Conditioning Boiler Water

PART TWO


| January/February 1992



Rt. 1, Box 163 Lengby, Minnesota 56651

Just recently at the Lake Itasca Threshing Show, I met an old friend, Al Johnson of Leonard, Minnesota. Al's an old hand at steam engineering, having owned many traction engines; he still has four of them now. He is an ex-Navy man and has been chief engineer at a local processing plant for years. Anyway, Al's quite knowledgeable with boiler water testing and treating, and he complimented me on my article in the September/October 1991 IMA pertaining to 'Conditioning Boiler Water.' Al stated that that article will probably only help one in a thousand, and I replied that if it can be of help to only one in a thousand it's well worth it. I also stated it's hard to find material relating to testing and treating boiler water, and that I want to share what I've compiled over the years with the readers of IMA.

Not too long ago I was talking to a chemist who represents the chemical company that supplies us with our testing and treating needs at the plant I work for. I stated that I read that water containing 85.5 ppm of calcium carbonate or less is considered soft. He said that it may be true for some applications but, when it comes to a steam boiler, anything over 1 gr./gal (17.12 ppm) is considered hard. Asking him about the pH factor, he stated that for each whole number it moves, the solution has doubled in concentration. An example: water that has a pH of 8 is alkaline, but a water of pH 9 is twice as alkaline. A water of pH 10 is twice as alkaline as 9. Mathematically the pH moves as a reciprocal of a log, thus lower numbers mean higher concentrations. Remember that pH represents hydrogen ion concentrations, which are acid. pOH represents hydroxyl ions, which are alkaline.

I want to explain a little more pertaining to the conductivity meter. It's a small box-like unit easily held in your hand. It has a meter on it calibrated in micro omhs and has a built-in container for the water to be tested. That container has two probes in it about 2 inches apart, thus the water's resistance will be measured in a 2 inch distance. Pure water has a high resistance to the flow of electricity; therefore, its conductivity, the reciprocal of resistance, is low. The purity of the water is frequently determined by measuring its conductivity, using a dip cell and conductivity bridge. The results are expressed in micro omhs of specific conductivity. The relation between micro omhs of specific conductivity and ppm of dissolved solids varies to some extent with the kind of dissolved solids in the water. The presence of carbon dioxide and ammonia in the water also affects the conductivity and the readings must be corrected accordingly in order to determine the total dissolved solids. In samples which do not contain carbon dioxide or ammonia or when corrections for them have been made, the approximate purity in ppm of dissolved solids can be determined by multiplying the conductivity in micro omhs by 0.55. In addition to checking relatively pure water, this method may, with suitably calibrated dip cells and conductivity bridges, be used to determine the concentrations of boiler water.

Here are some other uses of the conductivity meter:

Figuring boiler carry over Pull a water sample from the boiler and from the condenser outlet and measure their conductivity. The rule of thumb rough estimation of boiler water carrying over with the steam. TDS = Total Dissolved Solids. Conductivity of the condensate X 0.5 = ppm TDS. Conductivity of the boiler water X 0.85 = ppm TDS.