forced out the discharge valve. Then it passes through the  fourth-stage  intercooler  and  moisture  separator. Air is drawn into the fifth-stage cylinder on the piston  downstroke  and  is  compressed  and  discharged  on the upstroke. The discharge air passes through the fifth-stage  pulsation  bottle,  the  aftercooler,  the  moisture separator, a back-pressure valve, and a check valve before entering the ships’ HP piping. SAFETY  PRECAUTIONS Many hazards are associated with pressurized air, particularly   air   under   high   pressure.   Dangerous explosions  have  occurred  in  high-pressure  air  systems because  of  DIESEL  EFFECT.  If  a  portion  of  an unpressurized system or component is suddenly and rapidly  pressurized  with  high-pressure  air,  a  large amount of heat is produced. If the heat is excessive, the air may reach the ignition temperature of the impurities present in the air and piping (oil, dust, and so forth). When the ignition temperature is reached, a violent explosion will occur as these impurities ignite. Ignition temperatures may also result from other causes. Some are rapid pressurization of a low-pressure dead-end portion  of  the  piping  system,  malfunctioning  of compressor aftercoolers, and leaky or dirty valves. Air compressor accidents have also been caused by improper maintenance procedures. These accidents can happen  when  you  disconnect  parts  under  pressure, replace  parts  with  units  designed  for  lower  pressures, and install stop valves or check valves in improper locations.  Improper  operating  procedures  have  resulted in  air  compressor  accidents  with  serious  injury  to personnel  and  damage  to  equipment. You must take every possible step to minimize the hazards inherent in the process of compression and in the  use  of  compressed  air.  Strictly  follow  all  safety precautions outlined in the manufacturer’s technical manuals and in the NSTM, chapter 551. Some of these hazards and precautions are as follows: 1. Explosions can be caused by dust-laden air or by oil vapor in the compressor or receiver if abnormally high   temperatures   exist.   Leaky   or   dirty   valves, excessive  pressurization  rates,  or  faulty  cooling  systems may  cause  these  high  temperatures. 2.  NEVER  use  distillate  fuel  or  gasoline  as  a degreaser  to  clean  compressor  intake  filters,  cylinders, or air passages. These oils vaporize easily and will form a  highly  explosive  mixture  with  the  air  under compression. 3. Secure a compressor immediately if you observe that the temperature of the air being discharged from any stage exceeds the maximum temperature specified. 4.  NEVER  leave  the  compressor  station  after starting the compressor unless you are sure that the control and unloading devices are operating properly. 5. Before working on a compressor, make sure the compressor  is  secured.  Make  sure  that  it  cannot  start automatically or accidentally. Completely blow down the compressor, and then secure all valves (including the control or unloading valves) between the compressor and  the  receiver.  Follow  the  appropriate  tag-out procedures for the compressor control valves and the isolation valves. When the gauges are in place, leave the pressure  gauge  cutout  valves  open  at  all  times. 6. Before disconnecting any part of an air system, be sure the part is not under pressure. Always leave the pressure gauge cutout valves open to the sections to which they are attached. 7. Avoid rapid operation of manual valves. The heat  of  compression  caused  by  a  sudden  flow  of high-pressure air into an empty line or vessel can cause an  explosion  if  oil  or  other  impurities  are  present. Slowly crack open the valves until flow is noted, and keep the valves in this position until pressure on both sides has equalized.  Keep the rate of pressure rise under 200 psi per second. DEHYDRATORS The removal of moisture from compressed air is an important feature of compressed air systems. As you have   learned,   some   moisture   is   removed   by   the intercoolers  and  aftercoolers.  Air  flasks  and  receivers are provided with low-point drains so any collected moisture  may  drain  periodically.  However,  many shipboard  uses  for  compressed  air  require  air  with  an even  smaller  moisture  content  than  is  obtained  through these methods. Water vapor in air lines can create other potentially  hazardous  problems.  Water  vapor  can  cause control  valves  and  controls  to  freeze.  These  conditions can occur when air at very high pressure is throttled to a low-pressure area at a high-flow rate. The venturi effect  of  the  throttled  air  produces  very  low temperatures that cause any moisture in the air to freeze into ice. Under these conditions, a valve (especially an automatic   valve)   may   become   very   difficult   or impossible to operate. Also, moisture in any air system can cause serious water hammering (a banging sound) within the system. For these reasons, air dehydrators or 10-31