The Importance of Valves

1 Introduction

In general practice, it is universally accepted that pumps and piping require more attention when it comes to the design of a fluid transportation system. One can agree that the type of pump and piping mainly determines the type of fluid and pressure characteristics throughout the system. However, just as important is not only the type of valves used in the system but where these valves are located. Positioning a valve correctly can increase a system’s life expectancy, reduce maintenance costs, lower cavitation effects, and increase efficiency. This article will serve as a basis to broaden one’s knowledge of fluid mechanic principles and how to implement this knowledge practically for household and industrial use. Furthermore, discussing the importance of valve selection and placement provides general practice recommendations.

2 Why It Is So Important

During the design of a piping system, regardless of operating pressure, temperature, and the type of fluid being processed inside the system (these factors are confined to their own rules and specifications), valves are generally bought from a certified supplier and installed accordingly. Therefore, when selecting valves and locating them within the system, the following elements must always be taken into consideration:

2.1 General Valve Wear

One of the larger factors regarding valve selection is the amount of deterioration (“wear”) the valve must endure during its lifetime. Valve deterioration can possibly take place due to factors such as corrosion (how corrosive is the fluid flowing within the valve, as fluids such as water and acid have different chemical properties that need to be considered), abrasion (the roughness of the fluid flowing within the valve can remove material and drastically reduce life expectance) and working conditions (with large temperature variations during the operation of a valve one can come across difficulties with expansion and contraction factors).

2.2 Cavitation

Cavitation is a well-known phenomenon that causes pumps to fail catastrophically. However, it is also a serious problem for valves. Cavitation generates small but very powerful shock waves inside a valve, causing localized impact damage to the valve body and internals. Reducing cavitation can increase the efficiency and life expectancy of the valves inside one’s system. Section 4 of this article will elaborate more on cavitation and how it occurs in a valve section.

2.3 Flashing

Flashing occurs under the same conditions as cavitation. However, the result of flashing impacts a larger section of your system, while cavitation is localized damage. When flashing occurs, liquid and vapour flow through one’s system. This can cause severe damage due to the physical impact and the chemical reaction between the vapour and the pipe material. Reducing the possibility of flashing inside one’s system can decrease both corrosion and erosion. For all intents and purposes, this article will provide information on cavitation due to the fact that flashing and cavitation originate from the same principles and can be reduced using the same recommendations to be mentioned.

2.4 Hydraulic Hammer

A hydraulic hammer, also known as a water hammer, is generated inside a system when a moving fluid is suddenly stopped or forced to change direction. Physical waves moving rapidly throughout the entire system are created. These waves generate a large pressure surge and can cause instant failure of piping and components (such as valves). Closing a valve suddenly is also one of the main reasons a hydraulic hammer occurs. This sudden closure of a valve can result in a pressure increase upstream of the valve and a pressure drop on the downstream side of the valve, increasing the stress on the valve. Hydraulic hammering can also cause damage to pipe fixtures, loosen flanges, damage pumps and overstress pressure gauges. Therefore, hydraulic hammers are very dangerous in fluid transportation systems, and one would like to reduce or prevent them as much as possible.

Just consider the following:

You have a 150 mm (diameter) pipe with a 500 m length, which is full of water flowing at 4 m/s at ambient temperature.

The mass of the water is:

This means you have 8.84 tons of water moving at roughly 14.4 km/h. One can just imagine the stress acting upon a valve that is suddenly closed and causes this mass of water to stop suddenly. Reducing the water hammer inside one’s fluid transportation system can increase the valve’s life expectance, reduce the risk of a pipe rupture, reduce damage to pipe fixtures and decrease the risk of overstressing pressure measurement equipment.

3 Understanding the Fluid Mechanics

3.1 Brief Background

Fluids in motion or rest have been investigated since ancient Greece. When Archimedes studied fluids in static conditions and developed, the most renowned Archimedes’ principle (Roughly 250 BC) used to determine buoyance. The field of fluid dynamics has only advanced even further since the 1700s, thanks to Daniel Bernoulli, who studied how fluids react when they flow through different shapes of pipes and created Bernoulli’s principle.

3.2 Types of Fluids

In fluid mechanics, three main groups determine the characteristics of a fluid. These groups are as follows:

1. Liquids

Liquids are nearly incompressible fluids that offer great resistance against expansion and flow. They also take the shape of the container they are in; however, if the container is not completely full, a horizontal surface will indicate the liquid level inside the container. Liquids have the highest density among these three fluids.

2. Gases

A gas made out of one group of atoms is called a pure gas. When these different pure gas groups combine, a gas mixture results. One main factor that separates gases from liquids is the separation speed at which a gas or gas mixture can expand. When a container is filled with gas, it will expand until it is full, and if left without space restrictions, gas can expand infinitely.

3. Vapors

A vapour is fluid in the gas state at a temperature lower than its critical temperature. This means that if the pressure of the vapour is increased, it can condense to a liquid state without reducing the temperature of the vapour. Vapors have similar fluid characteristics to gases and fully occupy a volume in which it is contained.

3.3 Fluid Characteristics

Now, one can familiarize oneself with the three main types of fluids in most fluid mechanic systems. It is important to understand the different characteristics of these fluids and how they influence the design factors one should look at when designing or inspecting a fluid transportation system.

3.3.1 Density

Density can be understood as the total mass of a specific fluid inside a unit volume. The standard unit for density is  , which refers to the mass of a specific fluid in kilograms contained in a unit volume measured in cubic meters. The following formula illustrates how density is calculated.

The general density of water is a 1000 at atmospheric temperature , and is generally used as the basis of comparison between fluid densities.

The ideal gas law can describe a gas’s density relative to absolute pressure and temperature. Liquids are generally assumed to be mostly incompressible; however, gases are much more compressible than liquids. The following equation can be used to describe the density of only ideal or perfect gases.

3.3.2 Viscosity

Viscosity is a fluid characteristic that describes how easily a fluid flows or the resistance a fluid possesses against the flow. This characteristic is much needed because different fluids can have roughly the same density. However, the way they flow is completely different. The viscosity of a fluid is also directly connected to its temperature. For example, think of the way honey flows slowly, which is sort of heavy when at room temperature. Increasing the temperature of the honey, one will suddenly see a faster-flowing liquid, being compared to the cooler, thick-flowing honey at room temperature.

The SI (International System of Units) unit for viscosity is pascal-second (Pa.s), which is also equivalent to kilogram per meter per second . Viscosity can be determined using mathematical equations; however, in practice, the viscosity of a fluid is specified in pre-determined tables and standards.

3.3.3 Laminar and Turbulent Flow

When a fluid flows within a closed passage, there can only be two types of flow: laminar and turbulent. Both of these flow types depend on the fluid’s velocity and viscosity. Generally, when a fluid flows at a low velocity, one can witness a laminar or smooth, streamlined flow. As the fluid’s velocity increases, one can see more chaotic and unpredictable, called turbulent flow.

The higher the viscosity of a fluid, the more likely it would want to flow in a laminar manner, and the lower the viscosity of a fluid, the more likely it would want to flow in a turbulent manner. There is a transition phase where the flow goes from laminar to turbulent. The time it takes for the fluid to pass through this transition phase is not quick because it repeatedly transitions between laminar and turbulent before becoming fully turbulent or vice versa. How fluids mainly flow in practice is to be turbulent flow. For example, when a fluid is pumped, the pump’s impeller dramatically changes the fluid’s flow and transitions laminar flow into turbulent flow. Therefore, most of the time, one can expect turbulent flow at the outlet of a pump, and it takes some time for a fluid to transition from turbulent flow back to laminar flow.

The following two figures illustrate the difference between laminar and turbulent flow. Note the difference in streamlines.

Now that one can determine the difference between fluids inside a system, their different characteristics, and how these characteristics influence the fluid’s flow properties, one can use the following principles to understand how fluids work inside a fluid transportation system and determine the factors that influence a system’s successful operating life expectancy.

3.4 Reynolds Number

One can use the Reynolds number of a fluid to determine if the specific section being studied is in laminar or turbulent flow. The Reynolds number compares the fluid’s velocity (speed) and viscosity (thickness). It can also be used to determine when a fluid will transition from laminar to turbulent flow or vice versa. The Reynolds number of a fluid was introduced by George Stokes in 1851. The following equation can be used to determine the Reynolds number of a specific fluid being studied.

Once the specific fluid’s Reynolds number has been determined, the calculated Reynolds number is compared using the following range: If the calculated Reynolds number is below 2300, the fluid being studied is in laminar flow. If the calculated Reynolds number is between 2300 and 4000, the fluid will be busy transitioning from laminar flow to turbulent flow or vice versa. The fluid is in turbulent flow if the calculated Reynolds number is larger than 4000.

Calculating the Reynolds number at certain points throughout one’s system can largely benefit the system being studied or designed. Once the Reynolds number before and after the valve sections have been determined, one can establish whether there is laminar flow or turbulent flow within the studied section. This information can determine the best valve placements to ensure the desired flow is achieved inside a valve section.

3.5 Pascal’s Principle

First, we will look at Pascal’s Principle, which states that whenever a pressure change on a contained fluid occurs, this change in pressure will be transferred throughout the entire fluid so that the same change occurs everywhere else. This pressure (P) can be related to a force (F) being equally distributed over an area (A). This pressure can be expressed using the following formula.

Using the units for the force in Newton (N) and for the area in meters squared This equation will result in the calculated pressure to be in Pascals (Pa), hence Pascal’s Principle.

This principle is the basis of hydrostatic testing, a method used to pressure test valves, piping, and storage containers for leaks and determine a component’s or system’s strength. This test is executed by filling the pipe or vessel with water and then closing all possible outlets. The water inside the pipe or vessel is then pressurized to the pre-determined test pressure, which is usually the operating pressure multiplied by a safety factor of usually 1.5.

Once the fluid inside the enclosed pipe or vessel reaches the test pressure and there are no leaks, the tested specimen passes the hydrostatic test. The test fluid is then drained from the specimen, and if needed, the specimen is checked for cracks. This check for cracks can be done visually or with the help of scanners and different testing methods.

3.6 Bernoulli’s Principle

Somewhere, we all have heard that energy cannot be destroyed or created but can be transferred or changed from one form to another. This statement is known as the first law of thermodynamics or the energy conservation law. When we apply this statement to fluids, a fluid can mainly possess the following three energies.

1. Pressure energy – The pressure applied to a fluid can create this energy inside the fluid. Referring back to Pascal’s principle, the pressure applied to the fluid enables the fluid to transfer this pressure energy to the fluid’s surroundings.

2. Kinetic energy – The energy a fluid possesses due to its moving at a velocity throughout the system. This energy is also directly connected to the fluid’s density and can be determined using the following formula.

Referring back to our previous example in section 2.4, we see that the water inside the pipe has the following kinetic energy:

Therefore, the water inside the pipe has 8000 J of kinetic energy available to transfer onto its surroundings or to change into another form of energy.

3. Potential energy – The energy that a fluid possesses because there is an elevation difference between the fluid being studied and the reference point of the system. Because this energy is related to an elevation difference, it is connected to the earth’s gravitational pull; however, the density of the fluid plays an equally large role. The potential energy of a fluid can be calculated using the following equation.

Referring back to our previous example, with an added elevation difference of 20 meters. The water inside the pipe has the following potential energy.

Therefore, the water inside the pipe has 196 200 J of potential energy available to transfer onto its surroundings or to change into another form of energy. Comparing the fluid’s potential energy to its kinetic energy, one can see how an elevation difference in a system can create much more energy than pumping the fluid at a steady velocity without any elevation difference. Hence, gravitational feed dams and geezers were used before the pump was invented.

Daniel Bernoulli applied this energy conservation law to fluids. He determined that during steady flow, the sum of all the fluid’s different energies is the same at any two points in a single connected system. We can understand this principle better by saying that the sum of a fluid’s pressure energy, kinetic energy, and potential energy at a certain point remains constant when compared to another point in the same system. Bernoulli’s principle can be expressed as follows.

Where the subscripts 1 and 2 refer to two different points in the same system. The following figure illustrates Bernoulli’s principle in a practical example of a reducing-diameter pipe. Out of the figure, we can take that as the area of a component decreases, the pressure also decreases, but the velocity increases.

4 Flow Mechanics Inside Valves

Now that we have familiarized ourselves with the different types of fluids (liquid, gas, or vapour) that can flow within a fluid transportation system, their characteristics, and the principles that determine how these fluids flow, we can discuss how important it is to understand how fluids flow within a valve section of the system.

Bernoulli’s principle has taught us that as the area through which a fluid flows decreases, the velocity increases, and the pressure decreases. This decrease in pressure can cause the fluid pressure to drop below the vapour pressure of the specific fluid. The vapour pressure of a fluid is the balance pressure of a vapour above its liquid. This means that below this pressure, a fluid can evaporate or boil much easier at lower temperatures compared to usual operating temperatures. When the fluid evaporates, small air pockets are generated inside the fluid. When the fluid pressure is once again increased after the valve section where the area increases, these small air pockets implode (cavitation) and cause serious damage to the valve and fluid transportation system. When looking at this decrease in area in a valve section, one must remember that during the process where a valve is closed, the valve cross-sectional area is increasingly reduced until the valve is fully closed. Therefore, one must remember that during this process, the area of the valve section is reduced even more, resulting in higher fluid velocities and lower pressures. This increases the amount of cavitation and, in turn, the amount of damage to the valve and system.

Not only does this pressure decrease cause cavitation, but it can also lead to hydraulic hammering. During the process of cavitation, where these small air pockets are generated, it may occur that the system pressure does not increase enough or that the piping does not allow all these small air pockets to be removed from the fluid. This can create a large air pocket inside the piping system that can travel with the fluid along its path. When this air pocket reaches another closed valve or pipe component down the line, there will be a sudden increase in pressure due to the air pocket being pushed by the fluid behind it. Then, suddenly, the fluid slams against the closed component, causing a hydraulic hammer. This can generate large pressure spikes inside one’s system and lead to catastrophic failure of valves or system components. As mentioned in section 2.4, hydraulic hammering can also be generated when a valve is suddenly closed.

5 General Valve Recommendations

By understanding Bernoulli’s principle, we now fully grasp how processes like cavitation and hydraulic hammering occur in a system. We would now like to reduce the effects of these processes or eliminate them as far as possible. This section of the article will list a few recommendations that can be used to obtain the necessary results for one’s fluid transportation system while reducing the mentioned effects.

Recommendations to reduce the possibility of cavitation and hydraulic hammer:

1. When locating valves in a fluid transportation system or analyzing an existing system, one must remember that turbulent flow inside a valve section can reduce the valve’s operating effectiveness. Turbulent flow in valves also increases the probability of cavitation or hydraulic hammering. Therefore, we want to reduce the turbulent flow inside a valve section.

2. Generally, a rule can be applied to move a valve 5 pipe diameter away from its closest pump or piping equipment (elbows, t-pieces, etc.). For example, if a pipe diameter is 150 mm, the valve can be placed 750 mm away from its closest pump or piping equipment. This will increase the distance the fluid flows before entering the valve section and provide enough time to transition from turbulent to laminar flow. Increasing the probability of laminar flow inside the valve section and reducing the possibility of cavitation or hydraulic hammer.

3. As we have learned, high flow velocities can generate turbulent flow by increasing the fluid’s Reynolds number, hence decreasing system pressure and possibly increasing the occurrence of cavitation or hydraulic hammer. Increasing the diameter of a pipe section or valve diameter can reduce the flow velocity (a fluid velocity of 1.5 m/s is recommended to reduce the possibility of cavitation and hydraulic hammer) to reverse the mentioned effects. Ultimately, this will result in a smaller pressure drop over a valve section and increase the valve’s operating efficiency.

4. A large pressure drop over a valve section can also lead to cavitation or hydraulic hammering. One can reduce the pressure drop over a valve section by staggering the pressure downward before the specific section by using multiple valves; however, this could increase the total system cost. The pressure drop over a valve section can also be reduced by increasing the downstream pressure or by utilizing devices such as orifice plates.

5. Harder materials can also be implemented and used where valve cavitation is likely to occur. This will result in less damage or wear inside the valve body and increase the valve’s life expectancy.

6. Decreasing the fluid’s temperature will ensure it does not evaporate under high velocities when it reaches its vapour pressure, as discussed in section 4 of this article.

7. We have learned that when a valve is suddenly closed, the large inertia of the flowing fluid slams against the valve and causes hydraulic hammering. Ensuring that the plant personnel are properly trained to start up and shut down the fluid transportation system by closing valves over a longer period in a certain sequence can decrease the probability of hydraulic hammering.

8. Installing air release valves or surge tanks can remove unwanted air from the fluid transportation system and reduce the occurrence of hydraulic hammering. Air release valves should be installed in higher elevated sections of the system because air pockets are usually trapped or situated in these areas.

9. Swinging check valves are an excellent piece of equipment for reducing the effects of hydraulic hammering. The valve detects as soon as there is a reverse flow (the effect of the hydraulic hammer) and closes quickly. This valve is designed to close quickly and, therefore, can handle the fluid’s impact while bringing the reverse flow to a stop and ensuring minimal pressure surges are generated inside the system.

10. Luckily, we live in an age where one can analyze a system using accurate computer simulation software. One can use software such as AFT impulse or other available programs to model and simulate one’s fluid transportation system precisely. The software can determine pressure drops across a system and phenomena like hydraulic hammering. One must remember that the calculated results are only as accurate as the data entered into the software program. It is advised that after each run of the chosen simulation software, the results should be used to adjust the input parameters, and the simulation should be run again. Repeating this process a few times would result in more accurate outputs from the software package.

11. During the selection of valves for a system, one can consider the following factors to ensure that the correct valve is chosen for the desired purpose:

a. The type of fluid that will be flowing inside the valve section.
b. The desired function of the valve.
c. The minimum and maximum temperatures of the fluid inside the valve.
d. The material of which the valve stem, body and other components are manufactured.
e. The maximum allowable pressure drop across the valve section.
f. The fluid flow characteristics inside the valve section.
g. The shut-off capability and the amount of leakage when the valve is closed.

6 Summary

This article has served as a basis to broaden one’s knowledge of why valves are important in a fluid transportation system. It is generally accepted that pumps and piping require more attention in system design and even maintenance.

However, this article discusses the importance of reducing factors such as valve wear, cavitation, and hydraulic hammering in a fluid transportation system, specifically in valve sections.

Section 3 of this article discussed the basic principles of fluid mechanics to provide a better understanding of how fluids operate and the factors that influence the conditions under which these fluids operate. It also provided information regarding the three groups of fluids (liquids, gases, and vapours) that can be encountered in a fluid transportation system.

Furthermore, section 3 elaborated on these fluids’ characteristics (density and viscosity) and what factors influence them. Understanding and using these characteristics can help determine whether there is laminar flow or turbulent flow inside a valve section, which can help determine the system’s efficiency before making any calculations.

When determining the type of flow in the desired section, one must remember that there is a transition period between laminar flow and turbulent flow and that the system can accommodate this transition phase. The flow type in one’s valve section can be directly determined by calculating Reynold’s number and using the calculated and predicted outcomes to ensure the system operates at the desired conditions.

Pascal’s principle has taught us how pressure is transferred through the system by the fluid and how this pressure energy is transferred to the fluid’s surroundings. Bernoulli’s principle has taught us how this pressure energy, along with kinetic (velocity) energy and potential (elevation) energy, are utilized throughout the system.

Therefore, it is very important to ensure that this energy is utilized for the desired processes and not to feed phenomena like cavitation and hydraulic hammer, which can be harmful to one’s system.

Section 4 of this article has educated us on how cavitation and water hammer occur in a system. By understanding the origin of these phenomena, one can reduce or prevent their damaging results as far as possible.

Finally, section 5 of this article has provided us with the needed recommendations to ensure that when designing a fluid transportation system, what factors should be taken into consideration during the placement of valves, determining the pipe diameter, defining the fluid flow rates and how the system as a whole should be operated by trained personnel.

7 Conclusion

To conclude, all the different factors of fluid mechanics are shared in this article and how they determine how efficiently a fluid transportation system can perform. The most important thing after reading this article would be to apply this knowledge to a fluid transportation system that is being developed or already developed and to learn how the mentioned factors can influence the valves in the system through this process. Through this process, one will develop a ‘gut feeling’ of how valves are designed to function under certain conditions. Then, by selecting the correct valve for the desired purpose and placing this valve in the most suitable position, one can be assured that the valve will operate efficiently and not restrict the system’s performance.

8 References

Munson, Fundamentals of Fluid Mechanics 7th, John Wiley & Sons Inc, p. 796.
AVK, Advanced valves: Principles & Practice, Johannesburg: AVK.

Image references:

https://infrastructurenews.co.za/2019/03/14/iiot-a-focus-at-pumps-valves-pipes-africa/

https://www.dft-valves.com/blog/consequences-solutions-water-hammer/

https://www.chemicalprocessing.com/articles/2013/curb-control-valve-cavitation/