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外文原文
畢業(yè)設(shè)計(jì)外文原文
《Standard Handbook of Petroleum & Natural Gas Engineering 》
by Lyons, William C.; Plisga, Gary S.
Publication: Burlington, MA Elsevier, 2005
3.3 PUMPS
Pumps are a mechanical device that forces a fluid to move
from one position to another. Usually a pump refers to the
mechanical means to move incompressible (or nearly incompressible)
fluid or liquid. Pumps are our earliest machine and are to this day one of our most numerous mechanical devices. Pumps are a very essential part of the oil and gas industry. They are used throughout the industry, from drilling
operations through to final delivery to the customer.
3.3.1 Classifications
Pumps are classified into two basic classes, displacement
and dynamics. The most widely used pumps in the oil and gas industry
are reciprocating displacement pumps (in particular piston
plunger type), the rotary displacement pump, and the centrifugal
dynamic pump. Only these pumps will be discussed in detail.
The reciprocating and rotary positive displacement pumps
primary characteristic is that they have a nearly direct relationship
between the motion of the pumping elements and
the quantity of liquid pumped. Thus, in positive displacement
pumps liquid displacement (or discharge from the
device) is theoretically equal to the swept volume of the
pumping element. Figure 3.3.3 shows the typical positive
displacement plot of discharge rate Q (ft3/s) versus pressure
P (lbs/ft2) [3]. The discharge rate remains the same
(assuming a constant rate of rotation for the system) regardless
of the pressure in the flow. The pressure in the flow
is, of course, the result of resistance in the flow system
the pump discharges to. If the resistance increases, rotation
can be maintained and more force applied to each stroke
of the pump (i.e., power). This is why the reciprocating piston
plunger pump is also called a power pump. In practice,
pressure does have some influence on the capacity of these
pumps. This is because as the pressure increases, there is
some leakage of the seals in the system. This leakage is
somewhat proportional to the pressure, particularly beyond
some characteristic pressure related to the seals. The difference
between theoretical flow and the actual flow of a pump
is often referred to as slip. This slip is shown in Figure 3.3.3.
In the dynamic pump, in particular, the centrifugal pump,
the discharge rate Q is determined by the resistance pressure
P in the flow system the pump discharges to (assuming
some given speed of the pump). This is illustrated in Figure 3.3.4.
3.3.3 Reciprocating Pumps
The piston plunger pump is the simplest form of a positive
displacement pump. These pumps can be powered by
a variety of prime movers, internal combustion engines, and
electric motors (and in some cases, powered by a gas turbine
motor). In such applications, the separate pump unit is
connected to the prime mover by a power transmission.
The capacity of a pump is determined by the number of
plungers or pistons and the size of these elements (bore and
stroke). A reciprocating pump is usually designed for a specific
volumetric rate capacity and pressure capability. These
factors are set by the application. Once the volumetric rate
capacity and pressure capability are known, a designer can
determine the plunger piston bore and stroke the rotation
speed range and the power of the prime mover needed to
complete the system. Reciprocating pumps are fabricated in both horizontal andvertical configurations.
3.3.3.1 Single-Acting Pump
A single-acting pump has only one power (and discharge)
stroke for its pistons. Such a pump brings fluid into its chamber
through the inlet or suction value or the piston is drawn
backward to open the chamber. To discharge the fluid, the
inlet valve is closed and the outlet valve opened as the piston
is forced forward to push the fluid from the chamber into
the discharge line. The piston motion is accomplished by a
rotating crankshaft that is connected to the piston by a piston
rod much like an internal combustion piston engine. The
rotating crankshaft of the pump is rotated by the rotational
power of the prime mover (through a transmission) [7].
The single-action pump is usually available with three, five
and even seven pistons. The odd number of pistons allows
the pump to be rotationally balanced, and the use of at least
three pistons reduces the discharge pulsation of these single-acting
pumps. A three piston pump single-action pump is
called a triplex pump. A five piston, or seven piston single -acting
pump is called a multiplex pump.
3.3.3.2 Double-Acting Pump
Double-acting pumps have two power strokes. As a piston
of the pump is pushed forward, the fluid is discharged from
the forward chamber into the discharge line (much like a
single-action piston). But during this same stroke, the chamber
behind the piston (which contains the connecting rod)
is being filled via that chamber’s inlet valve (Figure 3.3.5).
When the forward power stroke is complete and the fluid discharged
fromthe chamber in front of the piston, the chamber
behind the piston is filled. The crankshaft continues to rotate,
requiring the piston to begin a rearward stroke. During this
stroke the fluid behind the piston is forced from its chamber
into the discharge line via the outlet valve and the chamber
in front of the piston refills via its inlet valve [7].
Double-acting pumps are usually available with one or twopistons.
A one-piston double-action pump is called a double-acting
simplex (since there are older single-action steam and pneumatic
driven simplex pumps).A two piston double-action pump is called a duplex pump.
3.3.3.3 Flow Characteristics
All reciprocating pumps have a pulsating discharge. This is
the result of the piston motion as it stops and reverses. At
this moment, the flow from that piston theoretically drops
to zero. Thus, the discharge curves as a function of time
are those illustrated in Figure 3.3.6.By having two or more pistons the pulsation of the discharge from the pump can besmoothed out and the magnitude of the pulsation reduced ifthe pistons motions are timed for proper dynamic balancingof the pump (Figure 3.3.7). For those pumps that have largepulsations, a cushion change (or accumulator) may be used
in the discharge line to reduce or eliminate the pulsations
(Figure 3.3.8).
Single acting mud pump piston?
Disclosed is a single acting mud pump piston assembly adapted for use in a mud pump mechanism including a piston and having an end portion with a shoulder reciprocatingly mounted in a cylinder. The assembly includes a circular flange mounted on the end portion in abuttment with the shoulder. A hub is removably mounted on the end portion in abuttment with the flange. A piston cap is mounted about the hub in abuttment with the flange. The assembly is held together by a washer and a nut engaging the end portion.
A piston assembly for use in a single acting mud pump including a cylinder and a piston rod reciprocatingly mounted in the cylinder, said piston rod including a cylindrical end portion and a radially outwardly extending shoulder, said piston assembly comprising: a circular planar flange having an outside diameter less than the inside diameter of the cylinder and having a bore through the center thereof, said bore having a diameter substantially equal to the diameter of said cylindrical end portion of said piston rod, said flange being adapted to be carried on said cylindrical end portion in abuttment with said shoulder; means for forming a seal between said flange and said piston rod; a circular planar hub having an outside diameter less than the outside diameter of said flange and having a bore through the center thereof, said bore having a diameter substantially equal to the diameter of said cylindrical end portion of said piston rod, said hub having an axial thickness, said hub being adapted to be carried by said cylindrical end portion of said piston rod in removable abutting relationship with said flange; a circular elastomeric piston cup having a body with an outside diameter substantially equal to the outside diameter of said flange and an outwardly flaring annular lip having an outside diameter greater than the inside diameter of the cylinder, said piston cup having a bore through the center thereof, said bore having a diameter substantially equal to the outside diameter of said hub, said piston cup having a central portion surrounding said bore having an axial thickness at least equal to the axial thickness of said hub, said piston cup being adapted to be removably carried about said hub in removable abutting relationship with said flange; a circular planar washer having an outside diameter greater than the outside diameter of said hub and an inside diameter substantially equal to the diameter of said cylindrical end portion of said piston rod, adapted to be carried by said cylindrical end portion of said piston rod in removable abutting relationship with said hub and the central portion of said piston cup; and a retaining nut adapted to threadably engage said cylindrical end portion of said piston rod to urge said washer into abuttment with said hub and said central portion of said piston cup.
A single acting mud pump mechanism which comprises: a cylinder having an inside diameter; a piston rod reciprocatingly mounted in said cylinder, said piston rod including a threaded cylindrical end portion and a radially outwardly extending shoulder; a circular planar flange removably mounted on said end portion in abuttment with said shoulder and having an outside diameter less than the inside diameter of the cylinder and having a bore through the center thereof, said bore having a diameter substantially equal to the diameter of said cylindrical end portion of said piston rod; means for forming a seal between said flange and said piston rod; a circular planar hub removably mounted on said end portion in abuttment with said flange and having an outside diameter less than the outside diameter of said flange and having a bore through the center thereof, said bore having a diameter substantially equal to the diameter of said end portion of said piston rod, said hub having an axial thickness; a circular elastomeric piston cup removably mounted about said hub and in abuttment with said flange and having a body with an outside diameter substantially equal to the outside diameter of said flange and an outwardly flaring annular lip having an outside diameter greater than the inside diameter of the cylinder, said piston cup having a bore through the center thereof, said bore having a diameter substantially equal to the outside diameter of said hub, said piston cup having a central portion surrounding said bore having an axial thickness at least equal to the axial thickness of said hub; a circular planar washer removably mounted about said end portion and having an outside diameter greater than the outside diameter of said hub and an inside diameter substantially equal to the diameter of said cylindrical end portion of said piston rod; and a retaining nut threadably engaged with said cylindrical end portion of said piston rod to urge said washer into abuttment with said hub and said central portion of said piston cup.The basic difference between reciprocating motion and circular motion The piston or plunger works within a watertight cylinder. Thebasic difference between a piston and a plunger should be notedA piston is shorter than the stroke of the cylinder;the plunger is longer than the stroke. Another distinguishing featureis that the packing is inlaid on the rim of the piston for a tight seal.When a plunger is used, the packing is moved in a stuffing boxlocated at the end of the cylinder to provide a tight seal.
Principles of Operation
In general (and with respect to the way that the water is handled),
reciprocating pumps may be classified as lift pumps or force pumps,
which in turn, are either single-acting or double-acting pumps.
Lift Pumps
A lift pump is a single-acting pump; it consists of an open cylinder
and a discharge or bucket-type valve (see Figure 5-3). An open
cylinder and a discharge or bucket-type valve in combination are the
basic parts of the lift pump—it lifts the water, rather than forces it.
In the lift pump, the bucket valve is built into the piston and moves
upward and downward with the piston.
A four-stroke cycle is necessary to start the lift pump in operation
(see Figure 5-4). The strokes are as follows:
_ Air exhaust—The piston descends to the bottom of the cylinder,
forcing out the air.
_ Water inlet—On this upward stroke, a vacuum is created.
Atmospheric pressure causes the water to flow into the
cylinder. After the pump has been primed and is in operation, the working
cycle is completed in two strokes of the piston—a downward stroke
and an upward stroke (see Figure 5-5). The downward stroke of the
piston is called the transfer stroke, and the upward stroke is called
the intake and discharge stroke, because water enters the cylinder
as the preceding charge of water is being discharged.
Force Pumps
The force pump is actually an extension of a lift pump, in that it
both lifts and forces the water against an external pressure. The
basic operating principle of the force pump is that it forces water
above the atmospheric pressure range, as distinguished from the lift
pump, which elevates the water to flow from a spout.
4.4 MUD PUMPS
Mud pumps consume more than 60% of all the horsepower
used in rotary drilling. Mud pumps are used to circulate
drilling fluid through the mud circulation system while
drilling. A pump with two fluid cylinders, as shown in Figure
4.4.1, is called a duplex pump. A three-fluid-cylinder pump, as
shown in Figure 4.4.2, is called a triplex pump. Duplex pumps
are usually double action, and triplex pumps are usually
single action. Pumps with six chambers are commercially
available as well (Figure 4.4.3).
Mud pumps consists of a power input end and a fluid output
end. The power input end, shown in Figure 4.4.4 transfers
power from the driving engine (usually diesel or electric) to
the pump crankshaft. The fluid end does the actual work of
pumping the fluid. A cross-section of the fluid end is shown
in Figure.4.4.5.
4.4.1 Pump Installation
4.4.1.1 Suction Manifold
The hydraulic horsepower produced by mud pumps depends
mainly on the geometric and mechanical arrangement of the
suction piping. If suction-charging centrifugal pumps (e.g.,
auxiliary pumps that help move the mud to the mud pump)
are not used, the pump cylinders have to be filled by the
hydrostatic head.
Incomplete filling of the cylinders can result in hammering,
which produces destructive pressure peaks and
shortens the pump life. Filling problems become more
important with higher piston velocities. The suction pressure
loss through the suction valve and seat is from 5 to
10 psi. Approximately 1.5 psi of pressure is required for
each foot of suction lift. Since the maximum available atmospheric
pressure is 14.7 psi (sea level), suction pits placed
below the pump should be eliminated. Instead, suction tanks
placed level with or higher than the pump should be used to
ensure a positive suction head. Figure 4.4.6 shows an ideal
suction arrangement with the least amount of friction and low
inertia.
A poorly designed suction entrance to the pump can produce
friction equivalent to 30 ft of pipe. Factors contributing
to excessive suction pipe friction are an intake connection
with sharp ends, a suction strainer, suction pipe with a
small diameter, long runs of suction pipe, and numerous fittings
along the suction pipe. Minimizing the effect of inertia
requires a reduction of the suction velocity and mud weight.
It is generally practical to use a short suction pipe with a
large diameter.
When a desirable suction condition cannot be attained,
a charging pump becomes necessary. This is a common
solution used on many modern rigs.
4.4.1.2 Cooling Mud
Mud temperatures of 150? can present critical suction problems.
Under low pressure or vacuum existing in the cylinder
on the suction stroke, the mud can boil, hence decreasing
the suction effectiveness. Furthermore, hot mud accelerates
the deterioration of rubber parts, particularly when oil is
present. Large mud tanks with cooling surfaces usually solve
the problem.
4.4.1.3 Gas and Air Separation
Entrained gas and air expands under the reduced pressure
of the suction stroke, lowering the suction efficiency. Gas
in water-base mud may also deteriorate the natural rubber
parts used. Gases are usually separated with baffles or by
changing mud composition.
4.4.1.4 Settling Pits
The normally good lubricating qualities of mud can be
lost if cuttings, particularly fine sand, are not effectively
separated from the mud. Adequate settling pits and shale
shakers usually eliminate this trouble. Desanders are used
occasionally.
4.4.1.5 Discharge Manifold
A poorly designed discharge manifold can cause shock
waves and excessive pressure peaks. This manifold should
be as short and direct as possible, avoiding any sharps turns.
The conventional small atmospheric air chamber, often furnished
with pumps, supplies only a moderate cushioning
effect. For best results, this air chamber should be supplemented
by a large atmospheric air chamber or by a
precharged pulsation dampener.
4.4.2 Pump Operation
4.4.2.1 Priming
A few strokes of the piston in a dry liner may ruin the liner.
When the pump does not fill by gravity or when the cylinders
have been emptied by standing too long or by replacement of
the piston and liner, it is essential to prime the pump through
the suction valve cap openings.
4.4.2.2 Cleaning the Suction Manifold
Suction lines are often partly filled by settled sand and by
debris from the pits, causing the pump to hammer at abnormally
low speeds. Frequent inspection and cleaning of the
suction manifold is required. The suction strainer can also
be a liability if it is not cleaned frequently.
4.4.2.3 Cleaning the Discharge Strainer
The discharge strainer often becomes clogged with pieces
of piston and valve rubber. This may increase the pump
pressure that is not shown by the pressure gauge beyond
the strainer. The strainer should be inspected and cleaned
frequently to prevent a pressure buildup.
4.4.2.4 Lost Circulation Materials
Usually special solids, such as nut shells, limestone,
expanded perlite, etc., are added to the drilling muds to fill
or clog rock fractures in the open hold of a well. Most of
these lost circulation materials can shorten the life of pump
parts. They are especially hard on valves and seats when
they accumulate on the seats or between the valve body and
the valve disc.
4.4.2.5 Parts Storage
Pump parts for high-pressure service are made of precisely
manufactured materials and should be treated accordingly.
In storage at the rig, metal parts should be protected from
rusting and physical damage, and rubber parts should be
protected from distortion and from exposure to heat, light,
and oil. In general, parts should remain in their original packages
where they are usually protected with rust-inhibiting
coatings and wrappings and are properly supported to avoid
damage. Careless stacking of pistons may distort or cut the
sealing lips and result in early failures. Hanging lip-type or
O-ring packings on a hook or throwing them carelessly into
a bin may ruin them. Metal parts temporarily removed from
pumps should be thoroughly cleaned, greased, and stored
like new parts.
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