Introduction
In the article, two basic requirements for trouble free operation and longer service life of centrifugal pumps are mentioned in brief.
1. PREVENT CAVITATION Cavitation of the pump should not occur throughout its operating capacity range.
2. MINIMIZE LOW FLOW OPERATION
Continuous operation of centrifugal pumps at low flows i.e. reduced capacities, leads to a number of unfavorable conditions. These include reduced motor efficiency, excessive radial thrusts, excessive temperature rise in the pumping fluid, internal re-circulation, etc. A certain minimum continuous flow (MCF) should be maintained during the pump operation.
Operating a pump under the condition of cavitation for even a short period of time can have damaging consequences for both the equipment and the process. Operating a pump at low flow conditions for an extended duration may also have damaging consequences for the equipment.
The condition of cavitation is essentially an indication of an abnormality in the pump suction system, whereas the condition of low flow indicates an abnormality in the entire pumping system or process. The two conditions are also interlinked such that a low flow situation can also induce cavitation.
The concept of cavitation is explored in detail under following topics:
1. Meaning of the term ‘cavitation’ in the context of centrifugal pumps.
2. Important definitions: Static pressure, Dynamic pressure, Total pressure, Static pressure head, Velocity Head, Vapor pressure.
3. Mechanism of cavitation.
4. General symptoms of cavitation and its effects on pump performance and pump parts.
5. Types of cavitation:
a. Vaporous cavitation
i. Classic cavitation
ii. Internal re-circulation cavitation
b. Gaseous cavitation
i. Air ingestion induced cavitation
7. Methods to prevent cavitation
The topics 1 to 4 are covered in detail in this part of the article. The topics 5 to 6 shall be explored in next part of the article.
Concept of Cavitation
Cavitation is a common occurrence but is the least understood of all pumping problems. Cavitation means different things to different people. Some say when a pump makes a rattling or knocking sound along with vibrations, it is cavitating. Some call it slippage as the pump discharge pressure slips and flow becomes erratic. When cavitating, the pump not only fails to serve its basic purpose of pumping the liquid but also may experience internal damage, leakage from the seal and casing, bearing failure, etc.
In summary, cavitation is an abnormal condition that can result in loss of production, equipment damage and worst of all, personnel injury.
The plant engineer’s job is to quickly detect the signs of cavitation, correctly identify the type and cause of the cavitation and eliminate it. A good understanding of the concept is the key to troubleshooting any cavitation related pumping problem.
Meaning of the Term "Cavitation" in the Context of the Centrifugal Pump
The term ‘cavitation’ comes from the Latin word cavus, which means a hollow space or a cavity. Webster’s Dictionary defines the word ‘cavitation’ as the rapid formation and collapse of cavities in a flowing liquid in regions of very low pressure.
In any discussion on centrifugal pumps various terms like vapor pockets, gas pockets, holes, bubbles, etc. are used in place of the term cavities. These are one and the same thing and need not be confused. The term bubble shall be used hereafter in the discussion.
In the context of centrifugal pumps, the term cavitation implies a dynamic process of formation of bubbles inside the liquid, their growth and subsequent collapse as the liquid flows through the pump.
Generally, the bubbles that form inside the liquid are of two types: Vapor bubbles or Gas bubbles.
1. Vapor bubbles are formed due to the vaporisation of a process liquid that is being pumped. The cavitation condition induced by formation and collapse of vapor bubbles is commonly referred to as Vaporous Cavitation.
2. Gas bubbles are formed due to the presence of dissolved gases in the liquid that is being pumped (generally air but may be any gas in the system). The cavitation condition induced by the formation and collapse of gas bubbles is commonly referred to as Gaseous Cavitation.
Both types of bubbles are formed at a point inside the pump where the local static pressure is less than the vapor pressure of the liquid (vaporous cavitation) or saturation pressure of the gas (gaseous cavitation).
Vaporous cavitation is the most common form of cavitation found in process plants. Generally it occurs due to insufficiency of the available NPSH or internal recirculation phenomenon. It generally manifests itself in the form of reduced pump performance, excessive noise and vibrations and wear of pump parts. The extent of the cavitation damage can range from a relatively minor amount of pitting after years of service to catastrophic failure in a relatively short period of time.
Gaseous cavitation occurs when any gas (most commonly air) enters a centrifugal pump along with liquid. A centrifugal pump can handle air in the range of ½ % by volume. If the amount of air is increased to 6%, the pump starts cavitating. The cavitation condition is also referred to as Air binding. It seldom causes damage to the impeller or casing. The main effect of gaseous cavitation is loss of capacity.
The different types of cavitation, their specific symptoms and specific corrective actions shall be explored in the next part of the article. However, in order to clearly identify the type of cavitation, let us first understand the mechanism of cavitation, i.e. how cavitation occurs. Unless otherwise specified, the term cavitation shall refer to vaporous cavitation.
Important Definitions
To enable a clear understanding of mechanism of cavitation, definitions of following important terms are explored.
· Static pressure,
· Dynamic pressure,
· Total pressure,
· Static pressure head,
· Velocity head, and
· Vapor pressure.
Static Pressure, ps
The static pressure in a fluid stream is the normal force per unit area on a solid boundary moving with the fluid. It describes the difference between the pressure inside and outside a system, disregarding any motion in the system. For instance, when referring to an air duct, static pressure is the difference between the pressure inside the duct and outside the duct, disregarding any airflow inside the duct. In energy terms, the static pressure is a measure of the potential energy of the fluid.
Dynamic pressure, pd
A moving fluid stream exerts a pressure higher than the static pressure due to the kinetic energy (½ mv2) of the fluid. This additional pressure is defined as the dynamic pressure. The dynamic pressure can be measured by converting the kinetic energy of the fluid stream into the potential energy. In other words, it is pressure that would exist in a fluid stream that has been decelerated from its velocity ‘v’ to ‘zero’ velocity.
Total pressure, pt
The sum of static pressure and dynamic pressure is defined as the total pressure. It is a measure of total energy of the moving fluid stream. i.e. both potential and kinetic energy.
Relation between ps, pd & pt
In an incompressible flow, the relation between static, dynamic and total pressures can be found out using a simple device called Pitot tube (named after Henri Pitot in 1732) shown in Figure 1.
In the article, two basic requirements for trouble free operation and longer service life of centrifugal pumps are mentioned in brief.
1. PREVENT CAVITATION Cavitation of the pump should not occur throughout its operating capacity range.
2. MINIMIZE LOW FLOW OPERATION
Continuous operation of centrifugal pumps at low flows i.e. reduced capacities, leads to a number of unfavorable conditions. These include reduced motor efficiency, excessive radial thrusts, excessive temperature rise in the pumping fluid, internal re-circulation, etc. A certain minimum continuous flow (MCF) should be maintained during the pump operation.
Operating a pump under the condition of cavitation for even a short period of time can have damaging consequences for both the equipment and the process. Operating a pump at low flow conditions for an extended duration may also have damaging consequences for the equipment.
The condition of cavitation is essentially an indication of an abnormality in the pump suction system, whereas the condition of low flow indicates an abnormality in the entire pumping system or process. The two conditions are also interlinked such that a low flow situation can also induce cavitation.
The concept of cavitation is explored in detail under following topics:
1. Meaning of the term ‘cavitation’ in the context of centrifugal pumps.
2. Important definitions: Static pressure, Dynamic pressure, Total pressure, Static pressure head, Velocity Head, Vapor pressure.
3. Mechanism of cavitation.
4. General symptoms of cavitation and its effects on pump performance and pump parts.
5. Types of cavitation:
a. Vaporous cavitation
i. Classic cavitation
ii. Internal re-circulation cavitation
b. Gaseous cavitation
i. Air ingestion induced cavitation
7. Methods to prevent cavitation
The topics 1 to 4 are covered in detail in this part of the article. The topics 5 to 6 shall be explored in next part of the article.
Concept of Cavitation
Cavitation is a common occurrence but is the least understood of all pumping problems. Cavitation means different things to different people. Some say when a pump makes a rattling or knocking sound along with vibrations, it is cavitating. Some call it slippage as the pump discharge pressure slips and flow becomes erratic. When cavitating, the pump not only fails to serve its basic purpose of pumping the liquid but also may experience internal damage, leakage from the seal and casing, bearing failure, etc.
In summary, cavitation is an abnormal condition that can result in loss of production, equipment damage and worst of all, personnel injury.
The plant engineer’s job is to quickly detect the signs of cavitation, correctly identify the type and cause of the cavitation and eliminate it. A good understanding of the concept is the key to troubleshooting any cavitation related pumping problem.
Meaning of the Term "Cavitation" in the Context of the Centrifugal Pump
The term ‘cavitation’ comes from the Latin word cavus, which means a hollow space or a cavity. Webster’s Dictionary defines the word ‘cavitation’ as the rapid formation and collapse of cavities in a flowing liquid in regions of very low pressure.
In any discussion on centrifugal pumps various terms like vapor pockets, gas pockets, holes, bubbles, etc. are used in place of the term cavities. These are one and the same thing and need not be confused. The term bubble shall be used hereafter in the discussion.
In the context of centrifugal pumps, the term cavitation implies a dynamic process of formation of bubbles inside the liquid, their growth and subsequent collapse as the liquid flows through the pump.
Generally, the bubbles that form inside the liquid are of two types: Vapor bubbles or Gas bubbles.
1. Vapor bubbles are formed due to the vaporisation of a process liquid that is being pumped. The cavitation condition induced by formation and collapse of vapor bubbles is commonly referred to as Vaporous Cavitation.
2. Gas bubbles are formed due to the presence of dissolved gases in the liquid that is being pumped (generally air but may be any gas in the system). The cavitation condition induced by the formation and collapse of gas bubbles is commonly referred to as Gaseous Cavitation.
Both types of bubbles are formed at a point inside the pump where the local static pressure is less than the vapor pressure of the liquid (vaporous cavitation) or saturation pressure of the gas (gaseous cavitation).
Vaporous cavitation is the most common form of cavitation found in process plants. Generally it occurs due to insufficiency of the available NPSH or internal recirculation phenomenon. It generally manifests itself in the form of reduced pump performance, excessive noise and vibrations and wear of pump parts. The extent of the cavitation damage can range from a relatively minor amount of pitting after years of service to catastrophic failure in a relatively short period of time.
Gaseous cavitation occurs when any gas (most commonly air) enters a centrifugal pump along with liquid. A centrifugal pump can handle air in the range of ½ % by volume. If the amount of air is increased to 6%, the pump starts cavitating. The cavitation condition is also referred to as Air binding. It seldom causes damage to the impeller or casing. The main effect of gaseous cavitation is loss of capacity.
The different types of cavitation, their specific symptoms and specific corrective actions shall be explored in the next part of the article. However, in order to clearly identify the type of cavitation, let us first understand the mechanism of cavitation, i.e. how cavitation occurs. Unless otherwise specified, the term cavitation shall refer to vaporous cavitation.
Important Definitions
To enable a clear understanding of mechanism of cavitation, definitions of following important terms are explored.
· Static pressure,
· Dynamic pressure,
· Total pressure,
· Static pressure head,
· Velocity head, and
· Vapor pressure.
Static Pressure, ps
The static pressure in a fluid stream is the normal force per unit area on a solid boundary moving with the fluid. It describes the difference between the pressure inside and outside a system, disregarding any motion in the system. For instance, when referring to an air duct, static pressure is the difference between the pressure inside the duct and outside the duct, disregarding any airflow inside the duct. In energy terms, the static pressure is a measure of the potential energy of the fluid.
Dynamic pressure, pd
A moving fluid stream exerts a pressure higher than the static pressure due to the kinetic energy (½ mv2) of the fluid. This additional pressure is defined as the dynamic pressure. The dynamic pressure can be measured by converting the kinetic energy of the fluid stream into the potential energy. In other words, it is pressure that would exist in a fluid stream that has been decelerated from its velocity ‘v’ to ‘zero’ velocity.
Total pressure, pt
The sum of static pressure and dynamic pressure is defined as the total pressure. It is a measure of total energy of the moving fluid stream. i.e. both potential and kinetic energy.
Relation between ps, pd & pt
In an incompressible flow, the relation between static, dynamic and total pressures can be found out using a simple device called Pitot tube (named after Henri Pitot in 1732) shown in Figure 1.
Figure 1: A Simple Sketch of a Pilot Tube
The Pitot tube has two tubes:
1. Static tube (b): The opening of the static tube is parallel to the direction of flow. It measures the static pressure, since there is no velocity component perpendicular to its opening.
2. Impact tube (a): The opening of the impact tube is perpendicular to the flow direction. The point at the entrance of the impact tube is called as the stagnation point .At this point the kinetic energy of the fluid is converted to the potential energy. Thus, the impact tube measures the total pressure (also referred to as stagnation pressure) i.e. both static pressure and dynamic pressure (also referred to as impact pressure).
The two tubes are connected to the legs of a manometer or equivalent device for measuring pressure.
The relation between ps, pd and pt can be derived by applying a simple energy balance.
As mentioned earlier, in the case of a fluid or gas the potential energy is represented by the static pressure and the kinetic energy by dynamic pressure. The kinetic energy is a function of the motion of the fluid, and of course it's mass. It is generally more convenient to use the density of the fluid (r) as the mass representation.
\ K.E = pd = ½ m v2 = ½ r v2
The corresponding pressure balance equation is
In place of the pressure terms as used above, it is more appropriate to speak of the energy during pumping as the energy per unit weight of the liquid pumped and the units of energy expressed this way are foot-pounds per pound (Newton-meters per Newton) or just feet (meters) i.e. the units of head. Thus the energy of the liquid at a given point in flow stream can be expressed in terms of head of liquid in feet.
The pressure term can be converted to head term by division with the factor ‘r g’. For unit inter-conversions the factor ‘r g/gc’ should be used in place of ‘r g’.
Static pressure head
The head corresponding to the static pressure is called as the static pressure head.
Static pressure head = ps / r g
Velocity head
The head corresponding to dynamic pressure is called the velocity head.
Velocity head = pd / r g = (r v2 / 2) / r g = v2/2g
From the reading hm, of the manometer velocity of flow can be calculated and thus velocity head can be calculated. The pressure difference, dP (pt – ps) indicated by the manometer is the dynamic pressure.
dP = hm (rm - r) g = r v2 /2
Velocity head = dP / r g = hm (rm - r) /r
Vapor pressure, pv
Vapor pressure is the pressure required to keep a liquid in a liquid state. If the pressure applied to the surface of the liquid is not enough to keep the molecules pretty close together, the molecules will be free to separate and roam around as a gas or vapor. The vapor pressure is dependent upon the temperature of the liquid. Higher the temperature, higher will be the vapor pressure.
Mechanism of Cavitation
The phenomenon of cavitation is a stepwise process as shown in Figure 2.
Figure 2: Phenomenon of Cavitation
Step One, Formation of bubbles inside the liquid being pumped.
The bubbles form inside the liquid when it vaporises i.e. phase change from liquid to vapor. But how does vaporization of the liquid occur during a pumping operation?
Vaporization of any liquid inside a closed container can occur if either pressure on the liquid surface decreases such that it becomes equal to or less than the liquid vapor pressure at the operating temperature, or the temperature of the liquid rises, raising the vapor pressure such that it becomes equal to or greater than the operating pressure at the liquid surface. For example, if water at room temperature (about 77 °F) is kept in a closed container and the system pressure is reduced to its vapor pressure (about 0.52 psia), the water quickly changes to a vapor. Also, if the operating pressure is to remain constant at about 0.52 psia and the temperature is allowed to rise above 77 °F, then the water quickly changes to a vapor.
Just like in a closed container, vaporization of the liquid can occur in centrifugal pumps when the local static pressure reduces below that of the vapor pressure of the liquid at the pumping temperature.
NOTE: The vaporisation accomplished by addition of heat or the reduction of static pressure without dynamic action of the liquid is excluded from the definition of cavitation. For the purposes of this article, only pressure variations that cause cavitation shall be explored. Temperature changes must be considered only when dealing with systems that introduce or remove heat from the fluid being pumped.
To understand vaporization, two important points to remember are:
1. We consider only the static pressure and not the total pressure when determining if the system pressure is less than or greater than the liquid vapor pressure. The total pressure is the sum of the static pressure and dynamic pressure (due to velocity).
2. The terms pressure and head have different meanings and they should not be confused. As a convention in this article, the term “pressure” shall be used to understand the concept of cavitation whereas the term “head” shall be used in equations.
Thus, the key concept is - vapor bubbles form due to vaporization of the liquid being pumped when the local static pressure at any point inside the pump becomes equal to or less than the vapor pressure of the liquid at the pumping temperature.
How does pressure reduction occur in a pump system?
The reduction in local static pressure at any point inside the pump can occur under two conditions:
1. The actual pressure drop in the external suction system is greater than that considered during design. As a result, the pressure available at pump suction is not sufficiently high enough to overcome the design pressure drop inside the pump.
2. The actual pressure drop inside the pump is greater than that considered during the pump design.
The mechanism of pressure reduction in the external and internal suction system of a pump system is explored next.
· Pressure reduction in the external suction system of the pump
A simple sketch of a pump ‘external suction system’ is shown in Figure 3.
Figure 3: External Suction System
Nomenclature used for Figure 3
r - Liquid density in lbm / ft3
G - Acceleration due to gravity in ft / s2
Psn - p refers to local static pressure (absolute). Subscript s refers to suction and subscript n refers to the point of measurement. The pressure at any point can be converted to the head term by division with the factor - r g
ps1 - Static pressure (absolute) of the suction vessel in psia
hps1 - Static pressure head i.e. absolute static pressure on the liquid surface in the suction vessel, converted to feet of head (ps1/ r g/gc). If the system is open, hps1 equals the atmospheric pressure head.
vs1 - Liquid velocity on the surface in the suction vessel in ft/s
hvs1 - Velocity head i.e. the energy of a liquid as a result of its motion at some velocity ‘vs1’. (v2s1 / 2g). It is the equivalent head in feet through which the liquid would have to fall to acquire the same velocity, or the head necessary to accelerate the liquid to velocity vs1. In a large suction vessel, the velocity head is practically zero and is typically ignored in calculations.
hs - Static suction head. . . . i.e. head resulting from elevation of the liquid relative to the pump centerline. If the liquid level is above pump centerline, hS is positive. If the liquid level is below pump centerline, hS is negative. A negative hS condition is commonly referred to as “suction lift”.
hfs - Friction head i.e. the head required to overcome the resistance to flow in the pipe, valves and fittings between points A and B, inclusive of the entrance losses at the point of connection of suction piping to the suction vessel (point A in Figure 1). The friction head is dependent upon the size, condition and type of pipe, number and type of fittings, valves, flow rate and the nature of the liquid. The friction head varies as the square of the average velocity of the flowing fluid.
ps2 - Absolute static pressure at the suction flange in psia
hps2 - Static pressure head at the suction flange i.e. absolute pressure of the liquid at the suction flange, converted to feet of head - ps2 / r g/gc
vs2 - Velocity of the moving liquid at the suction flange in ft/s. The pump suction piping is sized such that the velocity at the suction remains low.
hvs2 - Velocity head at suction flange i.e. the energy of a liquid as a result of its motion at average velocity ‘vs2’ equal to v2s2 / 2g.
pv - Absolute vapor pressure of the liquid at operating temperature in psia.
hpv - Vapor Pressure head i.e. absolute vapor pressure converted to feet of head (pv / r. g/gc).
Hs - Total Suction Head available at the suction flange in ft.
Note: As pressure is measured in absolute, total head is also in absolute.
The Pitot tube has two tubes:
1. Static tube (b): The opening of the static tube is parallel to the direction of flow. It measures the static pressure, since there is no velocity component perpendicular to its opening.
2. Impact tube (a): The opening of the impact tube is perpendicular to the flow direction. The point at the entrance of the impact tube is called as the stagnation point .At this point the kinetic energy of the fluid is converted to the potential energy. Thus, the impact tube measures the total pressure (also referred to as stagnation pressure) i.e. both static pressure and dynamic pressure (also referred to as impact pressure).
The two tubes are connected to the legs of a manometer or equivalent device for measuring pressure.
The relation between ps, pd and pt can be derived by applying a simple energy balance.
As mentioned earlier, in the case of a fluid or gas the potential energy is represented by the static pressure and the kinetic energy by dynamic pressure. The kinetic energy is a function of the motion of the fluid, and of course it's mass. It is generally more convenient to use the density of the fluid (r) as the mass representation.
\ K.E = pd = ½ m v2 = ½ r v2
The corresponding pressure balance equation is
In place of the pressure terms as used above, it is more appropriate to speak of the energy during pumping as the energy per unit weight of the liquid pumped and the units of energy expressed this way are foot-pounds per pound (Newton-meters per Newton) or just feet (meters) i.e. the units of head. Thus the energy of the liquid at a given point in flow stream can be expressed in terms of head of liquid in feet.
The pressure term can be converted to head term by division with the factor ‘r g’. For unit inter-conversions the factor ‘r g/gc’ should be used in place of ‘r g’.
Static pressure head
The head corresponding to the static pressure is called as the static pressure head.
Static pressure head = ps / r g
Velocity head
The head corresponding to dynamic pressure is called the velocity head.
Velocity head = pd / r g = (r v2 / 2) / r g = v2/2g
From the reading hm, of the manometer velocity of flow can be calculated and thus velocity head can be calculated. The pressure difference, dP (pt – ps) indicated by the manometer is the dynamic pressure.
dP = hm (rm - r) g = r v2 /2
Velocity head = dP / r g = hm (rm - r) /r
Vapor pressure, pv
Vapor pressure is the pressure required to keep a liquid in a liquid state. If the pressure applied to the surface of the liquid is not enough to keep the molecules pretty close together, the molecules will be free to separate and roam around as a gas or vapor. The vapor pressure is dependent upon the temperature of the liquid. Higher the temperature, higher will be the vapor pressure.
Mechanism of Cavitation
The phenomenon of cavitation is a stepwise process as shown in Figure 2.
Figure 2: Phenomenon of Cavitation
Step One, Formation of bubbles inside the liquid being pumped.
The bubbles form inside the liquid when it vaporises i.e. phase change from liquid to vapor. But how does vaporization of the liquid occur during a pumping operation?
Vaporization of any liquid inside a closed container can occur if either pressure on the liquid surface decreases such that it becomes equal to or less than the liquid vapor pressure at the operating temperature, or the temperature of the liquid rises, raising the vapor pressure such that it becomes equal to or greater than the operating pressure at the liquid surface. For example, if water at room temperature (about 77 °F) is kept in a closed container and the system pressure is reduced to its vapor pressure (about 0.52 psia), the water quickly changes to a vapor. Also, if the operating pressure is to remain constant at about 0.52 psia and the temperature is allowed to rise above 77 °F, then the water quickly changes to a vapor.
Just like in a closed container, vaporization of the liquid can occur in centrifugal pumps when the local static pressure reduces below that of the vapor pressure of the liquid at the pumping temperature.
NOTE: The vaporisation accomplished by addition of heat or the reduction of static pressure without dynamic action of the liquid is excluded from the definition of cavitation. For the purposes of this article, only pressure variations that cause cavitation shall be explored. Temperature changes must be considered only when dealing with systems that introduce or remove heat from the fluid being pumped.
To understand vaporization, two important points to remember are:
1. We consider only the static pressure and not the total pressure when determining if the system pressure is less than or greater than the liquid vapor pressure. The total pressure is the sum of the static pressure and dynamic pressure (due to velocity).
2. The terms pressure and head have different meanings and they should not be confused. As a convention in this article, the term “pressure” shall be used to understand the concept of cavitation whereas the term “head” shall be used in equations.
Thus, the key concept is - vapor bubbles form due to vaporization of the liquid being pumped when the local static pressure at any point inside the pump becomes equal to or less than the vapor pressure of the liquid at the pumping temperature.
How does pressure reduction occur in a pump system?
The reduction in local static pressure at any point inside the pump can occur under two conditions:
1. The actual pressure drop in the external suction system is greater than that considered during design. As a result, the pressure available at pump suction is not sufficiently high enough to overcome the design pressure drop inside the pump.
2. The actual pressure drop inside the pump is greater than that considered during the pump design.
The mechanism of pressure reduction in the external and internal suction system of a pump system is explored next.
· Pressure reduction in the external suction system of the pump
A simple sketch of a pump ‘external suction system’ is shown in Figure 3.
Figure 3: External Suction System
Nomenclature used for Figure 3
r - Liquid density in lbm / ft3
G - Acceleration due to gravity in ft / s2
Psn - p refers to local static pressure (absolute). Subscript s refers to suction and subscript n refers to the point of measurement. The pressure at any point can be converted to the head term by division with the factor - r g
ps1 - Static pressure (absolute) of the suction vessel in psia
hps1 - Static pressure head i.e. absolute static pressure on the liquid surface in the suction vessel, converted to feet of head (ps1/ r g/gc). If the system is open, hps1 equals the atmospheric pressure head.
vs1 - Liquid velocity on the surface in the suction vessel in ft/s
hvs1 - Velocity head i.e. the energy of a liquid as a result of its motion at some velocity ‘vs1’. (v2s1 / 2g). It is the equivalent head in feet through which the liquid would have to fall to acquire the same velocity, or the head necessary to accelerate the liquid to velocity vs1. In a large suction vessel, the velocity head is practically zero and is typically ignored in calculations.
hs - Static suction head. . . . i.e. head resulting from elevation of the liquid relative to the pump centerline. If the liquid level is above pump centerline, hS is positive. If the liquid level is below pump centerline, hS is negative. A negative hS condition is commonly referred to as “suction lift”.
hfs - Friction head i.e. the head required to overcome the resistance to flow in the pipe, valves and fittings between points A and B, inclusive of the entrance losses at the point of connection of suction piping to the suction vessel (point A in Figure 1). The friction head is dependent upon the size, condition and type of pipe, number and type of fittings, valves, flow rate and the nature of the liquid. The friction head varies as the square of the average velocity of the flowing fluid.
ps2 - Absolute static pressure at the suction flange in psia
hps2 - Static pressure head at the suction flange i.e. absolute pressure of the liquid at the suction flange, converted to feet of head - ps2 / r g/gc
vs2 - Velocity of the moving liquid at the suction flange in ft/s. The pump suction piping is sized such that the velocity at the suction remains low.
hvs2 - Velocity head at suction flange i.e. the energy of a liquid as a result of its motion at average velocity ‘vs2’ equal to v2s2 / 2g.
pv - Absolute vapor pressure of the liquid at operating temperature in psia.
hpv - Vapor Pressure head i.e. absolute vapor pressure converted to feet of head (pv / r. g/gc).
Hs - Total Suction Head available at the suction flange in ft.
Note: As pressure is measured in absolute, total head is also in absolute.
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thanks very much for any help. you really put some time into writing this post. i'm sure everyone really appreciate it, as i do.
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