Discussion on Pump Capacity of Speed ​​Regulation Water Supply System

Speed ​​governing water supply system used in our country, now has two years of history. After many years of practice, I found that it is exactly the capacity of the pump equipped with a direct impact on the stability of the speed control system and operational efficiency. The following, the author on the water supply system speed / constant speed pump capacity allocation issues are discussed.

1 speed pump operation efficiency analysis

Some articles have been discussed, that speed governor pump operation in the regulation process, you can always maintain the same operating efficiency, I believe that this view is not correct, in Figure 1.1 for qualitative analysis.

1.1 reference formula and applicable conditions

Pump head H is calculated as follows:
H = PR-PV + (V22-V12) / 2g + Δz - Formula (1.1)
H - pump head, m.
PR - pump outlet pressure gauge readings, meters.
PV - pump inlet pressure (vacuum) table readings.
V2, V1 - pump outlet, the entrance of the flow rate, m / s.
â–³ z - export, entrance meter installation height, m.
g - acceleration of gravity, m / s 2.
General engineering calculations lift simplified formula as (1.2).
H = PR-PV - Formula (1.2)

Equation (1.2) shows the relationship between the pump head and outlet pressure. The working curve of the pump are expressed as the flow Q - lift H curve, and look at the speed control of the pump, the water pump pressure PR as a control parameter, therefore, I use the pump flow Q - PR pressure curve Auxiliary analysis, for the difference, the curve of the curve points are called the condition curve and operating point.

1.2 speed pump running efficiency

According to equation (1.2), there are mainly two factors that affect the PR of the pump: one is the change of head caused by the change of water volume; the other is the change of PV at the inlet end. If the amount of water remains unchanged at this moment, the PR of outlet pressure assists Analysis, for the difference, the curve and points on the curve were called the condition curve and operating conditions.

1.2.1 variable speed water supply pump operating efficiency changes

Water supply changes, causing the pump head H changes, assuming the inlet pressure PV = O, then PR = H, the use of pump flow Q - outlet pressure PR curve (operating curve) analysis, as shown in Figure 1.1.

Constant pressure water supply pump speed adjustment mode, when the speed pump running at full speed, the condition curve is SH, the water pressure is PS, the working point is A, when the water supply decreased, the speed governor to adjust to reduce the pump operation Speed, then the pump working condition curve SL, operating conditions point B, the pressure remains unchanged.

A, B the same pressure point (equivalent to the same head), it is clear that they inequality point, according to the pump law, B point in the curve SH equivalent point C, the following relationship holds:

QB / QC = n <1-formula (1.1)
PB / PC = n2 <1-formula (1.2)
QA ~ QD - A ~ D corresponding to the amount of water, m3.
PC, PD - C, D corresponding pressure, m.
n - transfer rate.
PS - water supply to ensure pressure, meters.

According to Eqs. (1.1) and (1.2), point C corresponds to the flow rate (QC) and pressure (PC), flow rate (QS) and pressure (PB) which are greater than point B and the point between point A and point B Figure 1.1 shows.

If there is no governor, when the water supply drops, the working point of the pump is shown as point D in Figure 1.1. The efficiency points for each point are shown in the efficiency curve in Figure 1.1.

Curves ηB, ηL are pump efficiency curves corresponding to curves SB and SL.

According to Figure 1.1 analogy, when the water supply decreases, the efficiency of the pump moves to the left; relative to the operating point of the governor, the operating point is located in the right after the governor position, so that the water supply to reduce pump efficiency Decline, there is a governor pump operating efficiency is relatively higher than the governor without a pump.

1.2.2 pump inlet pressure changes on the efficiency of

When the inlet pressure increases, the outlet pressure remains constant, the pump head is down, this time does not take into account changes in the amount of water Q, pump working curve shown in Figure 1.2, SH, SL, respectively, before and after the change pump work Curve, A, B for the work point, as shown in Figure 1.1 analysis, point B of the equivalent point of C, their corresponding efficiency curve ηB, ηL shown.

Qualitative analysis of the results are as follows: With the inlet pressure increases, the pump speed decreases in a certain amount of water supply conditions, the efficiency of the pump gradually move to the right of the efficiency curve.

Integrated water supply and inlet pressure of two factors, reducing the water supply pump operating point to the left to move the water pressure to the right to move the pump operating point, the relative speed governor operating point, Speed ​​after the operating point is located in the right curve of the efficiency curve, so that the governor of the pump, the operating efficiency is relatively high without the governor of the pump.

2 at this stage speed pump and fixed-speed pump capacity ratio and the problems that arise

Because of the high price of the speed governor and the difficulty of maintenance, most of the water supply systems at this stage are equipped with multiple speed pumps (speed pumps). In almost all the designs at this stage, the speed difference between the speed pump and the speed pump is different Not large (some capacity ratio or even 1.1), according to my company's experience, such a capacity ratio is not appropriate, the main shortcomings in the following two aspects: First, resulting in unstable water supply system operation; Second, the transfer Speed ​​pumps are inefficient. To transfer / constant speed pump capacity of 1.1 for example analysis.

2.1 transfer, constant speed pump capacity of 1.1 system stability analysis

Water supply system The working mode of the water pump is shown in Fig. 2.1. When the water pump is used as the main pump, when its water supply reaches 100% of the rated water volume, the working curve is SA and the working point is A. The running speed of the water pump has reached If the water supply is further expanded, the water pressure will drop to a certain extent (for example, the working point is B and â–³ Q is 5% ~ 10%). The constant speed pump is put into operation. If the speed regulating pump And the constant speed pump capacity ratio of 1: 1, then the constant speed pump is put into operation, the water supply is fixed at 100% (working point A), forcing the pump speed curve SB, water supply is only â–³ Q (5% ~ 10%), the working point is C. As the constant speed pump capacity is set too large, the governor pump down to a very small range (only â–³ Q, about (5% ~ 10%), if the water supply slightly reduced (actually the amount of water is the moment Change occurs, this situation often occurs), the system is the implementation of fixed-speed pump shutdown operation, so that the fixed-speed pump frequent operation phenomenon.This is why the current stage of the water supply system is prone to frequent pump operation the main reason for the We have on the pump capacity ratio of non-excuse: 1 pressurized pump station to observe and found that the water supply is in the vote / cut the critical state of the pump, the pump cast / cut frequent operations.

Therefore, the speed and speed ratio of 1: 1 water supply system, due to the fixed speed pump capacity is too large, resulting in the system can not be stable operation, and, speed pump into operation after running speed in the inefficient zone, the operating efficiency is very Low, can not achieve energy-saving effect. .2 capacity ratio of 1.1 system operating efficiency

According to the above 1.2 on the speed of the pump operating efficiency analysis results, the speed of the pump operating efficiency decreases with the pump to reduce the water supply. After I analyzed the use of speed pump operation data, under normal circumstances when the speed pump out of 40% (speed pump because there is no speed control device, this data is 50%) below, the operation in the low-efficiency zone. 2.1 integrated analysis, constant speed pump is put into operation, the speed of the pump water supply is only 5 to 10%, no doubt, then speed pump running in the inefficient area, the operating efficiency is very low, can not be achieved energy saving effect.

3 reasonable transfer / constant speed pump capacity equipped

3.1 reasonable transfer / constant speed pump capacity ratio

From the perspective of improving the stability of the water supply system and the operation efficiency of the variable speed pump, the following methods are used to determine the ratio of the fixed speed pump to the fixed speed pump.

According to Figure 2.1, the adjustable pump of the variable speed pump can theoretically be in the range of 0-100%. However, according to the analysis of the operating efficiency of the variable speed pump in 2.2, the output of the pump is less than 40% and the efficiency of the pump is very low. The integrated pump Operational efficiency considerations, the pump speed can be adjusted actually about 60% of the water.

If the choice of constant speed pump parameters are: rated water pressure PS, water supply QC for QD half,

Ie QC = QD / 2 - (3.1)

Constant-speed pump capacity according to equation (3.1) to determine the working principle of the water supply system shown in Figure 3.1:

When the working point of the speed governing pump is B, the constant speed pump is put into operation. The curve of the constant speed pump working is SC and the working point is D. Since the QC is only 30%, the operation curve of the speed regulating pump SB and the water supply volume 70-80% , Running in the high-performance area. When the pump speed when the water is less than 40% (operating curve SL) set speed pump running at the lowest speed, if the water supply and then reduce the work point from E to F, the pressure rise, fixed-speed pump down. Speed ​​?? pump operating efficiency increased, and the rate of decrease of 30%, speed pump will not appear frequently operating phenomenon.

In summary, a reasonable transfer / constant speed capacity ratio of 1: 0.3, so that the ratio of capacity to promote the pump running in the high-efficiency area, and, due to the speed pump into the pump speed sufficient room to adjust, up And the reduction rate is about 30% of its rated capacity, the water supply system is not prone to frequent pump operation.

3.2 speed water supply system with pump capacity

Speed ​​governing water system The reasonable pump capacity is equipped as shown in the following table 3.1:

Table 3.1 Pumps with a reasonable capacity

According to the capacity shown in Table 2.1, the work process of the water supply system is as follows: The main pump to speed pump, water pressure drops to a certain extent 1 # constant speed pump start; if the water supply to increase again when the stop 1 # open 2 # ; Followed by 1 # and 2 # at the same time open (the amount of water can be added is 90%); open 3 #, 1 # +3 #, 2 # +3 #, 4 #, 1 # +4 #, 2 # +4 # , Followed by the amount of water can be added to 120%, 150%, 180%, 210%, 240%, 270%.

The number of pumps in Table 2.1 is maintained at three, with the principle of minimizing the number of pumps to open, so as to improve the operating efficiency of the system and ensure the stable operation of the water supply system. An important principle in starting (or stopping) a pump is that the capacity of a constant speed pump for each input (or withdrawal) is about 30% of the variable speed pump when the water supply is insufficient (or too large).

3.3 speed pump capacity to determine

From the above analysis we can see that the actual water supply pump speed and rated water supply closer to the speed of the pump operating efficiency is higher, which requires the selected speed pump capacity can not be too large, so that when the pump speed alone due to water supply Too little to run in inefficient zones, but as described in Section 3.2, the system's regulation depends on the capacity of the governor pump, which increases after the constant speed pump is put into operation, down by 30% of the governor pump's water supply The change rate of water supply per unit time is greater than this value, still can not guarantee to reduce the frequency of the water supply system switch, which in turn requires the pump capacity to be large enough. Therefore, the speed of the pump capacity should be considered from the above two aspects to determine the appropriate speed pump capacity.

4 Conclusion

At this stage of speed water supply system is unstable or even run low efficiency is a common problem, the future design of water supply pump room, the design department should consider the issues related to this. Through the above theoretical analysis and discussion of the pump capacity equipped, summarized as follows:
First, the main factors that affect the stable operation of the speed regulating water supply system and cause the pump to start and stop frequently are the unreasonable capacity ratio between the constant speed pump and the speed regulating pump, and the fixed speed pump capacity is too large. Pump capacity ratio of 1: 0.3 capacity to choose.
Second, the fixed speed pump capacity is too large, not only affect the stability of speed control water supply system operation, but also cause the low speed pump operation efficiency, according to our operating experience, sometimes speed control pump will appear zero efficiency operation Speed ​​pump water supply close to 0).
Third, to determine the speed of the pump capacity should be considered from two aspects, first of all, when the water supply alone with the rated water supply capacity can not be too low; the second is the constant speed pump input, water supply within the unit of time changes The rate can not be greater than 30% of the rated capacity of the speed regulating pump, otherwise it can not be guaranteed to reduce the frequent switching of the water pump.

Bibliography "Urban Water Supply Project" Yuan Shizuen Li Hongxi editor Jiang Naichang host Hunan Science and Technology Press 1990 published

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