Heater Tube Corrosion

Occasionally tube heater failures occur on steam heating jobs where corrosion appears to be the cause.While most complaints involve copper tubes, problems can also occur with steel and cupro-nickel. Tube corrosion problems and their causes will generally fall into four distinct catagories as detailed below. One factor that applies to all four catagories is that they are "system" problems, and as such, are beyond our control as a manufacturer. We want to offer our comments on each catagory along with recommendations in the hope that future problems can be minimized.

(1) In many cases, condensate will collect in a lower leg of a heater tube due to inadequate trapping. Improperly applied traps are often the cause. The condensate lies in the tube providing fertile ground for the formation of corrosive elements in the condensate that could damage the tube. The best remedy is to always provide adequate trapping, install properly-sized drip legs and use inverted bucket style steam traps to insure that no condensate lies in the tube for any length of time. Inverted bucket traps open only when they are completely filled with condensate. This means that they will work well if condensate has to be "lifted" to an overhead return from a unit heater or FAS. F & T (float and thermostatic) traps are a better choice where loads will vary and where there are concerns about noise and the handling of air.

(2) Corrosive condensate due to low pH (less than 6.0) and/or dissolved corrosive elements is a separate issue, and once a sample of the condensate has been analyzed, treatment of feed water will normally clear up the problem.

(3) If systems are not fitted with de-aerators, there may be air and/or carbon dioxide in the pressurized lines. One indication of the presence of these gases in the condensate, and in the case of dissolved oxegyn, pitting will occur on the sides of the tube at whatever level the condensate lies. The carbon dioxide will dissolve and then form Carbonic Acid which generally results in a sort of groove being formed along the bottom of the tube.

(4) Leaking steam shut-off valves that allow condensate to "drip" into a coil during summer shutdown (assuming the boiler runs at least intermittently) can cause tubes or even headers to corrode as the dripping impinges on a particular spot for a prolonged period.

In conclusion, good piping and trapping practices, de-aeration and non-modulating controls will go a long way to prevent heater tube corrosion.

**From LJWing "Technical Report", Issue 194A, Revised July 1997**