The rotating impeller of a centrifugal pump creates a suction at the pump inlet to draw in fluid, which is moved to the pump outlet by the impeller, where it is discharged. Positive displacement pumps have a constant torque characteristic, whereas centrifugal pumps demonstrate variable torque characteristics. What is positive displacement pumps types and advantages. Positive displacement pump is a collective term for all pumps operating according to the positive displacement principle. A positive displacement pump delivers a constant volume of liquid for each cycle, independent of discharge pressure or head. What are the main features and benefits of a positive displacement pump. A positive displacement pd pump moves a fluid by repeatedly enclosing a fixed volume and moving it mechanically through the system. Positive displacement pumps are most suitable where an accurate quantity of water must be supplied, and where the pressure at which the pump must deliver.
Chapter 22 pumps, fans, blowers, and compressors keith marchildon keith marchildon chemical process design, inc. Positive displacement pumps an overview sciencedirect topics. Beyond viscosity, other fluid characteristics relate to positive displacement pump selection and operation. Positive displacement pumps positive displacement pumps, in contrast, have very little slips, are selfpriming and pump against very high pressures, but their volumetric capacity is low. This is a special configuration of a positive displacement pump. Irrigation pumps, in general, are driven either by engines or electric motors. A positive displacement pump delivers a constant volume of liquid for each cycle independent of discharge pressure or head. Comparing 4 types of pd pumps the printed version of this article appeared as, selecting positive displacement pumps. A positive displacement pump is a machine where chambers fill suction and empty discharge each cycle of operation. Positive displacement pumps and their different types. This basic difference is evident in the pumps response to a systems.
Differences between the centrifugal pump and positive. Such types of pumps are basically designed for higher pressure application for example hydraulic fluid power application. Kingston, ontario david mody fluor canada kingston, ontario 1 introduction to fluid movers 717 2 liquid moverspumps 718 2. Positive displacement pumps with positive displacement pumps the flow rate q is linear dependent on the pump speed n. On a test stand the flow rate is determined for various speeds and total head i. Positive displacement an overview sciencedirect topics. Positive displacement pump characteristic curve engineers edge. A vendor manual for a specific pump usually contains a curve of pump flow rate versus pump. These pumps are also referred to as volumetric pumps as they pump the fluid handled in an enclosed volume. The low pulsation rate and gentle performance of this rootstype positive displacement pump is achieved due to a combination of its two 90 helical twisted. The volume is a constant given each cycle of operation. Lobe contact is prevented by external timing gears.
Technical requirements, performance limitations and terminology are presented to assist the person specifying a pd pump. Positive displacement pumps, unlike centrifugal or rotodynamic pumps, theoretically can produce the same flow at a given speed rpm no matter what the discharge pressure. The two main types of pumps are the centrifugal pump and the positive displacement pump. Before selecting the best pump for the job, consider the following. A positive displacement pump operates by alternating filling a cavity and then displacing a given volume of liquid.
What is positive displacement pumps types and advantages positive displacement pumps positive displacement pumps, in contrast, have very little slips, are selfpriming and pump against very high pressures, but their volumetric capacity is low. Positive displacement pumps properties of a pd pump. These pumps generally tend to be larger than equalcapacity dynamic pumps. The choice of pump for a given application depends largely on how the pump headflow characteristics match the requirement of the system downstream of the pump.
They share many similarities but the pumps are fundamentally different in design. A positive displacement pump operating against closed discharge valves continues to produce flow until the pressure in the discharge. A positive displacement pumps is a constant flow machine a positive displacement pump must never operate against closed valves on the discharge side of the pump it has no shutoff head like centrifugal pumps. Guide to reading a positive displacement pump curve pumps. In positive displacement pumps the energy is transferred to the fluid.
The piston displacement element forms a space with variable volume together with cylinder and cylinder cover. A centrifugal pump converts driver energy to kinetic energy in a liquid by accelerating the fluid to. Positive displacement pumps features and a positive displace. Positive displacement pumps operate by forcing a fixed volume of fluid from the inlet pressure section of the pump into the discharge zone of the pump. The 70u positive displacement double pump provides a compact design and low shear noise characteristics.
The pd pump range of operating conditions is presented in several. Among the first key elements is a relaunch of the centrifugalpump elearning course and the development of a new positivedisplacementpump course covering fundamentals, design and applications. Generally, we express density in units of pounds per cubic inch. Basically, the following four principles are involved in pumping water. Positive displacement pumps are generally used for specialist applications such as for pumping viscous liquids or liquids that contain suspended or fragile solids. Fundamentals of positive displacement pumps plant services. When using a positive displacement pump care needs to be taken to ensure the flow is not restricted causing pressures in excess of the pump. Check valves in the suction and discharge ports allow flow in only one direction. W2 t2 figure 1 net positive head availability for liquid pump. The metal vane pumps have characteristics similar to the gear pumps described above, but can be supplied with a method of varying the flow rate externally. The positive displacement pump delivers a constant volume of liquid against varying discharge pressure or head. Positive displacement pumps move fluids with pistons, gears or vanes and flowrate is a function of rotational speed and has little if any dependence on the pressure rise. For instance, the positive displacement pd pump provides a continuous flow more or less regardless of pressure, whereas the centrifugal pump pressure depends on the changing flow.
Thus, positive displacement pumps are constant flow machines. A person without significant exposure to pump selection is often experienced only with centrifugal pump curves. Now, pdps is a form of viking pump which can be further categorised into three. Apr 05, 2017 the characteristic curves of positive displacement pumps, 342017 lutfi alsharif. Positive displacement pumps pump rotodynamic positive displacement centrifugal rotary reciprocating axial flow mixed flow gear piston lobe diaphragm turbine sliding vane screw plunger blow case pump 32.
Centrifugal and positive displacement pumps cedengineering. Best practices in selecting and applying positive displacement. Positive displacement pumps the positive displacement pump operates by alternating of filling a cavity and then displacing a given volume of liquid. A common application of a positive displacement pump is that used to supply high pressure oil for hydraulic actuators such. Positive displacement pumps have a very close clearance between rotating and stationary parts and hence are selfpriming. For a newtonian fluid, the shear rate varies linerally from a maximum at the tube wall to zero at the center. A positive displacement pump delivers a constant volume of liquid for each cycle independent of discharge pressure. Positive displacement pumps features and a positive. Specific gravity specific gravity is used to compare the density of a product to the density of water. Piston compressors deliver compressible media such as gas or air. These pumps are typically not capable of such a high flow rate as say, a centifrugal pump, but they are capable of producing much higher pressures.
Another important distinction between these two sanitary process components is the effect viscosity. The characteristic curves of positive displacement pumps, 34. One such property is vapor pressure, the absolute pressure at which a liquid changes to a gas figure 4. This type of pump delivers a near constant flow rate regardless of the system pressure. These pumps generally tend to be larger than equal. Positive displacement pumps draw fluid into a compartment at the inlet and move it to an outlet for discharge, most typically using a rotary, reciprocating, or diaphragm method to move fluid. Only one of those pump technologies can achieve certain critical objectives for reliably pumping hydrocarbons with highly variable fluid properties and under varying operating conditions. Dec 07, 20 positive displacement pumps pump rotodynamic positive displacement centrifugal rotary reciprocating axial flow mixed flow gear piston lobe diaphragm turbine sliding vane screw plunger blow case pump 32. Therefore, the only factor that effects flow rate in an ideal positive displacement pump is the speed at which it operates. The universal pump design combines the innovation of our universeal shaft seal, the rugged simplicity of haights proven gear within a gear internal gear configuration, with the flexibility of a bolton, integral relief valve assembly. The positive displacement principle applies whether the pump is a. Positive displacement pumps an overview sciencedirect. Vanes that fit closely in rotor slots slide in and out as the rotor turns.
Positivedisplacement pumps operate by forcing a fixed volume of fluid from the inlet pressure section of the pump into the discharge zone of the pump. Positive displacement pumps are generally more tolerant of suction lifts than centrifugal pumps, and all tend to produce pulsating flows to some extent though a progressive cavity pump is better in this respect than a piston pump for example. Haight pump offers the 70u diu double pump, available with a variety of optional seals, bearings, and rotor materials. Selection of a positive displacement pd rotary pump is not always an easy choice.
Piston compressors are positive displacement machines. Several types of displacement pumps are available all working following the principle that a fixed amount of fluid is encapsulated and pushed to the pressure side of the pump. Positive displacement pumps deliver a definite volume of liquid for each cycle of pump operation. Specifying a positive displacement pump without prior training is a difficult task. These and future courses will be hosted within his new educational portal. Pdf positive displacement pumps are critical to applications ranging from drug. During the discharge stroke, the piston moves to the right, seating the check valve in the suction line and opening the check valve in the discharge line. Therefore, it can be frustrating to get the data required from a positive displacement curve. Review different pump types commonly used understand the difference between centrifugal pumps and positive displacement pumps understand the working principle of the tuthill. What to look for when selecting a sanitary process pump component. Pump shaft support bearings are located in the timing gear case.
Pumps and pumping system bureau of energy efficiency 118 figure 6. The operating principles of positive displacement pd pumps differ from centrifugal pumps. Positive displacement pumps are another category of pumps. Most pd pumps can be adapted to handle a wide range of applications, but some types are better suited than. This can have an impact on the speed of your sanitary processes. A positive displacement pump draws fluid into some type of compartment at the pump inlet and discharges the fluid at the pump outlet. Progressive cavity pumps lend themselves well to broad ranges of moderate discharge pressures, flow rates less than 100 gpm, and slurry percentages. The characteristic curves of centrifugal pumps plot the course of the following parameters against flow rate q. Positive displacement pumps are self priming because there will be very close clearance between rotary and stationary components of pump. However, a slight increase in internal leakage as the pressure. Types of positive displacement pumps are reciprocating, metering, and rotary pumps. Guide to reading a positive displacement pump curve.
The fkl is a circumferential piston pump, meaning that its rotors run in a channel described by the pump housing and builtin internal hubs. Displacement bodies periodically increase and decrease the working volumes, thus transferring energy to the fluid handled. Flow versus pressure characteristics for centrifugal. Atmospheric pressure, centrifugal force, positive displacement and movement of columns. Figure 21 positive displacement pump characteristic curve. Types of positivedisplacement pumps are reciprocating, metering, and rotary pumps. The flow resistance of the system in which the pump is operating will not. Pumps fluid by varying the dimension of an inner chamber.
The positive displacement pump can be classified as. Useful information on positive displacement pumps what is a positive displacement pump. The characteristic curves of positive displacement pumps, 342017 lutfi alsharif. The nearly vertical pressure stiff performance curve of rotary positive displacement pd pumps stems from a basic principle.
Liquid flows into the pumps as the cavity on the suction side expands and the liquid flows out of the discharge as the cavity collapses. Positive displacement pd pump technologies vary, as do the curves for these pumps. Application for metering fluids example, chemicals into a process, etc. In a positive displacement pump, slippage is negligible compared to the pump s volumetric output flow. Operate by forcing a fixed volume of fluid from the inlet pressure section of the pump into the discharge zone of the pump. Pumps operate by some mechanism typically reciprocating or rotary, and consume energy to perform mechanical work moving the fluid. By looking at the performance chart to the r you can see just how different these pump are. Fristam positive displacement pumps the fkl and fl ii are the two positive displacement pumps discussed in the manual. Centrifugal and positive displacement pumps course no. There are two different types of pumps, positive displacement and centrifugal.
Positive displacement progressive cavitytype pumps are the most commonly used type of pump used for slurry services in both upstream and downstream operations. Positive displacement pumps have very steep characteristics. Sliding vane pumps operate quite differently from gear and lobe types. Pump headflow curves are typically given for clear water. The pumping action is cyclic and can be driven by pistons, screws, gears, rollers, diaphragms or vanes. The positive displacement pumps can be divided in two main classes. Lobe pumps are similar to external gear pumps in operation, except the pumping elements or lobes do not make contact. About positive displacement pumps hydraulic institute.
These rotary lobe machines are typically used for polymer powder or pellet transfer and come in different sizes. Definitions and terms density density is the mass of a substance per unit volume. Pumps can be classified into three major groups according to the method they use to move the fluid. The characteristic curves shape is primarily determined by the pump type i. Centrifugal pump characteristics 21 of 34 duration. The maximum head h max is usually limited by a pressure relief valve or safety valve. A centrifugal pump converts driver energy to kinetic energy in a liquid by accelerating the fluid to the outer rim of an impeller. Suitable for application pump speeds up toapi674 1995andhydraulicinstitutespeedlimitations. They add energy directly to a movable boundary, which imparts the energy to the fluid. Pdf modeling a variable speed drive for positive displacement pump. A pump is a device that moves fluids liquids or gases, or sometimes slurries, by mechanical action. The characteristic curves of positive displacement pumps. A rotor with radial slots, is positioned offcenter in a housing bore. If the output port were plugged, pressure would increase instantaneously to the point that the pump s pumping element or its case would fail probably explode, if the drive shaft did not break first, or the pump s prime mover would stall.
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