Moving fluids plays a major role in the process of a plant. Liquid can only move on its own power, and then only from top to bottom or from a high pressure to a lower pressure system. This means that energy to the liquid must be added, to moving the liquid from a low to a higher level.
To add the required energy to liquids, pumps are used. There are many different definitions of the name PUMP but this is best described as:
Pump types generally fall into two main categories - Rotodynamic and Positive Displacement, of which there are many forms.
The Rotodynamic pump transfers rotating mechanical energy into kinetic energy in the form of fluid velocity and pressure. The Centrifugal and Liquid Ring pumps are types of rotodynamic pump, which utilise centrifugal force to transfer the fluid being pumped.
The Rotary Lobe pump is a type of positive displacement pump, which directly displaces the pumped fluid from pump inlet to outlet in discrete volumes.
In order to select a pump two types of data are required:
Different fluids have varying characteristics and are usually pumped under different conditions. It is therefore very important to know all relevant product and performance data before selecting a pump.
The science of fluid flow is termed 'Rheology' and one of its most important aspects is viscosity which is defined below.
The viscosity of a fluid can be regarded as a measure of how resistive the fluid is to flow, it is comparable to the friction of solid bodies and causes a retarding force. This retarding force transforms the kinetic energy of the fluid into thermal energy.
The ease with which a fluid pours is an indication of its viscosity. For example, cold oil has a high viscosity and pours very slowly, whereas water has a relatively low viscosity and pours quite readily. High viscosity fluids require greater shearing forces than low viscosity fluids at a given shear rate. It follows therefore that viscosity affects the magnitude of energy loss in a flowing fluid.
Two basic viscosity parameters are commonly used, absolute (or dynamic) viscosity and kinematic viscosity.
Absolute (or Dynamic) Viscosity
This is a measure of how resistive the flow of a fluid is between two layers of fluid in motion. A value can be obtained directly from a rotational viscometer which measures the force needed to rotate a spindle in the fluid.
This is a measure of how resistive the flow of a fluid is under the influence of gravity. Kinematic viscometers usually use the force of gravity to cause the fluid to flow through a calibrated orifice, while timing its flow.
Viscosity Variation with Temperature
Temperature can have a significant effect on viscosity and a viscosity figure given for pump selection purposes without fluid temperature is often meaningless - viscosity should always be quoted at the pumping temperature. Generally viscosity falls with increasing temperature and more significantly, it increases with falling temperature. In a pumping system it can be advantageous to increase the temperature of a highly viscous fluid to ease flow.
In some fluids the viscosity is constant regardless of the shear forces applied to the layers of fluid. These fluids are named Newtonian fluids. At a constant temperature the viscosity is constant with change in shear rate or agitation.
Typical fluids are: Water, Beer, Hydrocarbons, Milk, Mineral Oils, Resins and Syrups.
Most empirical and test data for pumps and piping systems has been developed using Newtonian fluids across a wide range of viscosities. However, there are many fluids which do not follow this linear law, these fluids are named Non-Newtonian fluids.
When working with Non-Newtonian fluids we use Effective Viscosity to represent the viscous characteristics of the fluid as though it was newtonian at that given set of conditions (shear rate, temperature). This effective viscosity is then used in calculations, charts, graphs and handbook information.
Types of Non-Newtonian Fluids
There are a number of different type of non-newtonian fluids each with different characteristics. Effective viscosity at set conditions will be different depending on the fluid being pumped. This can be better understood by looking at the behaviour of viscous fluids with changes in shear rate as follows.
Viscosity decreases as shear rate increases, but initial viscosity may be so high as to prevent start of flow in a normal pumping system.
Viscosity increases as shear rate increases.
Viscosity decreases with time under shear conditions. After shear ceases the viscosity will return to its original value - the time for recovery will vary with different fluids.
Viscosity increases with time under shear conditions. After shear ceases the viscosity will return to its original value - the time for recovery will vary with different fluids. As the name suggests anti-thixotropic fluids have opposite rheological characteristics to thixotropic fluids.
Viscosity decreases with time under shear conditions but does not recover. Fluid structure is irreversibly destroyed.
Need a certain applied force (or yield stress) to overcome 'solid-like structure', before flowing like a fluid.
The density of a fluid is its mass per unit of volume.
The specific weight of a fluid is its weight per unit volume.
The specific gravity of a fluid is the ratio of its density to the density of water. As this is a ratio, it does not have any units of measure.
The temperature of the fluid at the pump inlet is usually of most concern as vapour pressure can have a significant effect on pump performance. Other fluid properties such as viscosity and density can also be affected by temperature changes. Thus a cooling of the product in the discharge line could have a significant effect on the pumping of a fluid. The temperature of a fluid can also have a significant affect on the selection of any elastomeric materials used.
When considering a fluid flowing in a pipework system it is important to be able to determine the type of flow. Under some conditions the fluid will appear to flow as layers in a smooth and regular manner. This can be illustrated by opening a water tap slowly until the flow is smooth and steady. This type of flow is called laminar flow. If the water tap is opened wider, allowing the velocity of flow to increase, a point will be reached whereby the stream of water is no longer smooth and regular, but appears to be moving in a chaotic manner. This type of flow is called turbulent flow. The type of flow is indicated by the Reynolds number.
Velocity is the distance a fluid moves per unit of time
Fluid velocity can be of great importance especially when pumping slurries and fluids containing solids. In these instances, a certain velocity may be required to prevent solids from settling in the pipework, which could result in blockages and changes in system pressure as the actual internal diameter of the pipe is effectively decreased, which could impact on pump performance.
This is sometimes known as streamline, viscous or steady flow. The fluid moves through the pipe in concentric layers with the maximum velocity in the centre of the pipe, decreasing to zero at the pipe wall. The velocity profile is parabolic, the gradient of which depends upon the viscosity of the fluid for a set flow-rate.
This is sometimes known as unsteady flow with considerable mixing taking place across the pipe cross section. The velocity profile is more flattened than in laminar flow but remains fairly constant across the section as shown in fig. 2.1.7b. Turbulent flow generally appears at relatively high velocities and/or relatively low viscosities.
Between laminar and turbulent flow there is an area referred to as transitional flow where conditions are unstable and have a blend of each characteristic.
Fluids will evaporate unless prevented from doing so by external pressure. The vapour pressure of a fluid is the pressure (at a giventemperature) at which a fluid will change to a vapour and is expressed as absolute pressure. Each fluid has its own vapour pressure/temperature relationship. In pump sizing, vapour pressure can be a key factor in checking the Net Positive Suction Head (NPSH) available from the system.
Fluids Containing Solids
It is important to know if a fluid contains any particulate matter and if so, the size and concentration. Special attention should be given regarding any abrasive solids with respect to pump type and construction, operating speed and shaft seals.
Size of solids is also important, as when pumping large particles the pump inlet should be large enough for solids to enter the pump without 'bridging' the pump inlet. Also the pump should be sized so the cavity created in the pump chamber by the pump elements is of sufficient size to allow satisfactory pump operation.
Concentration is normally expressed as a percentage by weight (W/W) or volume (V/V) or a combination of both weight and volume (W/V).
Capacity (Flow Rate)
The capacity (or flow rate) is the volume of fluid or mass that passes a certain area per time unit. This is usually a known value dependent on the actual process. For fluids the most common units of capacity are litres per hour.
Pressure is defined as force per unit area: P = F A where F is the force perpendicular to a surface and A is the area of the surface. In the SI system the standard unit of force is the Newton (N) and area is given in square metres (m2). Pressure is expressed in units of Newtons per square metre (N/m2). This derived unit is called the Pascal (Pa).
Different Types of Pressure
For calculations involving fluid pressures, the measurements must be relative to some reference pressure. Normally the reference is that of the atmosphere and the resulting measured pressure is called gauge pressure. Pressure measured relative to a perfect vacuum is called "absolute pressure".
The actual magnitude of the atmospheric pressure varies with location and with climatic conditions. The range of normal variation of atmospheric pressure near the earth's surface is approximately 0.95 to 1.05 bar absolute (bar a). At sea level the standard atmospheric pressure is 1.013 bar.
Using atmospheric pressure as a zero reference, gauge pressure is the pressure within the gauge that exceeds the surrounding atmospheric pressure. It is a measure of the force per unit area exerted by a fluid, commonly indicated in units of barg (bar gauge).
Is the total pressure exerted by a fluid. It equals atmospheric pressure plus gauge pressure, indicated in units of bar a (bar absolute).
Absolute Pressure = Gauge Pressure + Atmospheric Pressure
This is a commonly used term to describe pressure in a pumping system below normal atmospheric pressure. This is a measure of the difference between the measured pressure and atmospheric pressure expressed in units of mercury (Hg).
Inlet (Suction) Pressure
This is the pressure at which the fluid is entering the pump. The reading should be taken whilst the pump is running and as close to the pump inlet as possible.
Outlet (Discharge) Pressure
This is the pressure at which the fluid leaves the pump. Again this reading should be taken whilst the pump is running and as close to the pump outlet as possible.
This is the difference between the inlet and outlet pressures. For inlet pressures above atmospheric pressure the differential pressure is obtained by subtracting the inlet pressure from the outlet pressure. For inlet pressures below atmospheric pressure the differential pressure is obtained by adding the inlet pressure to the outlet pressure. It is therefore the total pressure reading and is the pressure against which the pump will have to operate.
Power requirements are to be calculated on the basis of differential pressure.
The Relationship Between Pressure and Elevation
In a static fluid (a body of fluid at rest) the pressure difference between any two points is in direct proportion only to the vertical distance between the points. The same vertical height will give the same pressure regardless of the pipe configuration in between.
This term is generally used to describe a positive inlet pressure/head, whereby fluid will readily flow into the pump inlet at sufficient pressure to avoid cavitation
The static head is a difference in fluid levels.
Static Suction Head
This is the difference in height between the fluid level and the centre line of the pump inlet on the inlet side of the pump.
Static Discharge Head
This is the difference in height between the fluid level and the centre line of the pump inlet on the discharge side of the pump.
Total Static Head
The total static head of a system is the difference in height between the static discharge head and the static suction head.
This is the pressure drop on both inlet and discharge sides of the pump due to frictional losses in fluid flow.
This is the energy required to set the fluid in motion and to overcome any resistance to that motion.
Total Suction Head
The total suction head is the static suction head less the dynamic head. Where the static head is negative, or where the dynamic head is greater than the static head, this implies the fluid level will be below the centre line of the pump inlet (ie suction lift).
Total Discharge Head
The total discharge head is the sum of the static discharge and dynamic heads.
Total head is the total pressure difference between the total discharge head and the total suction head of the pump.The head is often a known value. It can be calculated by means of different formulas if the installation conditions are specified.
Manufacturers of processing equipment, Heat Exchangers, static mixers etc, usually have data available for pressure drop. These losses are affected by fluid velocity, viscosity, tube diameter, internal surface finish of tube and tube length.
The different losses and consequently the total pressure drop in the process are, if necessary, determined in practice by converting the losses into equivalent straight length of tube which can then be used in subsequent system calculations.
The pressure drop through the tubes, Valves and fittings is determined as equivalent tube length, so that the total pressure drop can be calculated.
Friction Loss Calculations
Since laminar flow is uniform and predictable it is the only flow regime in which the friction losses can be calculated using purely mathematical equations. In the case of turbulent flow, mathematical equations are used, but these are multiplied by a co-efficient that is normally determined by experimental methods. This co-efficient is known as the Darcy friction factor (fD).
The Miller equation given below can be used to determine the friction losses for both laminar and turbulent flow in a given length of pipe (L). The friction losses in a pipework system are dependent upon the type of flow characteristic that is taking place. The Reynolds number (Re) is used to determine the flow characteristic.
The relative roughness of pipes varies with diameter, type of material used and age of the pipe. It is usual to simplify this by using an relative roughness (k) of 0.045 mm, which is the absolute roughness of clean commercial steel or wrought iron pipes as given by Moody.
The term cavitation is derived from the word cavity, meaning a hollow space.
Cavitation is an undesirable vacuous space in the inlet port of the pump normally occupied by fluid. The lowest pressure point in a pump occurs at the pump inlet - due to local pressure reduction part of the fluid may evaporate generating small vapour bubbles. These bubbles are carried along by the fluid and implode instantly when they get into areas of higher pressure.
If cavitation occurs this will result in loss of pump efficiency and noisy operation. The life of a pump can be shortened through mechanical damage, increased corrosion and erosion when cavitation is present. When sizing pumps on highly viscous fluids care must be taken not to select too higher pump speed so as to allow sufficient fluid to enter the pump and ensure satisfactory operation.
For all pump application problems, cavitation is the most commonly encountered. It occurs with all types of pumps, centrifugal, rotary or reciprocating. When found, excessive pump speed and/or adverse suction conditions will probably be the cause and reducing pump speed and/or rectifying the suction condition will usually eliminate this problem.
Cavitation should be avoided at all costs.
Net Positive Suction Head (NPSH)
In addition to the total head, capacity, power and efficiency requirements, the condition at the inlet of a pump is critical. The system on the inlet side of the pump must allow a smooth flow of fluid to enter the pump at a sufficiently high pressure to avoid cavitation. This is called the Net Positive Suction Head, generally abbreviated NPSH.
Pump manufacturers supply data about the net positive suction head required by their pumps (NPSHr) for satisfactory operation. When selecting a pump it is critical the net positive suction head available (NPSHa) in the system is greater than the net positive suction head required by the pump.
NPSHa is also referred to as N.I.P.A. (Net Inlet Pressure Available) and NPSHr is also referred to as N.I.P.R. (Net Inlet Pressure Required). A simplified way to look at NPSHa or N.I.P.A. is to imagine a balance of factors working for (static pressure and positive head) and against (friction loss and vapour pressure) the pump.
Providing the factors acting for the pump outweigh those factors acting against, there will be a positive suction pressure.
The value of NPSHa or N.I.P.A. in the system is dependent upon the characteristic of the fluid being pumped, inlet piping, the location of the suction vessel, and the pressure applied to the fluid in the suction vessel. This is the actual pressure seen at the pump inlet. It is important to note, it is the inlet system that sets the inlet condition and not the pump.
It is important the units used for calculating NPSHa or N.I.P.A. are consistent i.e. the total figures should be in m or ft.
For low temperature applications the vapour pressure is generally not critical and can be assumed to be negligible.
Suggestions for avoiding cavitation:
Pressure "Shocks" (Water Hammer)
The term 'shock' is not strictly correct as shock waves only exist in gases. The pressure shock is really a pressure wave with a velocity of propagation much higher than the velocity of the flow, often up to 1400 m/s for steel tubes. Pressure waves are the result of rapid changes in the velocity of the fluid in especially in long runs of piping.
The following causes changes in fluid velocity:
The major pressure wave problems in process plants are usually due to rapidly closed or opened Valves. Pumps, which are rapidly / frequently started or stopped, can also cause some problems.
When designing pipework systems it is important to keep the natural frequency of the system as high as possible by using rigid pipework and as many pipework supports as possible, thereby avoiding the excitation frequency of the pump.
Effects of pressure waves:
Velocity of propagation
The velocity of propagation of the pressure wave depends on:
When for example, a Valve is closed, the pressure wave travels from the Valve to the end of the tube. The wave is then reflected back to the Valve. These reflections are in theory continuing but in practice the wave gradually attenuates cancelled by friction in the tube.
A pressure wave as a result of a pump stopping is more damaging than for a pump starting due to the large change in pressure which will continue much longer after a pump is stopped compared to a pump starting. This is due to the low fluid velocity which results in a relatively small damping of the pressure waves.
A pressure wave induced as a result of a pump stopping can result in negative pressure values in long tubes, i.e. values close to the absolute zero point which can result in cavitation if the absolute pressure drops to the vapour pressure of the fluid.
Pressure waves are caused by changes in the velocity of the liquid in especially long runs of tube. Rapid changes in the operating conditions of Valves and pump are the major reasons to the pressure waves and therefore, it is important to reduce the speed of these changes.
There are different ways to avoid or reduce pressure waves which are briefly described below.
Correct flow direction
Incorrect flow direction through Valves can induce pressure waves particularly as the Valve functions. With air-operated seat Valves incorrect direction of flow can cause the Valve plug to close rapidly against the Valve seat inducing pressure waves.
Correct flow directions in the process plant can reduce or even prevent pressure wave problems.
Damping of Valves
The pressure wave induced by a seat Valve can be avoided or minimised by damping the movement of the Valve plug. The damping is carried out by means of a special damper.
Speed control of pumps
Speed control of a pump is a very efficient way to minimise or prevent pressure waves. The motor is controlled by means of a soft starter or a frequency converter so that the pump is:
The risk of power failure should be taken into consideration when using speed control against pressure waves.
Equipment for industrial processes
There is various equipment available to reduce pressure waves such as:
These however, may not be suitable for hygienic processes and further advice may be required before they are recommended or used in such installations.