Water velocity

The most likely places for SCC to be initiated are crevices or areas where the flow of water is restricted. This is due to the buildup of corrodent concentrations in these areas. For example, chloride can concentrate from 100 ppm in the bulk water to as high as 10,000 ppm (1%) in a crevice. Deposits are initiating sites because of crevices formed beneath them. The low water velocities in shell-side cooling are also detrimental. 

Erosion Corrosion. Erosion corrosion is the increase in the rate of metal deterioration from abrasive effects. It can be identified by grooves and rounded holes, which usually are smooth and have a directional pattern. Erosion corrosion is increased by high water velocities and suspended soHds. 

Removal of Particulate Matter. The amount of particulate entering a cooling system with the makeup water can be reduced by filtration and/or sedimentation processes. Particulate removal can also be accompHshed by filtration of recirculating cooling water. These methods do not remove all of the suspended matter from the cooling water. The level of fouling experienced is influenced by the effectiveness of the particular removal scheme employed, the water velocities in the process equipment, and the cycles of concentration maintained in the cooling tower. 

High Water Velocities. The abiUty of high water velocities to minimize fouling depends on the nature of the foulant. Clay and silt deposits are more effectively removed by high water velocities than aluminum and iron deposits, which are more tacky and form interlocking networks with other precipitates. Operation at high water velocities is not always a viable solution to clay and silt deposition because of design limitations, economic considerations, and the potential for erosion corrosion. 

Environment Internal Treated cooling water adjusted with sulfuric acid for pH control and sodium hypochlorite added as a biocide pressure 50 psi (345 kPa), temperature 100-120°F (38-49°C), water velocity 7 ft/s (2.1 m/s), pH 8.0-8.4, sulfate 500-1000 ppm, chloride 100-450 ppm, total hardness 500 ppm External Steam and condensate... 

A pump lifts water from a lake. At the pump suction entry a foot valve is fitted. Determine the maximum static delivery height the water can be raised without cavitation taking place. The saturation pressure of water is 1.23 kPa at 10 °C and the dynamic viscosity is 1.3 x 10" kg m s T The suction pipe water velocity is 2.0 m s , the internal pipe diameter is 100 mm, and the pipe roughness is 0.03 mm. The resistance of the foot valve is 4.5. 

Venturi Scrubber A scrubber with water velocities of between 60 and 100 m s" or higher, which create shear stresses, breaking up a gaseous air stream to provide ef fective particulate removal. 

Check the water velocity in tubes. From Volume 1 ... 

Smeatnii s elegant experimental technique enabled him to deal with both hydraulic and mechanical friction losses, allowing him to calculate water velocity at the wheel and thereby determine an effective nr virtual head. Smeaton s experimental apparatus was a brilliant device that enabled him to measure the efficiency of the wateiwheel, alone rather than the overall efficiency of the experiment. Smeaton was able to conclusively shov that a water-wheel when driven by the weight of water alone, is about twice as efficient as when driven by the impulse of water. This demonstration ensured that British mills, wherever possible, from then on would be fitted with overshot or breastshot wateiwheels, rather than undershot. 

Water velocity in sump must be low, IK feet/second is good practice. 

Tube-side water velocities should be kept within reasonable limits, even though calculations would indicate that improved tube-side film coefficients can be obtained if the water velocity is increased. Table 10-24 suggests guidelines that recognize the possible effects of erosion and corrosion on the system. 

Estimate the number of tubes per pass to maintain minimum water velocity. 

Winter operation—In order not to allow the water velocity in the tubes to fall below 3 ft/sec in the winter, you may have to compromise with the selected unit as based on 90°F water. If the average four-month winter temperature drops to 70°F, the quantity of water required will be reduced as will the velocity through the tubes. The low velocity is the point of concern. Check to determine the prevailing conditions. 

Although the previously discussed unit will perform as required, it may be larger than necessary. The water velocity of 4.8 ft/sec is not as high as would be preferred. 

Tube-side film coefficient. Assume minimum water velocity of 5 ft/sec, using 1-in. X 14 BWG tubes. 

Finally, the condensate is often pumped from the receiver to the boiler house. Pumped condensate lines carry only water, and rather higher water velocities can often be used to minimize pipe sizes. The extra friction losses entailed must not increase back pressures to the point where pump capacity is affected. Table 22.10 can be used to help estimate the frictional resistance presented by the pipe. 

Warm and hot water heating systems 27/453 Design water flow temperature 27/454 Maximum water velocity 27/454 Minimum water velocity 27/454 System temperature drop 27/455 Use of temperature-limiting valves on emitters 27/455... 

Noise generation and erosion/corrosion considerations limit the maximum water velocity in pipework systems. Noise is caused by the free air present in the water, sudden pressure drops (which, in turn, cause cavitation or the flashing of water into steam), turbulence or a combination of these. 

High water velocities can result in erosion or corrosion due to the abrasive action of particles in the water and the breakdown of the protective film which normally forms on the inside surface of the pipe. Erosion can also result from the formation of flash steam and from cavitation caused by turbulence. Publishing data on limiting water velocities are in conclusive. Table 27.9 summarizes the available information. 

Minimum water velocities should be maintained in the upper floors of high-rise buildings where air may tend to come out of solution because of reduced pressures. High velocities should be used in down-return mains feeding into air-separation units located at a low level in the system. Table 27.10 can be taken as a guide. Water velocities shown in Tables 27.9 and 27.10 are indicative parameters only on the one hand, to limit noise problems and... 

On some group and district heating schemes, outlet-limiting valves which permit flow only when the water temperature has dropped to a specified low level are used. This procedure minimizes the water quantity to be pumped and permits indicative heat metering by water quantity alone. In such cases, care must be taken to size emitters to suit the available water temperatures. The effect of low water velocities through the emitter must also be taken into consideration, since the heat output of some convective appliances is greatly reduced under such conditions. 

The effect of deposits has been referred to in relation to organic matter. Oxygen depletion can, of course, also occur under other types of deposit. Water velocity plays some part here, as with a good flow deposition is less likely. 

The open-circuit potential of most metals in sea water is not a constant and varies with the oxygen content, water velocity, temperature and metallurgical and surface condition of the metal. 

The corrosion rates for both maraging steel and the low alloy steels in seawater are similar initially, but from about 1 year onwards the maraging steels tend to corrode more slowly as indicated in Fig. 3.32. The corrosion rates for both low alloy and maraging steel increase with water velocity . During sea-water exposure the initial attack was confined to local anodic areas, whereas other areas (cathodic) remained almost free from attack the latter were covered with a calcareous deposit typical of cathodic areas in sea-water exposure. In time, the anodic rust areas covered the entire surface. ... 

Slow moving sea-water-velocity 0-3-0-6 m/s. Location Harbor Island. Wrighlsville, Beach, N.C., USA. 

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