Conditioning Boiler Water

PART TWO

Content Tools

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.

ppm condensate TDS  x 100
ppm boiler water TDS
equals percentage of boiler water in steam

Figuring make-up feed in system Using the conductivity meter, pull a sample of the make-up water and a sample of the feedwater and measure their conductivity.

 Feedwater     = percentage of
Make-up water
make-up entering the system.

The meaning of micro omhs: from Ohm's Law, resistance in ohms equals the quotient of potential difference in volts divided by the current in amperes. The unit of trans conductance is the reciprocal of the resistance unit, the ohm, and is called mho (ohm spelled backwards). In a general sense:

Conductance =      1       resistance

and

1 ohm =1
1 ohm

Because the mho is too large a unit for practical use in expressing the trans conductance of a vacuum tube or other things in electronics, the micro omh is commonly used. Micro means one-millionth of the base unit:

1 MHO = 1,000,000 UMHOS
1 UMHO = .000001 in decimal version

An example: If the boiler water reads 2000 micro omhs, what is it in ohms?

   1   = .0005 + .000001 = 500 ohms
2000

Being we're dealing with reciprocals, it will be noted that the larger the micromhos, the smaller the ohms and the larger the ohms, the smaller the micromhos.

The Zeolite Softener: Water passes through the zeolite and the calcium, magnesium and other salts which produce hardness are converted into sodium compounds. The zeolite exchanges with the water, giving up sodium and receiving calcium. Solids are not removed from the water, but are converted into a form which renders the water soft. After this exchange has proceeded for some time, the sodium content of the zeolite becomes exhausted. It is then necessary to regenerate the softener. This is accomplished by passing sodium chloride (common salt water) through the zeolite. The sodium chloride replaces the sodium in the zeolite and removes the calcium. This process is referred to as regenerating the zeolite.

Natural zeolite is capable of exchanging from 3,000 to 5,000 grains of hardness per cubic foot. This means that from 300 to 500 gallons of 10 grain water may be softened with one cubic foot of zeolite before it must be regenerated. Synthetic zeolite has an exchange of from two to three times as great as green sand zeolite. The salt requirements for regeneration vary from 0.5 to 1.0 lb. per 1,000 grains of hardness removed. This varies widely depending upon the kind of zeolite used and the care in operation.

'Natural' or 'green sand' zeolite is mined and receives some processing to improve its effectiveness. This natural zeolite has the appearance of green sand.

Synthetic zeolite is made by precipitating compounds of sodium silicate and aluminum.

Boiler Blowdown Control: If the feedwater contains 4.5 gr./gal. of dissolved non-scale forming solids and it's necessary to hold the concentration of water in the boiler at 150 grains, what percent of the feed-water must be blown down? Neglecting the solids that may be present, what concentrations are added must be blown out. In this case, 3% of the feedwater would have to be blown out.

 4.5  = .03 x 100 = 3%
        150

Stack temperature reflects boiler efficiency. Fuel consumption increases 1% for every 40 degrees Fahrenheit increase in stack temperature. Thus it's important to keep both water and fire sides clean. Off hand I don't know the insulating value of scale, but soot has five times the insulating value of asbestos.

Testing the boiler water for alkalinity, blowdown control and alkalinity builder control, is sometimes abbreviated as P, M and OH. P equals phenolphthalein and M equals methyl orange, which are the titrating solutions. OH equals caustic, which is a widely used alkalinity builder in the boiler water. The P and M readings denote the alkalinity of the water. The M quantity is known as the 'total alkalinity.' The P is for indicating the alkalinity of the water. M is for boiler blowdown control, which is similar to the chloride or salinity tests. The OH is derived by multiplying the P reading times 2, and subtracting the M. These tests are usually expressed in ppm.

Parts per million (ppm) refers to a concentration of 1 in 1 million, that is to say, 1 lb. of salts in 1,000,000 lbs. of water. Another way of expressing solids in water is grains per gallon (gr./gal.), a grain being     1      lb.
                                                      7000

To convert from grains per gallon to parts per million, multiply by 17.12, that is to say, 1 gr./gal = 17.12 ppm. In closing once again, I hope this will be of help to the serious steam engineer with his boiler water testing and treating program. I hope that any readers with experience and knowledge in this field who can add to this will please do so!