Page 151 - Boiler_Operators_Handbook,_Second_Edition
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136                                                                               Boiler Operator’s Handbook

               associated with burner and circulating pump cycling.  from the interface for presence of Legionella at quarterly
                    City water temperature can vary significantly with  intervals after initial start-up and, if none is discovered,
               the season depending on the water source. If all water  annually thereafter. Annual testing should coincide with
               is supplied from wells then the temperature varies less.  heavy rains in the summer where the bacteria is most
               When the water is stored in reservoirs or lakes and tow-  likely to enter your system.
               ers the temperature can vary between 35° and 65°. If the     The process of checking for Legionella consists
               boiler operates at a fixed firing rate, as most do, the out-  of drawing a sample and sending it to a laboratory
               let temperature of the boiler will vary with the season.  for analysis. It requires a water sampling connection
               During burner operation the operator should note the  installed in the storage tank at the location indicated,
               temperature on the outlet thermometer (T2) regularly  just below the level of TC1. If the sample connection is
               and adjust the position of the throttling valve (TV) to  above the return line inlet it should penetrate the tank
               restore the desired tank temperature (±5°F) at least  as shown to ensure a sample of the interface is drawn.
               monthly. To increase the temperature the valve is closed  To ensure the operating personnel are not exposed to
               some, to lower the temperature the valve is opened  the bacteria (in the event it is there) they should wear
               further. Make the adjustment when the boiler operation  protective equipment recommended for this operation.
               has stabilized then wait a few minutes to see the results  A sample bottle should be placed such that the sample
               before adjusting the valve further.                  piping extends into the bottle to the bottom to minimize
                    Normally the boiler operating limit (OL) and high  splashing and generating aerosols while sampling. The
               limit (HL) do not function. However, when the boiler  sample should be drawn in the late afternoon or early
               operates  for extended  times  during periods  of  high  evening when demand is normally low and immedi-
               demand the operating limit could open its contacts be-  ately after the pump and boiler start operating (when
               cause the temperature gradient in the boiler changes.  the interface is near the level of the sample line.
               The operating limit should not be adjusted to the point     If the laboratory test indicates Legionella is in the
               that it controls the boiler (starting and stopping it) dur-  interface  it  should  be  flushed  from  the  storage  tank.
               ing normal operation.                                Connect a hose to the sample valve outlet and extend it
                    There is no provision to adjust the pressure in the  into a drum containing sufficient sodium hyperchlorite
               system. It should follow the supply water pressure. The  (Clorox™) to super treat a drum full of water. Turn off the
               safety relief valve should not be tested to determine if it  pump circuit breaker immediately after it starts to pre-
               operates. Operating personnel wearing proper protec-  vent pump and boiler operation temporarily then, after
               tive equipment should raise the lifting lever of the safety  a few minutes of drawing hot water from the building
               relief valve every three months to confirm that the valve  system, open the sample valve and close the pump circuit
               mechanism is free and the water flow passages are not  breaker. When hot water is flowing to the drum the sam-
               blocked. Testing of the safety relief valve should be re-  ple valve can be closed because the complete interface
               corded in the log.                                   was flushed to the drum. Repeat the procedure until a
                    The purpose of the high limit is to prevent over-  laboratory test of the interface does not show Legionella.
               heating of the boiler in the event the circulating pump     Even if Legionella does form in the storage tank in-
               fails or operating personnel inadvertently close a valve  terface it should not contaminate the hot water delivered
               in the piping that prevents flow through the boiler. Its  to the building unless the storage tank temperature is
               adjustment should be noted, lowered into the operat-  too low or hot water demands result in all the storage in
               ing range to ensure it functions to interrupt burner  the tank being consumed. In the latter case the interface
               operation, then restored to the original setting on an  flows into the building’s hot water distribution system.
               annual basis. The test of the high limit should be re-  Operating the system to maintain hot water in storage at
               corded in the log.                                   180°F for more than one half hour should kill all bacteria
                    The bacteria blamed for the deaths of several  except what’s in the interface. Blending valves should be
               members of the American Legion in Philadelphia is  installed to provide the maximum 120°F water for hand
               frequently found in water supplies. When exposed to  washing, bathing, etc.
               warm water in a confined environment it can flourish.     I’ve looked at a few service water heaters where
               It’s not the only one that can cause problems. The in-  thermal shock was determined to be the cause of their
               terface in the hot water storage tank always contains a  failure. Thermal shock is observed by anyone pouring
               level of water at the optimum temperature for that bac-  liquid into a glass of fresh ice. The ice cracks instantly,
               teria to grow and multiply. I suggest you sample water  even when the liquid is very close to freezing. Iron, steel,
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