Fluid thinking for water - Biwater

Simon Johnson, Senior Operations Engineer explains how they used Flowmaster to provide cost effective solutions to their customers.

The Challenge

Biwater Treatment designed and installed a pressurised filter and membrane plant at Broadford water treatment works (WTW). In brief the plant consists of a filtration unit positioned downstream of a pumping station which in turn is supplied, via a gravity fed suction main, with water from the Allt a Mhuilinn river, 1.2 km away.

During commissioning of the plant the site engineers observed that the primary filters frequently blocked which triggered a backwash cycle. On site investigations revealed that, following the on set of the backwash cycle, which caused a sudden increase in flowrate through the intake pumps, air was entering the system, collecting on the filter screen and causing further blockage.

“When the filters back-washed, the flow rate in the suction mains increased by 40% for 30 seconds before a stepwise decrease to the original flow rate. We suspected that the air was entering the main via the air valves” comments Simon Johnson.

The solution

Since the on-site investigations lead to the assumption that air was entering the system and the most likely route was through air valves situated at intervals along the suction main, the next stage was to prove this and then propose a cost-effective solution.

Once it could be numerically proven that the negative pressure transients caused the air valve on the suction main to open, the task was then to propose a solution. The simplest and most cost effective way came in the form of a bladder type surge vessel to absorb the rate of change of flow. Furthermore air release valves were fitted to the filter vessels.

“First of all we built a detailed Flowmaster model of the suction mains longitudinal profile from the river to the pumping station in order to quantify and prove that the negative pressure transient travels back along the suction main. The detail in this model uses exact physical parameters of the pipe wall, lengths, elevations and fluid properties. We have used Flowmaster for over 5 years on a range of projects and so we are confident in the accuracy of the results” explains Simon.

The Results

From the initial simulation that proved the pressure in the suction main was causing the air valves to open and identify which backwash flow rates were the cause, the model was then modified to include the bladder type surge vessel and predict the optimum size and connection diameters to the existing system. Figures 1 and 2 show the pressure transients at the most critical point before and after the surge vessel is added.

The addition of the surge vessel and air release valve on the filters at Broadford has reduced the frequency of the backwash cycle from typically 5 minutes to over 60 minutes and the ensured that clean safe water can be produced as economically as possible. The use of Flowmaster reduced the time to quantitatively identify the cause of reported problems and investigate cost effective solutions, faster and more accurately than traditional testing.