MechChem Africa September-October 2024

Why absolute pressure is important to abalone farming UNIDO Pump expert Harry Rosen of TAS Online talks about a recent pump assessment for a group of Abalone farmers in the Western Cape of South Africa, which involves pumping seawater at ever-changing sea levels in a suction lift application.

but both of these introduce further issues and complexity and were not considered. To maximise the available suction pres sure we need to minimise friction losses through the suction pipe, thus each pump has its own dedicated suction pipe. For minimum friction we go for the largest diameter pipe possible with the shortest length and as few bends as possible. We do not install any valves on the suc tion pipe, even if it means leaving out the non-return or foot valve so common in suction lift applications. It is better to re prime the pump every time it stops than to incur extra friction losses through the foot valve. Luckily, these pumps run 24 hours a day and only stop during planned maintenance or emergencies. The vapour pressure is insignificant for water where the water temperature is less than 30 °C. System design for abalone farming Since suction is the dominant feature of the application, we have designed the system to optimise the NPSH available (NPSHA) at the pump’s suction. We have also installed both suction and discharge pressure gauges for each pump to show exactly what is going on with the pumps. We selected a pump with the lowest pos sible NPSH requirement (NPSHR) at the duty required, which happens to be an NPSHR of 3. Pipe Friction • •

Suction lift applications require a level of planning and suitable engineering design to ensure that there is always a positive pressure pushing liquid into the suction of the pump. As much as I get fed up with them, pumps do not ‘suck’ in any sense of the word. Which means that they require a minimum absolute pressure on the suction side, or they will start cavitating and end up destroying themselves. The suction side pressure is called the Net Positive Suction Head available or NPSHA and is made up of the following three components: 1. Atmospheric pressure • One advantage of being at sea level as there is 101 kPa or 10 m of head available to push sea water into the pump. Up in Johannesburg, at 1 600 m above sea level, we only have 83 kPa to help out. A positive help in many applications but these pumping stations are mainly located at the maximum sea level or high tide mark. This reduces the risk of flooding during high seas but it means that for the majority of time, the sea is going to be at least 2.0 to 4.0 m below the pumps. During a spring low tide, the sea level will be even lower. • 2. Suction level: • A possible solution would be vertical turbine pump, which have the pump motor out of the water, while the impeller bowl is submerged in the sea. Submersible pumps are another option, •

I recently held a training course on pumps with a group of Abalone farmers in Gansbaai, a drive of around 40 minutes from Hermanus in the Western Cape. Besides being one of the most beautiful parts of the country to visit, Gansbaai is world renowned for its shark cave diving trips, which offer a chance to come up close and personal with a Great White. This industry is going through tough times as there are no white sharks to be seen all the way down the coastline. Apparently they are the favourite day-time snack of the Orca and whole groups of these killer whales ap parently swim up and down the SA coastline attacking and killing great white sharks. There are now far fewer sightings of these beasts in the waters around SA – maybe thanks to the Free Willy movie about saving an Orca whale. Abalone ¬ known in the Western Cape as Perlemoen – is an eastern delicacy farmed commercially in SA, with most of the natural abalone having been hunted to near extinc tion. Abalone farming requires pumping huge amounts of seawater through hundreds of tanks filled with abalone, supplying nutrients to the small creatures as well as removing their waste. Unfortunately two characteris tics of the sea make this a difficult pumping application. • The level of the sea water will generally be below the level of any pump station. This leads to a suction lift or negative suction application – excluding, of course, countries with areas of land below sea level, such as Holland, where sea water is pumped back into the sea. The level of the sea varies greatly, both over a single day – between low and high tides – as well as over a month – during spring low tides, for example, at full and new moon when the sun and the moon align. •

Figure 1: Mark up the suction pressure gauge with a red and green zone, clearly showing the point at which the pump begins to experience suction problems.

12 ¦ MechChem Africa • September-October 2024