In a recent white paper considering energy efficient pumping systems, Schneider Electric considers a pump system energy management approach for the energy efficiency of the pumping system and of its component parts.

In pumping systems, most inefficiency comes from a mismatch between the pump deployed and the actual system requirement, and the improper use of throttling valves and damper technologies to control the flow of liquids.  The way pumping systems are controlled plays a major role in how efficiency can be improved. Control systems themselves are composed of both hardware and software components. On the hardware side, variable speed drives are a primary enabler of high efficiency performance.

The following example compares two installations (one with a variable speed drive one with a fixed drive throttled system) in which static heads (height difference between the source and the end use) are different.

Figure 1 Throttle system

At fixed speed, it is necessary to add a throttle valve in the hydraulic circuit. This adjusts the flow by increasing or decreasing the flow resistance. This will modify the system curve; however the motor speed remains the same so the pump curve does not change. The flow rate is matched but the head is much higher than required resulting in poor energy savings.

In the figure, the static head represents 50% of the system head, and the pump is rated for the head and flow of the system. At 100% flow, the power consumed by the pump is the same at both fixed speed and with a variable speed drive. At 60% flow, the energy savings resulting in the variable speed drive use is 46%.

Figure 2 Variable speed drive

If a VSD is deployed, the system curve does not change. The pump curve is modified according to flow speed and affinity laws (rules of hydraulics that express the relationship between variables involved in pump performance such as head, volumetric flow rate, shaft speed, and power). Adjusting the speed matches the process requirement and results in significant energy savings.

In this case, the static head represents 85% of the system head, and the pump is oversized by 20%. In real world scenarios, 75% of pumps are oversized (by 10% to 30%) in order to meet anticipated lifetime peak production, to anticipate future needs, or to rationalize spare parts inventory. Therefore, a variable speed drive saves 20% of energy at 100% flow and saves 36% energy at 60% flow.

Energy savings depends on the static head: the lower is the static head, the bigger the energy savings (and speed variation range). For pumping to take place, it is necessary to generate enough power to overcome the static head. The friction head is the head required to push the liquid through the pipe and fittings. It depends on flow rate, pipe size, pipe length, and viscosity.

Pump efficiency

Changing the operating point on the pump curve also changes its efficiency. The pump performs at maximum efficiency at its full capacity, corresponding to its Best Efficiency Point (BEP). In terms of installation design and operation, the objective is to work as closely as possible to the BEP. By varying the speed, the pump efficiency remains roughly the same based on the new flow rate. At fixed speed, reducing the flow rate quickly deteriorates the pump efficiency, whilst adjusting the speed keeps the efficiency close to the BEP.

Determining pump efficiency goes part-way to identifying system performance levels: monitoring efficiencies via software can detect operating points that are not suitable for the pump and help to improve both system energy efficiency and reliability.

Best practice for Energy efficient pumping systems

The energy efficiency of a pumping system can be improved by implementing the following simple actions:

– Replace fixed drives with variable speed drives to boost the efficiency. Connected to a pump, a variable speed drive can control speed, pressure, and flow in conjunction with dynamic process and production requirements.

– Monitor production output and energy consumption data via software dashboards and use the information for rapid and cost effective decision-making.

– Monitor the operating point of the pump and its efficiency on a continual basis to visualize trends, leading to actions that improve efficiency, and verify the impact of improvements to the system.

– Use proper metrics to monitor efficiency in the systems and to compare efficiency performances of different pumps in multiple sites. A recommended key performance indicator (KPI) metric is the specific energy consumption metric (in kWh/ m3).

Go to the Schneider Electric website to download the white paper

See also 999 Automation article