Dummy's Guide to Apparent Water Loss Management - Meter Sizing

In my last post, I believe a point was made on how meter accuracy and in turn apparent water losses should be correctly estimated - using a combination of meter testing data at sufficient test flow rates and the consumption patterns. Today I want to share my perspectives and understanding on meter sizing and how it can be improved as meter sizing can be a major contributor to apparent losses.

Up sizing water meters unnecessarily by just one diameter size can lead to a 50% increase in apparent losses.

The problem of sizing water infrastructure at the household level such as water meters and service lines is a very old one and is largely done according to the empirical methodology developed by Hunter in 1940. The BMS65 Methods of Estimating Loads in Plumbing Systems (aka Hunters Curves) is the basis of most international plumbing codes. The Hunter Curves were developed using the binomial probability function based on the 99% probability that the demand of all running water fixtures in a household will not be exceeded. A nice presentation summarising Hunter’s Curve in the 21st Century can be found here.

In South Africa, the sizing of a water pipes in buildings should be done according to SANS 10252-1 where the probable demand is obtained by summing up the total possible demand from all fixtures raised to a factor (n) ranging between 0.5 to 0.8 relating to the probability of all fixtures being on at the same time. There is limited guidance in the selection of this factor. The output is used to size the connection pipe and other subsequent pipes, taking into account the headloss along the system. Then things start getting hairy. In many municipalities, the meter size matches the size of the connection and is likely amplified in areas where there is a fixed monthly charge (and/or meter connection fee) that correspond to the connection size. To illustrate why this is a problem; if we take the example in page 200 of SANS 10252-1, a household with a probable demand of 50l/min will require a connection pipe of 25mm. For this maximum flow of 3,000l/hr, a 15mm meter will do perfectly at its (typical) maximum flow rate of 3,000l/hr, and will measure better than a 25mm meter, particularly at low flow rates, minimising apparent losses. Metering has more to do with flow rates than nominal pipe diameters and the common lower flow rates matter as much, if not more, than the maximum flow rate! [FYI there are 40mm meters with a permanent flow rate of 10l/hr and others with a permanent flow rate of +40l/hr]. The reason why smaller flow rates matter is that in South Africa, and many other places, on-site leakage is a big issue and has a large component that occurs at low flow rates as explained here. In our example above, there is a chance that the apparent losses for that meter can be increased by up to 100% by simply installing a meter two sizes bigger than necessary.

The objective of having a meter is to be able measure all flows passing through the meter as accurately as possible - and not just the maximum flow rate.

A second issue is combined connections (both fire and, say, domestic usage) in multi-residential and other commercial complexes. The output of a rational fire design for the fire flow requirements must not be a pipe size but rather the required flow rate. In many cases, the fire flow requirements are much higher than the typical consumption flow rate requirements and one has to ask themselves whether they are willing to sacrifice accurately measuring the fire flow at the expense of the normal daily flow. Combination meters solve this problem but I have had numerous discussions with municipalities whose billing systems cannot "handle" combination meters but can handle a few meters for the same consumer. Such irony! My biased views on this matter are that;

• Use combination meters where they cannot be avoided - they have their issues as well. If you can't, then don't use combined connections but rather separate ones and size the meters accordingly. Do yourself a favour and flag fire connections on your database - this will come in handy later.
• The fire flow rate should be the maximum flowrate of the meter and never the permanent flowrate. Fire is never permanent.
• Size meters to fail from a measurement point of view, while delivering the required volume during a fire event and replace them immediately afterwards. You will not need to replace the vast majority of your meters due to fire events in any case - so why not?

Different studies have shown that the original Hunter's Curves lead to over-sized pipe infrastructure, as is the case with the design standards and guidelines in South Africa, with various calls to revise them. Importantly, water fixtures have since changed since 1940 and are far much more efficient and more municipalities encouraging the use of water efficient fittings and aerators. Such fixtures have much lower maximum flow rates. So in essence, we have not kept up as far as the input required for the sizing of meters and pipe infrastructure. For a utility committed to managing apparent water losses, an understanding of the consumption patterns, as discussed previous, becomes non-negotiable. This will involve the evaluation of water consumption at a high resolution (remember, consumers don't consume in averages) to be able to understand the pattern in general but also how peak flow rates vary with the number of fixtures in a household or other variable such as the number of units in a residential or business complex. Such information, together with the use of water end use studies that relate water consumption to a particular water fixture in the house, can help generate the current generation of Hunter's Curves and related relationships.

In summary, people responsible for sizing water meters need to re-evaluate the current assumptions under pinning the work they do. Fixture units related to Hunter's Curves, while valuable, together with local standards, have not kept up and need to be updated with more recent information on existing fixtures and the related consumption patterns of consumers. Invariably, an understanding of water consumption profiles is essential and will enable better sizing of meters and the management of apparent water losses (Thou shalt no thy consumers!). Focus must not only be on the maximum flow rate required but on how accurately can the entire flow profile be better measured. Most importantly, meter sizing should be about the flow rate, with the meter diameter being a secondary issue of practicality.