valve is installed to bleed off pressure at 341 psi. alarms  to  be  actuated.  Many  of  the  operating Another relief valve operates at 357 psi for rapid devices  discussed  earlier  in  this  chapter  can  be release of excess pressure. There is also a frangible used. Most commonly used are the heat-actuated disk designed to burst at 600 psi should the relief devices  (H.  A.  D.)  or  smoke  detecting  devices. valves  fail  to  control  pressure  buildups. Manual  controlling  devices  are  also  used  in Advantages/Disadvantages   of COZ  Systems carbon dioxide systems. Whether the agent release is  automatic  or  manual,  an  alarm  at  the  alarm system  control  unit  should  be  actuated. There  are  advantages  and  disadvantages  to Piping each type of carbon dioxide system. Low-pressure storage units have a liquid level gauge that con- Carbon  dioxide  fire  protection  system  pipe tinuously  monitors  the  amount  of  carbon  diox- and  fittings  are  selected  to  have  suitable  low ide  in  storage.  High-pressure  systems  require weighing  the  cylinders.  High-pressure  systems permit  storage  of  almost  the  exact  amount  of carbon dioxide required to protect a hazard area because  of  the  flexibility  and  selection  of  cylinders in 50-, 75-, or 100-pound sizes. The smallest low pressure  is  750  pounds.  High-pressure  systems require refilling and hydrostatic testing every 12 years. Low-pressure systems have no such require- ment. Pressures in high-pressure systems vary with the ambient temperature; this affects the discharge rate  of  the  system.  Low-pressure  systems  keep  the liquid  carbon  dioxide  at  0°F  and  300  psi  at  all times,  assuring  a  uniform  discharge  rate.  Another advantage of low-pressure systems is their ability to  allow  automatic,  simultaneous  discharge  for more than one hazard area on an engineered basis. Hose reels can also be attached to these systems to  operate  simultaneously  with  hazard  protection. temperature  characteristics  and  good  corrosion resistance  inside  and  out.  Ferrous  metals  are galvanized   steel,   copper,   brass,   and   other materials  having  similar  mechanical  and  physical properties  are  acceptable.  Copper  tubing  with suitable   flared   or   brazed   connections   is   also acceptable. Cast-iron (gray) pipe and fittings are not  used. Pipe  and  fittings  for  high-pressure  systems have a minimum bursting pressure of 5,000 psi. In low-pressure systems, pipe and fittings have a minimum  bursting  pressure  of  1,800  psi. Between  the  storage  tank  and  selector  valves, black steel pipe may be used because of the larger sizes involved and its airtightness. The  supply  piping  is  usually  routed  to  prevent unnecessary exposure to high temperatures from ovens or furnaces or to direct flame impingement A  reserve  supply  can  be  provided  by  increasing the  storage  unit  size  of  low-pressure  systems. High-pressure  systems  require  manifolding  and valving  arrangements  to  achieve  a  reserve  supply. Storage  of  the  carbon  dioxide  is  also  a  con- sideration in showing advantages or disadvantages of these systems. High-pressure systems require approximately  3  pounds  of  equipment  for  every pound.  Usually,  low-pressure  systems  require  less floor  space  for  storage  of  equal  amounts  of carbon  dioxide  as  compared  with  high-pressure systems.  In  many  instances,  low-pressure  storage containers  may  be  placed  outside  of  the  buildings. High-pressure   systems   allow   flexibility   in space  requirements  since  multiple  cylinder  banks may  be  stored  in  several  smaller  locations. Low-pressure  systems  require  one  large,  single area  for  the  refrigerated  storage  unit. Operating  Devices As  with  all  fire  protection  systems,  carbon dioxide systems must have operating devices for discharge of the extinguishing agent and to cause Figure 8-32.—Carbon dioxide nozzles. 8-26