Besides laboratory tests, theQP-curve-based estimation method has been applied to determine typical operating point locations of two pumping system in a paper mill. The operation of a primary water pump and a pulp pump was monitored by a frequency converter and a measurement computer for the time period of six months. In this section, estimation results are introduced as an example of how model-based estimation can provide additional information for the analysis of the pump operation.
The primary water pumping system consists of an Ahlström P-X80X-1 vertical wet pit (i.e., axial flow) pump, a Strömberg 550 kW induction motor and an ABB ACS 800 frequency converter. The nominal values of the vertical wet pit pump are 1.85 m3/s for the flow rate, 22 m for the head, 85 % for the efficiency and 740 rpm for the rotational speed, respectively. The water pump transfers cold water from the water station further to the mill, allowing the use of original pump characteristic curves for the analysis. The accuracy of the pump QH curve has been previously ensured by a test measurement run, but this has not been carried out for the pumpQP curve, possibly affecting the estimation accuracy of theQP-curve-based method. The rotational speed of the primary water pump is controlled according to the pressure measurement located away from the pump and a typical pressure reference value of 135 kPa. In practice, the reference pressure value and the pump head can differ from each other because of the varying head losses and water consumption before the pressure measurement in the steam power plant.
The pulp pumping system shown in Fig. 3.19 comprises a Sulzer ARP 54-400 centrifugal pump, an ABB 400 kW induction motor and an ABB ACS 600 frequency converter. The nominal operating values of the pulp pump are 675 l/s, 24 m, 85 % and 990 rpm, respectively.
The pumped fluid consists of water and wood fibres. However, the fluid consistency is only 1.5
% and it has the same density as water, which enables the use of the original characteristic curves for the analysis also in this case. The pulp pump is located next to the paper machine 3, and it transfers the pulp from a reservoir to the tanks near the paper machine. Also in this case, the rotational speed of the pump is controlled according to the pressure measurement and a typical reference value of 285 kPa. Different from the primary water pump, the location of the pressure measurement is closer to the pump, and there are no valves between the pump and the measurement location.
Fig. 3.19: Pulp pumping system in a paper mill. The pulp pump is located next to the paper machine 3, and it transfers the fluid from a reservoir to the tanks near the paper machine.
For both cases, estimations of the pump operating point locations were carried for a period of six months to ensure inclusion of the most typical operational states for the analysis. The rotational speed and shaft power estimates provided by the frequency converter were fetched and stored every five minutes to a personal computer, which was connected to the frequency converter. For both cases, approximately 50000 estimations were carried out during the measurement period. In both cases, a pre-filtered shaft power estimate (parameter 1.06, -3dB cut-off frequencyFc = 1.6 Hz; ABB, 2009) was used in the estimations. The rotational speed of the primary water pump was determined with the converter parameter 2.17, which is the unfiltered estimate of the motor rotational speed (ABB, 2009). For the pulp pump, a pre-filtered estimate of the rotational speed (parameter 1.02,Fc = 0.3 Hz) was used.
The estimation results for the primary water pumping system were analysed and then divided into two separate time periods based on the pump rotational speed and the resulting operating point locations. The first time period has a duration of one month and the second time period covers the remaining five months. The operating point locations of the first time period are shown in Fig. 3.20, in which the most typical operating point location is indicated by an asterisk. The normal distribution of the operating points (about 68 % of the shown points) around this location is demonstrated with an ellipse. In the first time period, the pump operating point locations varied all over the QH curve, and the pump rotational speed was around 655 rpm. The major part of the operating point locations lie at partial flows. Compared with the nominal efficiency of the pump, 85 %, the pump estimated efficiency was typically around 70
%. These results are also a good example of changing system characteristics, which can practically prevent the usage of the system-curve-based estimation method.
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30 %
50 % 60 %
70 %75 % 80 %
85 % 85 %
80 % 740 rp m
680 rp m 630 rp m
570 rp m
Flow rate (l/s)
Head (m)
Fig. 3.20: Estimation results of the first time period for the primary water pumping system. Since the pump duty varies greatly depending on the water consumption, the operating points are distributed over theQH curve. However, the major part of the operating points are located in the region with a reduced efficiency. It should be noted that the results may have been affected by the possible inaccuracy of the pumpQP curve.
The operating point locations of the second time period are shown in Fig. 3.21. Over this time period, the pump operating point locations were concentrated at partial flow rates, and the pump rotational speed was mainly around 605 rpm. Occasionally, a larger flow rate has been required from the pump, and the pump rotational speed has increased over 650 rpm. The system characteristics may also have varied as a function of time affecting the resulting operating point locations. Compared with the nominal efficiency of the pump, 85 %, the estimated pump efficiency was typically only 50 %, resulting in a notable efficiency improvement capability.
Hence, it has been recommended to the mill personnel that the operation of the pumping system and its energy efficiency should be studied further.
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30 %
50 % 60 %
70 %75 % 80 %
85 % 85 %
80 % 740 rp m
680 rp m 630 rp m 570 rp m
Flow rate (l/s)
Head (m)
Fig. 3.21: Estimation results of the second time period for the primary water pumping system. In this case, the typical pump efficiency has been only 50 %. It should be noted that the results may have been affected by the possible inaccuracy of the pumpQP curve.
The estimation results for the pulp pumping system are shown in Fig. 3.22. Its process function is to provide the pulp delivery into tanks, and hence it has a more uniform operating point location distribution than the primary water pump consisting of several fixed-speed pumpQH curves, which may indicate a constantly changing system static head. Related to this, Fig. 3.23 shows an example of operating points for the time period of two days. Also in this case, the most typical operating point locations are not in the pump’s best efficiency point, affecting negatively the total efficiency of the pumping system. Compared with the primary water pumping system, the typical estimated efficiency is however significantly closer to the nominal efficiency of the pump.
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60 % 65 % 75 %
82 % 85 %
85 % 83 % 990 rp m
1150 rp m 1080 rp m
900 rp m
Flow rate (l/s)
Head (m)
Fig. 3.22: Estimation results for the pulp pumping system. Process variations and normal distribution of the flow rate are considerably smaller compared with the primary water pump. In this case either, the pump is not typically operating in its best efficiency point.
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82 %
Flow rate (l/s)
Head (m)
Fig. 3.23: Estimation results for the pulp pumping system from the exemplary time period of two days.
Operating point population seems to comprise several fixed-speed QH curves, which may indicate a varying system static head.
In addition to the shown operating point group figures, estimation results allow the formation of duration curves for the pump flow rate and other operational values of the pumping system. For
instance, a duration curve can be formed for the pump efficiency to quantify the typical energy efficiency of the pump operation.