Economic aspects of speed control of pumps

The power requirement for flow control performed with speed control according to the drive alternatives in the three main groups A, B and C has in Table 8.4 been compared partly with a non-profit, completely lossless speed control, adjustable motor with 100% efficiency, and partly with flow control through throttle control and start-stop control .

The comparison applies to a specific case, where the system’s static head is zero, i.e. only pipe friction losses exist. For other operating cases, especially with an increasing proportion of static head, the power gain, using a frequency inverter for speed control, decrease. When the total head consists only of static head, in some cases 10 to 20% higher power consumption is obtained than for a pump with start-stop regulation.

Table 8.4 Comparison of power requirements for different methods for flow control of a centrifugal pump without static head, i.e. when there are only pipeline losses.

To determine the energy consumption, a summation of the power consumption over a period of time must be made using a duration flow diagram. In the absence of a duration chart, a rough estimate can be made using an average flow or better with the flow, which has a 50% duration. When studying the duration of the volume flow together with already selected pump performance, it is usually difficult to make a division of what are extreme flow requirements and what are more or less roughly dimensioning surcharges. The “normal” maximum flow is generally less than 80% of the pump flow. The flow with 50% duration is therefore usually quite low and can be judged to be:

The comparison applies to a specific case, where the system’s static boost height is zero, ie only pipe friction losses exist. For other operating cases, especially with an increasing proportion of static boost height, the power gains from speed control decrease. When the lifting height consists only of static height, in some cases 10 to 20% higher power consumption is obtained than for start-stop driving.

Table 8.4 Comparison of power requirements for different methods for flow control of a centrifugal pump without static head, ie when there are only pipeline losses.

To determine the energy consumption, a summation of the power consumption over a period of time must be made using a duration flow diagram. In the absence of a duration chart, a rough estimate can be made using an average flow or better with the flow, which has a 50% duration. When studying the duration of the volume flow together with already selected pump performance, it is usually difficult to make a division of what are extreme flow requirements and what are more or less roughly luxurious dimensioning surcharges. The “normal” maximum flow is generally less than 80% of the pump flow. The flow with 50% duration is therefore usually quite low and can be judged to be:

in process industry about 60% of maximum pump flow
within Water and Waste approx. 30% of maximum pump flow
for heat transport, plumbing and district heating, approx. 40% of maximum pump flow

Profitability and investment costs

With the energy economy of the speed control as a basis, possible electrical energy savings for pump operations in Sweden have been calculated in several places. The savings amount to 30-50% or 3-5 billion kWh / year, ie about as much electricity as a nuclear power plant produces. Perhaps more striking is that the investment costs for speed control are only a third of those for a nuclear power plant and less than a tenth for the same savings through the improvement of housing insulation.

However, as long as there is no legislation on speed regulation, the economy will be decisive in each individual case. Compared with throttle control, the pay-off times for speed control are usually 1-3 years and compared with start-stop control with a small proportion of static boost height 5-10 years.

Although speed control of pumps is the only possible control method in many special cases, where process or liquid does not allow any other control method, extensive financial calculations must generally be made before an investment decision on speed control. The calculations must take into account:

system requirements for flow evenness, flow equalization, pressure maintenance, etc
the duration of the flow over the life of the equipment
the appearance of the system curve
operational reliability, ie in general the need for spare units
superior control systems
environmental sensitivity, especially in humid environments with gases such as methane, ammonia and hydrogen sulphide
serviceability, especially regarding knowledge requirements of relevant personnel in several areas such as electrical and mechanical
energy costs
costs for speed converters
equipment life and discount rate

The appearance of the system curve and the duration of the flow determine whether the flow is to be distributed over several pump units, of which in turn only one may need to be speed-controlled. The need for spare units strongly influences an investment calculation as cost savings, primarily for energy, must amortize both operating and spare units. Compared with throttle control, speed control saves the control valve and, compared with start-stop control, the necessary magazine or pump sump volumes are reduced, which of course must be taken into account in a calculation.

Capitalization of annual costs for energy takes place in the usual way by calculating the present value of future costs. The present value of 1 Euro or Dollar, which falls out in the middle of each year during N years at the interest factor r constitutes F kronor, where F is the present value factor, which is determined from equation 8.14 or from figure 8.24.

Profitability and investment costs

Responses