A guide to differences between positive displacement pumps and centrifugal. Hydraulic pumps and motors are almost exclusively positive displacement devices. However, some loss in output is typically caused by the fluid slipping past the rotors. The power needed to drive a reciprocating pump depends on the maximum pressure, pump capacity and mechanical efficiency of the pump. Differences between the centrifugal pump and positive. Controlling pumps with adjustable speed drives section 3. Positive displacement pump design and calculation tajilapak.
Positive displacement pumps they cover gear, screw and reciprocating pumps in different configurations that guarantee reliable and efficient pumping processes. Efficiency in positive displacement pumps, efficiency increases with increasing pressure. A reciprocating pump is a type of positive displacement pump that uses a piston, plunger or diaphragm to drive pressure into the pumped fluid. Some positive displacement pumps use an expanding cavity on on the suction side and a decreasing cavity on the suction side expands and the liquid flows out of the discharges side. Positivedisplacement pumps are capable of developing high pressures while operating at low suction pressures. The second is to help the user analyze their pumping requirements, make an appropriate pump selection and size the pump, motor and drive for best performance. Typical overall efficiencies for different types of hydraulic pumps are shown in the table 1. The reciprocating group operates via pistons, plungers or diaphragms while rotary pumps use gears, lobes, screws, vanes or peristaltic action. A diaphragm pump also known as a membrane pump is a positive displacement pump that uses a combination of the reciprocating action of a rubber, thermoplastic or teflon diaphragm and suitable valves on either side of the diaphragm check valve, butterfly valves, flap valves, or any other form of shutoff valves to pump a fluid there are three main types of diaphragm pumps. A reciprocating positive displacement pump works by the repeated backandforth movement strokes of either a piston, plunger or diaphragm figure 1. The details of these specifications, along with their significance to pump selection, are described in engineering360s pump flow page. The two main types of pumps are the centrifugal pump and the positive displacement pump.
Mechanical efficiency would be 91% 329 360 x 100 91%. Pumps can be classified into three major groups according to the method they use to move the fluid. Changing the system pressure or head may have a dramatic effect on the flow rate in a centrifugal pump. It is best tackled by considering flow rate and pressure head requirements, the viscosity stiffness, corrosive and abrasive characteristics of the liquid being pumped, and finally mechanical efficiency. Mechanical seals are precision designed and manufactured products, yet one of the most common causes of failure in positive displacement pump types is due to the malfunction of its mechanical seal. A positive displacement pump draws fluid into some type of compartment at the pump inlet and discharges the fluid at the pump outlet. Changing the system pressure or head has little or no effect on the flow rate in a positive displacement pump. The amount of slip in a pd pump is affected by the system discharge pressure requirement and the fluid viscosity. Many pump designs fall into the positive displacement pd category, but, for the most part, they can be divided into two basic groups.
You will notice in the flow rate chart how the centrifugal pump loses flow as the viscosity goes up but the positive displacement pumps flow actually increases. The volumetric efficiency of rotary positive displacement pumps. What is the normal efficiency of a reciprocating pump. Numerous sizes, hydraulics and material combinations are particularly well suited for use in applications where uncompromising reliability and efficiency over a wide flow range are key. Positive displacement pumps were developed long before centrifugal pumps. The performance and efficiency of hydraulic pumps and. Recently, has been taking on the topics of sustainability and new advancements. By looking at the efficiency chart figure 3 you can see the impact of pressure changes on the pumps efficiency.
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. By looking at the efficiency plot on chart b you can see the impact of pressure changes on the centrifugal pump s efficiency. Liquid is positively displaced from a fixedvolume container. Additional pump models are available that utilize different designs but the concept holds true for all of them. 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. Mar 07, 2018 making the allimportant choice between a centrifugal pump or a positive displacement pump is not always straightforward.
The main difference between these types of pumps and centrifugal is that positive displacement pumps will move fluid at the same speed regardless of the. Positive displacement vs centrifugal pumps guide when to. Pumps operate by some mechanism typically reciprocating or rotary, and consume energy to perform mechanical work moving the fluid. Non positive displacement pumps are basically used for low pressure applications and are designed to work for a maximum pressure of 18 to 20 bar. Percent positive centrifugal e efficiency head 40% 45% 50% 55% 60% 65% 70% 75% 80% 55 80 105 feet of head percent positive centrifugal the pumps behave very differently whe. When to use a positive displacement pump by rodger jacoby, viking pump inc. Making the allimportant choice between a centrifugal pump or a positive displacement pump is not always straightforward. Efficiency is also reduced when pumping a light hydrocarbon that has some degree of compressibility. A nonpositive displacement pump can slip, making it a good choice for any application that does not put high pressure on the pumps outlet. Whilst centrifugal pumps are the most common type of pump installed due to their simplicity, positive displacement pumps are a solution that can handle. The operating principles of positive displacement pd pumps differ. To make a good choice between these pump types it is important to understand that the two types of pumps behave very differently.
If the purpose is high flow rates to distances and alot of places like cooling pump in engines where high pressure is not required, then the centrifugal pump will be good. Positive displacement pump or centrifugal which one do you. 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. Useful information on positive displacement pumps what is a positive displacement pump. Rotary positive displacement rpd pumps theoretically displace a fixed amount of fluid per revolution. Choosing the wrong pump could result in process failure and a poor return on investment. 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. Whereas in non positive displacement pumps, efficiency peaks at best efficiency point. Selecting a positive displacement pump using performance. Whats the difference between centrifugal and positive.
Let v be the average velocity of the flow in the line, a its crosssectional area and. Whereas in nonpositive displacement pumps, efficiency peaks at bestefficiencypoint. Continuing with the above example, the overall efficiency of the pump is 0. They cover gear, screw and reciprocating pumps in different configurations that guarantee reliable and efficient pumping processes. Positive displacement pumps are usually selected for their ability to handle high viscosity fluids at high pressures and relatively low flows as their efficiency isnt affected by pressure. Positive displacement pumps perform across a range of process conditions. Oct 17, 2016 another major difference between the pump types is the effect viscosity has on the capacity of the pump. When selecting positive displacement pumps, there are a few key performance specifications to consider, namely flow rate, pressure, power, and efficiency. Anderson process is proud to partner with longtime pump manufacturers who are leading the industry in positive displacement pump technology. 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. Understanding a positive displacement pump motor desertfox mechanical 8. Anderson processs offerings of positive displacement pumps combine mechanical efficiency with steady flow rates to deliver fluid transfer capabilities that are both costeffective and consistent.
Improving pump efficiency dont design it wrong invest in a good predesign decide how it will operate before messing around with layouts and equipment selection pick pumps to operate efficiently operate at conditions where the pump will actually operate most of the time consider variable speed drives or smaller pumps to. But for those who need a brush up on pump basics, we thought it would be a good time to discuss the differences between centrifugal and positive displacement pumps. In a piston pump, the first stroke of the piston creates a vacuum, opens an inlet valve, closes the outlet valve and draws fluid into the piston chamber. Both types offer a number of benefits and areas of application. Nov 12, 2016 positive displacement pump makes a fluid move by traping a fixed amount and forcing displacing that trapped volume into the discharge pipe. Positive displacement pump or centrifugal hayes pump, inc. When to use a positive displacement pump pump school. Reciprocating pumps have an overall efficiency that is generally over 85%, sometimes as high as 90%. How to calculate hydraulic pump and motor efficiency. A pump is a device that moves fluids liquids or gases, or sometimes slurries, by mechanical action. Depending on the type of pump and the liquid being handled, this happens with little influence from the back pressure on the pump. Sep 29, 2017 positive displacement pump working principle, is by providing a certain force, in the form of kinetic energy, at a fixed fluid volume from the inlet side to the pump outlet.
Volumetric efficiency not to be confused with mechanical efficiency is normally 95 to 97%. A positive displacement pump will in fact create fluid power proportional to speed cubed when hooked to the same fluid system that allows a centrifugal pump to create fluid power proportional to. The pumps behave very differently when considering mechanical efficiency as well. Pump displacement an overview sciencedirect topics. Nonpositive displacement pumps are basically used for low pressure applications and are designed to work for a maximum pressure of 18 to 20 bar. A positive displacement pump operating against closed discharge valves continues to produce flow until the pressure in the discharge. So if the pump has a displacement of 100 ccrev and is being driven at rpm its theoretical flow is 100.
The pumping action is cyclic and can be driven by pistons, screws, gears, rollers, diaphragms or vanes. The volumetric efficiency of positive displacement pumps increases with increased viscosity. The most significant efficiency losses are from mechanical and volumetric considerations which often amount to just a few percent. Positive displacement pump working principle, is by providing a certain force, in the form of kinetic energy, at a fixed fluid volume from the inlet side to the pump outlet. Small compact, lightweight design equals less pump and more performance. To allow a positive pumps rotors to rotate, small clearances must be maintained between the rotors and housing. Nonpositive displacement example if you designed a lift where a hydraulic cylinder is extended by flow from a nonpositive.
Positive displacement pumps an overview sciencedirect topics. Apr 27, 2015 it depends on the purpose or the required job. By looking at the efficiency chart figure 3 you can see the impact of pressure changes on the pump s efficiency. The pumps behaves different considering mechanical efficiency as well. The working principle is very different from the dynamic pump, which in theory, positive displacement pump will produce a fixed flow rate at a certain rpm even though the. Positive displacement pumps are generally more tolerant of suction lifts than. Choosing the right industrial pump to suit a specific application requires a choice between centrifugal and positive displacement pumps. This can have an impact on the speed of your sanitary processes. The pump can displace the computed pump displacement pd only if the formation can produce this much oil. Positive displacement pumps have a very close clearance between rotating and stationary parts and hence are selfpriming. Apr 10, 2018 choosing the right industrial pump to suit a specific application requires a choice between centrifugal and positive displacement pumps. The volumetric efficiency of rpd pumps depends on the percentage of slip to the total quantity displaced.
Understanding positive displacement pumps pumpscout. When to use a positive displacement pump michael smith engineers. Across the industry, professionals are constantly working to develop and implement the latest in pump and valve technology. Nonpositive displacement example if you designed a lift where a hydraulic cylinder is extended by flow from a nonpositive displacement pump, the performance would be terrible. Slip and efficiency positive pumps sometimes do not pump the full displacement for which they are rated because of a phenomenon called slip. What is the difference between the positive and non. Positive displacement pump working principle technology. The pumps behave very differently whe considering mechanical efficiency as well. Volumetric efficiency is determined by dividing the actual flow delivered by a pump at a given pressure by its theoretical flow. Theoretical flow is calculated by multiplying the pumps displacement per revolution by its driven speed. A positive displacement pd pump moves a fluid by repeatedly enclosing a fixed volume and moving it mechanically through the system. Each cycle encloses a certain volume of fluid and moves it mechanically through the pump into the system.
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. Changes in pressure have little effect on the pd pump efficiency but a dramatic one on the centrifugal. When to use a centrifugal or a positive displacement pump pd is not always a clear choice. How to calculate the power of a reciprocating pump it. Positive displacement pumps an overview sciencedirect. Energy efficiency considerations in pumps and pump stations. Flowserve positive displacement pumps comply either with asme or api standards. Overall efficiency is simply the product of volumetric and mechanical hydraulic efficiency. By design mechanical seals are friction contact devices and can be subjected to a very wide range and often hostile operating environments. They are commonly referred to as constantvolume pumps.
Positive displacement pump makes a fluid move by traping a fixed amount and forcing displacing that trapped volume into the discharge pipe. A positive displacement pump operating against a closed discharge valve continues to produce flow and the pressure in the discharge line increases until the line bursts, the pump is. A positive displacement pump must not operate against a closed valve on the discharge side of the pump, because it has no shutoff head like centrifugal pumps. What is the difference between the positive and nonpositive. Positive displacement pumps selection guide engineering360.
The second is to help the user analyze their pumping requirements, make an appropriate pump selection and size the. You must know these quantities to calculate the reciprocating pump power. By looking at the performance charts below you can see just how different they are. Slip is the loss of capacity as a percentage of the cylinder displacement because of volumetric efficiency, stuffingbox losses, and valve losses. Another important distinction between these two sanitary process components is the effect viscosity. Selecting a positive displacement pump using performance curves. The difference between the quantity displaced qd and the actual output quantity q is slip sq. When selecting, keep in mind that it is best to run a positive displacement pump at no more than 400 to 500 rpm. Pump performance fundamentals of positive displacement pumps. A positive displacement pump has a phenomenon known as slip, which is the recirculation of the pumped fluid from the discharge side of the pump back to the suction side of the pump. The volumetric efficiency of rotary positive displacement. Positive displacement pumps operate with a series of working cycles. The performance and efficiency of hydraulic pumps and motors.
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