Boiler Water Treatment Primer

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To paraphrase Coleridge's Rime (sic) of the Ancient Mariner, 'Water, water, everywhere nor any drop for the steam engine' is, every day, becoming more prophetic as our sources become contaminated. Just opening the nearest fire hydrant to fill the engine's tank could, in the long term, prove to be catastrophic. I am really not an 'expert' in the field of boiler water treatment but one who found it necessary to gain an understanding of the subject. Let me share that with you.

Impurities in water which form scale inside a boiler are hardness, silica and corrosion products. Scale is formed by impurities precipitating from solution directly on boiler plates and tubes or the suspended solids settling out and then being baked on these heat transfer surfaces. Scale causes failures of metal due to local overheating and also contributes to boiler corrosion.

The parameters of boiler design involved here are pressure and steam temperature i. e., saturated or superheated with pressure being the primary consideration. Since our interest is in traction engine and portable power boilers we will limit ourselves to the problems associated with 125 pound saturated steam operations.

By way of some background in the subject of water treatment, most waters available to Iron-Men will contain constituents that can be placed into three categories: temporary hardness, permanent hardness, and suspended solids. If the water source happens to be from a municipal supply and not some nearby creek then suspended solids can probably be neglected thus avoiding a costly and difficult coagulation type treatment step or, at least, a slow sand filter bed operation.

At a risk of over simplification we can define the two types of hardness by saying that temporary hardness elements in the water supply, and which cause excessive scale formation, are of a chemical nature that can be converted to compounds that will remain in solution in the boiler water through rather simple treatment steps to later be removed by hot blow-down on a regular basis. On the other hand, permanent hardness elements do respond to such simple treatment steps but require more difficult to achieve alkalinity control. If they are of sufficient amounts in the supply, they too can be a source of potential damage to the boiler. In general, the carbonates of calcium and magnesium constitute the temporary category while the sulphate form of calcium and magnesium constitute permanent hardness.

Those using large quantities of water and having need for extremely pure water either for process needs or very high pressure (2400 psig) boilers would need elaborate and costly treating plants. More modest needs can often be served by use of zeolite softners Hey, Culligan Man followed by storage tanks. Since this type of service can often be rented it would be an ideal answer except for the traveling habits of old traction engines and the temporary nature of many show grounds. This being the case, the most practical solution to our water treatment problem will undoubtly be in the form of a system of internal treatment within the boiler itself or in the engine mounted water tank. That will be the approach taken here.

In its simplest form this will consist of mixing the proper amount and type of chemicals in a 55 gallon drum and then adding this concentrated solution to the engine's water tank in measured amounts as the tank is filled from time to time. These amounts of solution are going to be in the neighborhood of two or three gallons of solution per 100 gallons of raw feed water.

Let us begin at the beginning which is to find out just what is in the water that will be fed to the boiler. Look up in the Yellow Pages of your telephone directory under 'water' and find the nearest place that provides water treatment service. Generally these will be dealers that will want to sell you water softeners and service, however, they do water analysis for a nominal charge, usually less than $20 per sample.

Using a CLEAN glass one quart container (they may provide you with one liter sample bottles) get a sample of the water to be tested. Fill the bottle completely and seal it to keep out air and contaminants. These people don't often do tests for boiler treatment and need to be told what it is you want, The report will have two parts; mineral content and bacteria logical. Under mineral content be sure to ask the following: alkalinity, hardness, and pH. These are the values that you will use in determining the treatment program.

A word about pH and ppm and grains at this point may be in order. That strange symbol, pH, stands for 'hydrogen ion concentration'. Zero pH is pure acid while 14 pH is totally basic or caustic. A pH of 7 is neutral. Parts per million is what ppm stands for. Sea water is about 35,000 ppm salt which is 3%, just to give you a feel for the numbers. You may find that hardness is given in grains or it may be reported in ppm. Again, a feel for the numbers; 10 grains hardness corresponds to 170 ppm and is usually reported as calcium carbonate. And, for those of us that haven't converted our feel for numbers to the metric system, a liter is near enough to a quart to give us perspective there.

Let us now take a specific example in which the report shows the following: alkalinity 130 ppm, hardness 120 ppm and pH at 7. We will take care of the temporary hardness first.

Calculations will show that this level of hardness can be controlled by adding 0.44 pounds of tri-sodium phosphate in the crystaline form per 1000 gallons of raw untreated feed water. (This is Na3 PO4 .12 H20 for the chemists among us.) Phosphate in this form has a solubility of 0.215 pounds per gallon. Mix 10 pounds of the phosphate in 50 gallons of water in a separate container such as an open top 55 gallon drum. This will be a saturated solution. Each time that the engine's water tank is filled add some of this concentrated solution at the rate of 2 gallons per 100 gallons of raw untreated water. The quantities have been rounded off for convenience and there is some surplus in order to maintain a phosphate residual in the boiler. We'll talk about testing for the proper level later.

Keep in mind that you will be increasing the amount of dissolved solids in the water in the boiler. These will become concentrated as the boiler steams and the control of both the proper residual and total dissolved solids in the boiler is by blowing down the boiler and adjusting the phosphate feed, i.e., gallons of mix per 100 gallons water.

Your water may have a greater or lesser hardness but your case can be proportioned to the numbers in the example. For instance, suppose that your hardness is 10% greater, then use 2.2 gallons of solution per 100 gallons of raw water.

The permanent hardness is next and for this treatment the boiler water alkalinity needs to be high enough to also precipitate the magnesium hardness. If the alkalinity shown in the report is higher number than the hardness number there is likely enough alkalinity to do the job. We need to have boiler water pH values in the 9.5 to 10.5 range and the best way to know the value is to run a test on a cold sample when the boiler is shut down If the figure is too low it may be necessary to add some soda ash to the mixture given above. We will discuss testing later. Keep in mind, however, that high pH values are the cause of foaming and, in the extreme, can lead to caustic embrittlement of the boiler steel.

There is one more bad actor that we need to contend with and that is oxygen. All natural water supplies contain more or less oxygen. Large boiler installations in, say, a power plant are equipped with de-aerating feed water heaters that will remove most of the oxygen. Here again, however, we will need to do our treatment on the engine with the equipment usually available. For this there are two chemicals available to do the job. One, and the cheapest, is sodium sulphite and the other is hydrazene (N2H4). The sulphite reacts with oxygen in the water to form sodium sulphate and the hydrazine yields nitrogen and water. Unfortunately, sodium sulphite is slow to react and ample time must be allowed. For example, adding the proper amount to the tank on the engine will require ten minutes for a 30% reduction and up to 30 minutes to do the job. It is possible to purchase sodium sulphite with a copper catalyst that greatly speeds up the reaction but it would have to be pumped into the boiler separately for it can not be mixed in with the phosphate solution due to the catalyst. So, use the tanks on the engine but allow enough time for the reaction to take place that gets rid of the oxygen that can cause corrosion of the boiler metal.

This time we are going to approach the determination of the amount of chemical to be used in a back handed way. That is we will test to see that we are maintaining the proper sulphite residual by tests on boiler water samples. If we keep the sulphite residual in the range of from 30 to 66 ppm we can be assured that we have reacted all of the oxygen in the boiler water which will have formed sodium sulphate. The reason for this approach is that oxygen values in the raw feed vary with time and our test report will not have given us the figures we need. Oxygen is reported as cc/liter or as ppm. One cubic centimeter (cc) of dissolved oxygen per liter is equal to 1.43 ppm and it takes 0.047 pounds of sulphite per 1000 gallons of water for 0.5 cc/liter (0.72 ppm) of dissolved oxygen. So, you see, we are not talking about very much sulphite something like a half a pound per 1000 gallons for a typical water whereas we have been talking about 4 pounds of phosphate per 1000. Let's see, 1000 gallons equals 8330 pounds of steam or about enough water to run a 25 horsepower engine at full load for a day's work. Add a little sulphite when mixing the phosphate, we'll check it later.

Now is the time to talk about testing to see if we are treating water properly. All the tests that we need can be done with simple equipment right out in the field. There are several supply houses that sell field test kits. I will mention only one since it is the one that I have always found to be most helpful to the small operator or hobbyist. You may wish to contact Hach Company, Sales Department, Post Office Box 389, Loveland, Colorado 80539. They have a toll free number 800-525-5940. Here are some of their kits just for your understanding of the subject.

Phosphate Test Kit: Model PO-19, catalog number 2248-00, shipping weight 4 pounds, cost $48.25. This kit is capable of handling two ranges of phosphate: 0-5 and 0-50. Sufficient materials are included to run 100 tests.

pH Test Kit: Model 17, catalog number 1470-10, shipping weight 1 pound, cost $59.50. This is a narrow range type kit in that it is specifically built for running tests in the 9.7 to 11.6 range which is the range of control we need. Sufficient materials are included for 100 tests.

Sulphite Test Kit: Model SU-5, catalog number 1480-02, shipping weight 3 pounds, cost $43.50. This kit is a dual range kit (0-20 and 0-200 ppm) with sufficient materials for about 100 tests.

Going back now to the temporary and permanent hardness treatment there is some reluctance to get too specific about residual quantities suitable for operation. Water treatment is somewhat of an art, in my view, though the science is well known. The reason for this statement is that, in the final analysis, there have to be some compromises. As long as you understand this then here are some suggestions. Run with a phosphate residual in the boiler of 20 ppm for if it is too high it can have an effect on pH and interfere with the permanent hardness reaction.

Since these engine operations are intermittent and generally not at full load, daily blow down is probably going to be often enough. Take a sample from the boiler blow down connection when the water is cold and use this for your tests. When the boiler comes up to pressure you can blow down and if there is a lot of water use then again during the day. You will find your own routine that suits your needs best after having done a few tests.

A word of caution for those with machines equipped with exhaust steam feed water heaters. The phosphate treatment requires temperatures above room temperatures for the reaction to take place. That means that some sludge formation could take place in the feed heater and, therefore, it should be drained daily to remove the sludge.

This is written simply as an introduction to the subject of water treatment, a primer, with the thought that those that want to delve deeper into the subject will do so if this sparks their interest. That is where I found myself some years ago when it was necessary to have an appreciation of the subject if only to understand what it was that the experts in the field were telling me. That is what I have just shared with you.