- 3 January 2008 -

Determining the real cost of powering
a pump

The cost of buying a pump is much higher than the price on the tag or in the catalogue. Understanding all the components that make up the total cost of ownership - initial costs, mechanical costs, miscellaneous costs and energy costs (the biggest category) is crucial to choosing the right pump for the job. Pete Noll, manager of learning and development for Peerless Pump Company, gives the specifics on calculating the real cost of a pump.

Pumps are the second most utilized machines in the world, and pumping systems account for nearly 22% of the world's electric motor energy demand. In some plants, more than 50% of electrical energy used by motors can be for pumping systems1. 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 programmes and attractive paybacks are available to companies that seek efficiency improvements within motor-driven systems, especially with pumps.

Experts know that there is 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 Cost) LCC of different solutions before installing new equipment or beginning a major overhaul. This evaluation will identify the most financially beneficial alternative.

Vertical turbine pumps

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 have a dramatic effect on wear and the overall mechanical reliability of the pump.

Recent developments in the pump industry and increasing awareness of the life cycle costs2 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 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, and 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.

Defining efficiency

Pumping efficiency can be explained in the following manner. A pump is a mechanical device that 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 550 ft-lb/sec or about 746 watts (0.746 kW).

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) 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 based on 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 pumps operating over a period of one year.

Assume electricity costs are US$0.10/ kWh, and you are evaluating two pump options being proposed by the manufacturer, A and B for a run time of 6000 hours/year/pump. The rating in our example is 6,000 gpm at 150 ft 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

= US$ 27, 159 x 7 pumps= US$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

= US$ 135, 636 x 7 pumps= US$949, 454 per year.

The net savings for operating Pump A instead of Pump B is US$59,341 US in a one-year period. If the price quote for Pump B was US$100,000 and for Pump A it was US$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 based on 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 costs1. 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 turbines

At Peerless Pump, we offer an electronic selection programme (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 programme? 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:

. US$55,918.48: Approximate cost of pump complete with motor

. US$570,241.77: Energy loss cost

. US$1,798,154.50: Cost of energy for lifting of fluid

. US$2,424,314.80: Sum of costs above

. 27.40 gWh: Energy used

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 three 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 the customer can realize the overall lowest operating costs.

By better understanding all the 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.

Contact

Pete Noll
www.peerlesspump.com
www.epumptraining.com

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