Optimising energy efficiency in hydraulic systems

Considering that approximately 20% of global electric energy production is consumed by motor-driven pump sets, they offer a large potential to save energy. High-power pumps are usually designed and purchased with a strong focus on life-cycle costs and low energy consumption, but the huge number of low-power pumps are usually only selected with a focus on low investment costs - even though they consume, due to their large population, a considerable share of the total consumed energy.

Low-energy pump system design is the key factor in the hand of the operator/planner, whereas variable speed control, trimming the impeller and high-efficiency motors are possible solutions that can be provided by the pump manufacturer, depending on the specific requirements of the individual system. A variable speed drive provides the lowest energy consumption in most cases and offers additional pump protection functions, but also adds complexity to the system and increases investment costs. Using motor-mounted variable speed drives, however, offers interesting new opportunities and potential for cost saving compared to cabinet-mounted solutions. High-efficiency motors reduce the energy consumption by 1-3%. Trimmed impellers save 10% energy, on average, compared to fixed diameter pumps without any changes in installation.

A detailed study of pumps operated in industry has been compiled by the US Department of Energy, assessing more than 2.4 million pumps from various industries. The total energy consumption of the pumps considered in the study was 142,000 GWh/year.

Clustering pumps by power range proves the guess of most pump operators. Half of the pumps in industry have a rated power of 4 kW or less, 32% of the pumps have a rated power from 4 to 15 kW. The upper curve in Figure 1 depicts the power consumed by the pumps per power range. Pumps in the power range from 4 to 15 kW consume almost 20% of the energy consumed by all pumps in total. As a consequence, 50% of the energy is consumed by pumps with a rated power of 40 kW or less - those are pumps which do not necessarily receive the closest attention during the selection process.

Figure 1: Population of pumps by power range and power consumed by power range for 2.4 million pumps operated in various US industries.1

The pump in the system

Pump selection is the result of cooperation between the process engineer, consultant and pump manufacturer. The energy consumption of a pump is influenced by different factors during this selection process.

Typically, the pump system design will start from the flow and head required by the process. Depending on the medium handled, and most often in open systems, the consultant will add some safety margin for incrustation. In almost all applications a general safety margin is also added to compensate for unexpected losses in the system design or a higher demand than anticipated by the process engineer.

The pump manufacturer will generally over-fulfil the specification and choose the next-larger pump size. Conservative engineering leads to continuously adding design tolerances, with the result that the pump will be operating at part load most of the time.

Figure 2: Influence of operating point on pump reliability.2

A field campaign by KSB with more than 2000 submersible pumps, measuring the electric output and the hydraulic data, found that 38% of the pumps have an overall efficiency of 40% or less. In most cases, the low efficiency was caused by a mismatch of pump and system rather than wear or a poor hydraulic design of the pump.

In addition to wasted energy, operating the pump at part or overload also increases the mechanical load on the pump and thus its wear.

Clearly, the hydraulic and mechanical design of the individual pump makes a difference, but the general tendency depicted in Figure 2 is valid for any centrifugal pump.

However, due to the tolerances and uncertainties involved in the different steps of process and system design, safety margins cannot be avoided. Typically, in service this leads to the pump head being throttled by means of a gate valve. The user is now faced with the situation in which he is paying for the energy being dissipated by the valve.

Figure 3 shows the typical effect of throttling a pump. Reducing the flow by 50% would lead to energy savings of 20%. But an energy saving of more than 80% would be possible, as the required system head has dropped considerably. Therefore, correctly controlling and minimising the safety margins in the hydraulic system design has a huge impact on energy consumption.

Figure 3: Flow control by throttling.

Improving the efficiency

When the system requirements, in terms of head and flow, are passed to the pump manufacturer there are a number of possibilities to optimise the energy consumption.

Variable frequency drives (VFDs)

Speed variation is the most efficient solution in terms of energy consumption. By reducing the speed, the head of the pump can be set to exactly meet the needs of the process. Tolerances and safety margins added during the design phase can thereby be absorbed. Incrustation causing additional pipe friction or a temporary overload can be accommodated on demand, as most frequency converters and pumps can be operated faster than synchronous speed.

Two factors limit the speed range. In order to ensure the health of the shaft seal it needs to be steadily operated in hydrodynamic friction. Therefore, most manufacturers limit the permanent operating speed to a minimum of 50% of the nominal speed.

In open systems with a static head (see Figure 4, right side), the minimum speed of the pump must be slightly greater than the static head. Further flow control can only be achieved by throttling.

Figure 4: Flow control by variable speed: closed system (left) and open system with static head limiting the control range (right).

Using inverters for pumps is almost state of the art, yet it can be cumbersome. Modern inverters are purpose-built computers with hundreds of settings, allowing them to drive an escalator, a stone mill or a pump with an appropriate control strategy. In order to capitalise on all the features, either the technician in charge of measurement and control needs to understand the pump, or the pump operator needs to be savvy on VFD parameterisation. In any case, it is time consuming and may require some trial and error.

Pump manufacturers believe that, in many cases, they are in a better position to offer a suitable drive supplied ready set up for operation. Using inverter technology they can provide pump-specific solutions for mounting and operating the VFD.

The parameter set and the terminology of the menus is adapted to pump applications. Instead of requiring the input of abstract parameters such as PID or anonymous limiting values, most inverters from pump manufacturers come preset with the H-Q/P-Q curves of the corresponding pump. Commissioning is reduced to entering a few parameters directly related to the application, such as a minimum flow.

Many pump-specific inverters also offer useful monitoring functions such as:

• Dynamic overload protection by speed limitation (motor speed is reduced along a defined curve)
• Characteristic curve control (Qmin, Qmax)
• Minimum flow stop
• Flow rate estimate
• Dry-running protection

These functions ensure the proper operation of the pump within the specified conditions and, thereby, reduce the wear and the downtime of pump and system.

Supplying the VFD together with the pump offers new opportunities for optimising the topology and the assembly. In many applications a VFD with a reinforced and sealed case can be placed on top of the motor out in the field. Instead of assembling and wiring a custom-designed cabinet on site, the VFD comes industrially preinstalled on the pump and preset with parameters. The cable between VFD and motor has to be shielded, due to the high-frequency harmonics generated by the VFD. With a cabinet-mounted VFD, the costly shielded cable has to be used for the long distance between switching room and motor, which, at in industrial production site, can easily be 100 m or more. Above a cable length of 50 m between VFD and motor, most VFDs require a filter to compensate for the capacitance of the cable, which further increases the cost of the cabinet mounting.

Figure 5: System topology of cabinet and motor-mounted VFD.

Clearly, a pump-mounted solution may not be ideal in a very rugged environment, such as a foundry, but for most applications in industry it is a viable solution.

Trimmed impellers match the specified operating point

Some manufacturers sell their pumps like cars: each order is custom built and tailored to the specification. Other manufacturers carry an inventory of pumps and supply the closest match, adding another positive safety margin and thereby increasing the energy consumption.

In order to make the inventory turn sufficiently, the number of variants stocked needs to be limited. The trimming diameter is just one parameter besides the pump size, material and seal code. Most manufacturers will offer three to five trimmed diameters per pump size that are usually matched to the rated motor power of the different frame sizes available. On average, a pump with a fixed impeller diameter will consume 10% extra power. But this value can be considerably higher in adverse cases, as depicted in Figure 6. If, for example, an application requires a power input of 46 kW to satisfy the hydraulic specification, a 45 kW motor cannot be used. A pump with trimmed impeller will come with a 55 kW motor and a 46 kW impeller. This pump is actually a 46 kW pump. If the pump manufacturer has stepped diameter impellers, the pump with 55 kW motor will come with an impeller designed for 55 kW. The pump will produce more head and, therefore, deliver more flow. In order to meet the hydraulic specification the operator will throttle the pump in the gate valve and operate the pump at part load.

Figure 6: Comparing trimmed impellers to typical stepped diameters yields a difference in power consumption of up to 15%.

Sourcing pumps with impellers trimmed to the duty point (on fixed-speed applications) will, on average, achieve a 10% lower energy consumption compared to pumps with typically stepped diameters because the pump output is matched to the system requirements. In the worst case - when the smaller motor is just not sufficient - the gain in efficiency can be more than 20%.

Trimming impellers can lead to another positive effect: usually pump manufacturers dimension the motor power rating of pumps to match the maximal absorbed power of the particular specific characteristic curve. With the reduction of the impeller diameter, the maximal absorbed power is being reduced as well. In some cases, the rated motor power and the motor size can be reduced, which leads to an additional cost reduction. For example, regarding the KSB ISO 5199 standard pump CPKN, the size 50-250 with full impeller diameter (260 mm) requires a maximum power input of 28 kW. According to the IEC norm, the pump will be supplied with a 30 kW motor (size 200 L). With the reduction of the impeller diameter to 240 mm (8% reduction), the max absorbed power decreases to 21.3 kW. In this case, a 22 kW motor (size 180 M) could be selected. As a consequence, even the size of the pump baseplate and the coupling can be reduced, which yields to further cost savings of about 5% in this case.

By Dr Dirk Kollmar and Christian Appel, KSB Germany

References:

1. US Department of Energy Office of Energy Efficiency and Renewable Energy, United States Industrial Electric Motor Systems Market Opportunities Assessment, 2002

2. Hodgson J, ‘Predicting Maintenance Costs Accurately’, Pumps & Systems, April 2004