Pump basics

Pumps are one of the oldest machines used by man for transporting energy from one form to another. It is not surprising that the there are many designs, resulting in some confusion when generic identification of a specific pump is attempted.

02/01/1999


Pumps are one of the oldest machines used by man for transporting energy from one form to another. It is not surprising that the there are many designs, resulting in some confusion when generic identification of a specific pump is attempted.

The simplest way to classify pumps is based on the method used to transmit power to the pumped liquid, mechanical principle behind this energy transfer, and mechanical device for moving fluid. These considerations result in two major pump classes: kinetic (dynamic) and positive displacement (Fig. 1).

Kinetic pumps -- mainly centrifugal -- develop pressure by radial force, dynamic lift, or momentum change. Energy is continuously imparted to the liquid in centrifugal pumps, resulting in radial, mixed, or axial flow, depending on the impeller design.

Positive displacement pumps discharge a given volume for each stroke of a reciprocating pump or revolution of a rotary pump. Energy is added in intermittent pulses by altering or displacing the confined volume of fluid in one or more cavities (Fig. 2).

Although it is difficult to compare the performance and limitations of various pump designs, common criteria are pressure and flow (Fig. 3). Reciprocating pumps can generate enormous pressure, centrifugal pumps offer practically unlimited capacity, and rotary pumps are in between.

Pumps should operate at or near the best efficiency point (BEP) (Fig. 4). Operation at any other point can cause surges and vibration. These reactions can result in bearing and seal problems, as well as excessive power usage. While published performance curves show smooth operating characteristics from zero to well past BEP, there are pumps that cannot function over these wide ranges.

Viscosity has a pronounced effect on pump performance. Manufacturer's recommendations should be followed on viscosity limits. Centrifugal pump performance depends on the impeller design. The influence of pump design on how performance varies with viscosity is greater in positive displacement pumps than in centrifugals, which cannot tolerate great variations in viscosity. The general effect on pump performance is gauged by the table on p 83.

Advantages of reciprocating pumps

- Operate with high efficiencies

- Handle varying pressures at a constant speed

- Produce momentary pressure spikes to clear pipeline obstructions

- Are self-priming and not bothered by air entrainment

Advantages of rotary pumps

- Provide continuous delivery with minor pressure fluctuations

- Handle a wide variety of viscosities

- Are self-priming

Advantages of centrifugal pumps

- Run at higher speeds to produce capacity, resulting in smaller sizes and lower costs

- Deliver fluid free from pressure fluctuations

- Operate at minimum flow without exceeding system pressure

- Operate at maximum flow without excessive power

- Designs match various system requirements

- Use simple foundations due to absence of vibration

- Handle various types of slurries consistently

- Can be direct driven

Effect of increasing viscosity on pump performance

Viscosity, SUS

100 250 500 750 1000

Flow reduction, %

3 8 14 19 23

Feet of head reduction, %

2 5 11 14 18

Power increase, %

10 20 30 50 65

Troubleshooting centrifugal pumps

Problem Cause Remedy

Pump not turning Driver not running Check fuses and circuit breakers

Keys sheared Replace

Drive belt slip Check and adjust

Coupling fault Check if slipping or broken; replace if necessary

Shaft or gears sheared Check; replace if necessary

Pump not priming Inlet valve closed Open valve

Inlet clogged or restricted Check and clear

Air leaks on suction side Replace seals; check lines for leaks

Liquid drained or siphoned from system Install check or foot valve to prevent draining

Worn impeller Inspect and replace; increase pump speed;

install foot valve

No discharge Lack of prime, see above Open vent cocks to release trapped air and fill

pump and suction pipe completely with fluid

Excessive suction lift Check pump inlet for clogging; check suction head

Excessive discharge head Check that valves are open; check piping for

blockage; check total head

Speed too low Check pump rpm

Pump clogged Check that impeller is not clogged

Wrong direction of rotation Check for correct rotation

Vapor lock Bleed suction pipe to clear air lock; check

that suction pipe is properly submerged

Relief valve not properly adjusted Check adjustment; check for dirt on valve seat

Air leak Check seals; check lines for air leaks

Low delivery Air leaks Check suction piping and pump for air leaks;

check pump gaskets

Vapor lock Check NPSH and fluid temperature to ensure

liquid in suction line is not flashing

Low NPSH or damage See above, and check suction pipe and foot valve

Clogged strainer Check and clean

Excessive inlet friction Suction line too small or too many fittings

adding fluid friction

Relief valve incorrectly set or jammed Check and adjust

Excessive system back pressure Reduce system friction

Worn impeller Inspect and replace

Worn wear rings Inspect and replace

Wrong direction of rotation Check direction of rotation

Constriction in suction line Check that foot valve is adequate; check

for obstructions

Wrong pump size Check pump specifications

Poor suction Check that suction pipe is properly submerged

and in position

Troubleshooting reciprocating pumps

Problem Cause Remedy

Low delivery High fluid viscosity Check that fluid viscosity is consistent with

required performance

Excessive fluid temperature Reduce speed and/or delivery; decrease suction head

Speed too low Check rpm against specification for pump performance

Overheating Stuffing box overheats Check that packing is not to tight or badly fitted; check

packing lubricant; check that packing is consistent with manufacturer's specifications; check cooling flow

Bearings overheating Check oil level or lubricant condition; check if correct

lubricant is being used; check bearing for misalignment or excessive tightness; check fittings and condition of oil seals; check that operating speed is not excessive

Fluid too viscous Reduce viscosity

Excessive pressure Reduce pump speed; increase delivery line size

Vibration and noise Cavitation Check operating conditions

Excessive fluid viscosity Check product suitability

Entrained air Check for air leaks

High vapor pressure fluid Check product/pump suitability

Improper pump assembly Check and correct

Unbalanced impeller Check for damage or clogging

Misalignment Check alignment with driver

Nonrigid mount Check mounting for rigidity

Bent shaft; faulty bearings Check and replace

Pump wear Strip down and check for wear

Relief valve chattering Adjust, repair, or replace

Excessive wear Misalignment Check alignment with driver

Nonrigid mount Check mounting for rigidity

Bent shaft Check and replace

Lack of lubrication Check quantity and quality of lubricant

Dirt in pump Use filter to remove

Corrosion Check that pump materials are compatible with fluid

being pumped

Operating pressure too high Check against recommendations for fluid viscosity

Operating speed too high Reduce speed or pressure

Abrasive in fluid Check product/pump suitability

Pump requires Speed too high Check against recommended rpm

excessive power Misalignment Check alignment of pump and driver and foundations

Internal friction Check for rubbing contact and clogging

Tight bearings Check bearing and packings (bearing temperature is a clue)

Lack of lubrication Check quantity and quality of lubricant

High fluid viscosity Check that fluid viscosity is not too high for

economic handling

Troubleshooting rotary pumps

Problem Cause Remedy

No discharge Not primed Prime to fill pump

Excessive suction lift Reduce suction lift or install larger suction pipe

Air leaks Check and correct; check gaskets

Blockage Check adjustment and setting

Excessive wear Check components for wear against manufacturer's tolerances

Wrong direction of rotation Check direction of rotation

Insufficient speed Check running speed

Low discharge Insufficient speed Check running speed

pressure or Wrong direction of rotation Check direction of rotation

reduced capacity Excessive suction lift Reduce suction lift or install larger suction pipe

Air leaks Check and correct; check gaskets

Air entrainment Reposition suction inlet

Relief or bypass valve Setting may be too low; check and reset

Excessive wear Check components for wear against manufacturer's tolerances

Excessive noise Misalignment Check alignment of driver, pump, and coupling

Internal damage Bent or broken rotor; replace

Unbalance Check rotor for static and dynamic balance

Air entrainment Reposition suction inlet

Air leaks Check and correct

Cavitation Check pump operating conditions

Excessive pressure Relief valve set too high; adjust to correct setting consistent

with pump rating

Deterioration Check for excessive wear or clearances on components

Excessive System pressure If system pressure is too high for pump rating, go to larger pump

discharge pressure Relief or bypass valve Check and reset valve for correct pressure

System throttled Discharge valve may be partially closed or system

partially blocked

Excessive wear Abrasive liquid Check that pump is suitable for handling liquid if abrasive

solids are present; check that filter or strainer are adequate

Distortion Check for piping loads being transmitted directly to casing

and correct

Excessive pressure Relief valve set too high; adjust to correct setting

Excessive speed Check that speed is consistent with pump specification

for viscosity of fluid handled

Excessive input Damage Check for bent or damaged shaft or other parts

power required Excessive pressure Relief valve set too high; adjust to correct setting

Excessive fluid viscosity Check speed rating against actual fluid viscosity;

reduce speed for higher viscosities

Excessive speed Check against pump rating for fluid viscosity being handled

Pump overheats Relief or bypass valve Check settings

Excessive speed for

fluid being handled Check that speed is consistent with rating for fluid viscosity

Excessive pressure Relief valve set too high; adjust to correct setting

Discharge throttled Looped flow through relief valve will cause overheating;

may be relieved by separate relief valve discharging to tank





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