c)screw pump

       A screw pump is a positive displacement pump that use one or several screws to move fluids or solids along the screw(s) axis. In its simplest form (the Archimedes' screw pump), a single screw rotates in a cylindrical cavity, thereby moving the material along the screw's spindle. This ancient construction is still used in many low-tech applications, such as irrigation systems and in agriculturural machinery for transporting grain and other solids.
Development of the screw pump has led to a variety of multi-axis technologies where carefully crafted screws rotate in opposite directions or remains stationary within a cavity. The cavity can be profiled, thereby creating cavities where the pumped material is "trapped".
In offshore and marine installations, a three spindle screw pump is often used to pump high pressure viscous fluids. Three screws drive the pumped liquid forth in a closed chamber. As the screws rotate in opposite directions, the pumped liquid moves aling the screws spindles.
Three-Spindle screw pumps are used for transport of viscous fluids with lubricating properties. They are suited for a variety of applications such as fuel-injection, oil burners, boosting, hydraulics, fuel, lubrication, circulating, feed and so on.
Compared to centrifugal pumps, positive displacements (PD) pumps have several advantages. The pumped fluid is moving axially without turbulence which eliminates foaming that would otherwise occur in viscous fluids. They are also able to pump fluids of higher viscosity without losing flow rate. Also, changes in the pressure difference have little impact on PD pumps compared to centrifugal pumps.

Working Of Screw Pump:-

Screw pumps also falls in the category of rotary pumps and have the same mechanism and principle. Screw pumps are the only type of pumps which are different from other rotary displacement pumps because of the axial flow of the fluid through their pumping elements.The fluid in screw pumps moves between the screw threads and is displaced axially as the screws rotate and mesh.
Due to the axial flow pattern and low internal velocities,the screw pumps have certain advantages in applications where churning and agitation of fluid is highly objectionable or not allowed.The inter-meshing of the screw threads on the rotors and the very fine clearances of the surrounding housing creates a seal between the pumps inlet and outlet.
Due to the rotation of the screws and the movement of fine clearances, pockets are formed in the helices of the screw.As the screw moves the pockets also move axially and act similar to a piston moving in one direction, while the fluid is moved axially from inlet to outlet.

The advantages and disadvantages of screw pumps

Advantages
1. Wide range of flows and pressures
2. Wide range of liquids and viscosities
3. Built-in variable capacity
4. High speed capability allowing freedom of driver selection
5. Low internal velocities
6. Self-priming with good suction characteristics
7. High tolerance for entrained air and other gases
8. Minimum churning or foaming
9. Low mechanical vibration, pulsation-free flow, and quiet operation
10. Rugged, compact design — easy to install and maintain
11. High tolerance to contamination in comparison with other rotary pumps

Disadvantages
1. Relatively high cost because of close tolerances and running clearances
2. Performance characteristics sensitive to viscosity change
3. High pressure capability requires long pumping elements                  

d)Bent axis pumps

       Bent axis pumps, axial piston pumps and motors using the bent axis principle, fixed or adjustable displacement, exists in two different basic designs. The Thoma-principle (engineer Hans Thoma, Germany, patent 1935) with max 25 degrees angle and the Wahlmark-principle (Gunnar Axel Wahlmark, patent 1960) with spherical-shaped pistons in one piece with the piston rod, piston rings, and maximum 40 degrees between the driveshaft centerline and pistons (Volvo Hydraulics Co.). These have the best efficiency of all pumps. Although in general the largest displacements are approximately one litre per revolution, if necessary a two-liter swept volume pump can be built. Often variable-
displacement pumps are used, so that the oil flow can be adjusted carefully. These pumps can in general work with a working pressure of up to 350–420 bars in continuous work.

2)Reciprocating Positive Displacement Pumps

       In the reciprocating positive displacement pumps the moving part forming the enclosed volume for fluid displacement has a reciprocating motion. Reciprocating pumps are mainly of two types based on the reciprocating part.

•    1 )Piston Pump
•    2) Diaphragm Pump

       Each type of these reciprocating pumps is available with various different features. Different variants of positive displacement pumps reciprocating pump are single discharge, double discharge, duplex and triplex.

1)Piston pump

Construction:-
A Piston Pump is very similar in construction to a Reciprocating Internal Combustion Engine. The basic pumping action is obtained by reciprocation of a piston in a cylinder. The cylinder has two valves, one inlet and one outlet valve. And they allow for only inwards and outwards movement of the liquid respectively. These valves are situated in inlet and outlet manifold respectively. The piston is connected to a crankshaft through a connecting rod. The Piston Pump has a liner made of leather or any other synthetic material to provide proper sealing between the moving surfaces of the piston and the cylinder.            

Piston Pump - Working:-
The reciprocating action of the piston is obtained by crankshaft arrangement and guide path provided by the cylinder. The piston is connected to the crankshaft through the connecting rod which is coupled with the crankshaft through a revolute joint situated at some distance from the crankshaft axis. This distance can be called as crank length or the eccentric distance. It is this crank length which determines the stroke length of a piston pump.
The crankshaft is coupled to an electric motor or an engine shaft. As the crankshaft rotates the piston reciprocates. In one cycle of the crankshaft the piston reciprocates once, that is, moves one stroke forward and one backward. The intake and discharge of the liquid by a piston pump is basically the same as reciprocating pumps in general as discussed in previous article under working of reciprocating pumps.

TypesOf Piston Pump

         a)Single Acting Piston Pump
         b)Double Acting Piston Pump
         c)Axial Piston Pump
         d)Radial Piston Pump

a)Single Acting Piston Pump
       Having liquid chamber on only one side of the piston and valve arrangement on that side only. The liquid is discharged from one side of the cylinder once in a crankshaft cycle, only in the forward stroke of the piston.
b)Double Acting Piston Pump
        Both sides of the piston have liquid chamber and the valve arrangement. Liquid is discharged from both the sides, from one side in the first half of the cycle and from the other side in the second half of the crankshaft cycle. As the liquid is discharged in both the forward and backward stroke, the discharge is more per cycle and also smooth as compared to the Single Acting pump.
c)Axial Piston Pump:
        Axial piston pump many designed with a variable displacement mechanism, to vary output flow for automatic control of pressure. There are various axial piston pump designs, including swashplate (sometimes referred to as a valveplate pump) and checkball (sometimes referred to as a wobble plate pump). The most common is the swashplate pump. A variable-angle swashplate causes the pistons to reciprocate.

                                                           

Applications Axial piston pump

Despite the problems indicated above this type of pump can contain most of the necessary circuit controls integrally (the swash-plate angle control) to regulate flow and pressure, be very reliable and allow the rest of the hydraulic system to be very simple and inexpensive.
Axial reciprocating motors are also used to power many machines. They operate on the same principle as described above, except that the circulating fluid is provided under considerable pressure and the piston housing is made to rotate and provide shaft power to another machine. A common use of an axial reciprocating motor is to power small earthmoving plant such as skid loader machines. Another use is to drive the screws of torpedoes.

b)Radial Piston Pump

A pump that is normally used for very high pressure at small flows.

Piston pumps are more expensive than gear or vane pumps, but provide longer life operating at higher pressure, with difficult fluids and longer continuous duty cycles. Piston pumps make up one half of a hydrostatic transmission.                        

                                                   

Applications Radial piston pump

•    Due to the hydrostatically balanced parts it is possible to use the pump with various hydraulic fluids like mineral oil, biodegradable oil, HFA (oil in water), HFC (water-glycol), HFD (synthetic ester) or cutting emulsion. That implies the following main applications for a radial piston pump:
•    machine tools (e. g. displace of cutting emulsion, supply for hydraulic equipment like cylinders)
•    high pressure units (HPU) (e. g. for overload protection of presses)
•    test rigs
•    automotive sector (e. g. automatic transmission, hydraulic suspension control in upper-class cars)
•    plastic- and powder injection moulding
•    wind energy

The Advantages,Disadvantages and Applications of piston pump:

Advantages
1. Parameter High: Rated high pressure, high speed, large power-driven pump
2. Efficiency, volumetric efficiency is 95% of the total efficiency of about 90%
3. Long life
4. Variable convenient form for
5 More unit power and light weight
6. Piston main components are compressive stress, strength of materials can be fully utilized
7. Piston pumps have a wide pressure range, can reach high pressures and the pressure can be controlled without an impact on the rate of flow. Piston pumps have a continuous rate of discharge. Pressure changes and discharge rate have minimal effect on performance. Piston pumps can maneuver viscous fluids, high gas volumes and solids, only if the valves are correctly designed.

Disadvantages
•    Piston pumps cost more per unit to run compared to centrifugal and roller pumps. The mechanical parts are prone to wear, so the maintenance costs can be high. The valves must be resistant to abrasives for large solids to pass through. Piston pumps are heavy due to their large size and the weight of the crankshaft that drives the pump.
Applications
•    Mobile equipment
•    Marine auxiliary power
•    Construction equipment
•    Oil field equipment
•    Machine tools
•    Metal forming and stamping
•    Packaging machinery

Diaphragm pump

A diaphragm pump is a positive displacement pump that uses a combination of the reciprocating action of a rubber, thermoplastic or teflon diaphragm and suitable non-return check valves to pump a fluid. Sometimes this type of pump is also called a membrane pump.
         
         

There are three main types of diaphragm pumps:

•    Those in which the diaphragm is sealed with one side in the fluid to be pumped, and the other in air or hydraulic fluid. The diaphragm is flexed, causing the volume of the pump chamber to increase and decrease. A pair of non-return check valves prevent reverse flow of the fluid.
•    Those employing volumetric positive displacement where the prime mover of the diaphragm is electro-mechanical, working through a crank or geared motor drive. This method flexes the diaphragm through simple mechanical action, and one side of the diaphragm is open to air.
•    Those employing one or more unsealed diaphragms with the fluid to be pumped on both sides. The diaphragm(s) again are flexed, causing the volume to change.
When the volume of a chamber of either type of pump is increased (the diaphragm moving up), the pressure decreases, and fluid is drawn into the chamber. When the chamber pressure later increases from decreased volume (the diaphragm moving down), the fluid previously drawn in is forced out. Finally, the diaphragm moving up once again draws fluid into the chamber, completing the cycle. This action is similar to that of the cylinder in an internal combustion engine.

Advantages and Disadvantages Of Diaphragm Pump

Advantages
•    Air diaphragm pumps are a very efficient type of transfer pump. These pumps can handle liquids with high, medium or low viscosities. They are also designed to pump liquids that feature a considerable amount of solids. This type of pump can be constructed out of materials that can handle pumping even the most acidic chemicals.
Disadvantages
•    Most air diaphragm pumps require around 20 standard cubic-feet per minute and 100 psi of air intake to operate efficiently. Also, these types of pumps tend not to pump very accurately at their bottom end. A functioning air diaphragm pump pulsates, and a dampener must be fitted onto the pump to reduce pulsing.

Diaphragm Pump Applications

•    1)Overview Of Edson's Diaphragm Pump Applications
Edson diaphragm pumps are primarily designed for the collection and transfer of waste liquids and pumping liquids with suspended solids. All of our diaphragm pumps are capable of handling liquids with large suspended solids without clogging.
•    2)Mohegan Sun Casino Wouldn't Gamble
They use Thermaco "Superceptor®" grease/oil separators with Edson International diaphragm pumps to extract the solids and grease in their waste water. Using the Thermaco system maintains a clean waste stream and keeps restaurants, like theirs, in compliance with local discharge limitations of oil, grease and like products. 
•    3)Oil Collection Stations Help In The Valley
Because of easy to use and dependable central waste oil collection stations, farmers from around the Sonoma Valley in California are now able to get rid of used motor oil drained from farm equipment in an evironmentally responsible manner.
•    4)Edson International Supplies The Pump That Supplies
BISCO Environmental Remediation System!!
Edson International recently sold BISCO Environmental our 120ELA-20-200 electric powered diaphragm pump. The exceptional dry suction lift and reliable "run dry" capabilities of this pump made it the right pump for the job. It is a natural fit for the smaller ground water remediation sights. 
•    5)Life Saving Pump
A heavy duty Edson emergency pump and an abundance of determination saved the life, and the race, for single-handed Transatlantic sailor, Neal Petersen.
•    6)Marine Boatbuilders' New PUMPOUT BOATS Feature
Edson 12 Volt Electric Diaphragm Pump
Introducing Marine Boatbuilders' newly designed 20' and 23' PUMPOUT BOATS using Edson 12Volt Electric, Diaphragm or Peristaltic, Pump Out Equipment. Marine Boatbuilders Co. specializes in the design of custom-made Work Boats, Patrol Boats and PUMPOUT Boats.