Pumps Flashcards
Pressure requirements
- Move the liquid from its original height to its final height
- Overcome any pressure in the destination vessel
- Overcome the pressure losses in the system due to piping, bends, valves etc.
Key factors in specifying a pump
- Quantity of liquid to be habdled
- Pressure (or head) requirements
- The nature of the fluid to be pumped
- The available power supply
- Continuous or intermittent usage
2 types of pump
- Centrifugal
- Positive displacement types
Viscosity does not change with shear stress
Newtonian
Shear thinning
Pseudoplastic
Shear thickening
Dilatant
works by delivering kinetic energy to the fluid and converting it to flow
centrifugal pump
moves a fixed volume against the system pressure
positive displacement pump
- most widely used type of pump for liquids
- the fluid is fed to the centre of a rotating impellor
- thrown outward by centrifugal action into a volute chamber
- kinetic energy gives the fluid velocity through discharge pipe
Centrifugal pump
Specifying a pump
- What is the material being pumped
- flowrates and pressure requirements
- what is the fluid temperature
- is the fluid corrosive
- what are the site standard process connections
- what certification is required
- is the pump variable speed, soft start, or drive on line
- is the pump motor single phase or three phase
- 50 or 60 Hz electrical supply
- is the suction flooded (primed)
- mechanical seals
- budget
Used for lower viscosity fluids
centrifugal pump
Fluid is fed to the centre of a rotating impellor and thrown outward by centrifugal action into a volute chamber. The kinetic energy imparted by this rotation gives the fluid velocity through discharge pipe
centrifugal pump
… are more efficient but not sanitary
closed impellers
main method for choosing a pump
pump performance curve
Describes the pressure and flow output that can be achieved by the pump
Pump performance curve
Point of highest efficiency of the pump. All points to the left and right of the BEP has a lower efficiency
B.E.P best efficiency point
The plot starts at zero flow. What the head at this point correspond to?
the shut off head of he pump
The head decreases until it reaches its minimum point. What is this point called?
the run out point
Represents the maximum flow of the pump. Beyond this the pump cannot operate
The rate at which the material is delivery by the pump
Flow
The pressure provided by the pump for that flow as measured in terms of height
head
Typically shown by plotting the head h, against flow Q
Pump performance
Can also plot power required P, and efficiency against Q on the same chart
Head falls as flow rate …
increases
Where efficiency reaches a maximum and then falls. This is the most efficiency point to run the pump
Duty point
If your requirements are …. the HEAD vs FLOW curve, the pump will be sufficient for your application
on or below
Why is pressure expressed in terms of height?
- pump will often be expect to a deliver fluid from a low position to a higher position
- we may already have a static head of liquid from the feeding vessel
Pumps in series
Increase in pressure but not flow
Pumps in parallel
Increase in flow but not pressure
Centrifugal pump advantages
- Single construction
- No valves required within the pump
- Steady delivery of fluid
- Low maintenance cost
- Relatively small compared with other pumps of similar capacity
- Can handle suspended solids in low concentration
Centrifugal pump disadvantages
- Single stage pump cannot develop a high pressure
- High efficiency only over a narrow range of conditions
- Needs to be primed with liquid
- Cannot handle high viscosity liquids
- Cavitation if not used properly
- Impellers can be damaged by solids
- Gases in liquid are a problem
the formation of an empty space within a solid object or body
cavitation
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How cavitation occurs
- Pumps create a low pressure at the inlet
- If liquid pressure at the pump inlet is below vapour pressure, bubbles form
- Further drop in pressure during movement through the pump
Pressure required to boil a liquid at a given temperature
vapour pressure
What NPSH stands for
Net Positive Suction Head
Minimum net pressure required at the pump inlet nozzle to prevent the pump cavitating
Net Positive Suction Head Required
The absolute pressure at the pump inlet and calculated for the system
Net positive suction head available
To avoid cavitation
NPSHA > NPSHR
Occurs when the pressure at the pump inlet is below the vapour pressure of the liquid
Cavitation
Motor is fixed speed
Direct on line (DOL)
Motor controlled by a frequency drive VFD
Variable Speed
Motor start (ramp up) speed can be controlled
Soft Start
Speed governed by number of poles and AC frequency
Fixed speed motors
Output from a fixed speed pump will follow a prescribed head vs flow profile. If this flow has to be controlled it can be “throttled” using a valve to reduce the flow. These are wasteful since energy from the pump is “lost”. An alternative method id to adjust the speed of the drive motor to the pump.
Variable Speed Drive
Used for lower flowrate but higher pressure applications
Positive displacement pumps
Suited for higher viscosity fluids
Positive displacement pumps
Generally self priming
Positive displacement pumps
Change in flow varies in proportion to …
speed
Change in head varies in proportion to
speed squared
Change in power varies in proportion to
speed cubed
These work by trapping fluid in fixed amounts and forcing it into the discharge tube.
Theoreticall can produce the same flow at a given speed no matter what the discharge pressure is.
Cannot safely operate against a closed system
No shut off head like centrifugal pumps
- Pressure will build if there is an obstruction
- Need a pressure relief system in the pump or downstream
Rotary, reciprocating or diaphragm type pumps
- liquid flows into the cavity and is trapped by the lobes as they rotate
- The liquid travels around the interior of the casb in the “pockets” between the lobes and the casing
- The meshing of lobes produces a change in pressure between the pump and discharge port causing the liquid to flow out
- Larger clearances than gear pump
Rotary lobe pump
Rotary lobe pump advantages
- good for solids and food applications
- sanitary and superior CIP/SIP capabilities
- No damage to particles, largest sized particles of any PD pump type
- No metal to metal contact
- Non pulsating discharge
Rotary lobe pump disadvantages
- not as good for higher viscosity as gear pump
- problems at low viscosity also
- Lower suction than gear pump
- A volume of fluid passes between the teeth of two meshing gears and their casing at a constant rate
- As the gears rotate and the meshed teeth separate, a partial vacuum is formed that fills with fluid
- As the gears continue to rotate, they trap the liquid and move it around the casing from the suction to the discharge point
Rotary gear pump
Advantages of rotary gear pumps
- Able to pump high viscosity liquids over large distances: good for high temperature application & not suited for low viscosity
- Bi-directional : can fill or empty a vessel
Disadvantages of rotary gear pumps
- limited to clean non abrasive materials
- Large footprint and relatively low capacity
- rotates to form chambers with the intermeshing of two screws inside the pump housing
- the chambers fill with fluid and move it form the suction side to the higher pressure discharge side of the pump
- they can handle non homogenous fluid and works regardless of lubricity and viscosity
Twin screw pump
- Most often used in situations that contian high gas volume fraction and fluctuating inet conditions
- some are reversible
- better for clean fluids - oil and fuels
Twin Screw pumps
type of positive displacement pump where there is a helical rotor turned inside helical stator - shaped to allow cavities
Internal clearnace so suited for delicate and high viscosity fluids
Steady flow
Reversible
Fllow rate is proportional to running speed
Higher Pressure up to 40 bar.
Flow rate up to 250 m^3/hr
Progressive cavity
Gear and twin screw have … clearances
smaller
Lobe and progressive have … clearances
bigger
Gear and lobe pumps have
….. footprints and run at
lower speeds than screw and
progressive cavity
bigger
- As the piston rod moves forward liquid is forced out through the discharge valves
- Simultaneously liquid enters the pump inlet
- As the piston rod begins the backward stroke the inlet valve machanically opens, permitting the liquid to continue its flow forward through the piston into the discharge chamber, until the stroke is completed
Piston pump
Positive displacement pumps
- rotary gear
- rotary lobe
- twin screw
- progressive cavity
- at the beginning of the stroke, the plunger displaces the liquid in the manifold chamber and forces the discharge valve open. There is a split second at the end of the stroke when bboth inlet and discharge valves are closed
- As the plunger rod begins its backward stroke, the inlet valve opens to allow more liquid into the manufold chamber, thereby keeping a smooth forward flow of liquid
Plunger pump
- Run via compressed air
- Intrinsically safe (ATEX)
- Low cost
- Generally small and portable
- Only small lifts possible
Compressed air powered double diaphragm pumps
- Diaphragm moves right and the volume of a left hand chamber is increased, the pressure decreases, and fluid is drawn into the chamber
- When the diaphragm moves back, chamber pressure increases from dereased volume, the fluid previously drawn in is forced out
- The right hand (opposing) sesction of the pump works in an alternating fashion
Diaphragm pump
- There is no limit to the outlet pressure if “dead headed”
- Some PD pumps produce pulsating flows
- Dampers fitted on the inlet to the pump as suction stabilisers to reduce vibration
- this mitigates against head loss due to rapid acceleration of fluid velocity on each stroke of the pump
- Flood suction (priming) is not always required for some types of PD pump
Pressure and delivery issues for PD pumps
Positive displacement pumps have … NPSH requirements
very low
Potential pump failures for centrifugal pumps
- Foreign objects
- Dry run
- Over load motor, end of curve runniing (high amps)
- Closed valve/ dead head pump (heat)
- Poor suction conditions (vibrations)
Potential pump failures for PD pumps
- Foreign objects
- Dry run
- Over pressure, no PRV (metal contact)
- Undersized Drive (gearbox & motor)
- Thermal Shock (metal contact)
- Poor suction conditions (vibrations)
Positive Displacement Advantages
- Handles almost all types of fluids - conventional and unconvential
- Fluid density does not influence pump performance
- Self priming
- Very wide operating range at overall high efficiency
- Very low NPSH requirements and not sensitive to vapour and cavitation
- Very good flow control
- Higher cost and footprint compared to centrifucal pumps
Weakest component of the pump
pump seal
Common cause of pump seal failure
Dry running of pump
