Boiler Blowdown

There is a long list of impurities to be found in water entering a boiler. However, in the dry cleaning and laundry industries, the major ones effecting the operation of boilers are dissolved solids (known as TDS, or total solids) and oxygen which is dissolved in the new, city water entering the return tank. This oxygen combines with carbon dioxide to form carbonic acid, a destroyer of the metal in the boiler and piping throughout the steam/return systems. The concentration of acids in boiler water is indicated by the Ph (Ph-is a scale used to specify how acidic or basic [or alkaline] a water-based solution is). Measuring TDS and Ph levels indicate the condition of the treated water entering as well as inside the boiler, allowing us to adjust the blow-down frequency as well as the volume and timing of boiler compound addition to the return tank to prevent boiler damage.

ESTABLISHING THE CORRECT BLOWDOWN PROCEDURE

The two most important factors in maintaining the proper boiler water chemistry is blowdown and compound addition. No matter how efficient the water treatment program, water entering the boiler (feed water) will contain impurities such as salts and suspended and dissolved solids. The process of boiling away water to generate steam leaves these impurities behind, accumulating inside the boiler during daily operations.  Blowdown uses gravity and steam pressure to expel this accumulated debris, detritus and other impurities before they adhere to and/or corrode the internal surfaces of your boiler and steam system piping.  Sometimes it seems like every boiler manufacturer and/or operator has their own idea about how and when to blow down their boiler. The recommendations I will be making are based upon several years’ experience in troubleshooting and repairing boilers of many different manufacturers in arguably the worst area for boiler water impurities in the country and may be differ from those recommended by the boiler manufacturer. When in doubt follow the manufacturer’s procedure.

WHEN TO BLOWDOWN

Under normal circumstances two blowdowns per day are sufficient; one in the morning the other early afternoon (Never leave the boiler or return tank empty overnight). While the boiler is shut down, much of the particulate matter, being heavier than water, will settle to the bottom of the boiler in an area where the blow-down is very effective. The morning blowdown would be shortly after the boiler is started up. Wait until the steam pressure rises to about 30-40 PSI then start the blowdown. For the afternoon blowdown shut down the boiler while you’re still using steam so boiler pressure drops quickly. When boiler pressure drops to about 30-40 PSI initiate the blowdown procedure.  You will quickly determine the length of time after a blow-down until steam pressure rises sufficiently to resume production. Use this recovery period to calculate the best timing to minimize production loss by timing blowdowns so the recovery period occurs during an employee break or lunch period would be shortly after the boiler is started up. Boilers used in the dry cleaning and small laundry operations will usually  have two to four blow down valve locations. First is the water column second is the boiler shell (pressure vessel). There may also be a large slow opening globe valve and a small valve at the lower end of the sight glass fixture which can also be used to drain water samples for testing.

In addition the most effective part of the blowdown is during the first several seconds. Therefore, short blowdowns are recommended. Blowdowns of about 15 seconds for the water column, 25 seconds for the boiler shell (pressure vessel) and 5 seconds for the sight glass (if there is a valve installed for that purpose). Locate the valves shown on the illustration. Slowly open the valve and start the timing after the valve is fully open (If the boiler is specified to run at pressures over 80 PSI there is generally a slow opening globe valve that must also be open during blowdown). Blowing all the water out of the boiler and/or draining the return tank and refilling with city water is NOT NECESSARY and a waste of water softening capacity and boiler compound. When blowdown is complete be sure to reset the valves for normal boiler operation.

Well that’s it for this month, next month I’ll be discussing how to measure the level of impurities present in your boiler water using inexpensive TDS and pH meters.