DCN February 2017 - Page 25

infrastrucure management system , Siemens investigated the cause of the problem back in 2009 . There are a number of effects to the environment in the data centre when the gas is released . The first is a slight drop in temperature in the room on discharge of the inert gas , but this is just a small transient change that does not affect the drives .

Next is an overpressure in the room . This happens because the extinguishing system needs to deliver a large amount of the inert gas quickly to protect IT equipment once fire is detected . Tests on several types of HDDs have shown , however , that even unrealistically high pressures and pressure gradients do not affect the performance of the drives .
The high pressures required for the fast response of the extinguishing system can also generate exceptional levels of noise as the gas passes through nozzles into the room . Indeed , in some applications the level of noise can exceed 140dB . Reaction of the HDDs to varying levels of noise was tested using a loud speaker system . These tests showed that an ‘ acoustic shockwave ’ can impact the performance of the disk drives with disruption to performance typically starting at 120dB . Depending on the drive technology and frequency of the noise , performance degradation was also seen at noise levels below 110dB . Noise in the frequency range of 500Hz to 8kHz was shown to have the greatest impact on the disk drives . However , noise below 500Hz and above 12.5kHz did not disturb operation of the drives .
Further trials were carried out using actual extinguish system discharges rather than noise from speakers . These showed that performance of the disk drives could be impaired but they did not show that there was any data loss or permanent damage to the drives . Extreme noise levels in excess of 140dB , however , could have the potential to cause irreparable damage to micro-mechanical systems such as the HDDs used in data centres . Such noise levels are generated by a direct discharge jet of a standard nozzle that has been historically used in fire extinguishing systems .
Types of noise damage So what damage can high levels of noise actually cause in a data centre ? Excessive noise levels or acoustic shock causes vibration of HDDs . This can affect the alignment of the head to the disk . At lower levels , problems with the correct reading and writing of the information on the disk surface can occur . If , however , the head comes into contact with the disk surface , it can scratch the magnetic layer and cause permanent damage . If the error correcting code ( ECC ) sees an ongoing high error rate , the self-diagnostic system will shut the HDD . Damage from initial contact of the head with the disk surface may further generate particles that trigger more crashes and ultimately total abrasion of the disk surface .
Silent extinguishing In response to the investigations into noise damage caused by fire extinguishing systems , silent extinguishing technology has been developed . This significantly lowers the noise levels as gas is discharged and combines selection of extinguishing agent with a different nozzle configuration that lowers noise levels .
Of course silencers could be added to standard discharge nozzles but the challenge is to lower noise without affecting the performance and efficiency of the extinguishing system . The Sinorix Silent Nozzle developed by Siemens , for example , creates a lower pressure drop ( and so lower noise ) as gas discharges to ambient conditions by using a two-stage gas flow expansion . It uses an orifice inside the nozzle which provides the first stage of pressure reduction but keeps the noise from the gas expansion inside the nozzle itself . This means the second expansion , which occurs as the gas moves outside the nozzle , has a much lower pressure drop to provide quieter system discharge . The design of the nozzle also uses many small jets rather than fewer larger ones which create a smoother discharge of the gas and lower the level of noise . In addition , the frequency of the noise is higher and less likely to degrade the performance of the HDDs .
The noise of a fire extinguishing system is further lowered through the use of constant discharge technology ( CDT ). By controlling the flow of the gas into the room , a constant mass flow can be obtained throughout the discharge period . This eliminates the peak flow and associated initial noise spike at the start of a discharge event . The use of CDT has further benefits as the size of overpressure flaps can be reduced by up to 70 per cent . By combining CDT with the use of the two-stage gas expansion nozzles , noise levels can be dramatically reduced and HDD performance protected . Still greater noise reduction can be achieved if the application allows for a longer discharge time ( up to 120 seconds ), room acoustics are optimised , and nozzle placement and flow patterns avoid direct discharge towards equipment .
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