positive displacement pump

 

displacement pump relies on the piston, plunger, diaphragm, gear or blade and other working parts to reciprocate or rotate in the pump body, so that the volume of several working chambers in the pump body changes periodically, and alternately sucks and discharges liquid A pump. It has the characteristics of low revolution, high efficiency, strong self-priming ability, stable operation, and preheating of some pumps. It is widely used in the transportation of high-viscosity media.

Volumetric pump Volumetric pump, energy through the action of force, periodically to the interface of 1 or more than 1 mobile effective accessories, liquid-equivalent to the volume, resulting in a direct rise in pressure, to transport the liquid through the valve or pipe fittings until the discharge pipeline required pressure device.

rely on pistons, plungers, diaphragms, gears or blades and other working parts in the pump body for reciprocating or rotary motion, so that the volume of several working chambers in the pump body changes periodically, and alternately inhale and discharge liquid. pump.

positive displacement pump relies on the working element to make reciprocating or rotating motion in the pump cylinder, so that the working volume alternately increases and decreases to realize the suction and discharge of liquid. Working element for reciprocating motion of the positive displacement pump called reciprocating pump, rotary motion called rotary pump. The suction and discharge process of the former are alternately carried out in the same pump cylinder, and are controlled by the suction valve and the discharge valve; the latter is through the rotation of the gear, screw, vane rotor or sliding vane and other working elements, forcing the liquid from The suction side is transferred to the discharge side.

positive displacement pump at a certain speed or number of reciprocations is certain and hardly changes with the pressure. The flow and pressure of the reciprocating pump have large pulsation, and corresponding measures to reduce pulsation need to be taken. Rotary pumps generally have no pulsation or only small pulsation. With self-priming ability, the air in the pipeline can be pumped out and sucked into the liquid after the pump is started. When starting the pump, the discharge pipeline valve must be fully opened; reciprocating pump is suitable for high pressure and small flow; rotary pump is suitable for medium and small flow and high pressure; reciprocating pump is suitable for conveying clean liquid or gas-liquid mixture. In general, the efficiency of positive displacement pumps is higher than that of dynamic pumps.

pump is the use of its working chamber volume changes to transfer energy.

 

positive displacement pumps

 

positive displacement pump is divided into two categories: reciprocating and rotary. Compared with reciprocating positive displacement pump, rotary positive displacement pump has no suction and discharge valve, and will not be like reciprocating pump. Due to the influence of high viscosity liquid on the normal operation of valve, the pump efficiency decreases rapidly with the increase of viscosity. Moreover, when the viscosity of the conveying liquid is increased, the decrease in the number of revolutions of the pump is smaller than that of the reciprocating pump. Therefore, when the viscosity of the conveying high-viscosity liquid or the liquid changes greatly, the rotary solvent pump is more suitable than the reciprocating displacement pump. Rotary positive displacement pump points: gear pump, rotary piston pump, screw pump, and sliding vane pump and other types.

 

gear pump

gear pump is a rotary positive displacement pump, a kind of rotor pump. There are a pair of meshing gears in the pump housing, one of which is a driving gear and the other is a driven gear, which is driven by the meshing of the driving gear. There is a small gap between the gear and the pump housing. As the gears within the pump rotate, they force liquid through the pump by a process that creates suction and voids within the system.
Fluid entering the pump receives energy through grooves in the gears, thereby driving the fluid to the output area. Because the gears are rotating in a particular direction, this prevents backflow of liquid. Gear pumps are usually used to transport viscous liquids that do not contain any solid substances, such as oil, grease, polymer, synthetic resin, rubber slurry, adhesive, emulsion, fat, cod liver oil, paint, shampoo, cosmetics, etc.

 

screw pump

rely on the change of the volume of the pump chamber when the screw rotates to inhale and transport water. There are single screw, twin screw and multi screw types. In agriculture, single screw pumps are used, and their pump chambers consist of a steel screw and a rubber sleeve fixedly installed in the pump housing. A single-pitch screw rotates in a sleeve having a double-pitch inner spiral, and the cavity formed therebetween moves from the suction end to the outlet end, thereby forming a continuous flow of water. Because of its simple structure, small size, easy disassembly and assembly, reliable work, and good self-priming performance, it is mostly used in mobile sprinkler irrigation systems.

 

diaphragm pump

Used for low lift, small flow water lifting operations, by the pump body, in and out of the water pipe, in and out of the water valve, diaphragm and push-pull rod and so on. The pump body may consist of one or two pump chambers. In a diaphragm pump having two pump chambers, the diaphragm is arranged in the center of the pump body, or the two diaphragms are respectively arranged outside the two pump chambers. When working, two people manipulate the push-pull rod connected with the diaphragm by hand to push the diaphragm to press in and open the reciprocating motion, so that the volume of the two pump chambers is alternately expanded and reduced. When the pump cavity expands, the pressure decreases, the inlet valve opens, the outlet valve closes, and the water flows into the pump cavity from the inlet pipe; when the pump cavity shrinks, the pressure increases, the inlet valve closes, the outlet valve opens, and the pump cavity The water flows out from the drain pipe, and the two pump chambers alternately absorb water and drain.

diaphragm pump, also known as the control pump, is the main type of actuator, which receives the control signal output by the modulation unit. Diaphragm pumps operate with power to change fluid flow. Diaphragm pumps are generally composed of actuators and valves. Diaphragm pump according to the power used by the actuator, can be divided into pneumatic diaphragm pump, electric diaphragm pump and liquid diaphragm pump three. In the absence of electricity, pneumatic diaphragm pump or liquid diaphragm pump is the ideal choice.
Diaphragm pumps are used to transport chemicals with high flash points, such as volatile solvents and corrosive chemicals. Pneumatic diaphragm pump is a widely accepted diaphragm pump. The pump has two chambers with diaphragms, and air is transferred between the two chambers through a valve. This change in air creates a pressure that moves the fluid from one side of the pump to the other.

 

tie rod piston pump

is driven by animal power prime movers, wind or internal combustion engines, and is often used on grazing grounds to lift water from wells. It is composed of pump cylinder, piston, inlet and outlet pipes, inlet and outlet valves, pull rods and transmission devices. The piston is driven by the pull rod connected to it and reciprocates up and down in the pump cylinder. When the piston moves upward, the inlet valve opens, the water in the inlet pipe enters the pump cylinder, and the outlet valve is closed at the same time, and the water above the piston is driven upward; when the piston moves downward, the inlet valve is closed and the outlet valve is opened. The water in the pump cylinder rises to the top of the piston from the outlet valve, so that the water is repeatedly fed and lifted, so that the water is continuously discharged from the drain.

 

lobe pump

lobe pumps include pairs of rotating "cams" that are similar to gears, even though they do not touch. When the cams rotate together, they create suction, drawing liquid into the pump. When liquid enters the pump housing, the liquid is trapped because the cam rotates in only one direction.
The liquid continues to be pushed towards the output area of the pump. Since there is no contact between the cam and the cam and the pump housing, this type of pump can handle dense liquids and liquids containing some solid substances, such as molasses, cooking oil, chocolate paste, paint, Ink, asphalt, paraffin, grease, clay, etc.

 

plunger pump

plunger pump belongs to a specific type of positive displacement pump and is a reciprocating pump, consisting of a hydraulic end and a power end. It generates suction or discharge pressure through a plunger (piston). The drive end drives the plunger to move backwards, creating a gap and vacuum that draws fluid into the pump housing. When the plunger moves forward, the chamber will be pressurized and the fluid drawn into the pump will be expelled.
A valve is also installed in the pump to prevent fluid from flowing back from the inlet. This type of pump is used for the simple movement of liquid materials and to facilitate the work of more complex pumps.
The plunger pump is suitable for conveying various media with small flow and high pressure. When the flow rate is less than 100 m3/h and the discharge pressure is greater than 10 MPa, it has high efficiency and good operation performance.

 

Positive Displacement Pump

 

What Is 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. The pumping action is cyclic and can be driven by pistons, screws, gears, rollers, diaphragms, or vanes.

 

Positive displacement pumps add energy to a fluid by applying force to the liquid with a mechanical device such as a piston or plunger. A positive displacement pump decreases the volume containing the liquid until the resulting liquid pressure equals the pressure in the discharge system.

That is, the liquid is compressed mechanically, causing a direct rise in potential energy. Most positive displacement pumps are reciprocating pumps in which the linear motion of a piston or plunger in a cylinder causes the displacement.

In rotary pumps, another common positive displacement pump, a circular motion causes the displacement. There are several manufacturers of positive displacement pumps which are often found in high-pressure services.

How Does Positive Displacement Pump Work?

A positive displacement pump will draw fluid into the pump chamber at an inlet valve and then discharge it through an outlet valve. Typically, this may involve a rotary, reciprocating, or diaphragm system that moves the fluid through the pump. It is a repeating cycle that generates consistent flow rates. Different pumps will use different methods to achieve this action.

A positive displacement pump makes a fluid move by trapping a fixed amount of the fluid and forcing (displacing) that trapped volume into a discharge pipe or discharge system.

 

Some positive displacement pumps use an expanding cavity on the suction side and a decreasing cavity on the discharge side. Liquid flows into the pump as the cavity on the suction side expands and the liquid flows out of the discharge as the cavity collapses. The volume remains constant through each cycle of pump operation.

Positive Displacement pumps do not use impellers but rely on rotating or reciprocating parts to directly push the liquid in an enclosed cavity until enough pressure is built up to move the liquid into the discharge system.

The pump does not rely on raising the velocity of the fluid as the centrifugal pump does by moving the liquid through the impeller. Consequently, the fluid velocity inside a positive displacement pump is much lower than that of a centrifugal pump. This is often a desirable feature for certain applications, such as when needing to pump a media containing fragile solids.

Positive Displacement Pump Characteristics

Positive displacement pumps, unlike centrifugal or roto-dynamic pumps, can theoretically produce the same flow at a given speed (RPM) no matter what the discharge pressure. Therefore, positive displacement pumps can be regarded as constant flow devices. However, a slight increase in internal leakage as the pressure increases can prevent a truly constant flow rate.

In application, a positive displacement pump must not be allowed to operate against a closed valve on the discharge side of the pump, because it has no shutoff head like a centrifugal pump.

With the pump operating against a closed discharge valve, it will continue to produce flow and the pressure in the discharge line will increase until the pipeline can either fracture or the pump can become severely damaged, or both.

 

To prevent this, a relief or safety valve on the discharge side of the positive displacement pump is therefore often necessary. This relief valve can be positioned either internally or externally to the pump. The pump manufacturer normally has the option to supply internal relief or safety valves.

An internal valve is usually only used as a safety precaution, but an external relief valve in the discharge line, with a return line back to the suction line or supply tank, will provide increased safety.

Types Of Positive Displacement Pumps

The main different types of Positive Displacement Pumps are;

• Progressive Cavity Pump (PC Pump)
• Gear Pump - Internal & External
• Vane Pump (Impeller Pump)
• Rotary Lobe Pump
• Screw Pump
• Diaphragm Pump
• Peristaltic Pump (Hose pump)

Positive displacement pumps, which lift a given volume for each cycle of operation, can be divided into two main classes, reciprocating and rotary. Reciprocating pumps include piston, plunger, and diaphragm types; rotary pumps include gear, lobe, screw, vane, and cam pumps.

 

 

• Progressive Cavity Pumphas a rotor rotating within a housing called a stator. The rotor is always metallic and the stator is made up of a rubber type of material. It looks somewhat like a screw thread - the fluid is between the cavities and the rotary motion of the rotor forces the fluid through from one end to the other. It has a low to moderate capacity, low to high pressure, good solids handling capability, one seal, low shear, constant flow and a low pulsation.
• Gear pumpsare available in 2 configurations; 1) Internal gear which has a moderate capacity, low to moderate pressure, max 14 Bar/ Spl 17 Bar, no solids handling capability, one seal and has a constant flow. 2) External gear has a moderate capacity, moderate to high pressure, Standard 20 Bar /Spl 250 Bar. It has one seal, no solids handling capability and a constant flow.
• Vane Pump(Impeller Pump) has moderate capacity, low pressure, moderate solids handling capability, one seal and a constant flow.
• Rotary Lobe Pumphas moderate to high capacity, low to moderate pressure, good solids handling capability, two/four seals, a constant flow and moderate pulsation.
• Screw Pump- the screw pump has multi versions known as multi screw pumps featuring moderate to high capacity, high pressures, only lubricative liquids, no solids handling capability, one seal and a constant flow.
• Diaphragm Pump- Air Operated Diaphragm Pump has low to moderate capacity, low to moderate pressure, very low efficiencies, no seal and high pulsation.
• Peristaltic Pump (Hose pump) has moderate capacity, low pressure, good solids handling capability, low shear, no seal and high pulsation.

5 Reasons To Choose A Positive Displacement Pump

The decision to choose a positive displacement pump over a centrifugal is not always a clear one. They each have very different behaviors and if not familiar with them, it may be difficult to understand how a positive displacement pump could fit into your process.

Positive displacement pumps are a more efficient choice than centrifugal pumps in some situations. If any of the conditions or applications below are in your process, a positive displacement pump should probably be used.

1. High Viscosity

Centrifugal have issues pumping viscous liquids, becoming very inefficient at even modest levels. Positive displacement pumps, on the other hand, have no problem moving thick liquids.

2. Low Flow

Oftentimes, a centrifugal pump is run off its BEP when a lower flow is desired, much to the pump's detriment. Running a centrifugal off its BEP can cause excessive energy consumption, damage to the pump, and overall poor performance.

A positive displacement pump, however, is well suited for these conditions, providing a constant flow of fluid at a given pump speed. If you're trying to get the pressure or flow you need by operating a centrifugal pump off its best efficiency point (BEP), a positive displacement pump may be a better choice.

3. Metering

Positive displacement pumps are an ideal choice for metering applications. They deliver constant flow, allowing them to easily meet process requirements. Some common types of positive displacement pumps used for metering are:

 

• Peristaltic
• Gear
• Diaphragm
• Plunger
• Piston

4. High Pressure Requirements

Positive displacement pumps are excellent for applications that require high pressure, with some models producing over 1,000 psi (2,300 ft). Due to the positive displacement pump's design, however, if kept in operation against a closed discharge valve, it will continue to build pressure until the line bursts, the pump is damaged, or both.

5. Shear Sensitive Liquids

Centrifugal pumps generally operate at higher speeds when compared to positive displacement pumps. the higher speeds can shear liquids, making centrifugal pumps a poor choice for liquids like tomato paste and latex paint. Positive displacement pumps operating at lower speeds can be gentler on products and are usually preferred in these types of applications.

Applications Of Positive Displacement Pumps

These pumps are commonly used to pump high viscosity fluids where precise dosing otherwise high force output can be necessary. Not like centrifugal pumps, the outputs of these pumps are not affected by force thus they also have chosen in any condition where the supply is unequal. The best positive displacement pump examples are piston, plunger, diaphragm, gear, lobe, screw, and vane.

• Piston and Plunger pumpsare used to pump low viscosity fluids, paint spraying, oil production, and high force washing.
• Diaphragm pumpcan be used for metering, spraying, treatment of water, oils, and paints.
• Gear pumpsare used for pumping the high viscosity fluids within the petrochemical, food industries, paints, oils, etc.
• Lobe pumpis used in food and chemical industries pharmaceutical, biotechnology, sanitary, etc.
• Screw pumpis used in fuel transferring, production of oil, irrigation, etc
• Vane pumpis used in low viscosity liquids, fuel loading, & transmission, etc.

A positive displacement (PD) pump is used to move a liquid frequently with a set volume, with the help of valves otherwise seals by moving it automatically throughout the system. The action of pumping is repeated & can be driven by screws, pistons, lobes, gears, vanes, diaphragms. These pumps are mainly used where highly viscous fluids are required.

Advantages Of Positive Displacement Pumps

There are many benefits of positive displacement pumps for varying applications. Below we outline the main reasons many operations choose to use them.

 

• PD pumps are sometimes called constant-volume pumps because they maintain a constant speed and flow. Even if the system pressure varies, the flow remains constant.
• PD pumps can handle a variety of fluid types: high, low and variable viscosity; shear sensitive fluids; solids; and liquids with a high percentage of air or gas entrainment.
• Their capacity is not affected by the operation pressure.
• They are excellent for applications with flows below 100-gpm and above 100-psi.
• They can be 10 to 40 points more efficient than centrifugal pumps when handling viscous fluids.
• PD pumps are able to self-prime.
• They can be designed as a sealless pump.

Disadvantages Of Positive Displacement Pumps

• These are less efficient pumps than dynamic pumps.
• Low efficiency.
• These can't deliver pulsating free flow.
• These are not best for high-pressure applications.
• They generate high noise.
• Low discharge capability.