Best Practices: Pipe and Valve Selection for a Cooling System

Submitted by Ray Herrera || Valin Corporation
When selecting the pipes and valves to be included in a cooling system, it is important to understand the options available — and the possible outcomes associated with each selection. The selection process is often overlooked or, at least, not given proper attention by designers and contractors when putting together a cooling system. Many factors can impact the effectiveness, longevity and quality of the overall cooling system, and each of these factors should be examined when choosing the best pipes and valves for the application. In most cases, valves, piping and sensors are all key factors to a cooling system’s effectiveness and overall quality.

Within the industrial processes that require a cooling system, there is often a portion of the process that is running hotter than it should. In these types of situations — where a significant amount of cooling is needed — cooling towers often are used. A major issue with cooling towers is that they are open to the atmosphere, thereby exposing them to outside elements. Along with the potential for external contamination, it typically is required to treat the water with a series of decontaminants that will aid in the slowing of corrosion and erosion caused by the chemicals and foreign particles in the water that is used to cool the system.

Cooling towers are not the only methods used to combat the effects of an overheated system. Closed systems also may be used for smaller cooling applications.

Choosing the Piping for the Cooling System

Choosing the best piping for the application is an important part of the cooling system assembly process. Five types of piping can be used for a cooling system, including:
  • Tubing
  • Iron
  • Steel
  • Stainless steel or exotic alloys
  • Plastic

The size and type of piping that should be used in the system is determined by factors such as:
  • The desired velocity through the system
  • Compatibility with the fluid and the chemicals that need to be introduced into the system to ensure efficiency

The size of the piping is important, but it easily can be determined by how much water needs to flow through the system at any given time. Higher grades of piping material come with a higher cost but will help with longevity and reliability. The compatibility of the piping system is important because water will be running through it at a near-constant flow rate. As the flow rate of water increases, a more chemically resistant pipe material is required to better hold up to corrosion.

If there is corrosion within the piping system, there can be a number of issues, including:
  • Leaks within the piping system where water can seep through
  • Corrosion buildup in the pipe

Corrosion will inevitably reduce the flow area of the water. This corrosion issue is more predominant in small systems. For example, a 1” pipe with 0.5” corrosion will show the effects of the corrosion more than a larger pipe system.

The size of the pipe is important because calculations need to be made for the pipe diameter to correctly fit with the overall system. The calculations of the pipe diameter need to be correct so that the velocity of the system is optimized and efficient. If velocity is minimized throughout the system, then the water will be able to freely run and cool proper areas. Additionally, there will be less corrosion if the velocity is kept to a minimum.

The last element to consider when looking at the piping in a cooling system is the introduction of chemicals in the water. In an open-atmosphere system, chemicals typically are introduced to control the pH level of the water. If chemicals are not introduced and the pH levels are not monitored, corrosion is likely in the piping system, causing a slow flow of water. In closed systems, there tends to be fewer minerals in the water. Consequently, closed systems can use aggressive fluids such as deionized water, requiring the use of plastic piping or stainless steels rather than iron or steel.

Choosing the Valve Type for the Cooling System

Regarding cooling systems, the valves that are selected typically are going to be flow control valves and isolation (shutoff and bypass) valves. There are important questions to consider before picking the valve for the cooling system.

The first thing to consider is the flow capacity. When considering the size required by the cooling application, it is easy to narrow down the valve that should be chosen. An important thing to remember in valve selection is the pipe size.
  • What flow capacity is needed for the cooling system?
  • What size piping will be used?

Temperature and pressure are the next critical pieces to look at when determining the control valves to use for a cooling system. The valve pressure/temperature rating must be within the system design parameters. The first factor that should be considered is the maximum pressure that the cooling system is going to have to hold to determine the valve rating. Along with pressure comes the capacity of the system. Consider the following three options when examining the pressure level and relative capacity of the cooling system:
  • Small-diameter pipe (0.5 to 2”): globe control valve
  • Mid-diameter pipe (3 to 6”): segmented ball valve
  • Large-diameter pipe (8” and above): high performance butterfly valve (HPBV)

Different types of isolation valves also are considered based on pressure and related capacity:
  • Small-diameter pipe: two- or three-piece ball valve
  • Mid-diameter pipe: three-piece or  flanged ball valve
  • Large-diameter pipe: butterfly valve

The capacity of the system as it relates to the diameter of the piping is an important aspect to consider when putting the cooling system together.

The size of the overall cooling system is something that also should be considered. The smaller the system size, the more accurate it will likely need to be. For instance, the cooling system in a CPU liquid cooler is one that should be concise and accurate because there is a finite amount of space that the water is working with to cool the variables along the piping system.

Conversely, the larger the cooling system, the less accurate the valve needs to be. This is not to say that the accuracy of the valve is not important in large systems; instead, it is simply a contrast when looking at the smaller and more concise systems. For example, if a power plant needs to have a cooling system, a turbine will be able to work with a much larger valve than that of an automobile’s engine.

One final thing to understand when selecting a valve is the factors that make up the valve’s accuracy. The factors that make one valve more accurate than another include construction of the valve, complexity of internal components and higher engineered designs

The preciseness of the valve is important. It could possibly be a needless expenditure if an extremely precise control valve is purchased when a less precise valve type could have done the job just as effectively. The more precise a valve, the more one can expect to spend on the overall cost of the cooling system. An important figure to determine is the percentage of inaccuracy (or margin of error) that a system can endure while still being effective and efficient.

Lastly, it is important to narrow down valve selection by taking a look at the heat transfer fluids such as water that are going to be involved in the cooling process system. Crucial questions to ask include:
  • What is the source of the fluid?
  • What is the target water temperature?
  • To what degree does the system need to be cooled down?
  • What is the temperature of the water when it is introduced into  the system?
  • Is the water clean?
  • Is the water going to be treated before it reaches the valve?

The materials of construction selected for the cooling system and valves will determined by these questions. Select the body materials based on the strength needed in the system (pressure and temperature rating); the internal and external environment of the system (open or closed); and the resistance to corrosion or erosion of the fluid being used to cool the system.

In conclusion, when engineers take a look at cooling systems, it is often the case that the system may look good on paper but not when it is finally assembled. Some things to look for include:
  • Insufficient runs of pipe before or after a valve or instrument
  • Various elbows or bends that are too close to valves. They can cause performance/control problems
  • Vertical elevations

Sometimes, the vertical pipes will be flowing downward, and the flow is more difficult to control when gravitational pull is considered. If this occurs, it cannot be guaranteed that there is a full pipe downstream of the valve. Conversely, if the water is flowing upwards, it will always have a full pipe.

Valves, pipes and controls throughout the cooling system are all critical to its overall efficiency and effectiveness. It is important to take a close look at each step in the process and perform an evaluation on the overall design of the system before putting it to work in the field.

Article featured in Process Cooling Magazine.
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