Pump Handbook by Igor J. Karassik, Joseph P. Messina, Paul Cooper, Charles C. Heald - 3rd edition
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6.1.5 GAS TURBINES 6.95
able units because they are too numerous and are continually being upgraded and added to.
A listing can be found in Sawyer’s Gas Turbine Catalog, which is published annually.
Pipeline Service Crude oil pipeline service has seen a tremendous growth in the use
of gas turbine drivers since the mid-l950s, and this trend will continue as crude oil pro-
duction becomes more and more remote from its markets. The advantages of the gas tur-
bine for this application are as follows:
1. Installed cost is usually lower than that of a corresponding reciprocating engine.
2. Variable-speed operation allows maintenance of a specific discharge pressure under a
wide range of operating conditions, thus achieving maximum flexibility.
3. Normal gas turbine control system is easily adapted to unattended operation and
remote control.
4. Operating experience has proved the gas turbine to have a high degree of reliability.
5. Gas turbines can be packaged into modules for ease of transportation and erection.
Water Flooding An important aspect of oil production is the use of secondary recovery
methods to increase the output of crude oil reservoirs whose pressure does not allow the
crude to flow freely to the surface. Flooding the reservoir with high-pressure water has
been a primary technique for years. The development of high-pressure centrifugal pumps
has allowed increased water flow into oil fields.
As most oil production is located in remote areas, the packaged gas turbine driver has
become increasingly popular. Figure 9 shows a typical 3300-bhp (2460-brake kW) split-
shaft gas-turbine-driven centrifugal water flood pump package. A cutaway view of a typi-
cal 1100-hhp (820-brake kW) gas turbine is shown in Figure 10; note the reduction gearing
at the exhaust end for direct driving of a pump. Supporting systems, such as starter, lubri-
cation oil pumps, governor, and fuel oil pumps, are driven off the accessory pods located at
the air inlet end. Offshore platform installations require drivers with a minimum vibra-
tion as well as small, unbalanced inertial forces. The gas turbine fits both of these descrip-
tions very well.
Cargo Loading Another interesting application of this prime mover is in the field of
cargo loading, where units are currently in operation charging tankers with crude oil.
Selection of a gas turbine pumping unit with critical speeds above the normal operating
range allows great flexibility of operation, particularly during final topping operations.
FIGURE 9 A 3300-bhp (2460-bkW) gas turbine-driven waterflood package (Solar Division of International
Harvester)
6.96 CHAPTER SIX
FIGURE 10 Cutaway view of 1100-bbp (820-bkW) split-shaft gas turbine driver (Solar Division of International
Harvester)
Application Considerations The application of gas turbine drivers can vary from a sim-
ple driver for one pump operating at constant flow and discharge pressure to a multiplicity
of units operating at variable speed on a pipeline. For the purposes of this discussion, it is
assumed that the pumping system has been analyzed, a pump selection has been made, and
all possible operating conditions have been analyzed so brake horsepower (brake kilowatt)
and speed requirements are known.
The brake horsepower (brake kilowatt) output of the gas turbine must equal or exceed
that required by the pump.This output can be determined by the use of specific performance
curves similar to Figure 7 as corrected for elevation (Figure 8). Gear losses as necessary are
added to the brake horsepower (brake kilowatt) required by the pump. Intermediate brake
horsepower (brake kilowatt) and speed requirements should then be checked against a gas
turbine output versus speed curve (Figure 4).
A torsional analysis of the combined unit is made (usually by the gas turbine manu-
facturer) to ensure the absence of any critical speeds in the operating range. Table 1 is a
typical data sheet recommended for use when purchasing a gas turbine to drive a cen-
trifugal pump.
6.1.5 GAS TURBINES 6.97
TABLE 1 Typical data sheet for gas turbine driven centrifugal pump
Information from purchaser Information from manufacturer
Pumping requirements
Service__________________________________
Liquid __________________________________
Pumping temp. __________________________
Capacity (total) normal/max__________/____
No. pumps operating _____________________
Specific gravity at pump temp. ____________
Viscosity at pumping temp. _______________
Total head_______________________________
NPSH available__________________________
Pump type, materials, and accessories
Site conditions
Elevation, ft (m) _________________________
Range of site ambient temperature
Dry bulb Wet bulb
Design, °F (°C) ________ ________
Maximum, °F (°C) ________ ________
Minimum, °F (°C) ________ ________
Atmospheric air
Dust below 10 mm, ppm __________________
10 mm and above, ppm____________________
Corrosive constituents:
Sulfur, ammonia, ammonium salts,
salt or seacoast, other __________________
Noise specifications
City, state, federal, other__________________
Emission specifications
City, state, federal, other _________________
Utilities available at site
Steam: Pressure, lb/in
2
(kPa) gage ______________________
Temp., °F (°C) ___________________
Quantity, lb/h (kg/h) _____________
Electricity
V Phase Cycles
ac ______ ______ ______
dc ______ ______ ______
Cooling water
Source___________quality_______________
Supply temp.________min_______max
Supply press, lb/in
2
(kPa) gage
Max return, °F (°C) ____________________
Pumping requirements
Service__________________________________
Liquid __________________________________
Pumping temp. __________________________
Capacity (total) normal/max__________/____
No. pumps operating _____________________
Specific gravity at pump temp. ____________
Viscosity at pumping temp. _______________
Total head_______________________________
NPSH available__________________________
Bhp (brake W)
required _____normal _____max
Speed, rpm _____normal _____max
Efficiency _____normal _____max
Pump type, materials, and accessories
Gas turbine excluding gear
Design Max Min
Dry bulb temp.,
°F (°C) ______ ______ ______
Output, hp (kW) ______ ______ ______
Heat rate,
Btu/hp h
(J/kWh) (LHV) ______ ______ ______
Output shaft
speed, rpm ______ ______ ______
Turbine inlet
temp., °F (°C) ______ ______ ______
Gear loss ______ ______ ______
Net hp (kW) ______ ______ ______
Total utility consumption
Cooling water, gpm (1/s) __________________
Electric power, kW ac/dc __________________
Steam, lb/h (kg/h) ________________________
Compressed air, standard
ft
3
/min (m
3
/min)________________________
Shipping data
Aux.
Turbine items
Shipping wt, tons (kg) ______ ______
Max erection wt, tons (kg) ______ ______
Max maint. wt, tons (kg) ______ ______
Length, ft-in (m) ______ ______
Width, ft-in (m) ______ ______
Height, ft-in (m) ______ ______
FURTHER READING __________________________________________________
Baumeister, T., and Marks, L. Standard Handbook for Mechanical Engineers. 8th ed.
McGraw-Hill, New York, 1978.
Combustion Gas Turbines for General Refinery Services, API Standard 616, 1982.
Dacy, I. R. “Gas Turbines Used on Arctic Pipelines.” Oil Gas J. 200:26, 1973.
Proposed Gas Turbine Procurement Standard, ASME.
Sawyer, J. W., ed. Sawyer’s Gas Turbine Engineering Handbook. 3rd ed. Gas Turbine Pub-
lications, Stamford, CT, 1985.
Schiefer, R. B. “The Combustion of Heavy Distillate Fuels in Heavy Duty Gas Turbines.”
ASME 71-GT-56, 1971.
Shepherd, D. G. Introduction to the Gas Turbine. 2nd ed.Van Nostrand, Princeton, NJ, 1960.
6.98
CHAPTER SIX
TABLE 1 Continued.
Information from purchaser Information from manufacturer
Source. Adapted from API Standard 616.
Fuel
Gas ____________liquid_________________
Analysis attached
Accessory items required (see list in
right-hand column)
Accessories included (as required by pur-
chaser)
Inlet air filter ___yes ___no
Inlet air silencer ___ ___
Exhaust silencer ___ ___
Exhaust duct ___ ___
Starting equipment ___ ___
Load gear (if required) ___ ___
Driven pump ___ ___
Coupling ___ ___
Fire protection system ___ ___
Equipment enclosure ___ ___
Baseplate or soleplates ___ ___
Combined turbine-pump ___ ___
Lub. oil system ___ ___
Main lube pump ___ ___
Auxiliary lub. pump ___ ___
Lub. reservoir ___ ___
Lub. filter ___ ___
Lub. oil cooler ___ ___
Unit control panel ___ ___
Auxiliary motor ___ ___
Control center ___ ___
6.2.1
EDDY-CURRENT COUPLINGS
W. J. BIRGEL
S. A. MOLL
6.99
SECTION 6.2
SPEED-VARYING
DEVICES
DESCRIPTION _______________________________________________________
The eddy-current coupling is an electromechanical torque-transmitting device installed
between a constant-speed prime mover and a load to obtain adjustable-speed operation.
Generally ac motors are the most commonly used pump drives, and they inherently oper-
ate at a fixed speed. Insertion of an eddy-current slip coupling into the drive train will
allow desired adjustments of load speeds.
In most cases, the eddy-current coupling has an appearance very similar to that of its
driving motor except that the coupling has two shaft extensions. One shaft, which operates
at constant speed, is connected to the motor, whereas the other shaft, providing the
adjustable-speed output, is connected to the load. A typical self-contained air-cooled eddy-
current slip coupling is shown in Figure 1.
The input and output members are mechanically independent, with the output magnet
member revolving freely within the input ring or drum member. An air gap separates the two
members,and a pair of antifriction bearings maintain their proper relative position. The mag-
net member has a field winding that is excited by direct current, usually from a static power
supply. Application of this field current to the magnet induces eddy currents in the ring. The
interaction between these currents and magnetic flux develops a tangential force tending to
turn the magnet in the same direction as the rotating ring. The net result is a torque avail-
able at the output shaft for driving a load.An increase or decrease in field current will change
the value of torque developed, thereby allowing adjustment of the load speed. Field current,
output torque, and load speed are usually not proportional. Therefore load torque character-
istics and speed range must be known for proper adjustable-speed drive size selection.
Most eddy-current couplings have load speed control.An integral part of this system is
a magnetic pickup that provides an indication of exact output speed and enables control
of load speeds within relatively close tolerances regardless of reasonable variations in load
6.100 CHAPTER SIX
FIGURE 1 Cutaway of an Ampli-Speed eddy-current slip coupling (Electric Machinery Mfg.)
torque requirements. Slip-type adjustable-speed drives without load speed control will
experience fluctuations in load speed when load torque requirements vary.
FUNDAMENTALS_____________________________________________________
The eddy-current coupling, like many other adjustable-speed drives, operates on the slip
principle and is classified as a torque transmitter. This means that the input and output
torques are essentially equal, disregarding frictional and windage losses.The motor input
power is equal to the sum of load power and what is known as slip loss.This slip loss is the
product of slip speed, which is the difference between motor and load speed, and the trans-
mitted torque.
The various relationships may be expressed as follows:
In USCS units
In SI units
In USCS units
In SI units
In USCS units
In SI units
This may be further expressed as follows:
Slip loss 1kW2
rpm
3
T
9545
Slip loss 1hp2
1rpm
1
rpm
2
2 T
5250
rpm
3
T
5250
Load kW
rpm
2
T
9545
Load hp
rpm
2
T
5250
Motor kW
rpm
1
T
9545
Motor hp
rpm
1
T
5250
6.2.1 EDDY-CURRENT COUPLINGS 6.101
In USCS units
In SI units
Or
In USCS units
In SI units
where rpm
1
motor speed at designated load
rpm
2
load speed at designated load
rpm
3
slip speed at designated load
T load torque, ft lb (N m)
An eddy-current coupling must slip in order to transmit torque. The normal minimum
value of slip for a centrifugal pump application is usually 3%, but values from 1 to 4% are
common. The above formulas hold true regardless of the type of load involved.
The efficiency of an eddy-current coupling can never be numerically greater than the
percentage that the output speed is of the input motor speed. This effectively takes into
consideration only the slip losses, and a true efficiency value must also include frictional,
windage, and plus excitation losses. The frictional and windage losses are constant for a
fixed motor speed and therefore increase in significance with speed reduction. Excitation
losses, on the other hand, decrease with reduction in output speed. The overall effect of
these losses is an efficiency versus speed relationship that is somewhat linear with effi-
ciency values anywhere from 1 to 4 points less than the output speed percentage.
LOAD CHARACTERISTICS _____________________________________________
In the application of slip couplings for continuous pump loads, both variable- and constant-
torque requirements are encountered.
Variable-Torque Loads Variable-torque loads are those where the torque increases
with the speed and varies approximately as the square of the speed while the load power
varies approximately as the speed cubed. The centrifugal pump fits into this classifica-
tion. To be specific, the above torque-power-speed relationship exists only where the fric-
tion head is the total system head, as would be the case if a centrifugal pump were
pumping from and to reservoirs having the same liquid levels or in a closed loop.
The various relationships of load power, motor power, and slip loss applying to a typi-
cal variable-torque load are shown in Figure 2
—
again frictional and windage losses have
been disregarded.
Static heads, which usually exist in centrifugal pump systems, do not significantly
affect the selection of a suitable slip coupling for a specific requirement. Pump efficiency,
on the other hand, can be quite an important factor when it decreases significantly with
speed reduction. Pumps with relatively flat efficiency curves are most desirable for
adjustable-speed duty.
Centrifugal pumps usually operate against some static head. This causes the pump
torque to follow a closed discharge characteristic until sufficient speed is reached to cause
the resultant discharge head to equal or exceed the static head.This is equivalent to a closed
discharge or normally unloaded condition. The pump load then follows a different curve to
the full-load condition at minimum slip. These characteristics are illustrated in Figure 3.
Curve OAB indicates the closed discharge power. At point A the static head is overcome, and
Slip loss 1kW2
rpm
3
rpm
1
motor kW
Slip loss 1hp2
rpm
3
rpm
1
motor hp
Slip loss 1kW2
rpm
3
rpm
2
load kW
Slip loss 1hp2
rpm
3
rpm
2
load hp
6.102 CHAPTER SIX
FIGURE 3 Percent efficiency, motor load loss power versus pump speed in percent of motor speed for a static
head and friction pumping system
FIGURE 2 Load, power, and slip characteristics for a friction-only pumping system
6.2.1 EDDY-CURRENT COUPLINGS 6.103
FIGURE 4 Power, torque, and losses for a constant-torque load
the pump load then rises along curve AC; this causes a significant difference in slip loss as
shown. Under conditions as indicated, where static head is not exceeded and water does not
start to flow until 80% of full speed is reached, the slip loss never exceeds 10% of the full-load
rating. In general, this is true of a pump with high static head.
Constant-Torque Loads Constant-torque loads are those requiring essentially con-
stant torque input regardless of operating speed. Positive displacement pumps generally
are of this type. The load characteristics showing the division between slip loss and load
power are illustrated in Figure 4. Note that the driving motor output does not change,
regardless of load speed and power.
The slip loss characteristics make slip couplings undesirable for large power loads if
any appreciable speed range is required. However, in relatively small units, the simplicity
and ease of speed control will frequently justify the use of slip couplings instead of more
efficient but more complex speed-control systems.
The constant-torque-load capacity of a slip coupling is largely limited by slip loss and,
to a lesser degree, by breakaway torque and minimum slip. Because slip loss is directly
proportional to slip, the desired speed range will definitely limit the torque, and therefore
the power, that can be transmitted. In some cases, breakaway torque is important as the
static friction may be quite high. It is usually recommended that at least 150% of starting
torque be available for any constant-torque load.
CONSTRUCTION _____________________________________________________
Eddy-current couplings are available in both horizontal (Figure 5) and vertical (Figure 6)
configurations as might be required for any pump mechanical arrangement. Horizontal
machines in smaller sizes are frequently close-coupled to the drive motor in what is known
as integral construction. Larger sizes are usually flexibly coupled to the drive motor and
pump load.
6.104 CHAPTER SIX
FIGURE 6 Vertical centrifugal pumps driven by 40-hp (30-kW), 1750-rpm induction motors and eddy-current
couplings (Electric Machinery)
FIGURE 5 Horizontal centrifugal pump driven by a 1000-hp (746-kW), 1780-rpm induction motor through a
stepup gear and an eddy-current coupling (Electric Machinery)
Vertical motors and slip couplings are close-coupled to limit overall height and to pre-
vent vibration problems. Pump hydraulic thrust requirements can be accommodated,
when necessary, in much the same way as in constant-speed motor applications. Because
of mechanical limitations, thrust bearings are frequently located in the bottom of the
adjustable-speed drive.