Module 3 : Positive Displacement Pumps

Question 1

What is the purpose of a pump?

Answer 1

Pumps move liquids or gasses in a piping system.

Question 2

How do pumps function?

Answer 2

Pumps move fluid through a system by displacing a volume by mechanical or physical action.

Question 3

What are the two categories of pumps?

Answer 3

1. Positive Displacement Pumps
2. Centrifugal pumps

Question 4

How do positive displacement pumps work?

Answer 4

• Positive displacement pumps operate on the principle that liquid is displaced by mechanically varying the size of the chambers within the pump body.
• Fluid is drawn into a chamber which is increase in volume (creating suction).
• Fluid is discharged when the chamber volume is decreasing (creating pressure)

Question 5

What are the characteristics of positive displacement pumps?

Answer 5

• Do not need to be primed to ensure that fluid will enter the pump
• Pressure generated can be fairly high, so must ensure discharge line and valves are all open prior to operating the pump. If the outlet is choked off, pressure will increase to infinity (until something breaks/explodes in the system)
• To protect the system, pump must be fitted with a relief valve at the outlet to allow fluid to recirculate to the suction side of the pump
• Flow rate and volume can be varied.

Question 6

Reciprocating Positive Displacement Pumps

Answer 6

• Simplest form of positive displacement pump
• Can be single or double action, operated by linear or rotary motion
  Construction: use an actuating handle, connecting rod, and piston/cylinder arrangement.

Question 7

Reciprocating Positive Displacement Pumps (single action)

Answer 7

Operation (single action): The movement outward from top dead center (TDC) creates a vacuum opening a pressure-activated spring controlled suction valve, thus filling the chamber with fluid. At bottom dead center (BDC), the pressure in the cylinder matches the pressure of the incoming liquid and the valve closes. As the piston moves back towards TDC, the volume in the cylinder is reduced and pressure increases until the discharge valve opens.

Question 8

Reciprocating Positive Displacement Pumps (double action)

Answer 8

Operation (double action): similar to single action, but the pump will use the space above and below the piston as pumping chambers. The volume displaced will be lower in the space containing the piston rod.
This is not commonly used due to capacity limitations.

Question 9

Reciprocating Positive Displacement Pumps (rotary motion)

Answer 9

Operation (rotary motion): the handle is replaced by a crankshaft attached to the connecting rod. As the shaft rotates, it converts rotary motion to reciprocating motion and actuates the piston.

Question 10

Reciprocating Positive Displacement Pumps (air vessel)

Answer 10

• Air vessel fitted at discharge to dampen the momentary pressure pulse (pressure fluctuation) felt when piston is changing directions at bottom dead center.
• Repeated pulsations in a pumping system is noisy and will cause piping, fittings, valves or machines to fail prematurely

Question 11

Reciprocating Positive Displacement Pumps (advantages)

Answer 11

• Ability to move fluids laden with abrasives
• Ability to pump large particles
• Ease of operation and maintenance
• Reliability
• Ability to operate over a wide range of pressures and flow rates.

Question 12

Reciprocating Positive Displacement Pumps (uses)

Answer 12

• Bilge pumping systems
• High pressure fuel pumps supplying fuel to the injectors of a diesel engine
• Handy for fuel injection and hydraulics

Question 13

Rotary Positive Displacement Pumps

Answer 13

• The second type of positive displacement pumps
• Use rotary motion to vary the volume of the chamber and displace the liquid.
  Four basic categories:
  1. Gear Pumps
  2. Screw Pumps
  3. Vane Pumps
  4. Progressive Cavity Pumps

Question 14

1. Gear Pumps

Answer 14

Two categories:
A) External Gear Pumps
B) Internal Gear Pumps

Question 15

A) External Gear Pumps

Answer 15

Purpose: Petroleum and oil systems, small hydraulics. Not for high pressure
Material: the gears are made of carbon steel and hardened for wear resistance.
The fluid being pumped lubricates the gears so should not be run dry.
Construction: two gears mesh together within a casing. The gears can be driven by an independent supply or directly from a piece of machinery. Generally, one gear is driven by the prime mover (the driver) and the other will follow (the idler). The gears are matched to each other and are not interchangeable.
Operation: As the teeth disengage with one another, the space that is formed between the teeth and the casing increases in volume causing a pumping action. As the teeth mesh again, the space becomes progressively smaller causing a suction action.
Flow is regulated by the size of the cavity between the teeth and the casing, and the speed of the gears
The gear teeth extend outwards but they do not come into direct contact with the casing -> there’s a small amount of clearance and this leads to slip.
Can handle liquids with a small amount of suspended solids but this will cause increased wear between the fixed and moving parts.

Question 16

Externally cut gears

Answer 16

Teeth point out

Question 17

Slip

Answer 17

Liquid leaking back towards the inlet port.
Slip is directly proportional to viscosity: low viscosity leads to more slip.

Question 18

B) Internal Gear Pumps

Answer 18

Purpose: Found as lift pumps for fuel oil supply systems or as fuel transfer pumps. Same function as an external gear pump but takes up less space.
Material:
Construction: two gears within a casing, but one gear has internally-cut teeth (this will be the larger, external gear, called the rotor gear) and the other has externally-cut teeth (this will be the smaller, internal gear, called the idler).
A crescent shaped insert in the bottom of the pump acts as a seal between the suction and the discharge sides of the pump.
Operation: As the gears rotate, small pockets are formed on the suction side. The fluid flows into these pockets and travels around to the discharge side of the pump.

Question 19

2. Rotary Screw Pumps

Answer 19

Summary: Same principle as gear pumps however two or three long screw shaped shafts are substituted for the gears.
Purpose: Often used for lube oil pumps in engine lubrication systems, could also be used as fuel transfer pumps.
Material: The shafts are fabricated of carbon steel, machined to close tolerances, and chromed.
Construction: The screw shafts are mated together and held in place inside a long casing by bearings on each end. One of the screws (the driver) is driven off a motor and the other(s) are either gears to the drive or driven by directly touching the driver.
If desiring to increase the capacity and discharge rate, either add another screw to the casing or modify the construction of the pumps. A third screw is placed between the other two screws, called nesting. The nested screw becomes the driven screw and the other two are driven by it. The design can also be modified to have suction at both ends and discharge in the middle.
Operation: Clearance between the screws and the casing are minimal so there is no slip. As the screws rotate, cavities are formed on the suction side. As rotation continues, the cavity moves down the length of the pump until reaching the discharge side where fluid is forced out.

Question 20

3. Rotary Vane Pumps

Answer 20

Purpose: often used in hydraulic systems. Cannot be used with water
Material: the vanes may be constructed of carbon steel, plastic, or graphite.
Construction: Consists of a casing with a cylindrical rotor offset from its center. Slots are cut into the rotor and are fitted with vanes. The vanes are fitted tightly to the ends of the casing and held in positive contact with the casing by centrifugal force or springs behind the vanes.
Self-priming and fitted with relief valve.
Operation: The rotor is driven by the prime mover. As the cavity increases in size at the inlet, fluid is drawn in. As the space between the casing and the vanes decreases, fluid is pushed out.
Balanced vane pumps are double acting - two pumping actions occur for every complete rotation of the cylinder. This is done by making the casing oval in shape.

Question 21

3. Rotary Vane Pumps - rubber impeller

Answer 21

In some types of rotary vane pumps, a rubber impeller is attached to the rotating shaft to allow for use with water. The pump casing is round with a crescent shaped insert to aid with the varying size of the cavity. As the impeller rotates, a cavity is formed on the suction side. Fluid is carried around to the discharge side where the crescent causes the cavity to collapse, forcing discharge.
Most often used as a cooling pump on small engines and air compressors.
Cannot be run dry for any length of time.
Trade name: Jabsco pump

Question 22

4. Progressive Cavity Pumps

Answer 22

Summary: use a rotor in the form of an auger that rotates within a synthetic stator where volume progressively increases and decreases to create flow.
Purpose: Mainly used for pumping bilge waste or on sewage systems because it is capable of handling fluids with large amounts of suspended solids. Good for fluids that we don’t want to agitate. Low capacity and will fail if outlet is choked off.
Material: Helical screw rotor fabricated from chromed carton steel. Flexible rubber casing called a stator. The stator is encased within a steel casing for strength and to maintain form during operation
Construction:
Operation: As the rotor turns, a cavity forms at the suction end and traps the fluid. As the rotor continues turning, the cavity travels down the length of the casing to the outlet, where the fluid is discharged.
Cannot operate at high speeds.

Question 23

Axial Piston Pump

Answer 23

Summary: PDP with an odd number of pistons arranged in a circular array within a housing called a cylinder block, rotor or barrel.
- Combine the lesser amount of slip found in piston pumps with the increased capacity of a rotary pump.
Purpose: high pressure hydraulics

Question 24

Bent Axis Pump

Answer 24

Summary:
Purpose: Commonly used in hydraulic circuits
Material:
Construction: A drive shaft fitted with a flange turns inside an angled housing. Circumferentially attached to the flange are a number of pistons which fit inside individual cylinders of a free-floating circular cylinder block (inside the angled housing)
Operation: when the drive shaft is rotated, the attached pistons turn the cylinder block and move from the inner point of the cylinder to the outer point. The movement of the pistons varies the size of the cavity, causing suction and discharge.

Question 25

Swash Plate Pumps (variable displacement pumps)

Answer 25

Purpose: Can vary the displacement of the pump allowing pumping in either direction - steering gears and buoy cranes
Material:
Construction: Variable-angle swash plate the causes pistons to reciprocate a greater or lesser distance per rotation, allowing output flow rate and pressure to be varied.
Operation: Designed with a variable displacement mechanism to vary output flow for automatic control of pressure.

Question 26

Radial Piston Pump

Answer 26

Purpose: Normally used for very high pressure at small flows. More expensive than gear or vane pumps, but will last longer at higher pressure, with difficult fluids, and longer continuous duty cycles. Very rare to see
Material:
Construction: a center barrel has holes drilled radially from the center. Pistons are fitted inside each hole, with their outer ends attached to an offset fixed plate. The center barrel is attached to a drive shaft and rotates about its axis.
Operation:
- As the piston travels towards the 3 o’clock position, it starts to move outward in its cylinder - the cavity expands, drawing in fluid
- When the piston reaches 12 o’clock, it has travelled out in the barrel to its maximum distance
- As the piston travels towards 9 o’clock, the cavity starts to contract, pushing out fluid
- When the piston reaches 6 o’clock, all fluid has been pushed out and the cycle repeats