault Tree Analysis are designed to make precisely this calculation, turning an asset failure rate to a system failure rate – a quantity representing the reliability in failures per unit time. These techniques are available and soƒware is also available to help us. The failure rate estimates at asset level are not so commonly available. The big databanks that store data and regularly re-assess failure rates are mostly maintained by the oil industry. Water data is scarce and we generally have to assume the equivalents in the oil industry have similar failure rates – a weak assumption. We may get some supporting Reliability is a means with which to measure the industry's resilience both now and in the future TEcHKNoW • Reliability engineering is a collection of techniques to help determine whether an item or system is going to function or not • It can split between two approaches: the physical and the actuarial • Oil and gas and the nuclear industry employ the techniques of reliability routinely Techniques and software such as Reliability Block Diagrams and Fault Tree Analysis can help turn an asset failure rate to a system failure rate evidence from company failure records but there is not enough of this data. Reliability is a means with which to measure our resilience both now and in the future as required by Ofwat. Using standardised or observed rates of failure and calculating the chances of the rare simultaneous failures needed to cause system failure, we can arrive at an objective measure. This helps with many decisions including those difficult calls involving safeguarded systems. If the pump fails, the standby kicks in and there is no consequence, so there is no risk, so how can we justify replacing the pumps? Well, there is still system risk which will increase as the pumps age – and reliability engineering can quantify it. So not only does reliability allow us to satisfy the requirements of the law but it may help us see where we have enough standby and where we need more. And that is a benefit worth chasing. Alec Erskine is senior principal consultant at MWH, now part of Stantec.