CO2 Pumping in Nuclear Power Stations

CO2 Pumping in Nuclear Power Stations

Liquid CO2 (carbon dioxide) is used in processes across many industries. However, pumping it presents an array of challenges both to equipment/systems manufacturers and plant managers because CO2 must be transferred at a controlled combination of low temperature and high pressure to minimise vapour forming. For the pump manufacturer, the inherent compressibility and poor lubricating properties also places special demands on pump design.

Within the nuclear power industry, CO2   is the main form of temperature cooling for advanced gas cooled nuclear reactors during the reaction process. The CO2 to top up the reactors is stored at two pressures: at 20 bar to receive supplies from road tankers and at 50 bar in order to be able to top up the reactors and refuelling plant. Pumps are required to transfer the liquid CO2 from 20 bar to 50 bar.

In order to overcome the problems with the original installed rotary pumps used for CO2 pumping at its Heysham and Torness nuclear power stations British Energy, part of EDF Energy, is currently installing four purpose-designed Cat Pumps triplex positive displacement pumpsets at each power station. The eight pumps are the Cat 6861K Series and have been specially adapted to handle liquid CO2.

“The main problem experienced at both power stations when transferring liquid CO2 was the very short operating life of the existing rotary pumps,” reports Cat Pumps UK general manager Brian Hubbard, “and these pumps required frequent maintenance and eventual replacement.”  Aware of the Company’s considerable experience in the design and provision of high performance positive displacement triplex pumps for CO2, British Energy contacted Cat Pumps during 2005 with a view to resolving the problem, who in turn invited Star Refrigeration to partner them in the contract.

Each nuclear reactor is cooled using CO2 which is held in a high pressure tank at 50 bar. This is replenished by pumping liquid CO2 from low pressure tanks where it is stored at 20 bar a. The Cat Pumps 6861K pumps are configured to handle a flow rate of 6 tonnes/hour. For reactor topping-up duties only one of the four pump systems is in service, with the other three being on stand-by. During refuelling the CO2 storage system is segregated into two parts. In this mode there is a pair of pumps in each part – one duty and one on auto-standby. In order to ensure the CO2 is maintained in its liquid phase, it is sub-cooled. Both Heysham and Torness reactor sites are also installing Star Refrigeration R404A air-cooled refrigeration plant. The sub-cooler heat exchanger lowers the temperature of the CO2 from the low pressure CO2 bulk storage tanks from –16ºC down to –18ºC prior to the inlet to the CO2 liquid pumps.

Mounted in purpose-designed skid frames, the Cat Pumps Model 6861K triplex plunger pumps feature a forged-billet fully machined 316 stainless steel pump-head for optimum liquid and process compatibility, strength and corrosion resistance. The plunger seals are lubricated only by the liquid CO2 and run on concentric, high density polished alumina ceramic plungers. These are extremely smooth and hard, ensuring excellent seal life. There is no requirement for additional seal flushing liquids.

In the Cat Pumps Model 686K pumps, the seal arrangement is configured to predict future seal life and allow planning of routine servicing. The three high pressure seals are connected to a leak detection system for CO2 monitoring and the three low pressure back up seals are installed in a way that minimises the risk of moisture build up inside the seal spacers, which could otherwise result in ice formation.

The high pressure CO2 pump-head is completely oil-free with a spacer section and extended plunger rod and Nitrile Elastomers/PTFE seals between the CO2 pump end and the oil lubricated crankcase end, preventing cross-contamination. The pump is gear driven via a 12kW TEFC motor built to the demanding BE electrical and mechanical specifications and the gearbox assembly runs in its own separate oil lubrication regime.

“Star worked closely with Cat Pumps at the pre-order stage to complete a detailed system design feasibility study,” reports Douglas Hamilton of Star Refrigeration. “This close collaboration continued during the later design stages where Cat Pumps’ contribution was significant and important in satisfying the exact requirements looked for by BE to provide a robust and reliable solution for the nuclear industry.”

Reports indicate that the first two pumpsets designed and fabricated by Cat Pumps to have been installed at Torness are performing well.  Two more have been installed at Heysham, and Torness is currently installing their third unit. These have proved to be a vast improvement on the rotary pumps that they are replacing. As a result of the improved performance and reliability of the pumps, British Energy is experiencing a valuable reduction both in the amount of time and expense incurred in pump maintenance, and in the effect on the refuelling programmes.

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