A power pump is a positive displacement machine consisting of one or more cylinders, each
containing a piston or plunger. The pistons or plungers are driven through slider-crank
mechanisms and a crankshaft from an external source. The capacity of a given pump is
governed by the rotational speed of the crankshaft.
Unlike a centrifugal pump, a power pump does not develop pressure; it only produces a
flow of fluid. The downstream process or piping system produces a resistance to this flow,
thereby generating pressure in the piping system and discharge portion of the pump. The
flow fluctuates at a rate proportional to the pump speed and number of cylinders. The
amplitude of the fluctuations is a function of the number of cylinders. In general, the greater
the number of cylinders, the lower the amplitude of the flow variations at a specific rpm.
All power pumps are capable of operating over a wide range of speeds, thereby making
it possible to produce a variable capacity when coupled to a variable speed drive. Each pump
has maximum suction and discharge pressure limits that, when combined with its maximum
speed, determine the pump’s power rating. The pump can be applied to power conditions
that are less than its maximum rating but at a slight decrease in mechanical efficiency.
The power pump is a positive displacement device. When operating, it will continue to
deliver flow independent of the pressure in the discharge piping system. Unlike a centrifugal
pump, a power pump will not “deadhead” or “go back on its curve” in response to increasing
discharge pressure. When this pressure exceeds the design limits of the pump,
mechanical failure—often catastrophic—will result.For this reason, all piping systems incorporating
power pumps must have discharge pressure relief devices to limit the pressure in
the piping system and avoid pump failure. These devices must be located between the discharge
connection on the pump and the first isolation valve in the piping system.
This section has been organized to provide sufficient power pump theory to properly
select a pump for most applications. It is recognized that the proper pump metallurgy for
the pumped fluid must be used, but due to the vast number of possible fluids, selection of
pump metallurgy is outside the scope of this section.
The subject of Net Positive Suction Head (NPSH) is mentioned in several places in this
section and is covered in more detail in Section 3.4, “Displacement Pump Performance,
Instrumentation, and Diagnostics.” For basic power pump selection, it is only necessary to
understand that the NPSH available from the suction system must be sufficiently above
the NPSH required by the pump to operate properly.
containing a piston or plunger. The pistons or plungers are driven through slider-crank
mechanisms and a crankshaft from an external source. The capacity of a given pump is
governed by the rotational speed of the crankshaft.
Unlike a centrifugal pump, a power pump does not develop pressure; it only produces a
flow of fluid. The downstream process or piping system produces a resistance to this flow,
thereby generating pressure in the piping system and discharge portion of the pump. The
flow fluctuates at a rate proportional to the pump speed and number of cylinders. The
amplitude of the fluctuations is a function of the number of cylinders. In general, the greater
the number of cylinders, the lower the amplitude of the flow variations at a specific rpm.
All power pumps are capable of operating over a wide range of speeds, thereby making
it possible to produce a variable capacity when coupled to a variable speed drive. Each pump
has maximum suction and discharge pressure limits that, when combined with its maximum
speed, determine the pump’s power rating. The pump can be applied to power conditions
that are less than its maximum rating but at a slight decrease in mechanical efficiency.
The power pump is a positive displacement device. When operating, it will continue to
deliver flow independent of the pressure in the discharge piping system. Unlike a centrifugal
pump, a power pump will not “deadhead” or “go back on its curve” in response to increasing
discharge pressure. When this pressure exceeds the design limits of the pump,
mechanical failure—often catastrophic—will result.For this reason, all piping systems incorporating
power pumps must have discharge pressure relief devices to limit the pressure in
the piping system and avoid pump failure. These devices must be located between the discharge
connection on the pump and the first isolation valve in the piping system.
This section has been organized to provide sufficient power pump theory to properly
select a pump for most applications. It is recognized that the proper pump metallurgy for
the pumped fluid must be used, but due to the vast number of possible fluids, selection of
pump metallurgy is outside the scope of this section.
The subject of Net Positive Suction Head (NPSH) is mentioned in several places in this
section and is covered in more detail in Section 3.4, “Displacement Pump Performance,
Instrumentation, and Diagnostics.” For basic power pump selection, it is only necessary to
understand that the NPSH available from the suction system must be sufficiently above
the NPSH required by the pump to operate properly.
SELECTION THEORY
Power Brake horsepower (bhp) is a function of a pump’s capacity, differential pressure,
and mechanical efficiency. It is an essential criterion for selecting the drive components
but is not valuable for pump selection. A large pump operating well below its design rating
can meet the same horsepower requirements as a smaller pump running at a higher
speed. Unless the application requires a derated pump, it is usually more economical to
select a pump at the upper end of its design rating.
The brake horsepower for the pump is
and mechanical efficiency. It is an essential criterion for selecting the drive components
but is not valuable for pump selection. A large pump operating well below its design rating
can meet the same horsepower requirements as a smaller pump running at a higher
speed. Unless the application requires a derated pump, it is usually more economical to
select a pump at the upper end of its design rating.
The brake horsepower for the pump is
Capacity The capacity Q is the total volume of fluid delivered per unit of time. This fluid
includes liquid, entrained gases, and solids at the specified conditions.
Displacement Displacement D, gpm (m3/h), is the calculated capacity of the pump with
no slip losses. For single-acting plunger or piston pumps, this is
includes liquid, entrained gases, and solids at the specified conditions.
Displacement Displacement D, gpm (m3/h), is the calculated capacity of the pump with
no slip losses. For single-acting plunger or piston pumps, this is
Pressure The pressure Ptd used to determine brake horsepower is the differential pressure
or discharge pressure minus the suction pressure. In most applications, the suction
pressure is small relative to the discharge pressure. However, when pumping some compressible
liquids, such as methane and propane, the suction pressure may be 20 to 30%
of the discharge pressure. For accurate brake horsepower calculations, always include the
suction pressure. Figure 1 shows a typical performance curve for a power pump.
or discharge pressure minus the suction pressure. In most applications, the suction
pressure is small relative to the discharge pressure. However, when pumping some compressible
liquids, such as methane and propane, the suction pressure may be 20 to 30%
of the discharge pressure. For accurate brake horsepower calculations, always include the
suction pressure. Figure 1 shows a typical performance curve for a power pump.
Slip Slip (S) is the loss of capacity due to internal and external pump leakage. External
leakage occurs primarily through the stuffing box via the packing. Internal leakage is primarily
the backflow past the suction and discharge valves. Backflow occurs when a valve
remains open for a fraction of a second as the plunger or piston reverses direction. A small
amount of leakage may occur across the piston in a double acting pump from the high
pressure side to the low pressure side. Slip is expressed as a percentage loss of the suction
capacity and is typically 1% to 4%:
leakage occurs primarily through the stuffing box via the packing. Internal leakage is primarily
the backflow past the suction and discharge valves. Backflow occurs when a valve
remains open for a fraction of a second as the plunger or piston reverses direction. A small
amount of leakage may occur across the piston in a double acting pump from the high
pressure side to the low pressure side. Slip is expressed as a percentage loss of the suction
capacity and is typically 1% to 4%:
Fluid viscosity, pump speed, and discharge pressure can all have an effect on slip,
which is shown in Tables 1 and 2.
which is shown in Tables 1 and 2.
Mechanical Efficiency The mechanical efficiency of a power pump is
where Pin is the input power from the driver, bhp (kW).
The mechanical efficiency of a power pump is the sum of all the frictional losses in the
fluid and power ends.These include the plungers and packing, the crossheads, the rod seals,
and the bearings. The efficiency of a single acting pump often exceeds 90%, while a doubleacting
piston pump will be 88% due to the additional piston and rod seals. If the pump is
equipped with internal gearing, an additional 2% loss is common.
Most power pumps are designed to accept a range of plunger or piston sizes. When the
larger plungers are used, the increased diameter of the packing/seals and plunger/liners
will result in higher frictional losses than with smaller components. As a rule, doubling the
plunger/piston diameter will decrease the mechanical efficiency by 8%. Mechanical efficiency
is also affected by speed and, to a lesser extent, by developed pressure, as indicated
in Tables 3 and 4.
The mechanical efficiency of a power pump is the sum of all the frictional losses in the
fluid and power ends.These include the plungers and packing, the crossheads, the rod seals,
and the bearings. The efficiency of a single acting pump often exceeds 90%, while a doubleacting
piston pump will be 88% due to the additional piston and rod seals. If the pump is
equipped with internal gearing, an additional 2% loss is common.
Most power pumps are designed to accept a range of plunger or piston sizes. When the
larger plungers are used, the increased diameter of the packing/seals and plunger/liners
will result in higher frictional losses than with smaller components. As a rule, doubling the
plunger/piston diameter will decrease the mechanical efficiency by 8%. Mechanical efficiency
is also affected by speed and, to a lesser extent, by developed pressure, as indicated
in Tables 3 and 4.
Speed Pump speed, or, more correctly, stroke rate, is one of the most critical selection
criteria for power pumps. The rotating and reciprocating parts of the power end, as designated,
are often capable of speeds twice that of the actual pump rating. The maximum
pump speed is determined by the design of the fluid end, the hydraulic capability of the
anticipated suction system, and the required life of the plungers, packing, and valves. Most
power pump standards limit the plunger speed from 140 to 280 ft/min (0.71 to 1.42 m/s).
The plunger speed is
criteria for power pumps. The rotating and reciprocating parts of the power end, as designated,
are often capable of speeds twice that of the actual pump rating. The maximum
pump speed is determined by the design of the fluid end, the hydraulic capability of the
anticipated suction system, and the required life of the plungers, packing, and valves. Most
power pump standards limit the plunger speed from 140 to 280 ft/min (0.71 to 1.42 m/s).
The plunger speed is
All pumps have a minimum speed limit, usually determined by a decrease in the adequate
lubrication to the bearings in the power end.
Volumetric Efficiency Volumetric efficiency (VE) is the ratio of the discharge volume
to the suction volume, expressed as a percentage, plus the slip. It is proportional to the
ratio r and the developed pressure where r is the ratio of the internal volume of fluid
between valves when the plunger or piston is at the top of the peak of its back stroke (C
D) to the plunger or piston displacement (D) (see Figure 2).
lubrication to the bearings in the power end.
Volumetric Efficiency Volumetric efficiency (VE) is the ratio of the discharge volume
to the suction volume, expressed as a percentage, plus the slip. It is proportional to the
ratio r and the developed pressure where r is the ratio of the internal volume of fluid
between valves when the plunger or piston is at the top of the peak of its back stroke (C
D) to the plunger or piston displacement (D) (see Figure 2).
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