Pumps are the second most utilized machines in the world. From studies carried out, it was shown that pumping systems account for nearly 22% of the world’s electric motor energy demand. In certain plants over 50% of electrical energy used by motors can be for pumping systems (i). In our global marketplace, margins have been squeezed to the point where companies must operate at the highest levels of efficiency to maintain long term success. Anywhere pumps are employed to move liquid, there is a potential to conserve energy.
Industrial, municipal and many other sectors of world economies are energy intensive. Many savings programs and attractive paybacks are available to companies that seek efficiency improvements within motor-driven systems, especially with pumps.
Experts know that there’s more to buying a pump than the initial cost of the pump. Many organizations only consider the initial cost of a system. It is in the best interest of the plant designer or manager to evaluate the (Life Cycle Costs) LCC of different solutions before installing major new equipment beginning a major overhaul. This evaluation will identify the most financially beneficial alternative. |
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Vertical turbine pumps for example can be high energy pumps with either large flows or high heads or a combination of both. Accordingly the motors to drive these pumps can be relatively large. These pumps are generally used on water services and operate at 3600 rpm which can also have dramatic effect on wear and the overall mechanical reliability of the pump.
Recent developments in the pump industry(i) and increasing awareness of the life cycle costs(ii) amongst users have prompted a growth in concern about energy usage from pumps. Vertical Turbine pumps are an important type, especially in the high power levels seen where the movement of large volumes of water are required, such as in municipal applications and process water systems. These pumps run 24-hours a day typically, hence power consumption is a vital consideration.
Vertical turbine pumps have a few unique features that can be specified in order to help with the reduction of overall power consumption and improved mechanical reliability.
Mitered discharge heads, reduced bearing spacing, larger diameter line shafts, column size, keyed couplings, dual bowl bearings, lateral seal rings will improve the mechanical reliability of the pump and ease of maintenance. In looking at a 20-year life cycle costs optimizing these components can reduce the overall cost of operating these pumps.
Pumping efficiency can be explained in the following manner. A pump is a mechanical device which delivers useful work when external power is applied. Work is defined as raising a given weight of fluid a given height, and power is the time rate at which work is done. One horsepower is equal to 550ft-lb/sec or about 746 watts (0.746kW ).
The liquid horsepower delivered by a pump (WHP) =
(Lbs. of liquid raised per min.) x ( H in feet) / 33,000=
(GPM x Head (feet) x S.G.) / 3,960
WHP: water horsepower
S.G.: specific gravity, for water is 1.0
BHP: horsepower required to drive the pump
Head: feet
Pump Efficiency: as expressed as a dimensionless fraction= Output/ Input = WHP/ BHP
Pump Efficiency is the Net Efficiency Value when considering the Bowl (centrifugal pump itself) Efficiency, Column Losses, and Discharge Head Losses
BHP: GPM x Head x 1.0 / 3960 x Pump Efficiency
Electrical HP input to motor = BHP/ Motor Efficiency
= GPM x Head / 3,960 x Pump Efficiency x Motor Efficiency
Kw input to motor = BHP x 0.746 / Motor Efficiency
= GPM x Head x 0.746 / 3,960 x Pump Efficiency x Motor Efficiency
Wire to Water Efficiency= Pump Efficiency x Motor Efficiency
Kw consumed = GPM x Head x 0.746 / 3,960 x Wire to Water Efficiency
Total energy cost will be, of course, the number of kilowatts consumed in a given time period multiplied by the cost per kilowatt. The standard by which electrical consumption is charged to the customer is the kilowatt hour or Kwh. Therefore, you may evaluate on the basis of one hour of operation or any hourly multiple. (Many companies also include demand charges in their power evaluations, which, for high power verticals can be significant, since this provides them with a “truer” energy consumption number)
When doing an energy evaluation of a pump/motor assembly it is important to determine the “wire to water” efficiency in real dollars. Below is an evaluation of seven (7) pumps operating over a period of one year:
Assume electricity costs are $ 0.10/ Kwh, and you are evaluating two pump options being proposed by the manufacturer, A & B for a run time of 6000 hours/year/pump. The rating in our example is 6,000 gpm @ 150 feet TDH
Pump A has a guaranteed wire to water efficiency of 80%, whereas Pump B is only 75% then
Electrical cost for one year, Pump A= (6, 000 hrs )
6,000 x 150 x 0.746 ( $0.10/hr )/ 3,960 x .80
= $ 127, 159 x 7 pumps= $890, 113 per year
Pump B has a guaranteed wire to water efficiency of 75%, then
Electrical cost for one year, Pump B= ( 6,000 hrs )
6,000 x 150 x 0.746 ( $0.10/hr )/ 3,960 x .75
= $ 135, 636 x 7 pumps= $949, 454 per year
The net savings for operating pump A instead of pump B is $59,341 in a one year period. If the price quote for pump B was $100,000 and for pump A it was $120,000, then, obviously, pump A would be a better choice. It would recover the higher initial cost in less than one year of operation.
Efficiency evaluation compares bids on the basis of total cost, not just initial cost. Maintenance costs are generally not comparable between manufacturers when the main components, like line shafts, coupling styles, bearing spacing, column size, and bearing materials and style are not the same. Power consumption is a very real portion of total operating cost, and in some applications accounts for 45% of the total life cycle costs(i). The savings over a 20 year period in a few points of wire to water efficiency can be significant.
The vertical turbine operating efficiency is a function of the surface roughness, internal clearances, desired curve shape, mechanical shaft seal losses, column and discharge head losses and the staging effect of multiple bowls. The overall pump efficiency will be less than the attainable bowl efficiency due to hydraulic losses in the column piping, discharge elbow losses, and bearing losses. Because of this variability in this style pump, the selection of these components needs to analyzed when selecting a vertical turbine for a particular application.
This has become such an important issue that Peerless Pump Company has DOE certified instructors on-staff and conducts in-depth technical review of this subject matter in their Total System Evaluation and Pump System Assessment Training workshops.
Vertical Turbine Optimization:
At Peerless Pump we offer an electronic selection program (RAPID) which provides the technical tools to optimize pump selection when considering both the initial costs and the operating costs.
How to offer an optimized pump with an electronic selection program?
First, you want to show the structure of main pump components of the offered pump (especially the ratio cost of hardware to cost of losses)
Cost of main pump components and their losses:
- Motor
- Discharge head
- Column
- Bowl assembly
The following are the values determined for this pump:
- $55,918.48: Approximate cost of pump complete with motor
- $570,241.77 Energy loss cost
- $1,798,154.50 Cost of energy for lifting of fluid
- $2,424,314.80: Sum of costs above
- 27.40Gwh: Energy used
Standard Pump Example
As you can see from the above example, the key areas to lower the total life cycle costs are in the following areas
- Higher efficiency of bowl assembly
- Higher diameter of column
- Fabricated discharge head with 3 segments instead of 90° or cast one
- Higher efficiency of motor
Proper selection of vertical turbine pumps and the design of the pumping system is complex and requires considerable engineering expertise, the use of electronic system design tools, and electronic pump selection tools to realize the lowest life cycle cost.
It is extremely important that a pump supplier provide the above technical data so that a proper pump selection can be made on a comparable basis and to realize the overall lowest operating costs.
By better understanding all components that make up the total cost of ownership, operators will be able to dramatically reduce energy, operational and maintenance costs. Excessive waste and energy usage are important factors in global environmental pollution.
- Variable Speed Pumping: A Guide to Successful Applications, Hydraulic Institute, Europump
- Pump Systems Matter is an organization that is focuses on Energy Savings, Efficiency and Economics of pumps and pumping systems. Industrial pumping systems account for nearly 25% of industrial electrical energy demand. Pump Systems Matter aims to help lower the energy needs of North America while improving the bottom-line profitability of businesses by providing pump users with strategic, broad-based energy management and performance optimization solutions. Pump Systems Matter, www.pumpsystemsmatter.org
- Pump Life Cycle Costs: A Guide to LCC Analysis for Pumping Systems”, Hydraulic Institute, Europump, and the US Department of Energy’s Office of Industrial Technologies
- Hydraulic Institute Standards, www.pumps.org.
Notes: (iv)
Factors that can affect efficiency are:
- Effects of Accessories
- Mechanical seals produce drag that varies with seal design and setting. The variation in seal drag can affect efficiency and power
- Packing must be tightened manually to minimize leakage. Because this is a manual process there is a significant variation in the resulting drag between the packing and shaft which can affect power and efficiency.
- Surface Roughness
- Efficiency increases due to improvements in waterway surface finish are very dependent on pump specific speed and size. Generally, surface finish improvements are economically justifiable for small and low specific speed pumps
- Internal Clearances
- Pump wear ring clearances can have a major influence on efficiency, particularly for low specific speed pumps ( less than Ns= 1500 (29)
- Mechanical Losses
- Bearings, lip seals, mechanical shaft seals, packing etc all consume power and reduce pump efficiency.
- Impeller Trim Diameter
- Reduction in efficiency due to impeller diameter trim must be expected. Efficiency reductions can range from 1 to 6 points for impeller diameter trim to 80% of the maximum diameter. High specific speed pumps generally have greater reductions in efficiency due to trim than low specific speed pumps.
- Staging Effect
- Due to hydraulic losses at the inlet and discharge of the pump, single stage attainable efficiency could be as much as 6 points below the bowl efficiency. This difference reduces as the number of stages increases. Typically this correction applies to 4 stages and less.
Other Factors Affecting Quoted Pump Efficiencies
- Liquid being pump
- Pump speed
- Blade angle setting on adjustable pitch pumps
- Materials of construction
- Standard of finish
- Testing tolerances
- Application of Penalties on Performance Guarantee
(Pete Noll is the Manager of Learning and Development for Peerless Pump Company {www.peerlesspump.com}. With more than 30-years experience in the pump industry Pete has conducted more than 600-workshops and seminars on a variety of subjects and is a certified DOE/PSAT instructor. Pete can be reached at pnoll@peerlesspump.com) |
