The Knowledge Bank at The Ohio State University

Ohio State Engineer

Title: Boiler Tube Performance

Creators: Miele, Anthony A.

Issue Date: 1945-02

Publisher: Ohio State University, College of Engineering

Citation: Ohio State Engineer, vol. 28, no. 3 (February, 1945), 10, 18.

URI: http://hdl.handle.net/1811/36127

Boiler Tube PerformanceBy ANTHONY A. MIELE

Boiler tube performance may be affected by oneor more of the following canditions:

1. Foaming2. Corrosion3. Scale formation4. Caustic embrittlement5. Sooty surfaces

The above conditions not only affect the life ofthe tube but also the thermal conductivity of thetube.

Foaming which is caused by soluble sodiumsalts and is aggravated by solids, oil or grease.This causes a mass of bubbles to form in thesteam space. The result is that thermal conduc-tivity is reduced tremenduously. The concentra-tion of sodium salts must be reduced below 200 to300 gr per gallon in order to eliminate foaming.

If the boiler tube has been rid of scaling, aifdthe feed water contains oxygen, corrosion of thetube will result. This is very harmful becauseit not only lowers the thermal conductivity but thelife of the tube may be shortened.

Corrosion in as short a time as one week maycause the boiler tube to burn out and thus resultin a shut down of the boiler. A shut down of aboiler is very expensive and extreme care shouldbe taken in order to reduce corrosion.

Because oxygen in the feedwater causes corro-sion, it should be removed by one of the two meth-ods: 1. De-activator. 2. De-aerator.

The De-activator is rarely found but it is alarge tank filled with layers of iron sheets overwhich the water passes and corrodes the iron.This corrosion of the sheets absorbs the oxygen inthe water so therefore the water is free of oxygenbefore entering the boiler.

In the De-aerator, the boiler water enters aclosed heater and flows by gravity through a float-regulated valve into the separator tank where areduced pressure exists. The water, when flow-ing over the heating tube, partially flashes intovapor thus releasing the non-condensable gases.An air ejector lifts the vapor into a surface con-denser where the vapor is condensed and the airpasses through the ejector.

Corrosion may be reduced by having a thin lay-er of scale form on the tube surface. If there isoxygen in the water, it will not be able to come

in contact with the metal because of this thin lay-er of scale so therefore corrosion is prevented atthe cost of a lower thermal conductivity.

The deposit of layers of salt on the surface ofthe boiler tube is known as scale formation. Thelayer is baked and hardened so therefore the ther-mal conductivity is reduced tremendously.

In lower pressure boilers the scale formationmay be prevented by the use of soda ash but athigher pressures it decomposes so that the desiredreaction between the soda ash and the iron doesnot take place.

In high pressure boilers, phosphate salts areused to prevent scale formation.

Scale formation like sludge, is greatly reducedif the feedwater were pure water, evaporatedfeedwater or lime-soda treated water. Thesemethods are not entirely adequate because thereis always a leakage of some raw water into theboiler.

Caustic embrittlement is a term applied to thecondition of boiler metal in which small hairlineintercrystalline cracks appear. These cracksare usually found around high stress riveted jointwhich below the water line. This embrittlementis due to the presence of caustic soda and thecracking is due to sodium hydroxide.

As the hot gases pass around the tube, sootdrops and cakes on the surface of the tubes. Thissooty surface lowers the thermal conductivity.This condition may be prevented by blowing ex-cess air through the tube thus carrying the sootaway.

Most of the conditions mentioned above arefunctions of the feedwater. An analysis of thefeedwater should be taken because it differs evenwithin a given locality. With the aid of the wateranalysis, a treatment of this raw water should bemade so that it would give the best condition inthe boiler.

In many places evaporators are used to rid thewater of suspended solids and some of the soluablesalts. Distilled water from these evaporatorsis used especially for make up water.

It is quite evident that there must be a closewatch on the feedwater so as to assure fine tubeperformance. It is not unusual to find tubes filledsolid with sludge and scale, and it is obvious thatsuch tubes are useless.

(Continued on page 18)

P a g e 1 0 T h e O h i o S t a t e E n g i n e e r

BOILER TUBE PERFORMANCE

(Continued from page 10)

Bursting of tubes is caused many times by overheating of the metal due to the poor thermal con-ductivity of the tube. This poor thermal con-ductivity may be caused by scale or sludge.

In a closed plant cycle, the problem of feed-water is reduced to that of make up water so thatit offers no danger to the tubes but the make upwater should pass through an evaporator beforeentering the boiler thus assuring good feedwater.In this type of a cycle the tube should give finerperformance because it should have a high ther-mal conductivity and a longer life.

In an open cycle the problem of feedwater islarge and important. Because of imperfect treat-ments the tubes will eventually become full ofscale and sludge. The performance of these tubeswill not be as good as those in a closed cycle.

The choice of treatment for a certain type offeedwater must be done with care. Years ago,water treatment was an art because the facts in-volved were not known but today water treatmentis being placed on a scientific basis.

P a g e 1 8 T h e O h i o S t a t e E n g i n e e r

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