Page 58 - ITA Journal 3-2018

Technical Papers
signs of wear at an early stage,” reports BINE Information Service, published by the FIZ Karlsruhe – Leibniz Institute for Information Infrastructure GmbH, which is in parts funded by Germany’s Federal Ministry for Economic Affairs and Energy (BMWi). There were no specialpumpsavailableforgeo- thermal applications.
Valves prone to failure
As such, pumps were highly prone to failure, some pumps already broke down after just a couple of months. In the Unter- haching geothermal power plant, ten ESP pumps (electro-sub- mersible pump) had “given out”, as VDI Nachrichten, the news service of the The Association of German Engineers (VDI), reported several years ago. More often than enough, broken frequency converters, bearings, damaged bearingsorhotwindingswereto blame. “These failures really cost money”. Around one million euro need to be invested, per pump.
Not only the technology, but also the dimensions make pumps for geothermal applications a very expensive component. The rather new pump in the Freiham geo- thermal plant is 45 metres long, and has a total weight of 45 tons, including the rod. A heavy-duty crane was needed to lower it 600 metres below the surface. Here, the pump conveys thermal water with a temperature of approxi- mately 90° Celsius from a depth of 2,500 metres to the surface.
High temperatures, aggressive gases
The production depths alone indi- cate the conditions pumps have to cope with, and make under- standable, why pumps from the oil and gas industry aren’t the best of choices. High temperatures
and feed rates, aggressive gases and crystallised salts demand an optimal performance from pumps.
Solutions need to be found for special pumps in geothermal applications – numerous manufac- turers are working vigorously on hard-wearing components. Baker Hughes is one such example. The manufacturer of special geo- thermal pumps constructed a high-temperature test loop on its company site, in order to test entire pump systems under real-life conditions. “Field tests have yielded further insights into structural weak points in the aggregates used to date,” reports BINE in its Projektinfo 3/2016. Scientists had salvaged broken pumps from German geothermal plants,takenthemapartandana- lysed the causes of defect. “On the basis of this data, they then developed modi ed prototypes which promise signi cantly higher ef ciency and a longer service life”. Here, they concentrated on the conditions found in hydro- thermal plants in South Germany, where thermal water reaches tem- peratures between 120 and 140° C, and is highly calciferous. Baker Hughes collaborated with the geo- thermal plants in Oberhaching, Dürrnhaar, Sauerlach, Grünwald and most of all Unterhaching for this task, reports BINE.
Temperature and pressure uctuations
With hardly any lubricant proper- ties and containing dissolved lime, the thermal water can have grave consequences for components. The limestone “is then deposited on all components, particularly the hotter bearings. These depos- its prevent lubrication and cooling of the bearings, which then leads to overheating and destruction of
the pump shaft,” explains BINE. To make things worse, a layer of limescale acts as insulation, pre- venting heat dissipation from the motor.
“The high absolute water temper- atures and the cyclical temper- ature and pressure uctuations whicharisewhentheplantisata standstill or operates at a reduced capacity are a source of stress for pump bearings, seals and motor insulation. “Thermal water furthermore contains dissolved gases. During the course of time, these diffuse into the oil with which the motor is lled. When a pressure drop occurs, the volume of the gases increases and pushes the oil out of the motor”.
Redesigned bearings structure
In general, the pump rates in geothermal plants vary – depend- ing on the current heat demands oftheplantandheatgrid.“Asa result, the pumps are frequently subject to start and stop pro- cedures or phases with reduced capacity,” explains BINE. As such, the resulting variation of torque, motor temperature and pres- sure cause extra stress. In order to monitor pumps deep below the surface and initiate mainte- nance early enough, measurement sensorsarerequiredthatcanwith- stand high temperatures, and can transfer a large number of varia- bles with a high data transfer rate.
In the end, the outcome was pos- itive. The motor’s bearings struc- ture was entirely redesigned – and there have been no more failures. “A pump with optimised bearings has been operating in a southern German plant for over 8 months without any failure occurring. The weak points found to date in the insulation have been recti ed,” states the news service.
ITAtube Journal No3/October 2018
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