Page 55 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Pump Best Practices Be st Practice 2.2

Pump drivers are normally electric motor or steam turbine.
Choice of driver type is usually based on plant utility balance
plus reliability evaluation of each type to perform within the
operating system.
Motors can be sized by several methods:
Name plate rating large enough to cover the complete range of
pump performance curve.
Size of motor based on system curve analysis to establish
maximum horsepower required at intersection of system curve
and pump performance curve.
API Standard 610 has guidelines for sizing motor drives. A
margin of 125% is recommended for motors equal to or less
than 18.6 kW (25hp), 115% for 22.4 kW–56 kW (30 to 75 hp) and
Fig 2.2.7 Suction recirculation flow pattern 110% for motors rated 75 kW (100hp) or more.
Steam turbine drives are normally sized for the power required at
pump rated condition. This is possible because turbines can
accommodate increased power loads more readily than electric
Centrifugal pumps require bearings to carry radial/axial loads
motors.
Bearing alternatives: Anti-friction, hydrodynamic ring oil
lubricated or hydrodynamic pressure lubricated
Oil lubricated (anti-friction) bearings are used in majority of Fig 2.2.10 Driver sizing guidelines
process pumps to carry loads
Pressure lubricated hydrodynamic bearings are normally used
for high pressure, high horsepower, high speed applications
Criteria in API Standard 610 for pressure lubrication: when Most commonly applied centrifugal pump – most applications
product of pump rated horsepower and rated speed in rev/min is Total head limited to 380 mm (15 inch) impeller diameter @3600
greater than 2.0 million rpm (approximately 183 meters [600 feet] head). Larger diameter
Pressure lubrication systems can either be integral or separate, impellers operate at lower speeds
but should include, as a minimum, an oil pump, reservoir, filter, Low, medium, high temperature (with cooled bearings, stuffing
cooler, controls and instrumentation
box)
Ring oil lubrication may be applied to hydrodynamic journal Relatively low NPSH required for single suction impeller
bearings in less severe service (when dN factor is less than All process services with proper materials selection
300,000. A dN factor is the product of bearing, size (bore) in Center of gravity of impeller is outside bearing span
millimeters and the rated speed in revolutions per minute).
Axial thrust
Fig 2.2.8 Bearing application guidelines Fig 2.2.11 Single stage, single suction overhung impeller
characteristics

Mechanical seals are often used in pumps handling hazardous as
well as non-hazardous liquids that must be contained within the
unit. Gaining acceptance as alternative to single stage overhung
The single seal arrangement is most widely used in process pump
industry. Total head limited to approximately 122 meters (400 ft)
Single seal design consists of a rotary face in contact with Low temperature applications only
a stationary face. Relatively low NPSH required
For most services, a carbon face mating against tungsten carbide Limited to approximately 150 kW (200 H.P.)
is satisfactory. Most designs utilized do not incorporate bearings (they use the
Seals offer the advantages of long life, low maintenance and high motor bearings to position the pump shaft)
reliability. Note: Designs are now available that incorporate an anti-friction
In general, seals handling light specific gravity liquids at low bearing in the pump housing
temperature and high vapor pressure give most problems in the
field.
Materials for cold service seals must be suitable for temperatures Fig 2.2.12 Single stage in-line pump characteristics
of start-up, cool down and running; the atmospheric side must be
held above 0°C (32°F) to prevent ice formation; and there must be
enough liquid at the seal surfaces.
Successful operation of any seal depends largely on correctly Use for high pressure applications greater than 13.730 kpa
specifying liquid conditions of vapor pressure, temperature specific (2000 psi)
gravity, etc. Unlimited head capability
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API Standard 682 is an excellent resource for overall mechanical Maximum flow approximately 114 M /hr (500 gpm) (5 plungers)
seal application guidelines. Limited to approximately 1119 kW (1500 bhp)
The pressure in the seal chamber (stuffing box) must be at least Constant volume capacity
25psig above the pump suction pressure. Produces pulsations and requires pulsation dampners

Fig 2.2.9 Mechanical seal application guidelines Fig 2.2.13 Plunger-PD pump characteristics

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