Because  the  cleaning  is  more  complete,  it  is possible  to  examine  the  unit  thoroughly  for defects, such as cracks and corrosion pitting. cleaners. A more thorough job is accomplished because the acid reaches areas inaccessible to mechanical Less dismantling of the unit. Lower cost and labor. ACIDS FOR CLEANING.—The  following  acids are used to clean boilers: hydrocholoric acid, phosphoric acid, sulfamic acid, citric acid, and sulfuric acid. HYDROCHLORIC ACID is most frequently used for boiler cleaning because it has a relatively low cost and satisfactory  inhibitors  are  available.  Also,  the  chemical reactions  of  the  hydrochloric  acid  with  the  boiler  deposits usually result in soluble chlorides. PHOSPHORIC ACID can remove mill scale from new boilers. With  this  acid,  the  boiler  can  be  fired directly  without  producing  noxious  or  corrosive  fumes. Direct  firing  produces  good  circulation  and  distribution of  the  cleaning  solution. Another   advantage   of phosphoric acid cleaning is that the metal surfaces resist corrosion  after  cleaning. When   cleaned   with phosphoric acid, you must protect metal surfaces from surface  corrosion  during  draining  and  before neutralization. SULFAMIC ACID is available in powder that must be placed in solution.   The powdered acid is easier and safer to handle than liquid acids in carbons. It does not produce  noxious  fumes  as  it  dissolves  and  it  is  less corrosive than hydrochloric acid, especially at higher concentrations  and  temperatures. CITRIC ACID AND SULFURIC ACID are used for removing  boiler  waterside  deposits.  Sulfuric  acid  is economical and easily inhibited. However, a danger is that the sulfuric acid can form insoluble salts, such as calcium  sulfate. INHIBITORS.—Without inhibitors, acid solutions attack  the  boiler  metal  as  readily  as  they  attack  the deposits. With the addition of suitable inhibitors, the reaction   with   the   boiler   metal   is   greatly   reduced. Inhibitors used include arsenic compounds, barium salts, starch, quinolin, and pyridin. Commercial inhibitors are sold under trade names by various chemical concerns. Other  inhibitors  are  manufactured  by  companies  that furnish complete acid cleaning services. SAFETY PRECAUTIONS—When  acid  cleaning a  boiler  installation,  you  must  observe  SAFETY precautions as follows: Slowly pour the acid into water when mixing the solutions. Use goggles, rubber gloves, and rubber aprons when  handling  acids. After acid cleaning, be sure to thoroughly flush out all  of  the  tubes  that  are  horizontal  or  slightly sloping. Obstructions in these tubes can cause poor circulation, overheating, and failure of tubes when  the  unit  is  placed  in  service. Do not exceed the specified acid and inhibitor allowable   temperature.   The   inhibiting   effect decreases  with  the  temperature  rise  and  the probability  of  acid  attack  of  the  boiler  metal increases. Provide competent chemical supervision for the cleaning  process. Close all valves connecting the boiler with other piping  or  equipment. Provide adequate venting for safe release of acid vapors. Before acid cleaning, replace all brass or bronze parts temporarily with steel or steel alloy parts. CAUTION NEVER POUR WATER INTO ACID. Do not chemically clean boilers with riveted joints. During acid cleaning, hydrogen gas can develop through the reaction of the acid on the boiler metal. Some of the generated gas becomes part of the atmosphere inside the boiler, and the remainder is absorbed  by  the  boiler  metal,  then  liberated gradually.  Because  hydrogen  air  mixtures  are potentially explosive, be careful when opening a unit for inspection after acid cleaning. Until the atmosphere within the boiler pressure parts has been definitely cleared of explosive gases, do NOT use  open  flames.   flashlights,   lighting   equipment. or anything that might produce a spark near the openings to the pressure parts. Do NOT enter the boiler. The unit can be cleared of explosive gases by thoroughly flushing the unit with warm water with a positive overflow from the highest vent openings. The water temperature should be as near 1-39