CMS installations are put to the test
CMS has just undergone two important tests: a spectacular test of the fire extinguishing system in the underground cavern (photo below) and, on the surface, a strength test on the plug over the main shaft, which will bear the weight of the detector components when they are lowered into the CMS hall.
A feat of strength for CMS
The plug over the CMS shaft, which will be required to bear the weight of the various detector sub-assemblies when they are lowered into the experiment hall, has just passed a strength test.
The plug, a huge 2.2-metre-thick rectangular block of reinforced concrete measuring 15 x 20 metres and weighing 2000 tonnes, underwent its first strength test on 15 May.
The plug's construction itself represented a major technical challenge for the Technical Support Department's Civil Engineering Group (TS-CE), as it comprises a 142-tonne steel frame. 'It's unquestionably the biggest single-piece mobile concrete structure our group has ever had to deal with', explains Jean-Luc Baldy, Head of Civil Engineering at CERN.
The plug will play a vital role when the CMS detector is lowered into the experiment hall. The various CMS sub-assemblies are being put together on the surface and will be lowered into the cavern this summer. Each one, weighing between 270 and 2000 tonnes, will be shifted onto the plug over the shaft, then lifted by the crane recently installed (see Bulletin No. 11/2006). The plug will then slide open on its rails to expose the top of the shaft. Finally, the crane will lower the detector components 100 metres down to the bottom of the shaft.
Before commencing the manoeuvres, the teams tested the strength of the plug by placing steel blocks with a total weight of 2500 tonnes on it. 'This is equivalent to 125% of the maximum load on the block', says Lucien Veillet, who is in charge of coordinating CMS assembly at Point 5. A few hours later, the surveyors from the TS-SU Group measured the deformations at four vital points of the structure. The results exceeded expectations as the maximum deformation was 2.8 mm, much less than the expected 12 mm resulting from the theoretical calculations.
The next step will be to test the crane by lifting the plug with an additional 800 tonne load, in other words a total load of 2800 tonnes. The equipment will then be ready for the lowering operations to begin. 'The first detector components, the two 270-tonne forward calorimeters (HF), will be lowered down at the end of July', explains Lucien Veillet. It will be followed by thirteen other sub-assemblies, including the central part that will support the solenoid, weighing a total of 2000 tonnes.
CMS gets foamy
The safety system aimed to extinguish fires was tested in the CMS cavern. This very impressive foam test proved a great success.
The enormous rumbling heard 100 m under the earth on Friday, 12 May, was not the start of a foam party at CMS. The Safety Team looked on from the second tier of the CMS underground cavern as it reechoed to the sound of water rushing through the two huge pipes overhead and the air was filled with a mixture of water and foam. A minute later it was a winter wonderland, as fluffy puffs of foam came shooting out of the twelve foam blowers lining the upper cavern walls on both sides.
In less than two minutes 7 m3 of water mixed with a small percentage of foaming liquid, was transformed into 5600 m3 of foam and discharged into the cavern.
This was the first foam test conducted in a physics experiment hall, and hopefully the last. If a run-away fire were to break out in the CMS cavern the High Expansion Foam System foam would be used as a last resort to save the cavern, parts of the metallic structure and particle detector.
According to Stefano Fratianni, Project Engineer for the Safety System, these kinds of systems are used to protect extremely large premises, such as automobile factories and aircraft hangars, but the CMS system is characterized by some innovative technical solutions. The system is completely passive, the pressure is not generated by pumps, but by the 100 m water column between the surface reservoir and the underground cavern. The Safety Team dealt with many unique requirements that never had before been met by the industry, including the height of foam to be produced, the distance between the different components of the system, the resistance to activated materials and the large amount of smoke. The engineers had to consider that foam tends to collapse when produced in a similar environment, so they designed the system to fill approximately 120 000 m3, even though the volume of the cavern is just 40 000 m3. It takes just seven minutes to fill the entire space.
Kiddie Italia supplied the system components while ZEC, a Polish company, took care of the pipelines. 'Foam was the natural choice for this sort of space. We needed a kind of foam that doesn't deteriorate for a long time,' explained Fratianni. Kidde Italia foam can stay in the system, ready for use for up to 15 years.
'Fire is one of the biggest threats to CMS, especially since it is an underground installation. We therefore took unprecedented precautions for a particle physics detector to mitigate this threat. I am very happy to see that the Foam System is performing to our fullest satisfaction,' said Christoph Schäfer, Group Leader in Matters of Safety for CMS.
