Lyons W.C. (ed.). Standard handbook of petroleum and natural gas engineering.2001- Volume 1
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Pumps
465
Oouble acting
I
I
Single acting
Figure
3-51.
Pressure-time curves
for
single-acting and double-acting pumps (one
piston)
[21].
I
Simplex double acting
I
Cylinder
No.
1
+Cylinder
No.
1
4
+Cylinder
No.
2
+Cylinder
No.
2
4
Duplex
double acting
FCylinder
No.
14
I-
Cylinder
NO.
3
4
+Cylinder
NO.
2
4
Triplex single acting
Figure
3-52.
Simplex (double-acting), duplex (double-acting), triplex (single-
acting), pressuretime curves
[Zl].
Calculations
There are several important calculations that are needed in order to properly
evaluate and select the appropriate reciprocating piston pump
[
171.
These calculations
are used in conjunction with Equations
3-32-3-37.
The capacity
of
a reciprocating piston pump,
q
(in3/min), is
q
=
V,(
1-S)N
(3-38)
where
Vd
is the displacement
of
the pumps piston in in.',
S
is the slip fraction, and
N
is the speed
of
the pump in rpm.
The displacement
V,
(gals)
of
a single-acting pump is
nA,=,
v,
=-
23
1
(3-39)
466
Auxiliary Equipment
Figure
3-53.
Example pulsation dampener
[21].
where n is the number of pistons,
AP
is the cross-sectioned area of the piston in in.2,
and
Ls
is the length of the piston stroke in in.
The displacement
V,
of a double-acting pump is
n(2A,
-
a)L,
23
1
v,
=
(340)
where a is the cross-sectioned area of the piston rod in in.2.
The pressure p used in Equation 3-32 is the differential developed pressure
(across the pump inlet and outlet). Since the inlet suction pressure is usually
small compared to the discharge pressure, the discharge pressure is used. Thus,
this is the application resistance pressure in most cases. Figure 3-54 shows a typical
reciprocating pump performance.
The slip
S
is
the fraction of suction capacity loss. It consists of the volumetric
efficiency loss fraction le, the stuffing box loss fraction ls, and the valve
loss
fraction
1".
The slip
S
is
s
=
le
+
1,
+
1"
(3-41)
The volumetric efficiency loss le is
le
=
1
-
e"
(3-42)
where eV is the volumetric efficiency. The volumetric efficiency is the ratio of discharge
volume to suction volume and is expressed as a percentage. It is proportional to the
ratio rand the developed pressure (see Figure 3-55). The ratio r is the ratio of internal
volume
of
fluid between valves when the piston is at the top of its back stroke to the
Pumps
467
Figure
3-54.
BHP
VI.
GPM PLUNGER
%%
(BASED ON
ZERO
SLIP1 DIA GAGE
Z'/e'
D1A.-
1.265
2.
0IA.-
1,425
m
60
0
50
c'
3
40
z
1'1~"
D1A.-
2,500
8
30
1
1
k"
OlA.-3,6SO
4
20
i'/e"
DIA.-4.510
is
MA.-5.?00
14,''
DIA.-
1,865
L
w
0
m
0
10
20
30
40
50
60
PUMP
BRAKE
HORSEPOWER
CRANKSMAFT
RPM
io0
200
300
400
500
Typical power-pump performance (courtesy Ingersoll-
.Rand
CO.).
s
2:
U
z
w
u
2
a
LA
LA
W
i-
W
J
0
>
s
DISCHARGE
PRESSURE,
LB/INz
Figure
3-55.
Volumetric efficiency
(courtesy
Ingersoll-Rand
Co.).
piston displacement (see Figure
3-56).
Because volume cannot readily be measured at
discharge pressure,
it
is
taken
at
suction pressure. This will result in a higher
e,
due
to
fluid compressibility which is neglected.
The stuffing box loss
1,
is usually negligible.
The valve
loss
Ip
is the loss due to the flow of fluid back through the valve during
closing.
This
is
of the order
of
0.02
to
0.10
depending on the valve design.
468
Auxiliary Equipment
C+D
r
Figure
3-56.
Ratio
r
(courtesy
Ingersoll-Rand
Co.).
The mechanical efficiency em of a power pump at full load of pressure and speed
is
0.90
to 0.95. When the pump has a single built in gear as part of the power frame,
the mechanical efficiency is 0.80 to
0.95.
Table
3-25
shows the typical effects
of
speed
and pressure on the mechanical efficiency of a power pump
[17].
Rotary
Pumps
Another important positive displacement pump is the rotary pump. This type
of
pump is usually of rather simple construction, having no valves and being
lightweight. Such pumps can be constructed to handle small fluid capacities (i.e.,
less than a gal/min) to very large fluid capacities (i.e.,
5,000
gal/min or greater).
Rotary pumps are designed to operate at
1,000
psi discharge pressure, but the
normal rotary pump design is for pressure of
25
to about
500
psi with mechanical
efficiencies of 0.80
to
0.85.
Table
3-25
Effects
of
Speed and Pressure
on Mechanical
Eff
iclency
[17,21,22]
Constant Speed
'Yo
full-load Mechanical
developed pressure efficiency
20
40
60
80
100
82
88
90.5
92
92.5
Constant Developed Pressure
Mechanical
'Yo
speed efficiency
44
50
73
100
93.3
92.5
92.5
92.5
Pumps
469
Rotary pumps are classified in two basic groups and several subgroups. These are:
Single-rotor
-vane
-piston
-flexible vane
-screw
Multiple-rotor
-lobe
-gear
-circumferential piston
-screw
These pumps require the maintenance of very close clearances between rubbing
surfaces for continual volumetric efficiency. Some of the important pumps are
discussed.
In general, rotary pumps with discharge pressure of up to
100
psi are considered
low-pressure pumps. Rotary pumps with pressure between
100
and
500
psi are consid-
ered moderate-pressure pumps. Pumps with pressure beyond
500
psi are considered
to
be high-pressure pumps.
Rotary pumps with volume capacities up to
50
gal/min are considered to be small-
volume capacity pumps. Pumps with volume capacities from
50
to
500
gal/min are
moderate-volume capacity pumps. And pumps with volume capacities beyond
500
gal/min are large-volume capacity pumps.
Gear
Pumps
Gear pumps are rotary pumps in which two
or
more gears mesh
to
provide the
pumping action. Usually one of the gears
is
capable of driving the other(s). The
simplest form of this
type
of rotary pump is the gerotor pump. Figure
3-57
shows a
typical gerotor pump configuration. These pumps
are
often used for small-volume
capacity applications where space is quite limited
[HI.
Such gear pumps can be used
on
a
rotating shaft such as a crankshaft
to
provide oil lubrication of critical moving
machine parts.
Figure
3-58
shows an external gear pump. This is an example
of
a
double- (multiple)
rotor pump. Such a gear pump has a drive gear and a driven gear that are encased in
housing with minimum clearance between the housing and the tips
of
the gears. The
Discharga
poh
.Suction
port
-Internal
mar
'
srmnl
Figure
3-57.
Gerotor
type
pump
[17].
470
Auxiliary Equipment
Figure
3-58.
Gear pump (double rotor type)
[17].
simplest type of this pump uses spur gears. The large number of gear teeth in contact
with the casing minimizes leakage around the periphery. The spur-gear type of pump
is limited by its characteristics
of
trapping liquid. This occurs on the discharge side at
the point of gear intermesh. This results in a noisy operation and low mechanical
efficiency, particularly at high rotary speeds. Gear pumps can also be constructed
with single
or
double-helical gears with
15-30"
angles
(0.26-0.52
rad). Such a helical
(or
even a herringbone) gear construction
will
nearly eliminate the problems
of
liquid
trapping but increase the leakage.
One-Tooth Difference.
These pumps are commonly known as gerotor pumps (see
Figure
3-57).
An outlet gear is mounted eccentrically with the outlet casing actuated
by an internal gear rotating under the action of the central shaft (keyway connected).
The internal
gear
has one less tooth than the outside ring gear. There is clearance
between the outside impeller gear and its casing and the inner gear fixed to the shaft.
As
the shaft rotates, the inner gear forces the outer gear to progress around the shaft
at a rate slower than the shaft rotation. The liquid trapped between the two gears
is
forced from the space between the two gears as the rotation takes place.
Two-Teeth Difference.
In this type of pump an abutment on one side plate is used to
fill the clearance between the external and internal gear. This construction reduces
leakage but involves the use of an overhung internal gear. Such a gear arrangement
limits the application of these pumps to small and moderate-volume capacity pumps.
Circumferential Piston
Figure
3-59
shows the typical cross-section of this type of pump. In this pump
liquid is pumped
by
the action of the rotation of the two eccentrically located piston
surfaces. There is no contact between the piston surfaces.
Vane Pump
This example
of
a single-rotor type pump makes use
of
an eccentric shaft on which
are mounted several sliding vanes (see Figure
3-60).
The vanes are forced against the
Pumps
471
Figure
3-59.
Circumferential-piston
pump (double rotor)
[17].
Figure
3-60.
Sliding vane pump (single
rotor)
[
171.
bore of the housing by springs and the centrifugal force of the rotor rotation. The
vanes are usually made of materials that will not damage the surface of the bore (e.g.,
bronze and bakelite). This type of pump is useful for small and moderate-volume
capacities and low pressure. This is due to the rather low speeds such pumps must be
operated at. High speeds result in rapid wear
of
the vanes.
Screw
Pumps
Screw pumps are available in single or double-rotor (multiple) types.
Figure
3-61
shows the cross-section of the typical single-rotor screw pump. In
this pump there is a single helical threaded rotating element.
As
the screw rotates
the liquid progresses axially down the pump. This pump produces continuous
flow with relatively little pulsation or agitation of the fluid. Screw pumps of this
type are quiet and efficient. They are available with high-speed, high-pressure,
and large-volume capacities.
Figure
3-62
shows the cross-section of the typical multiple-rotor screw pump. This
screw pump incorporates right-hand and left-hand intermeshing helices on parallel
shafts with timing gears. Such pumps are available with high-speed, high-pressure,
and large-volume capacities.
For all screw pumps, flow is continuous.
Figure
3-61.
Single screen pump (single rotor)
[17].
472
Auxiliary Equipment
A
Figure
3-62.
Two-screw
pump (double
rotor)
[17].
Calculations
There are several important calculations which are needed to properly evaluate
and select the appropriate rotary pump
[
171.
These calculations
are
used in conjunction
with
Equations
3-32-3-37.
The capacity of a rotary pump operating with zero slip is called displacement
capacity,
a
(gpm). Thus, the actual capacity of a rotary pump
q
(gpm) is
or
dN
q=--ql
(344)
where
Vd
is the pump displacement per revolution in in.3,
N
is the pump speed in
rpm, and
q,
is the pump
loss
in gpm.
Pump horsepower
P
is the power actually imparted to the fluid
by
the rotary pump
in Equation
3-32.
The definitions for the various power terms and efficiencies for the rotary
pumps are the same as those discussed above for the reciprocating pump; namely,
Equations
3-33-3-37.
The volumetric efficiency eV for the rotary pump is
q
e,
=
-
qd
or
(3-45)
(usually expressed as a percentage).
Centrifugal
Pumps
The centrifugal pump is the most important pump of the dynamic class of
pumps for the oil and gas industry. Other pumps in this class are covered in other
references
[17,18].
Pumps
473
In its simplest form, a centrifugal pump consists
of
a
rotating impeller (with radical
vanes) rotating at a rather high speed. The rotating impeller is encased in a rigid
housing that directs the liquid within the pump (see Figure
3-63)
[17].
Liquid is
supplied to the inlet
that
feeds the liquid
to
the center section of the rotating impeller.
The rotational motion of the impeller forces the liquid, via the centrifugal forces, to
move radically outward with the aid of the stationary diffuser. The rigid housing
around the impeller guides the high-velocity fluid around the inside of the housing
and out of the outlet of the pump.
The capacity of this type of pump depends on the pressure head against which the
pump must act (see Figure
3-49).
When the liquid within the impeller is forced radially outward to the diffuser, a
major portion of the velocity energy is converted into pressure energy
by
the stationary
diffuser vanes (see Figure
3-63).
This can also be accomplished by means of a volute,
which is a part of the casing design (see Figure
3-64)
[17].
Centrifugal
Pump
Classiflcations
Centrifugal pumps with diffusion vanes
are
called
diffusion
pumps
or,
more recently,
vertical
turbine pumps.
Those pumps with volute casings
are
called
volute
pumps.
Centrifugal pumps can also be classified
by
the design of the impeller. Centrifugal
pumps
may
have radial-f low impellers, axial-f low impellers, and mixed-f low impellers
(both radial-flow and axial-flow).
Pump impellers are further classified as to the inlet flow arrangement such as
single suction (which has a single inlet on one side) and double suction (which has a
double inlet on each side of the impeller).
Impellers can be further classified with regard to their physical design: a closed
impeller has shrouds
or
sidewalls enclosing the fluid flow, an open impeller has no
shrouds
or
sidewalls, and a semiopen impeller is a mix of the closed and open design.
Another centrifugal pump classification is whether the pump is a single-stage pump
(the pressure head is developed
by
a single impeller)
or
a multistage pump (the pressure
head is developed by two
or
more impellers).
Centrifugal pumps can be further classified by physical design or axially split,
radially split and whether the axis of rotation of the impeller(s) is vertical
or
horizontal.
ser
Figure
3-63.
Typical
diffuser-type pump
[17].
474
Auxiliary Equipment
FLOW
LINE
POINT
OF
ENTRANCE
TO
IMPELLER VANES
200
(66
3.500
2;
2
6:
5
b
GPM
FT TOTAL HEAD
RPM
IN SUCTION DIAM.
IN
DISCHARGE DIAM.
IN
IMPELLER
DIAM.
IN IMPELLER
WIDTH
-1
SWTION
FLANGE
DEVELOPED PATH
I
DISCHARGE
FLANGE
Figure
3-64.
Typical
volute-type
pump
[17].
Horizontal pumps can be classified according as end suction, side suction, bottom
suction, and top suction.
In
applications, centrifugal pumps can be supplied with liquid via piping, or
the pump may be submerged. Vertical pumps are called dry-pit
or
wet-pit types.
The wet-pit pump (submerged) discharges up through a pipe system to some point
above the pump.
Calculations
There are important calculations that are needed to properly evaluate and select
the appropriate centrifugal pump
[20].
There are several similar relationships
for
centrifugal pumps that can be used if
the effects of viscosity of the pumped fluid
can
be neglected. These relate the operating
performance of any centrifugal pump
for
one set of operating conditions to those of
another set of operating conditions, say conditions, and conditions
2.
The volumetric flow rate q (gpm) is related to rate q, (gpm) and the impeller
speeds
N,
(rpm) and
N,
(rpm) by