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Stay Cool in a Crisis

Nov. 15, 2002
When water-based process coolers and chillers fail or underperform, temporary cooling equipment can avert a plant shutdown

Process cooling is integral to the operation of power plants, process reactors, low-temperature processes, pharmaceutical operations and many other applications. The larger-duty setups tend to be operated with water, which can pick up heat in one section and transfer it to another. Waste heat can be disposed of with cooling towers, which transfer the heat from the water to open air, or with chillers, which run various types of refrigeration processes and vent unwanted heat.

Modern cooling systems require relatively little maintenance and are dependable, provided they are sized correctly for the actual process conditions. But a host of situations ," ranging from out-of-normal process conditions to catastrophic breakdowns ," can give the plant operators few choices other than shutting down. Only rarely does a plant have a backup cooling system in place. If the plant must rebuild the cooling system, the downtime could translate into months.

Those providers serving the chemical processing industry have responded to this situation by developing portable, temporary cooling equipment. The systems can be operated from existing plant power or from additional temporary power generators.

A bank of temporary cooling towers processes 64,000 gpm of refinery cooling water. The refinery's existing cooling tower had collapsed.

Rebuilding a typical industrial-scale cooling tower of, for example, a 15,000-gallon-per-minute (gpm) capacity can take six months or more. Even if a contractor is readily available to cut that time to weeks instead of months, temporary cooling equipment provides a desirable option during the rebuild period. In other cases such as routine plant turnarounds, temporary equipment provides a means to keep a plant running while maintenance is being performed.

Tower repair/rebuild

Next to emergency outages, the most common known use of temporary cooling towers is for supplying cooling water during the repair or replacement of a permanent cooling tower. Temporary cooling towers allow this work on the fly, permitting the plant to maintain full production.

This scenario not only brings about positive financial consequences for the plant, it also provides a safer work environment for the repair crew. Workers do not have to service an active tower, where they likely would have to negotiate their way along a poorly lit, slippery and potentially rotten structure.

When basin repairs or structural members below the water line are required, a temporary cooling tower might be the best solution. A temporary cooling tower can be installed complete with pumps, piping and other required equipment to allow workers complete, safe and unrestricted access to all areas of the primary tower. At the same time, the plant can enjoy uninterrupted cooling tower performance.

For example, if a three-cell tower ," designed to cool 20,000 gpm from 110F to 90F with a 78F wet bulb ," required one of its cells to be taken out of service for repairs, the cold-water temperature would rise from 90F to 98F, and the hot water temperature would rise from 110F to 118F. In a temperature-sensitive process, this could result in as much as a 40 percent reduction in production. In other less-temperature-sensitive processes such as steam turbines, a combination of increased fuel costs and reduced production would result.

Both cases more than justify the cost of the temporary cooling towers. To take one of the three cells out of service without any reduction in plant operations or efficiency, it would be necessary for the wet bulb to drop from 78F to approximately 59F.

Fig. 1 shows a three-cell cooling tower with a temporary cooling tower. The repair crew can work on each cell of the permanent tower, in turn, with the temporary unit providing the necessary cooling.

Figure 1. Repair Application


Temporary cooling towers also offer a quick fix to underperforming or overworked cooling towers. In some cases, the addition of temporary cooling capacity is a straight calculation of added operating costs for higher efficiency; justification of a capital cost does not enter into the equation.

Plants can augment current systems by diverting a portion of the hot water from the return line before it reaches the primary cooling tower. The diverted water then is sent to a temporary cooling tower, where it is cooled and then fed back into the primary cooling tower basin.

Augmentation is most effective when the primary and temporary cooling towers are working together. In other words, the plant first must establish the performance of the existing tower before it can accurately determine the amount of augmentation required.

For example, if a cooling tower is to cool 20,000 gpm from 110F to 90F at a 78F wet bulb, the plant would determine the impact of augmentation by reducing the water flow to the primary tower and maintaining the same temperature range. The cooling tower manufacturer should be able to provide information detailing the expected tower performance at varying water flow rates. The information will be in the form of curves or a printed table such as that shown below.

If the plant determines the optimum temperature reduction would be 3F, it would need to size the temporary cooling tower to cool 20 percent or 4,000 gpm from 107F to 87F at a 78F wet bulb. The primary tower and the temporary tower should be engineered to work as one. Any variation of this approach would require one of the towers to achieve a very close approach to the wet bulb temperature and will likely require more equipment and produce less desirable results.

With any augmentation, plants should consider:

How the reduced water flow will impact the water distribution of the primary tower. If too much water is diverted from the primary tower, the water distribution will be affected and might reduce part of the benefit provided by the temporary cooling towers. Rarely is it possible to augment more than 25 percent of the design water flow.

Prevailing winds. The temporary tower likely will be lower in profile and could potentially send its hot discharge air into the air inlet of the primary tower. It should be positioned cross- or downwind when possible.

Tower elevation. In some cases, the temporary cooling tower inlet might be higher than that of the primary tower and might require the plant to pinch back the valves to each cell of the primary tower to ensure adequate water to the temporary cooling tower. It is unlikely that the elevation difference will impact pump performance significantly; however, the difference should be considered.

Basin elevation. Some temporary towers can be elevated to allow the cooled water to be gravity fed from the temporary tower basin into the permanent cooling tower basin. If the primary tower basin is too high and pumps are required to pump the water from the temporary tower basin to the primary tower basin, the plant should consider special automated valves and a level controller.

Fig. 2 shows a typical cooling tower augmentation.

Figure 2. Typical Augmentation

Isolating exchangers

Regardless of how well the primary cooling tower is performing, one or more exchangers might not have adequate water flow or might be more sensitive to temperature than others. When an exchanger is partially fouled, additional cooling water and colder temperatures can compensate enough to meet production demands until the plant can schedule an outage to correct the problem.

In these cases, the affected exch-anger(s) can be isolated from the primary tower and placed on a designated temporary cooling tower. The temporary cooling tower and pump can be sized to meet the new or desired requirements. This setup not only will relieve the limitations of the affected heat exchangers, but also will remove part of the heat load of the primary tower. When a portion of the heat load is removed from the primary tower, the tower will provide more and colder water to the exchangers still being served.

Fig. 3 illustrates how the limited equipment can be separated from the primary cooling tower.

Figure 3. Isolating Exchangers

Once-through cooling systems

At manufacturing facilities with large cooling needs, the use of once-through cooling systems is common. Many of these plants once were served reliably by once-through cooling, but now find they have severe limitations. Temporary cooling towers can be an effective tool in relieving these limitations.

Thermal discharge (environmental cooling).

Almost all once-through cooling systems have very strict environmental permits. These permits establish guidelines that limit the water volume that can be removed from a water source and the temperature at which the water can be returned to that source.

In some cases, a plant that depends on once-through cooling becomes limited in the hot summer months by the maximum discharge temperature limit. The restrictions often are magnified during low river/lake levels or drought conditions.

Temporary cooling towers ," meeting discharge, intake or total replacement cooling needs ," can serve as a quick, cost-effective solution to once-through cooling problems.

The most common and least intrusive solution is the installation of a temporary cooling system at the plant discharge area. A portion of the hot discharge water is pumped from the discharge canal to the temporary cooling towers, where it then is cooled and redeposited downstream with the remaining discharge water. The thermal requirements for most of these applications can usually be met by treating approximately 10 percent of the total water flow.

When a river is being used for the cooling source and the plant is limited by reduced water availability or excessively high inlet temperatures, intake cooling becomes attractive.

The temporary solution is installed in much the same way as the discharge cooling system. A portion of the discharge water is pumped from the discharge canal to the temporary cooling towers, where it is cooled and then redeposited into the plant intake area. This solution solves both the limited intake water supply and the excessively high intake temperatures. However, this option is usually more expensive than the discharge cooling option because the water temperatures required for intake normally are cooler than those required for the thermal discharge limit.

A third alternative ," the most radical choice ," is to completely remove the plant from its once-through cooling source and place it on a temporary cooling tower until normal water levels or temperatures return.

Preparing for disruptions

To better prepare for temporary cooling system disruptions, plant personnel should:

Have a contingency plan in place.

Know the cooling system, including its water flows, operating temperature ranges, wet-bulb temperatures, power requirements, heating or cooling demands and water sources.

Properly maintain the cooling tower and process water, and keep good records. Monitor the condition of parts that degrade (such as the wooden structure of the tower), and check the chemistry of the process water (fouling, scale, dissolved solids) and, if used, the water treatment system.

The cooling tower's manufacturer should be able to provide much of this information, especially that concerning maintenance. A water-treatment consultant can provide information on water-quality maintenance. In addition, a vendor of temporary cooling systems can provide information about contingency planning or process optimization.

Creating a contingency plan

A temporary cooling system is rarely a drop-in replacement for the installed unit, especially if the process conditions have changed since the cooling tower installation. Thus, chemical facilities can save time and headaches when they need a temporary system by creating a contingency plan.

A contingency plan should incorporate a number of elements, including:

Expected lead time for delivery, setup and operation. You must do more than merely ascertain that certain equipment can be available at a certain time. After all, just having the equipment on-site does not solve the problem. You need to verify how long it will take to deliver, connect, set up and begin operating the equipment. Some vendors can provide turnkey service; others cannot.

A company that can provide a turnkey service will simplify the process. The chosen emergency provider should be one that keeps all required workers' safety training up to date. If safety training is not kept current, it is likely that at least one additional day of downtime will occur.

Piping. What size piping and how many piping connections are required for the temporary solution? The appropriate valves and flanges should be put in place before the emergency, not after.

Drawings.The drawings should include, at the very least, a plot plan showing the location of all the temporary equipment, including cooling towers, pumps, piping, electrical distribution, transformers, generators and accessories.

Power requirements. The contingency plan should outline power requirements, voltage requirements, power supply source(s), generator requirements and related details.

Equipment availability. Any company providing temporary towers has a limit to its fleet size and might not be able to fulfill a contingency plan at the time of need. Make sure the company you select to be your emergency provider will have enough equipment to support your requirements and/or select an alternate supplier. Check equipment availability periodically, especially in the summer.

Electrical distribution. The plant must spell out how many load centers and how much cable will be required, what electrical classifications must be met and similar details.

Pumps. Will pumps be required? If so, what flow rate and head pressures are needed? Can the same supplier take care of the pumps?

Make-up water. How much make-up water will be needed and what will be its source?

If the contingency is such that a cooling tower constructor will be repairing or rebuilding the permanent cooling tower, special attention should be paid to the placement of the temporary equipment. If access to the primary tower is too restricted, the repair or rebuild could take longer and increase rental and construction costs.

Working with the supplier

Temporary cooling towers can be rented for a period ranging from one day to multiple years. Most suppliers of temporary cooling towers offer significant price reductions based on the length of time the equipment is needed. Rates are usually based on daily, weekly, monthly, four-month, six-month and one-year pricing, so plant operators must communicate the expected duration of use when asking for quotes. Pricing also is dependent on the size of the project and the time of year, as well as the thermal duty, pipe quantity, pump sizes, power source distance and the supplier.

The average space required is approximately 100 square feet per 1,000 gpm. Many designs require minimum distances between each temporary unit to allow proper air inlet; others are more modular and do not. Some units have elevated water basins that allow much of the piping and pumps to be placed underneath the towers to reduce space requirements. CP

Carpenter is a business development manager with Aggreko Process Services, Houston. He has several years of experience in process design, optimization and operations and maintenance in the gas processing, refining and petrochemical industries. Contact him at [email protected]. Childers is manager of Aggreko Cooling Tower Services and has 14 years of experience in cooling tower design, manufacture, construction/installation and project management. He can be contacted at [email protected].

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