A Positive Displacement Pump has an expanding cavity on the suction side and a decreasing cavity on the discharge side. 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. The volume is a constant given each cycle of operation.
The positive displacement pumps can be divided in two main classes
The positive displacement principle applies whether the pump is a
A Positive Displacement Pump, unlike a Centrifugal or Roto-dynamic Pump, will produce the same flow at a given speed (RPM) no matter the discharge pressure.
A Positive Displacement Pump must never operate against closed valves on the discharge side of the pump - it has no shut-off head like Centrifugal Pumps. A Positive Displacement Pump operating against closed discharge valves continues to produce flow until the pressure in the discharge line is increased until the line bursts or the pump is severely damaged - or both.
A relief or safety valve on the discharge side of the Positive Displacement Pump is absolute necessary. The relief valve can be internal or external the pump. An internal valve should in general only be used as a safety precaution. An external relief valve installed in the discharge line with a return line back to the suction line or supply tank is highly recommended.
Reciprocating Pumps
Typical reciprocating pumps are
Plunger pumps consists of a cylinder with a reciprocating plunger in it. In the head of the cylinder the suction and discharge valves are mounted. In the suction stroke the plunger retracts and the suction valves opens causing suction of fluid into the cylinder. In the forward stroke the plunger push the liquid out the discharge valve.
With only one cylinder the fluid flow varies between maximum flow when the plunger moves through the middle positions, and zero flow when the plunger is in the end positions. A lot of energy is wasted when the fluid is accelerated in the piping system. Vibration and "water hammers" may be a serious problem. In general the problems are compensated by using two or more cylinders not working in phase with each other.
In a diaphragm pump the plunger pressurizes hydraulic oil which is used to flex a diaphragm in the pumping cylinder. Diaphragm pumps are used to pump hazardous and toxic fluids.
Rotary Pumps
Typical rotary pumps are
In a gear pump the liquid is trapped by the opening between the gear teeth of two identical gears and the chasing of the pump on the suction side. On the pressure side the fluid is squeezed out when the teeth of the two gears are rotated against each other.
A lobe pump operates similar to a gear pump, but with two lobes driven by external timing gears. The lobes do not make contact.
A progressive cavity pump consist of a metal rotor rotating within an elastomer-lined or elastic stator. When the rotor turns progressive chambers from suction end to discharge end are formed between the rotor and stator, moving the fluid.
Explain the primary difference between a positive displacement pump and a centrifugal pump. Give an example of a practical application for each pump.
1. A positive displacement pump has the following characteristic(s) (a) It pumps a certain quantity of fluid depending the exit pipe diameter (b) It discharges a fixed mass of the fluid per revolution (c) It ejects a fixed volume of the fluid (d) It is all of the above (a) - (c) choices 2. Some good examples for a positive displacement pump are: (a) An impeller pump (b) A propeller pump (c) A centrifugal pump (d) None of the above...
An engineer has designed a system in which a positive-displacement pump is used to pump water from an atmosphere tank into a pressurized tank operating at 150 psig. A control valve is installed between the pump discharge and the pressurized tank. With the pump running at a constant speed and stroke length, 350 gpm of water is pumped when the control valve is wide open and the pump discharge pressure is 200 psig. (a) If the control valve is pinched...
How to size the positive displacement pump? what equation do I need to use? Is it the same equation as the centrifugal pump? Is there have any recommended book for calculation pump?
A standard pump with a displacement of 1.60 in/rev uses a 1,800 rpm electric motor. What is the theoretical flow rate in gallons per minute assuming 100% efficiency? (4 significant figures).
Name: Student No.: 1. The pump represented by the graph below has a displacement of 3 in /rev. Determine- (12 pts.) Efficiencies at 3000 psi - Overall efficiency (6) Volumetric Efficiency,% 100 90 1 - - - - 1 1 1 1 1 1 1 - - - 1 80 - - - - - 70 - - - EFFICIENCY, % 60 V 1 50 40 30 20 0 500 1000 2000 2500 3000 1500 SPEED, N (a) The volumetric...
4. (a) (i) For a pump what is the mathematical formula for the Net Positive Section Head (NPSH (2 marks) (ii) What is the meaning of NPSHrequired and describe, giving an example test-rig setup, how it is obtained by pump manufacturers (8 marks)
value: 10.00 points Match the following turbomachines with their corresponding technical classifications. 1. A fuel pump in a car 2. A wind mill and a power plant steam turbine 3. A household fan and an aircraft propeller 4. A fluid coupling transmission 5. An eductor Axial flow fan Axial flow turbine Positive displacement pump Liquid-jet-pump Double-impeller energy transmission device
Displacement 1 is in the yz plane 65.4 ° from the positive direction of the y axis, has a positive z component, and has a magnitude of 5.31 m. Displacement d2 is in the xz plane 38.3 ° from the positive direction of the x axis, has a positive z component, and has magnitude 2.23 m. What are (a) did2 . (b) the x component of dixd2 .(e) they component of dixd2 , (d) the z component of dix d2...
Displacement D1 is in the yz plane 58.8o from the positive direction of the y axis, has a positive z component, and has a magnitude of 4.34 m. Displacement D2 is in the xz plane 27.2o from the positive direction of the x axis, has a positivezcomponent, and has magnitude 2.23 m. What are (a) Dot product of D1 and D2 (b) the x component of D1 x D2 , (c) the y component of , (d) the z component...