A loss of cooling water can result in a loss of process cooling and shutdown…
Treated Cooling Water
Heat exchangers in treated cooling water service should be designed and operated consistent with the water treatment. The temperature limits and thermal resistance of corrosion inhibitor films affect the design and operating limits of cooling water exchangers.
Cooling tower water is normally acidified to suppress alkaline scaling, and concentrated (“cycled”) close to but not exceeding the solubility limit of the least soluble salt at the maximum design tube wall temperature. Chlorination is normally used to suppress biological fouling. These procedures eliminate natural causes of water fouling.
Corrosion inhibitors are added to cooling tower water to provide a protective barrier between the oxygen saturated water and carbon steel components of the system. orrosion inhibitor films are effective in piping and on the tube side of heat exchangers. Movement of tubes in baffle holes prevents effective film formation on the outside (shell side) of baffled tubes. Where shell side water is used, tube material should be selected to resist oxygenated water.
Thermal resistance of corrosion inhibitor films is significant and should be included in heat exchanger design. Recommended design resistance for corrosion inhibitor films is given in Figure 300-1. Figure 300-1 is based on HTRI data obtained in member plants, and includes the effects of imperfect control in the commercial environment.
The temperature limits of corrosion inhibitor films are more important than their thermal resistance. The protective films break down between 160°F and 220°F and result in rapid fouling and loss of corrosion protection. The interface temperature between the metal and the inhibitor film should be kept below 160°F throughout the exchanger. Figure 300-2 expresses this criterion in a plot of maximum process emperature versus the process-side heat transfer coefficient for various water-side velocities. The curves are based on 120°F bulk water.
The shell side heat transfer coefficients for liquid hydrocarbons from Figure 200-4 are superimposed on Figure 300-2 for convenience. Cooling liquid hydrocarbons at economic velocities is not a problem in the usual temperature range.
Condenser design within the inhibitor stability limits is difficult when condensing temperatures are over 200°F, since the condensing heat transfer coefficients in inlet regions are usually between 500 and 1000 Btu/hr×°F×ft2. Air cooled condensers are preferred in those cases.
Figure 300-2 can also be used to evaluate the effect of throttling water to control condenser duty. Throttling reduces water velocity, increases water temperature, and is likely to exceed inhibitor stability limits if the condensing-side temperature is over 160°F. Bypassing part of the process fluid around the condenser is a better way to control a water cooled condenser.
Corrosion inhibitor film resistance at economic liquid velocity is not large enough to justify the use of all alloy cooling water systems without corrosion inhibitors.
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