As mentioned in part 1 of this series last month, since 2007 two diaphragm metering pumps from Lewa GmbH have been used inside the Large Hadron Collider (LHC), the largest and most sophisticated accelerator in the world, which is operated by the famous CERN research institute. These pumps ensure a uniform flow rate, which is essential for continuous cooling and disturbance-free operation.
DIAPHRAGM METERING PUMPS
CERN chose diaphragm metering pumps from Lewa for a number of reasons, especially because no oil can be tolerated in the detector cooling circuit, because oil can start to solidify under the influence of radiation and may then cause a blockage in the thin cooling lines. At least for CO2, a compression cycle was impossible as no oil-free compressor for CO2 exists on the market. Therefore, it is only possible to adopt a pumped loop operated by an oil-free pump. Rotary oil-free pumps require lubrication by the circulating refrigerant fluid, but CO2 is a very poor lubricant. “That’s why we use three LEWA ecoflow diaphragm pumps in the detector loop. Unlike other pump types, they have the benefit of not requiring lubrication by the fluid,” according to Hans Postema, the senior mechanical engineer at CERN who is developing CO2-based cooling systems, together with the cooling team of the Detector Technology group.
Since the collision energy in the LHC will be significantly increased later this year, more powerful Lewa LDE-1 diaphragm metering pumps were needed in order to effectively cool the Compact Muon Solenoid (CMS) detectors. These pumps have a remote head design in order to prevent warming of the CO2 or cooling of the oil, which could lead to formation of gas bubbles and require pumping activity to stop.
So a membrane pump was the best choice for long term reliability. Furthermore, the Lewa pumps are known for their ability to deliver the completely uniform flow rate that is necessary for continuous, stable, two-phase cooling. This approach to cooling removes heat by exploiting the phase change of CO2 from liquid to vapor. It has the benefit of using significantly less coolant and much smaller pipes than in single-phase cooling. In other respects, handling the coolant is not so easy: “Liquid temperatures can be as low as -58 degrees Fahrenheit (-50 degrees Celsius), which is within the critical range because CO2 begins to solidify at -70.6 degrees Fahrenheit (-57 degrees Celsius). We are currently working within a range of +68 to -40 degrees Fahrenheit (+20 to -40 degrees Celsius) for testing purposes. In most cases the temperature is around -22 Fahrenheit (-30 degrees Celsius),” according to Mr. Postema. However, for the specific case of the ATLAS IBL detector an operational range extended down to -40 degrees Fahrenheit (-40 degrees Celsius) was required: in this case a two-stage primary chiller was adopted, with carefully in-house designed controls in order to provide the pump with the correct sub-cooling level even at these low temperatures. By taking all this into consideration, the robust membrane pumps were the best choice for long term reliability.
REMOTE HEAD DESIGN KEEPS CRITICAL CONDITIONS AT BAY
A different detector at the LHC, known as the Compact Muon Solenoid (CMS) experiment, is involved in the discovery of the Higgs boson, the search for evidence of super symmetries, and the study of what happens when heavy ions collide. The tracker used in this experiment contains 25,000 silicon sensors, each of which must be cooled individually. A major advantage of CO2 cooling becomes apparent in this situation: due to the high level of compression, the volume of vaporized CO2 remains very low, allowing the use of very thin tubing with a diameter of just 2 mm. As a result, very little material is needed despite having several hundred cooling tubes.
From 2015, the collision energy in the LHC, operated at 7 TeV during its first run, shall be increased to 13 TeV and subsequently to as high as 14 TeV. With the increased number of collisions to be recorded, a more powerful silicon detector will be installed in 2016. For this, a new CO2 cooling system, recently commissioned, will be put in operation. This system will have a total dissipated power of 15 kW, much higher than the LHCb and the ATLAS ones (of the order of 2 kW). For the new plant engineers have chosen Lewa LDE-1 diaphragm metering pumps with a remote head design. The pump head has a cooling jacket and is constructed of 1.4571 type stainless steel. The displacement movement is transferred by way of a liquid column, also known as the hydraulic rod, contained in the connection line. The plunger puts the rod into an oscillating motion, which is forwarded to the valve head. The check valve responds to pressure and alternates between open and closed, inducing a unidirectional, pulsating flow of the fluid in the valve head.
In total, three remote-head pumps were delivered. One was installed in the prototype of a 15 kWh system and has already been tested successfully. The two others will be used redundantly in the two final systems involved in the CMS experiment where they will pump the fluid without adding heat.
In this way, the remote head design ensures that the displacement system stays out of the critical range in order to protect the system and the surrounding environment. It also prevents warming of the CO2 or cooling of the oil, which would result in the formation of gas bubbles and cessation of pumping action, a common problem with standard pumps.
“In two-phase cooling, the CO2 must be close to its boiling point, as it tends to vaporize at warmer parts of the pump. That’s why the low heat input into the fluid is important. This in particular means that diaphragm metering pumps with remote head design can bring substantial advantages to the plant performance,” according to Marc Geiselhart, managing director at the Swiss Lewa subsidiary.
INITIAL TESTS SUCCESSFUL
Since the failure of a pump during an experiment would be very costly in terms of time and money, additional Lewa-specific features help ensure reliability. For example, the two-layer PTFE diaphragm prevents contamination of the CO2 in case one layer of the diaphragm becomes damaged. In addition, an integrated pressure switch triggers immediate shutdown of the pump in the event of leakage. If requirements change, the flow rate can be regulated by remote stroke adjustment via two check valves.
The prototype for the new 15 kW system has already been installed and has successfully passed its initial test. The system is ten times larger than the one that uses Lewa standard pumps. The two CMS systems have also been fully assembled and are under commissioning. They will run redundantly in the actual experiment. In a subsequent scale-up, even more powerful pumps will be required, but engineers are currently waiting on results from the most recent experiment. ◆
For More Information:
LEWA GmbH is the world’s leading manufacturer of metering pumps and process diaphragm pumps, as well as complete metering packages for process engineering. LEWA develops technologies and provides solutions for the vast array of applications among its customers. Products are primarily used in the oil and gas industry, in the area of gas odorization, refineries, and petrochemistry, but also in the manufacture of plastics, detergents, and cleaning agents. For more information, visit www.lewa-inc.com
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MODERN PUMPING TODAY, October 2016
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Tracking the Origins of the Universe: Part 2 of 2
