Water Impurities in Boiler Water Treatment

Transcription

1 Water Impurities in Boiler Water Treatment Natural water contains a variety of impurities, which can drop out of solution when heated, leaving behind deposits, which interfere with proper boiler performance, inhibiting heat transfer and water flow. Water is an excellent solvent, dissolving gases from the air, gases from organics in the soil, suspended matter from the earth, and minerals, chiefly calcium carbonate, magnesium carbonate, calcium sulfate, magnesium sulfate, silica (sand), sodium chloride, sodium sulfate, and small quantities of iron, manganese, fluorides, aluminum, etc. Waters which contain a large proportion of calcium and magnesium are considered "hard to wash with", hence the name hard water. The amount of hardness in natural water can vary from several parts per million to over 500 parts per million. Since calcium and magnesium compounds are relatively insoluble in water, when heated, they tend to precipitate (fall out) of solution, causing scale and corrosion. Since deposits insulate the pipes, they prevent the efficient transfer of heat, causing overheating. If the overheating is severe enough and long enough, the metal fails. Boiler tube deposits can also cause plugging or partial obstruction of boiler tubes, also causing overheating. Corrosion can occur under the deposits, potentially leading to leaks in the tubes. Natural waters also contain varying levels of oxygen and carbon dioxide, which act as oxidizers, causing both weakening and failure of metal and corrosion byproduct, which add to the boiler deposits. RESAT EGYPT uses oxygen scavenging chemicals CORROTAN series to eliminate oxygen from the system and alkalinity boosters, such as Alkalin-RB series, to maintain the water in the range of 100 ppm to 600 ppm alkalinity. Daily testing by the boiler attendant allow the amount of chemical to be adjusted as needed. Condensate system corrosion is caused by carbon dioxide and oxygen carried into the system by the steam. Condensate corrosion is controlled by neutralizing amines, which neutralize the corrosive effect of the gases found in the return condensate piping and filming amines, which form a protective film on the interior surfaces of the return condensate piping. The STEAMATE series contains a variety of amine formulas, selected by your RESAT EGYPT representative to compliment your particular water and its problems. When possible, it is best to obtain boiler feed water from ground-water, as this water is more consistent in composition and contains less suspended matter than surface supplies, which are modified by rainfall and erosion. A 0.5mm. Deposit of scale on the waterside surface increases fuel consumption by 9%; while a 1mm. deposit of scale on the waterside increases fuel consumption by 16%. By measuring the difference between the stack temperature and the temperature of the water can indicate the amount of scale that may be present, assuming that the combustion side of the boiler is operating efficiently (if not, FRS-100 can solve most fireside problems). ١

2 Water Testing There are two general methods used to determine the levels of impurities and the type and amount of treatment necessary to eliminate corrosion, scale and loss of heat transfer ability. These are titration and colorimetric tests. The colorimetric is the easiest to perform for plant engineers, so compliance with routine testing is usually higher with this type of test. The colorimetric test is based upon the development of a color in a sample proportional to the amount of the substance present. Concentration of substance in the sample is found by comparing its color with color standards of know concentration. Some comparators have color standards in sealed glass ampoules (CHEMetrics): others (Palintest and La MOTE) use standard colored disks. To perform a titration test, standard solutions from a burette, drop test kit, or calibrated container are added to the water sample. When the endpoint is reached, usually signaled by a color change in the sample, the burette solution level is noted or number or drops counted, if a drop test is used. The reading is proportional to the amount of the particular impurity present. There are three options to determine water "hardness", including gravimetric analysis for calcium and magnesium, colorimetric titration and soap tests, of which the latter two are the most popular. Conductivity is another method of measuring the total solids found in a water sample. Pure, distilled water, is a poor conductor of electricity, therefore the less dissolved solids in the water, the lower will be the conductivity. The instrument used for this measurement is a conductivity meter and the results are measured are micromhos (µ.mhos). When taking a sample for water testing, please observe the following: Use a clean, rubber-stoppered, resistant glass bottle or a new plastic bottle Either thoroughly wash and rinse used bottles or use new bottles for samples Before taking samples through metal lines and valves, allow water to run for several minutes to wash out the system. This is necessary to make certain the sample taken is representative and not contaminated with a disproportionate amount of any one impurity. Rinse the container thoroughly with the water that is to be analyzed before taking a sample. ٢

3 How Does A Boiler Work? A boiler is water containing vessel which transfers heat from a fuel source (oil, gas, coal) into steam which is piped to a point where it can be used to run production equipment, to sterilize, provide heat, to steam-clean, etc. The energy given up by the steam is sufficient to convert it back into the form of water. When 100% of the steam produced is returned to be reused, the system is called a closed system. Examples of closed systems are closed steam heating, hot water heating, and "one-pipe" systems. Since some processes can contaminate the steam, so it is not always desirable to feed the condensate back into the boiler. A system that does not return the condensate is called an open system. The two main types of boilers are: Fire tube - the fire or hot gases are directed through the inside of tubes within the boiler shell, which are surrounded by water. The tubes are arranged in banks so that the gases can be passed through the boiler up to 4 times before passing out the stack. This system exposes the maximum heat transfer surface to the water. Fire tube boilers are also known as shell boilers and can produce up to approximately 750 hp or 57 Ton of steam per hour. 80% of boilers in use are of this configuration. A subtype of this boiler is the packaged boiler, shipped complete with fuel burning equipment, mechanical draft equipment, automatic controls and accessories and is designed to function automatically with a very minimum of attention. It is particularly important to prevent scale formation in this type of boiler. Water tube - the fire or hot gases are directed to and around the outside of tubes containing water, arranged in a vertical position. Water tube boilers are usually rectangular in shape and have two or more drums. The separation of steam and water takes place in the top drum, while the bottom drum serves as a collection point for sludge. This system is usually used when more than 750 hp or 57 Tons of steam per hour, are needed. There are other designs with special configurations, adapting them to particular applications. ٣

4 Boiler Ratings and Boiler Load The oldest method of rating boilers, still used to rate small boilers, is by horsepower (hp). One horsepower is defined as the ability to evaporate 34.5 lbs of water into steam at 212 deg. F and above. Large boiler capacity is generally given in lbs of steam evaporated per hour, under specified steam conditions. Maximum continuous rating is the hourly evaporation that can be maintained for 24 hours. To convert horsepower (hp) into lbs of steam: Multiply hp x 34.5 Example: 100 hp x 34.5 = 3450 lbs of steam per hour To convert lbs of steam to hp: Divide steam per hour by 34.5 Example: 8625 lbs of steam 34.5 = 250 hp boiler Note: 1 lbs = Kg. Another measure is the BTU (British thermal unit). 33,472 BTU equals 1 hp To convert BTU into hp, divide the BTU rating by 33,472 Example: 8,368,000 BTU 33,472 = 250 hp Boiler load - The horsepower, lbs of steam per hour, or BTU is the rating indicating the maximum capacity of a boiler. When a boiler operates at its maximum rated capacity, it is referred to as maximum load. If the load varies from hour to hour, it operates at a varying load. Load and load variations can influence the amount of chemicals required for treatment and the treatment controls required. Treatment Options If you do not treat the water used by your boiler, boiler shutdowns for expensive cleaning will be required to remove the buildup of scale. If the corrosion is sufficient, you may also need to replace your pipes. ٤ Boiler water carryover, the contamination of the steam with boiler water solids, can occur if conditions of excessively high suspended and dissolved solids are present in the water. If the steam is used to sterilize, as in a hospital or food process plant, the solids can cause critical problems by depositing out at the point where the steam is used. In many industrial plants, steam is used directly on the product produced, so that deposits carryover can cause the shutdown of the plant, until the problem can be remedied. Maintaining the cycles of concentration at a low level and using antifoam chemicals can prevent this problem.

5 You can combine chemical treatment with the removal of solids known as blow-down. Softening agent chemicals, which react with calcium and magnesium to produce a non-adherent, very mobile and readily dispersed sludge, are added to the water to maintain the water at "0" hardness. The non-adherent sludge is removed by either manually wasting boiler water to the drain (bottom blow-down) and/or surface or skimmer blow-down. The water wasted is replaced with fresh water, which further dilutes the amount of solids. The combination of this process of dilution and adding chemicals keeps the system in control. Regulating blow-down. The two tests used to regulate the frequency and volume of blow-down is chloride level and specific conductance. These tests should be run daily by the boiler attendant, who regulates blowdown to keep the solids within limits prescribed by your RESAT EGYPT service representative. Since chloride does not react with the chemicals in the water treatment, the cycles of concentration can be calculated by testing this substance. Example: If the makeup chlorides are 20 ppm and boiler water chlorides are 100 ppm, the boiler is at 5 cycles of concentration. If makeup chlorides are at 30 ppm and the boiler water is at 120 ppm, the boiler is at 4 cycles of concentration. The second test used for regulating blow down is specific conductance. A conductivity meter is used to measure the conductivity of the "make up" water as compared to the conductivity of the boiler water. The ratio of the two figures is the "cycles of concentration". Example: If the makeup water conductivity is 300 µmhos and boiler water conductivity is 2100 µmhos, equals 7 cycles of concentration. Corrosion control is maintained by monitoring ph and/or alkalinity. Test strips and meters are available to measure ph and test kits are available from La Motte, Palintest and CHEMetrics. Does the Boiler Need Treatment when it is Out-of-Service? Unless idle boilers are stored properly, they can corrode badly. Wet lay up of a boiler is possible as long as the ambient temperature remains above freezing. Before wet storage, the boiler should be inspected, cleaned if necessary and refilled to the normal water level with de aerated feed water. Add the correct dosage of treatment and apply heat for one hour. If the super heater is drainable or the boiler does not have a super heater, allow the boiler to cool slightly after firing. Then, before a vacuum has a chance to form, completely fill the unit with de aerated feed water. After filling the boiler completely, connect a surge tank, such as a 250 Kgs. drum ٥

6 containing a solution of CORROTAN, or a nitrogen tank at 0.4 bar pressure, to the steam drum vent. This supply will compensate for volumetric changes due to temperature variations. Leave the drain between the non-return and main steam stop valves open wide. Tightly close all other drains and vents. Test the boiler water once a week from the shut-down boiler, and add extra treatment, as necessary, to maintain minimum levels. When chemicals are added, use an external pump to circulate the boiler water or reduce the water level to the normal operating level and steam the boiler for a short time. Then follow the above directions to lay it up again. If the super heater cannot be drained, fill it with de aerated water and treat it in the same proportion as the boiler. For dry lay up, the boiler should be drained, cleaned and dried out. Place an absorbent material, such as hydrated lime or silica gel in trays inside the boiler and seal it to prevent air from seeping into the boiler. Periodic replacement of the drying agent may be required during a login storage period. What is the Plant Control Test Log Sheet? The plant control test log is the book of forms in which to record the daily tests that are performed to monitor the system and adjust its treatment. Your RESAT EGYPT service representative will prescribe the parameters that should be maintained to keep your system in proper control and will then examine these logs when he makes his regular service visit. If boiler load varies, slight adjustments may be made by the boiler attendant to keep the chemical treatment within the limits set by the RESAT EGYPT representative. A sample RESAT EGYPT plant control log sheet is on-line. ٦

7 PLANT CONTROL TEST LOG SHEET Boilers Water Systems Company: Plant Type: Plant Location:. Date: Parameters ph TDS (ppm) Total Hardness (ppm) Row Water Pretreatment Water Feed Boiler Tank Boiler No 1 Boiler No 2 P1 Alkalinity (ppm) P2 Alkalinity (ppm) Total Alkalinity (ppm) Tannin / sulfite (ppm) Chlorides (ppm) Concentration Factor Corrosion Rate Treatment in Use:... Fuel Burned/Day Steam Rate/Day Makeup Rate/Day ٧