Table of Contents
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 systems life expectance, reduce maintenance costs, lower the effects of cavitation and increase the system’s efficiency. This article will serve as a bases to broaden one’s knowledge of fluid mechanic principles and how to implement this knowledge in a practical manner, being for household or industrial use. Furthermore, discussing the importance of valve selection, placement and providing recommendations for general practice.
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 with 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”) that the valve must endure during its life time. Valve deterioration can possible 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 needs 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).
Cavitation is well known phenomenon that causes the catastrophic failure of pumps. However, cavitation is a serious problem for valves as well. Cavitation generates small but very powerful shock waves inside a valve that generates 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.
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 takes place there is a combination of liquid and vapor flowing through one’s system. This can cause severe damage due to the physical impact and the chemical reaction between the vapor 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 originates from the same principles and can be reduced using the same recommendations to be mentioned.
2.4 Hydraulic Hammer
Hydraulic hammer or also known as 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 why 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 a valve. Hydraulic hammering can also cause damage to pipe fixtures, loosen flanges, damage pumps and overstress pressure gauges. Therefore hydraulic hammer is a very dangerous phenomena in fluid transportation systems and one would like to reduce or prevent it as much as possible.
Just consider the following:
You have a 150 mm (diameter) pipe with a 500 m length and this pipe 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 all of a sudden. Reducing 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 has been investigated from the days of 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, especially in the 1700’s thanks to Daniel Bernoulli. Whom 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 there are three main groups that determine the characteristics of a fluid. These groups are as follow:
Liquids are nearly an incompressible fluid that offers great resistance against expansion and flow. A liquid also takes the shape of the container it is in, however if the container is not completely full there will be a horizontal surface which indicates the liquid level inside the container. Liquids have the highest density among these three fluids.
A gas made out of one group of atoms is called a pure gas. When these different pure gas groups combine, the result is a gas mixture. One of the main factors that separate gases from liquids is the separation speed that a gas or gas mixture can expand at. When a container is filled with a gas, it will expand until the container is completely full and if left without a space restriction, a gas can expand infinitely.
A vapor is a fluid in gas state, at a temperature lower than its critical temperature. This meaning that if the pressure of the vapor is increased it can condense to a liquid state without reducing the temperature of the vapor. Vapors have similar fluid characteristics to gases and fully occupies a volume which it is contained in.
3.3 Fluid characteristics
Now that 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 the influence the design factors one should look at when designing or inspecting a fluid transportation system.
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 be used to describe the density of a gas with regards to absolute pressure and temperature. In general liquids are assumed to be mostly incompressible, however gases are much more compressible being compared to liquids. The following equation can be used to describe the density of only ideal or perfect gases.
Viscosity is a fluid characteristic that describes how easily a fluid flows or the resistance a fluid possess against flow. This characteristic is much needed due to the fact that different fluids can have roughly the same density, however the way they flow are completely different. The viscosity of a fluid is also directly connected to its temperature. For example think of the way honey flows slowly and 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), but it 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 is flowing within a closed passage there can only exist two types of flow, laminar or turbulent flow. Both of these types of flow are dependent on the fluid’s velocity and viscosity. In general, when a fluid is flowing at a low velocity one can witness laminar or smooth streamlined flow. As the velocity of the fluid is increased one can see more chaotic and unpredictable flow 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 flow to turbulent flow. The time it takes for fluid to pass through this transition phase is not quick, due to the fact that the fluid transitions repeatedly between laminar and turbulent before becoming fully turbulent or vice versa. The manner in which fluids mainly flow in practice are to be turbulent flow. For example, when a fluid is pumped the impeller of the pump dramatically changes the flow of the fluid 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 shows an illustration between the difference of laminar and turbulent flow. Note the difference in streamlines.
Now that one can determine the difference between fluids inside a system, the different characteristics of these fluids and how these characteristics influence the flow properties of the fluid. One can use the following principles to understand how fluids work inside a fluid transportation system and determine the factors that influences 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 is a comparison between 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 Reynolds number of the specific fluid has been determined the following range has been determined to compare the calculated Reynolds number with. If the calculated Reynolds number is below 2300 then the fluid being studied is in laminar flow. If the calculated Reynolds number is between 2300 and 4000 the fluid being studied is busy with a transition from laminar flow to turbulent flow or vice versa. If the calculated Reynolds number is larger than 4000 then the fluid is in turbulent flow.
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 if there is laminar flow or turbulent flow within the studied section. This information can be used to determine where the best valve placements could be to ensure that the desired flow is achieved inside a valve section.
3.5 Pascal’s Principle
First we are going to take a look at Pascal’s Principle which states that whenever a pressure change on 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 for the calculated pressure to be in pascals (Pa), hence Pascal’s Principle.
This principle is the basis of hydrostatic testing. Hydrostatic testing is a method used to pressure test valves, piping and storage containers for any leaks and to determine the strength of a component or system. 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 specimen being tested passes the hydrostatic test. The test fluid will then be drained from the specimen being tested and if needed the specimen will be checked for cracks. This check for any 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 that a fluid possesses due to the fact that it is moving at a velocity throughout the system. This energy is also directly connected to the fluids density and can be determined using the following formula.
Referring back to our previous example in section 2.4 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 due to the fact that 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 gravitational pull of the earth, 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 fluids potential energy to its kinetic energy one can see how an elevation difference in a system can create a whole lot more energy than pumping the fluid at a steady velocity without any elevation difference. Hence, the use of gravitational feed dams and geezers before the invention of the pump.
Daniel Bernoulli applied this energy conservation law to fluids and 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 being 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 vapor) that can flow within a fluid transportation system, the different characteristics of these fluids 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 decreases through which a fluid is flowing, the velocity increases and the pressure decreases. This decrease in pressure can cause the fluids pressure to drop below the vapor pressure of the specific fluid. The vapor pressure of a fluid is the balance pressure of a vapor above its liquid. Meaning that below this pressure a fluid can evaporate or boil much easier at lower temperatures compared to usual operating temperatures. When the fluid evaporates it generates small air pockets inside the fluid and when the fluid pressure is once again increased after the valve section where the area increases these small air pockets implode (cavitation) and causes 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 lead to hydraulic hammering as well. 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 for 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 and then suddenly the fluid slamming against the closed component causing 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
Through our understanding of Bernoulli’s principle we now fully grasp how process 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 operating effectiveness of the valve. Turbulent flow in valves also increases the probability of cavitation or hydraulic hammering. Therefore, we would like to reduce the amount of turbulent flow inside a valve section.
2. In general, a rule can be applied to move a valve 5 pipe diameters away from its closest pump or piping equipment (elbows, t-pieces, etc.). For example, if one has a pipe diameter of 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 provides enough time for the flow to transition from turbulent flow 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 that 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. By increasing the diameter of a pipe section or valve diameter one 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 valves 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 result in an increase of 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. One can also implement and use harder materials where cavitation in valves are 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 fluids temperature will ensure that the fluid does not evaporate under high velocities when the fluid reaches its vapor 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. By 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 one can decrease the probability of hydraulic hammering.
8. By installing air release valves or surge tanks one can remove the unwanted air from the fluid transportation system and reduce the occurrence of hydraulic hammering. It is recommended that air release valves are 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 that can be used to reduce the effects of hydraulic hammering. The valve detects as soon as there is a reverse in flow (effect of hydraulic hammer) and closes quickly. This valve is designed to close quickly and therefore can handle the impact of the fluid, while bringing the reverse flow to a stop and ensuring that there are minimal pressure surges 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 precisely model and simulate one’s fluid transportation system. The software can be used to 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 ran 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 that the valve stem, body and other components are manufactured of.
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.
This article has served as a bases to broaden one’s knowledge of why valves are important when it comes to a fluid transportation system. It is generally accepted that pumps and piping require more attention when it comes to the design and even maintenance of a system.
However, this article has discussed how important it is to reduce factors such as valve wear, cavitation and hydraulic hammering in a fluid transportation system as a whole and specifically on 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 provided us with information regarding the three groups of fluids (liquids, gasses and vapors) that can be encountered in a fluid transportation system.
Furthermore, section 3 has elaborated on the characteristics (density and viscosity) of these fluids and what factors influences these characteristics. Through understanding and using these characteristics one can predict if there is laminar flow or turbulent flow inside a valve section which can help determine the system’s efficiency before doing 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 be designed to accommodate this transition phase. The type of flow in one’s valve section can then also be directly determined by calculating the Reynold’s number and using the calculated results along with the predicted outcome 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 surrounding. While Bernoulli’s principle has taught us how this pressure energy along with kinetic (velocity) energy and potential (elevation) energy is utilized throughout the system.
Therefore, it is very important to ensure that this energy is utilized for the desired processes and not to feed phenome 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 and through understand the origin of these phenomena one can reduce or prevent their damaging results in a system 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.
To conclude, on all the different factors of fluid mechanics shared in this article and how they determine how efficient a fluid transportation system can perform. The most important thing after reading this article would to be apply this knowledge onto a fluid transportation system that is being developed or which is already developed and to learn through this process how the mentioned factors can influence the valves in the system. 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 as a whole.
Munson, Fundamentals of fluid Mechanics 7th, John Wiley & Sons Inc, p. 796.
AVK, Advanced valves: Principles & Practice, Johannesburg: AVK.