Prevention and Treatment for Carryover-Foaming and Priming

There are two kinds of solids present in most boiler water-the dissolved solids, or substances that are in solution, and suspended solids. Suspended solids are finely divided solid particles floating around in the water. This is material left over after the scale-forming and corrosive salts have been changed into sludge by chemical treatment.

When a boiler is steaming, the feedwater continuously carries dissolved mineral matter into the boiler. However, the steam leaving the boiler carries very little mineral matter with it. The concentration of dissolved solids in the boiler water, therefore, keeps building up unless properly controlled by continuous or intermittent blowdown.

In water tube boilers, concentrations are generally highest at the place where (he mixture of steam and water from the tubes spills over into the steam drum. Where total concentrations are not reduced sufficiently by the bottom blow, another blowdown line should be installed to remove water from the drum at the point where TDS (total dissolved solids) concentrations are the highest. This blowdown is generally operated continuously when the boiler is in service and is called a continuous blowdown.

The best remedy for foaming and priming carryover is the proper blowdown of TDS. The continuous blowdown should be regulated to maintain the TDS at 3,000 to 4,000 ppm. The greater the TDS that can be carried without trouble, the less water, fuel, and chemicals required or wasted in the TDS blowdown.

CHEMICAL TREATMENT DETERMINATION

Because raw water conditions vary so greatly with locale, it is impossible to recommend a single, specific water treatment. Whenever possible, a water treatment chemist should be consulted and his or her recommendations for the chemical treatment of boiler water should be followed. The degree of success of any water treatment program depends upon how well the recommendations for treatment are monitored. When the services of a qualified water treatment chemist are obtained, his or her recommendations should include the following:

l The treatment formula

l The treatment ingredients

l Instructions to the boiler operator in the use of the treatment

l Periodic visits to the plant to check on the results of the treatment plan

When the operator follows instructions and uses the proper blowdown procedure, scale and sludge in the boiler are reduced to a minimum. Blowdown limits the amount of dissolved and suspended solids in the boiler water.

Consulting a chemist is an ideal situation. Seabees seldom operate under ideal situations, particularly during contingency operations. How do you determine the initial chemical treatment for a boiler, and then, how do you establish an effective treatment program? Some general guidelines follow.

The first determination you have to make is the steaming rate of the boiler, expressed in pounds per hour. This is a fairly simple computation. You first determine the boiler horsepower (bhp); then multiply the result by 4.5 pounds. For example, if you have a 100 horsepower boiler operating at one-half fire, your steaming rate is 1,725 pounds of steam per hour.

1 BHP = 34.5 lb steam/hour 100 x 34.5 = 3,450 steam/hour at high fire 3,450 1/2 =1 .725 lb steam/hour at one-half fire

To determine the initial chemical dosage, you must know the hardness of the raw water. A chemist can tell you this; however, in the field you must determine it by experimentation. The harder the water, the more phosphates you must add during treatment to obtain correct phosphate residuals. The example that follows assumes zero hardness of the raw water and uses a 1,725-pound steaming rate. 1. Mix the following chemicals in 28 gallons of water:

a. 1 1/4 pounds of sodium sulfate

b. 1/2 pound of trisodium phosphate

c. 1/2 pound of caustic soda

2. Adjust the chemical feed rate to 3 gallons per hour (allows for 8- to 10-hours of steaming).

The chemical dosage varies with the steaming rate of the boiler. To establish your water treatment program, use the following steps every hour of operation for the duration of your initial chemical batch.

1. Determine the hourly steaming rate

2. Test for phosphate residual (30-60 ppm)

3. Test for sulfite residual (25-50 ppm)

4. Test for pH (9.5 to 11.5)

5. Test for TDS (3,000 to 4,000 ppm)

You should make a log entry of these test results every hour. This establishes a history of the test results. At the completion of the initial chemical dosage, you can either add or subtract chemicals, based on your log. It may take several batches fed over an 8 to 10 hour period to get a consistent chemical requirement for boiler water treatment. Once the boiler has stabilized and treatment test results remain reasonably balanced, testing may be required only every 4 hours.

At this time you can chart your chemical requirements, based on load demand of the boiler. By establishing this history through experimentation, your operators are able to treat the boiler water with fairly accurate results. At this time note that boiler blowdown has a big effect on your treatment program. Proper blowdown practices cannot be overemphasized. Too little blowdown causes TDS readings to be high; too much blowdown causes a high demand for chemicals and results in lost efficiency of the boiler.

[ Back ] [ Home ] [ Up ] [ Next ]

Privacy Statement - Press Release - Copyright Information. - Contact Us - Support Integrated Publishing