Respuestas frente la actual situación del agua

m/sec ft/sec

Carbon Steel 1.8 to 3.0 6.0 to 10.0

Admiralty 1.2 to 2.4 4.0 to 8.0

Cupro nickel 1.2 to 3.6 4.0 to 12.0

FIGURE 28. RECOMMENDED COOLING WATER VELOCITIES

Sidestream Filtration

Continuous sidestream filtration of one to five percent of the recirculation rate has been used to reduce fouling. Proper selection of a filter to remove the particles in a cooling tower is required. When used in conjunction with good chemical treatment, sidestream filters have been effective. However, they are not a replacement for the proper control of corrosion, scale, and biofouling.

Dispersants and Surfactants

Dispersants and surfactants are useful for minimizing general fouling. The chemicals suspend the foulants, preventing their deposition and allow removal through blowdown.

Cleaning General Deposits

On-line or off-line cleaning are required in all cooling tower systems. Acids, chlorine, abrasive salts and materials, sponge balls and other substances have been used to clean systems on-line. On-line cleaning should only be attempted with experienced personnel with consideration for the impact of the cleaning on potential plugging, corrosion and safety. Hydroblasting is generally sufficient to clean systems off-line. If hydroblasting is insufficient, several options for mechanical and chemical cleaning are available. Testing is the best means for selecting the optimum cleaning procedure.

Monitoring and Control Required to Operate Cooling Water Systems

Chemical Feed Equipment

Chemicals are available in wet and dry forms. The choice is usually based on economics and location. The majority of chemicals fed to cooling water systems are in liquid form. The feed system should be furnished as a prefabricated, skid-mounted package with pump, tank, tank stand, shut-off valves, suction piping, agitator, and accessories in place. Where dilution is required, two day tanks are recommended.

Instrumentation commonly includes level indicators on tanks, pump discharge pressure indicator, flow sensors, and a draw down tube to measure feedrate. The control cabinet should contain pump stroke control, on-off pushbuttons and indicators, tank level indicators, and alarms.

Tanks must be sized based on delivery schedules and shelf-life of products. Duplicate positive displacement pumps are commonly provided. Piping and tanks should be

designed for easy cleanout. In large cooling towers, the feed pump is often automatically controlled based on the flow of make-up water.

pH and Blowdown Controllers

Automatic pH and blowdown controllers are recommended for all large cooling towers, especially when nonchromates are required. Accurate control of pH is required for the reliable control of scale and corrosion. The pH controller activates the feed of acid (usually sulfuric), or base (usually soda ash), depending on the alkalinity in the make-up water. pH probes should be equipped with alarms to indicate any low pH excursion. The automatic control of blowdown can usually be justified based on the resulting savings in water and chemicals. Blowdown controllers operate based on the conductivity of the recirculating water. The blowdown valve is opened when the conductivity exceeds a preset limit and closes when the conductivity is low.

ORP (oxidation reduction potential) probes are relatively new devices which can be used to sense residual chlorine in water and can be used to control the feed of chlorine.

Saudi Aramco DeskTop Standards 50

Frequency of Chemical Analysis

The recirculating water in a process cooling tower should be monitored routinely for corrosion inhibitor, chlorine, concentration of salts (usually conductivity), and process leaks, and monitored often for the ion which limits the cycles of concentration (e.g., calcium or silicate). Complete water analyses should be conducted twice per year to detect any long-term problems or trends. Large process towers which are not equipped with on- line monitors may require testing as frequently as every four hours. In well automated towers it may be sufficient to test once per day. Corrosion coupons should be evaluated quarterly, and microbiological counts should be run quarterly or more frequently. In all cases, more frequent monitoring is recommended if problems are detected.

Generally, closed cooling systems can be monitored quarterly. Once-through systems should be monitored for chlorine at least daily.

GLOSSARY

acid A chemical that lowers the pH by reacting with the

alkalinity

algae Simple aquatic plant life requiring sunlight for growth.

alkalinity The total of carbonate, bicarbonate, and hydroxide ion impurities expressed in mg/l as CaCO3.

anodic inhibitor A chemical added to cooling water which reduces corrosion by forming a protective film at the anodic surface of a metal.

bacteria One cell microbiological life which grow everywhere in cooling systems; excessive growth forms slime deposits.

biocide A chemical which is toxic to biological life.

biological fouling In cooling water, accumulations of algae and bacteria on components.

blowdown Water removed from a cooling system used to control the concentration of impurities in the recirculated water.

cathodic inhibitor A chemical added to water which reduces corrosion by forming a protective film at the cathodic surface of the metal.

chlorination The addition of a chemical containing free chlorine which is used to prevent biological fouling.

chromate Chemical containing hexavalent chromium. It was used as an anodic inhibitor until it was found to cause cancer.

cycles of concentration The number calculated by dividing the concentration of salts in the recirculated cooling water by the

concentration of salts in the make-up water.

deposit External material adhering to the original components in a cooling system.

Saudi Aramco DeskTop Standards 52 induced draft Pulling of atmospheric air through a cooling tower by

fans.

precipitation The formation of insoluble salts from soluble salts due to a chemical change.

Ryznar Stability Index A number based on the Langelier Index which indicates either corrosion or scaling.

silica The chemical silica dioxide. An inorganic impurity in water expressed as SiO2.

slime Insoluble organic matter of a viscous gelatinous composition normally resulting from microbiological growth which traps other suspended impurities in the cooling water.

solubility The amount of salt dissolved in water.