- 4 July 2007 -

Life cycle energy cost savings through careful system design and pump selection

Much of a pump's initial cost is not through the initial purchase, but in the energy it uses throughout its life. However, proven steps can be taken to reduce these costs considerably when designing a pump system. Here Jukka Tolvanen, product manager from ABB's low-voltage AC drives factory in Helsinki, explains why it is important that all aspects of ownership cost are explored, not just initial purchase and installation.

Pumps are the single biggest user of electricity in the European Union, consuming 160 TWh/a of electricity and accounting for 79 million tonnes of CO2. Centrifugal pumps account for 80% of all industrial pumps and about 90% of the life cycle cost (LCC) of a pump system goes to energy. Pump systems almost always require variation of the flow rate. The most common flow control methods are:

Throttling

This is the most common solution. The flow is controlled through increasing losses in the system by closing the valve. However, increasing losses means wasting energy - producing 70% flow requires up to 90% of the energy used at full speed.

On/off control

On/off control is often used in cases where stepless control is not necessary, for instance when keeping the pressure in a tank between preset limits. The pump is either running or stopped and average consumption is the same as average running time, i.e. 70% consumption for 70% average flow.

Variable speed control

Using a variable speed drive (VSD) is the only control method that reduces the speed of the pump and with that, the flow generated. Only the necessary flow is produced. Reducing the speed of the pump reduces both the flow and the head of the system, which can result in staggering energy savings - producing 70% flow may only require 45% of the energy at full speed.

Life cycle costs

VSDs are crucial when limiting the life cycle costs (LCC) of a pump station. The investment cost of the equipment needed (AC drives, motors and pumps) is relatively low when compared to total life cycle cost.

Energy is the largest part of the LCC, especially if pumps run more than 2000 hours per year. With a VSD, it is easy to check that your pumps are not running at higher speed or for longer hours than needed.

Energy saving

There are seven simple ways to save energy when designing a pump system:

. Design systems with lower capacity and total head requirements.
Do not assume these requirements are fixed.

. Avoid allowing for an excessive margin of error in capacity and/or total head. It typically will be less expensive to add pumping capacity later if requirements increase.

. Despite the tendency to emphasize initial cost, you will save in the long run by selecting the most efficient pump type and size at the onset.

. Use variable-speed drives to avoid losses from throttle valves and bypass lines, except when the system is designed with high static heads.

. Use two or more, smaller pumps instead of one large pump so that excess pump capacity can be turned off.

. Use pumps operating as turbines to recover pressure energy that would otherwise be wasted.

. Maintain pumps and all system components in virtually new condition to avoid efficiency loss.

Low maintenance

Maintenance is the other main cost component. Active control using a VSD works as a means of preventive maintenance, limiting maintenance costs in many ways.

The cost of unexpected downtime and lost production is a very significant part of the LCC and can rival the energy costs. VSDs cut maintenance and repair costs not only of the pump, by:

Water hammer

Water hammer is caused by rapid changes in flow. These flow changes are followed by rapid pressure transients that can damage pipes, pipe supports and valves, causing leakage. Variable speed drives allow you to gradually ramp the acceleration at a safe rate to avoid hammering.

Less stress

Less stress on the electricity supply is achieved by soft-starting the pump motor, giving much lower peak current than direct-online starting.

Cavitation

Cavitation occurs whenever the static pressure drops below the liquid vapour pressure, causing bubbles to collapse with a very high impact force. This force causes surface damage inside the pump. With a variable speed drive, it is possible to monitor the pressure of the incoming pipeline and take steps to avoid cavitation.

Redundancy

Redundancy can be offered by using parallel pumps. If one pump fails, the remaining pumps can continue uninterrupted. During normal duty, the load can be shared between the pumps to achieve the most energy efficient operation. You can also control the running times of each pump to get service breaks at the most convenient times.

Life cycle cost

A life cycle cost (LCC) analysis is a method of calculating the cost of a system over its entire life span. The analysis of a typical system includes:

. initial costs

. installation and commissioning costs

. energy costs

. operation costs

. maintenance and repair costs

. down time costs

. environmental costs

. decommissioning and disposal costs

Many organizations only consider the initial purchase and installation cost of a system. It is in the fundamental interest of the plant designer or manager to evaluate the LCC of different solutions before installing major new equipment or carrying out a major overhaul.

In addition to the economic reasons for using LCC, many organizations are becoming increasingly aware of the environmental impact of their businesses, and are considering energy efficiency as one way to reduce emissions and preserve natural resources.

Life cycle analysis for pumping systems shows that:

. 5% of industrial energy goes to pumps

. 90% of the total cost of owning a pump comes from energy consumption

. Pump energy consumption can generally be reduced up to 20%

Pump curves

When considering implementing variable speed control, the pumping system curves should always be considered. The pump performance curves show the technical performance of the pump. The horizontal axis shows the flow rate and the vertical axis shows the head and power generated. A system curve, normally plotted below the pump curve, describes the static head and resistance of the pipeline. The operating point of the pump is at the intersection of the system curve and the pump curve.

The greatest savings will occur in systems where the static head is relatively low and the friction losses are high, as the pump will not drop significantly in efficiency. Where high static heads are involved, the savings will be lower, as the speed turndown may be less.

Control loop

With variable speed drive control, the pump becomes part of the control loop. For instance, the level in the tank can be controlled by a variable speed drive controlling the pump to generate only the pressure needed to discharge the required amount of liquid. The level in the tank can be maintained using input from a level transducer, ensuring smooth and accurate level control.

Component selection

The selection of pumps, motors and drives is based on the process information. The motor and AC drive are dimensioned to run the pump under normal pump operation conditions.

Pump selection

The general requirements for pump selection are:

. Working conditions

. Capacity, suction and discharge pressures with variation ranges

. Maximum differential pressure for the pump casing

. Exceptional starting, stopping and other running conditions

. Liquid specification with density, temperature etc.

. Suction conditions such as suction head, suction pipe losses etc.

. Pump construction material

. System description, e.g. single, parallel or serial connection with some other pump

. Special conditions in the mounting space

Quite often the required capacity can be achieved by several different pump types with the same or different nominal speed. The selection has to be made, for instance, between a higher speed pump with low initial cost, and a lower speed pump with lower maintenance costs. The selected pump is the smallest capable for the duty point required. If we know that there is no need for higher capacity either now or in the future, there is no need to choose a bigger pump. The bigger pump leads to higher initial and operational costs.

Far too often, users only ask for the flow when acquiring a pump, then select the least costly option. In most cases, this is false economy. By considering the full lifecycle of the pump, significant savings can be made.

www.abb.com

 

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