/font> -
Help for Webmasters « back to results for ""
Below is a cache of http://industrial-energy.lbl.gov/files/industrial-energy/active/0/LBNL-55836.pdf. It's a snapshot of the page taken as our search engine crawled the Web.
The web site itself may have changed. You can check the current page or check for previous versions at the Internet Archive. Yahoo! is not affiliated with the authors of this page or responsible for its content.
Variable Speed Pumping: A Guide to Successful Applications; Executive Summary
V
ARIABLE
S
PEED
D
RIVES
A W
AY
T
O
L
OWER
L
IFE
C
YCLE
C
OSTS
A G
UIDE

TO
S
UCCESSFUL
A
PPLICATIONS
A G
UIDE

TO
S
UCCESSFUL
A
PPLICATIONS
V
ARIABLE
S
PEED
P
UMPING
V
ARIABLE
S
PEED
P
UMPING
E
XECUTIVE
S
UMMARY
U.S. Department of Energy
Energy Efficiency and Renewable Energy
Industrial Technologies Program
uropump
V
ARIABLE
S
PEED
D
RIVES
A W
AY
T
O
L
OWER
L
IFE
C
YCLE
C
OSTS
Bringing you a prosperous future where energy is
clean, abundant, reliable, and affordable Table of Contents
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Pumping Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Selection Process New Systems . . . . . . . . . . . . . . . . . . . . . . . 8
Selection Process Retrofitting to Existing Equipment . . . . 10
Benefits of VSDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Potential Drawbacks of VSDs . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Estimating Pumping Energy Costs . . . . . . . . . . . . . . . . . . . . . . 12
Capital Cost Savings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Financial Justification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Example: Variable Speed Drives Fitted on a Primary
Feed Pump and Product Transfer Pump in a Refinery . . . . . . . 14
Acknowledgment
Variable Speed Pumping A Guide to Successful Applications, Executive Summary is the result of a
collaboration between the Hydraulic Institute, Europump, and the U.S. Department of Energys
(DOE) Industrial Technologies Program. Executive Summary
Introduction
Pumping systems account for nearly 20% of the worlds energy used by electric
motors and 25% to 50% of the total electrical energy usage in certain industrial
facilities. Significant opportunities exist to reduce pumping system energy con-
sumption through smart design, retrofitting, and operating practices. In particu-
lar, the many pumping applications with variable-duty requirements offer great
potential for savings. The savings often go well beyond energy, and may include
improved performance, improved reliability, and reduced life cycle costs.
Most existing systems requiring flow control make use of bypass lines, throttling
valves, or pump speed adjustments. The most efficient of these is pump speed con-
trol. When a pumps speed is reduced, less energy is imparted to the fluid and less
energy needs to be throttled or bypassed. Speed can be controlled in a number of
ways, with the most popular type of variable speed drive (VSD) being the variable
frequency drive (VFD).
Pump speed adjustment is not appropriate for all pumping systems, however. This
overview provides highlights from Variable Speed Pumping A Guide To Successful
Applications, which has been developed by Europump and the Hydraulic Institute
as a primer and tool to assist plant owners and designers as well as pump, motor,
and drive manufacturers and distributors. When the requirements of a pump and
system are understood, the user can consult this guide to help determine whether
variable speed pumping is the correct choice. The guide is applicable for both new
and retrofit installations and contains flowcharts to assist in the selection process.
Pumping Systems
A proper discussion of pumping considers not just the pump, but the entire pump-
ing system and how the system components interact. The recommended systems
approach to evaluation and analysis includes both the supply and demand sides of
the system.
Pumping System Hydraulic Characteristics
In a pumping system, the objective, in most cases, is either to transfer a liquid from
a source to a required destination, e.g., filling a high-level reservoir, or to circulate
liquid around a system, e.g., as a means of heat transfer. Pressure is needed to
make the liquid flow at the required rate and this must overcome losses in the sys-
tem. Losses are of two types: static and friction head.
Static head, in its most simple form, is the difference in height of the supply and
destination of the liquid being moved, or the pressure in a vessel into which the
pump is discharging, if it is independent of flow rate. Friction head (sometimes
called dynamic head loss), is the friction loss on the liquid being moved, in pipes,
valves, and other equipment in the system. This loss is proportional to the square
of the flow rate. A closed-loop circulating system, without a surface open to atmo-
spheric pressure, would exhibit only friction losses.
1
Pumping applica-
tions with variable-
duty requirements
offer great potential
for energy savings,
improved perfor-
mance, and reduced
life cycle costs.
The ratio of static
to friction head over
the operating range
influences the ben-
efits achievable
from VSDs. Variable Speed Pumping A Guide To Successful Applications
Most systems have a combination of static and friction head. The ratio of static
to friction head over the operating range influences the benefits achievable from
VSDs. Static head is a characteristic of the specific installation. Reducing this head
whenever possible generally reduces both the cost of the installation and the cost
of pumping the liquid. Friction head losses must be minimized to reduce pumping
cost, but after eliminating unnecessary pipe fittings and length, further reduction
in friction head will require larger diameter pipes, which adds to installation cost.
Pump Types
Proper selection of pumps, motors, and controls to meet the process requirements
is essential to ensure that a pumping system operates effectively, reliably, and effi-
ciently. All pumps are divided into the two major categories of positive displace-
ment (PD) and rotodynamic.
PD pumps can be classified into two main groups: rotary and reciprocating.
Rotary pumps typically work at pressures up to 25 Bar (360 pounds per square inch
[psi]). These pumps transfer liquid from suction to discharge through the action
of rotating screws, lobes, gears, rollers, etc., which operate within a rigid casing.
Reciprocating pumps typically work at pressures up to 500 Bar. These pumps dis-
charge liquid by changing the internal volume. Reciprocating pumps can gener-
ally be classified as having a piston, plunger, or diaphragm, displacing a discrete
volume of liquid between an inlet valve and a discharge valve. The rotary motion
of the driver, such as an electric motor, is converted to the reciprocating motion by
a crankshaft, camshaft, or swash-plate.
The performance of a pump can be expressed graphically as head against flow
rate. The rotodynamic pump has a curve where the head falls gradually with
increasing flow. However, for a PD pump, the flow is almost constant whatever the
head. It is customary to draw the curve for PD pumps with the axes reversed, but
for comparison, a common presentation is used here for the two pump types.
Figure ES-1.
Performance curve for a
rotodynamic pump
Figure ES-2.
Performance curve for a positive
displacement pump
Flow Rate
Head
Flow Rate
Head
2 Executive Summary
For a PD pump, if the system resistance increases, i.e., the system curve is moved
upwards, the pump will increase its discharge pressure and maintain a fairly con-
stant flow rate, dependent on viscosity and pump type. Unsafe pressure levels
can occur without relief valves. For a rotodynamic pump, an increasing system
resistance will reduce the flow, eventually to zero, but the maximum head is lim-
ited. Even so, this condition is only acceptable for a short period without causing
problems. Adding comfort margins to the calculated system curve to ensure that
a sufficiently large pump is selected will generally result in installing an oversized
pump. The pump will operate at an excessive flow rate or will need to be throttled,
leading to increased energy use and reduced pump life.
Many pumping systems require a variation of flow or pressure. Either the sys-
tem curve or the pump curve must be changed to get a different operating point.
Where a single pump has been installed for a range of duties, it will have been
sized to meet the greatest output demand. It will therefore usually be oversized,
and will be operating inefficiently for other duti