To combat this issue, all oxygen shutoff valves are slow,  opening valves designed to decrease velocity.  [Figure 16-27] Additionally, technicians should always open all oxygen  valves slowly. Keeping oxygen from rushing into a low- pressure area should be a major concern when working with  high-pressure gaseous oxygen systems. Oxygen cylinder valves and high-pressure systems are often  provided with a relief valve should the desired pressure  be exceeded. Often, the valve is ported to an indicating or  blowout disk. This is located in a conspicuous place, such as  the fuselage skin, where it can be seen during walk-around  inspection. Most blowout disks are green. The absence of  the green disk indicates the relief valve has opened, and the  cause should be investigated before flight.  [Figure 16-28] The two primary types of chemical oxygen systems are the  portable type, much like a portable carry-on gaseous oxygen  cylinder, and the fully integrated supplementary oxygen  system used as backup on pressurized aircraft in case of  pressurization failure.  [Figure 16-29]  This latter use of solid  chemical oxygen generators is most common on airliners. The  generators are stored in the overhead PSU attached to hoses  and masks for every passenger on board the aircraft. When a  depressurization occurs, or the flight crew activates a switch,  a compartment door opens, and the masks and hoses fall out in  front of the passengers. The action of pulling the mask down  to a usable position actuates an electric current, or ignition  hammer, that ignites the oxygen candle and initiates the flow  of oxygen. Typically, 10 to 20 minutes of oxygen is available  for each user. This is calculated to be enough time for the  aircraft to descend to a safe altitude for unassisted breathing.  Chemical oxygen systems are unique in that they do not  produce the oxygen until it is time to be used. This allows safer  transportation of the oxygen supply with less maintenance.  Chemical oxygen-generating systems also require less space  and weigh less than gaseous oxygen systems supplying  the same number of people. Long runs of tubing, fittings,  regulators, and other components are avoided, as are heavy  gaseous oxygen storage cylinders. Each passenger row  grouping has its own fully independent chemical oxygen  generator. The generators, which often weigh less than a  pound, are insulated and can burn completely without getting  hot. The size of the orifice opening in the hose-attach nipples  regulates the continuous flow of oxygen to the users. Figure 16-28.  An oxygen blowout plug on the side of the fuselage  indicates when pressure relief has occurred and should be  investigated. Figure 16-29.  An oxygen generator mounted in place in an overhead  passenger service unit of an air transport category aircraft.  Figure 16-27.  This high-pressure oxygen system shutoff valve has  fine-pitch threads and a regulating stem to slow the flow of oxygen  through the valve. A soft valve seat is also included to assure the  valve closes completely.  16-16 Chemical Oxygen Systems Fine-pitch threads Soft seat Regulating stem