The Käppala Association was formed in 1957 for processing and measuring sewage from municipalities north and east of Stockholm. In April 2000, the King of Sweden inaugurates a modern plant expansion. Today, it serves 11 municipalities and is one of the world’s most environmentally friendly and advanced water purification plants.
The plant is located in Käppala, Lidingö, an island east of Stockholm, in the inner part of the famous Stockholm Archipelago. Wastewater from 11 different municipalities, approximately 550.000 people, hospitals, offices, and also industries travels a 60-kilometer system of tunnels. It also travels through three pumping stations before entering the treatment plant. Then, it takes only 24 hours before leaving the plant as pure water. For properly measuring sewage quantities, a special magmeter is in use, which can measure partially filled pipes.
The wide area of Stockholm divides into 24 sub-areas from where effluent intakes. The quantities coming in from these sub-areas require measuring sewage flows very accurately. The customer requires a measurement accuracy of better than 1% of full scale in a partially filled pipe. However, the flow rate fluctuates drastically during the day. The filling rate can be as low as 10% of the pipe size. In the winter season, the temperature of the wastewater can be low. This can cause fats to build up easily at low flows and create flow disturbances, which can result in a lower accuracy.
The tunnels through which the wastewater runs is from natural rocks. Here, a lot of highly corrosive H2S forms, which puts restrictions on the materials for construction. This is particularly critical for the electrodes when they are not always wetted. Another customer H2S requirement is that the measurement should take place in a closed system. In turn, this creates a safe and clean working environment for the maintenance engineers. Therefore, operators reject an open channel measurement solution.
The customer chooses a TIDALFUX flow meter, from KROHNE, because it meets all operation requirements. In total, the customer installs 24 TIDALFLUX magmeters with various diameter sizes, ranging from DN 600 up to DN 1000. The TIDALFLUX offers an abrasion resistant liner and relies on wetted electrodes at a height of 10% from the pipe bottom. This is a major advantage when comparing to a competitive flow meter for partially filled pipes, which requires multiple electrodes. In partially fill mode, most of the electrodes are in contact with the highly corrosive H2S environment. As a result, the instrument breaks down in a very short period.
With the TIDALFLUX, capacitive plates and high frequency electronics measure the filling level. It uses patent, integrated level sensors, located behind the liner, which have no contact with the liquid and are therefore insensitive to fat and oil floating on the surface. A thin wall reducer pipe will create a smooth passage and prevent fat build up at the transition from customer pipe to the flow sensor. TIDALFLUX can reliably measure flows between 10% and 100% of the pipe cross section.
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