JP2010084697A - Pump cavitation-free control device and pump cavitation-free control method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a control device and method for a direct feed water supplying system of large rate of flow structured so that a parallel operation is conducted with variable speed pumps, capable of controlling the rate of flow of each pump so that the rate is no higher than the cavitation limit. <P>SOLUTION: A means is provided to compute the in-service pump suction head NPSH<SB>av</SB>from an approximating expression to give a synthesized loss head with respect to the rate of flow of a plurality of counterflow preventing check valves in parallel connection and the sensing value of a pressure sensor for sensing the service water main pipe pressure placed upstream of the check valves. The approximating expression of the requisite pump suction head NPSH<SB>re</SB>given by the pump maker is determined, followed by computing the condition that the in-service NPSH<SB>av</SB>-requisite NPSH<SB>re</SB>>margin head, from the obtained NPSH<SB>av</SB>and NPSH<SB>re</SB>, and if this condition is not met, the frequency of an inverter is held at the value at this time, and the pump speed is controlled at or below the cavitation limit rate of flow by inhibiting the pump(s) from being accelerated. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention is a large-flow direct feed water system configured such that a variable speed pump driven by a variable voltage, variable frequency power supply performs parallel operation, and is controlled by constant discharge pressure control or estimated terminal pressure constant control. In this case, even in a large flow rate water supply operation when the main pipe pressure drops to the specified minimum value or a pressure close to the minimum value, the pump flow rate can be controlled to be less than the cavitation limit, and a safe water supply operation can be performed. The pump cavitationless control apparatus and the pump cavitationless control method are controlled as described above.

  When the pumps in the direct feed water supply system are operated in parallel and supplying a large amount of water, cavitation may occur in the pump when the water mains pressure decreases for some reason. The generation of the cavitation is because the pressure loss of the check valve for backflow prevention placed on the suction side of the pump becomes very large in a large flow rate operation. Further, to prevent the harmful effects caused by the cavitation, the pump operation is interlocked with the pump suction pressure. However, conventional cavitation protection is difficult because the pressure sensor is installed upstream of the check valve for preventing backflow.

  In order to solve the above problems, the present invention reduces the pump speed to the cavitation limit even when the pump is supplying a large amount of water by parallel operation and the water mains pressure is reduced to a specified minimum value. An object of the present invention is to provide a pump cavitationless control device and a pump cavitationless control method that automatically control the flow rate to be equal to or lower than the flow rate.

In the present invention, an approximate expression representing the combined loss head with respect to the flow rate of a check valve for backflow prevention that is connected singly or in parallel is obtained and set. Further, according to the present invention, the utilization pump suction lift (NPSH _av ) is used as the utilization pump suction lift (NPSH _av ) from the approximate expression and the detected value of the water main pressure detection pressure sensor placed upstream of the check valve for preventing the backflow. NPSH _av ) calculating means is provided. On the other hand, the present invention obtains an approximate expression of the necessary pump suction head (NPSH _re ) given by the pump manufacturer and sets it. The present invention is, from the determined utilization pump suction heads (NPSH _av) and required pump suction heads (NPSH _re), utilizing the pump suction heads (NPSH _av) - required pump suction heads (NPSH _re)> margin lift calculated If this condition is not satisfied, the pump speed is controlled below the cavitation limit flow rate by holding the inverter frequency at the current value and preventing acceleration of the pump.

ADVANTAGE OF THE INVENTION According to this invention, when a pushing head is low, the cavitation problem resulting from the loss head of the check valve for backflow prevention which is a problem in the large flow rate operation of the direct feed water supply system can be solved. In particular, according to the present invention, the loss head of the check valve for backflow prevention is represented by a quadratic approximate expression of the flow rate, the used pump suction lift (NPSH _av ) corresponding to the pump lift is calculated, and the necessary pump suction lift is calculated. By expressing (NPSH _re ) by a quadratic approximation formula of the flow rate, the cavitation limit flow rate for the actual indentation head is calculated continuously, so that the indentation lift is calculated from the cavitation limit flow rate set at the conventional specified minimum head. Since the cavitation limit flow rate automatically increases in response to the recovery of water, there is an effect that the feed water flow rate can be automatically increased.

  According to the present invention, when the check valve for backflow prevention is maintained, the selection switch for the check valve for backflow prevention is provided for each check valve for backflow prevention. Never become.

According to the present invention, when the check valve for backflow prevention is removed for maintenance, the used pump suction lift (NPSH _av ) is calculated from the combined loss characteristics of the remaining check valve for backflow prevention. Since the pump is controlled to operate at a speed equal to or lower than the cavitation limit flow rate by comparing with the pump suction lift (NPSH _re ), it is safe against the occurrence of cavitation.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 shows a constant estimated end pressure configured to perform parallel operation of three variable speed pumps driven by a variable voltage and variable frequency power supply, to which the cavitationless control method for a direct feed water system pump of the present invention is applied. The overall configuration of a direct water supply system that performs control is shown. In FIG. 1, the embodiment of the present invention does not directly relate to, for example, pump alternate operation sequence control or pump pressure holding operation sequence control. The estimated terminal pressure constant control circuit 31 in FIG. 1 is a known one, and a block diagram of the cavitationless control circuit 41 of the present invention is added thereto.

  The operation of the estimated terminal pressure constant control circuit 31 will be described with reference to FIG. The pressure setters 31 to 4 are the minimum lift setters for setting the minimum lift h0 (p.u.) of the estimated terminal pressure constant control, and the output Δh (p) of the pipe friction loss lift calculators 31 to 6 (HEAD). u.) is added to generate a target head hS (p.u.) for constant terminal pressure constant control.

The target head hS is compared with the output hI of the discharge pressure detector 11, the deviation is amplified by the PID or PI controllers 31 to 1, and the output fS * is converted to D / via the control limiters 31 to 8 described later. A is given to converters 31-2, 31-3, 31-4. Each D / A converter 31-2, 31-3, 31-4 has a frequency command f1S to inverters 21, 22, 23.
, F2S and f3S are given, and the speeds of pump 1, pump 2 and pump 3 are adjusted. That is, the estimated terminal pressure constant control circuit 31 constitutes one system.

The pipe friction loss lift calculators 31 to 6 (HEAD) receive the square q * ² of the estimated flow rate from the flow calculation circuits 31 to 5 (FLOW), and set the friction loss lift Δh to Δh = (1−h0).
) Calculate with q * ² . The flow rate calculation circuits 31 to 5 (FLOW) receive the net pumping pressure hP obtained by subtracting the net pushing pressure hSUN from the pump discharge pressure hI and the inverter command frequency fS * as input, and a lift-flow rate approximate quadratic equation To calculate the square of flow rate q * ² and its square root q *. Here, the pump net pushing pressure hSUN is obtained by subtracting the check valve loss lift hSU0 for backflow prevention from the pushing pressure hSU.

  Although the details of the sequence circuits 31 to 7 (SEQ) are not shown in the figure, the sequence control such as the parallel and disconnection control of the pump or the pump holding pressure control and the pump stop control necessary for the automatic operation of the direct feed water supply system is performed. A control block to be executed. The sequence circuits 31 to 7 (SEQ) indicate that RUN1, RUN2 and RUN3 are signals for operating the inverters 21, 22, and 23, ZERO is the FLS signal for the flow switch 10, and the water supply flow rate has fallen below the specified value. A signal to be detected is shown.

  The pumps 1, 2, 3 are driven by electric motors 14, 15, 16 controlled by inverters 21, 22, 23, respectively, as shown. As shown in the figure, the pumps 1, 2, and 3 are connected to the discharge side via check valves 4, 6, and 8 and shut-off valves 5, 7, and 9, and the suction side is a suction common pipe. Are connected in parallel. The pressure tank 13 is placed to hold the feed water pressure when the pump is stopped.

  Check valves 24-1, 25-1, 26-1 for backflow prevention are backflow prevention valves that prevent backflow from the water supply system to the water main when the pump is stopped. It is shown. The check valves 24-1, 25-1, 26-1 for backflow prevention are maintenance cutoff valves on both sides, 24-2, 24-3, 25-2, 25-3, 26-2, 26-3. Are provided so that the check valves 24 to 1, 25 to 1, and 26 to 1 for backflow prevention can be maintained by closing these shut-off valves.

  The water main pressure detector 12 is placed upstream of the check valves 24-1, 25-1, 26-1 for backflow prevention. In addition, the backflow prevention valve 27 is a bypass pipe arranged so that when the water main pressure is equal to or higher than the specified pressure, the pump is stopped and water can be supplied only by the water main pressure. The frequency limiters 31 to 8 (LIM) placed on the output side of the PID or PI controllers 31 to 1 are controlled by the control signal fLIM of the cavitationless control circuit 41 of the present invention. As described later, in normal operation, the control signal fLIM is “0”, and the limit operation of the frequency limiters 31 to 8 (LIM) is not performed. According to the above description, according to the configuration of FIG. 1, it is possible to carry out the parallel operation system of the pump and the direct feed water operation of the estimated terminal pressure control.

FIG. 2 is a block diagram illustrating an operation of cavitationless control according to an embodiment of the present invention. Blocks 41 to 1 that set an approximate expression for the check valve loss head expressed by the secondary expression of the flow rate correspond to the selected state of the check valve selection switches 41 to 2, 41 to 3, and 41 to 4 for preventing backflow. Thus, the combined loss lift hL (pu) of the check valve for backflow prevention is set as shown in equation (1).
hL = C1 · q * ² + C2 · q * + C3 ---- (1)
Where C1, C2, C3 = constant q * = Q * / QN = estimated flow rate (m ³ / min) / rated flow rate of pump (m ³ / min) (pu)
hL = HL / HN = Check valve combined loss lift (m) / Pump rated lift (m) (pu)

  For example, when three check valves for backflow prevention according to the present invention are arranged in parallel, C1 = 0.0128, C2 = 0.0198, and C3 = 0.0619 are obtained from the measured data. For example, when two check valves are arranged in parallel, other than the three check valves for preventing the reverse flow are arranged in parallel, the selected check valves for preventing the reverse flow determine constants C1, C2, and C3 from the measured data. it can. For example, in the configuration of the two check valves for preventing the backflow, data such as C1 = 0.0248, C2 = 0.0330, and C3 = 0.0652 were obtained.

FIG. 3 is a table showing examples of set constants when the check valve for backflow prevention in the embodiment of the present invention is selected. As shown in FIG. 3, the blocks 41 to 1 for setting the approximate expression of the check valve loss head represented by the secondary expression of the flow rate are constants C1, C2, C3 as shown in FIG. A table set with is created. For example, the check valve selection switches 41-2, 41-3, 41-4 for backflow prevention are all “ON”, that is, 3
When the check valves for the backflow prevention of the base are connected in parallel, d31 is set for C1, d32 for C2, and d33 for C3. Here, d is a setting constant set in advance in 21 memories.

  The table shown in FIG. 3 is created when the loss characteristics of the check valves for preventing backflow connected in parallel are different from each other. In order to provide C1, C2, and C3 data for all combinations, It shows the case where a total of 21 memories are required. However, when there is little variation in the loss characteristics of the check valve for backflow prevention connected in parallel, the table is throttled in three cases: three in parallel, two in parallel, one in case, It can be simplified to use a total of nine memories.

In the present invention, the check valve for preventing the backflow is temporarily removed for maintenance by setting a constant of a quadratic expression approximating the combined loss head of the check valve in advance as a table. Even in this case, manually turn off the check valve selection switch for backflow prevention.
The calculation circuit for the combined loss lift of the check valve for backflow prevention can be automatically corrected so as to match the actual use state. The estimated value of the flow rate shown in the above set secondary equation When q * (pu) is given, the combined loss hL (pu) of the check valve for backflow prevention at the flow rate is outputted to the outputs of the blocks 41 to 1.

Therefore, the net pushing head hSNET (pu) is calculated by subtracting the output hL (pu) of the blocks 41 to 1 from the pushing head hSU (pu). The net pushing head hSNET (pu) is obtained by adding NPSH _av (pu) by adding atmospheric pressure ha (pu) = atmospheric pressure HA (m) / pump rated head HN (m). On the other hand, the required pump suction lift (NPSH _re ) is indicated by the pump manufacturer. Based on this, based on the pump rated head and the rated flow rate, the secondary expression of pu display is approximated as shown in the expression (2).

NPSH _re = D1 ・ q * ² + D2 ・ q * + D3 ---- (2)
NPSH _re = Necessary pump suction head (pu) = Necessary pump suction head (m) / Pump rated head HN (m) (pu)
D1, D2, D3 = Constant Necessary pump suction lift (NPSH _re ) calculation blocks 41 to 5 (REH) perform the calculation of equation (2), and are given by the pump manufacturer corresponding to the pump used. The pu value is calculated from NPSH _re (m), and constants D1, D2, and D3 that approximate the curve are calculated and set in advance. Therefore, when the estimated value q * (pn) of the flow rate is given to the calculation blocks 41 to 5 (REH), a necessary pump suction lift NPSH _re (pu) is obtained as an output thereof. The estimated value h00 (pu) of the friction loss head of the pump suction side pipe is set to a value obtained by adding a head tolerance (pu) for executing cavitationless control. Then, the comparators 41 to 6 perform the calculation of the following equation (3).

NPSH _av (pu) −NPSH _re (pu) −h00 (pu) ≦ 0 ---- (3)
When the expression (3) is established, the comparators 41 to 6 set their outputs, fLIM, to “1”. The signal is transmitted to the limiters 31 to 8 (LIM), and the inverter frequency is held at the current value. In other words, when the pump suction head NPSH _av used is less than the necessary pump suction head (NPSH _re ) + the margin head, fLIM is set to “1” to maintain the pump speed and prevent the pump flow rate from increasing. . Therefore, the pump is safely operated at a cavitation limit flow rate or less.

FIG. 4 is a different embodiment of the present invention and explains the relationship between the used pump suction lift (NPSH _av ) (m) and the necessary pump suction lift (NPSH _re ) (m) when there are two check valves for preventing backflow. It is a figure for doing. The calculation is made for the case where the check valve for preventing backflow has two pushing heads of 10 m, 15 m and 18.14 m. As shown in the drawing, in this example, the cavitation limit flow rate is almost zero at the pushing lift 10 m, indicating that the operation cannot be performed at the pushing lift 10 m. However, when the indentation head is 15m, it increases to about 190l / min, and when the indentation head is 18.14m, it increases to about 260l / min. As shown in this example, it can be seen that the decrease in the indentation head greatly affects the cavitation limit flow rate.

In the present invention, even when such a check valve combined loss lift is large, the large combined loss lift is set in the cavitation-less control circuit by the above-described method, and the actual pumping lift is detected to use the pump suction lift. Since (NPSH _av ) is calculated, safe operation without cavitation can be performed.

Applying the cavitation-less control method for a direct feed water system pump according to the present invention, the control of the estimated end pressure is performed to perform parallel operation of three variable speed pumps driven by a variable voltage and variable frequency power source. The overall configuration of the direct water supply system is shown. (Example 1) It is a block block diagram explaining the operation | movement of cavitationless control in the Example of this invention. It is a table which shows the example of the setting constant at the time of selecting the check valve for backflow prevention in the example of the present invention. In different embodiments of the present invention, the relationship between the used pump suction lift (NPSH _av ) (m) and the necessary pump suction lift (NPSH _re ) (m) when there are two check valves for preventing backflow is described. FIG.

Explanation of symbols

1, 2, 3 ... Pump 4, 6, 8 ... Check valve 5, 7, 8 ... Shut-off valve 11 ... Discharge pressure detector 12 ... Main water pressure detector 13. .... Pressure tanks 14, 15, 16 ... Motors 21, 22, 23 ... Inverter 31 ... Constant terminal pressure constant control circuit 41 ... Cavitationless control circuit

Claims (4)

Two or more variable speed pumps driven by an electric motor controlled by a variable voltage / variable frequency power source are operated in parallel, and the pressure of the variable speed pump is controlled by a constant discharge pressure control or an estimated terminal pressure constant control, In a pump cavitationless control device in which two or more check valves for backflow prevention are installed in parallel on the suction side of the pump controlled by the frequency of the inverter,
A pressure sensor that is installed on the upstream side of the check valve for preventing backflow and detects the indentation pressure on the upstream side;
A head approximation formula computing means for obtaining a quadratic equation relating to a flow rate approximating the combined loss lift of the check valve for backflow prevention;
Utilization pump suction lift (NPSH _av ) for computing the utilization pump suction lift (NPSH _av ) from the indentation pressure detected by the pressure sensor and the combined loss lift of the check valve for backflow prevention calculated by the lift approximate expression calculation means Computing means;
Necessary pump suction lift (NPSH _re ) calculating means for calculating a quadratic approximate expression of the pump-specific required pump suction lift (NPSH _re ) given to the pump;
A comparison means for comparing a result obtained by subtracting a calculation result of the necessary pump suction lift (NPSH _re ) calculation means from a calculation result of the used pump suction lift (NPSH _av ) calculation means, and a pump-specific margin lift;
When the comparison result by the comparison means is the same as or smaller than the pump-specific margin, control means for holding the inverter frequency and preventing the pump flow rate from increasing;
A pump cavitationless control device comprising at least   2. The pump cavitation according to claim 1, wherein each of the check valves for preventing backflow is provided with a selection switch, and can be individually separated by selection of the selection switch and can be maintained or replaced while the pump is in operation. Less control device. While the check valve for backflow prevention is disconnected by the selection switch, the calculation of the pump suction lift NPSH _av -Necessary pump suction lift (NPSH _re ) ≤ Margin lift is performed, and the pump flow rate is increased by maintaining the inverter frequency. 3. The pump cavitationless control device according to claim 1, wherein the pump cavitationless control device prevents the pump cavitation. Two or more variable speed pumps driven by an electric motor controlled by a variable voltage / variable frequency power source are operated in parallel, and the pressure of the variable speed pump is controlled by a constant discharge pressure control or an estimated terminal pressure constant control, In the pump cavitationless control method in which two or more check valves for backflow prevention are installed in parallel on the suction side of the pump controlled by the frequency of the inverter,
The upstream pressure is detected by a pressure sensor installed upstream of the check valve for backflow prevention,
Obtain a quadratic equation for the flow rate that approximates the combined loss lift of the check valve for backflow prevention,
Calculating a pump suction lift (NPSH _av ) from the indentation pressure detected by the pressure sensor and the combined loss lift of the check valve for backflow prevention calculated by the lift approximate expression calculation means;
Calculate a quadratic approximate expression of the necessary pump suction lift (NPSH _re ) specific to the pump given to the pump,
Use pump suction lift (NPSH _av )-Necessary pump suction lift (NPSH _re ) ≤ margin lift,
A pump cavitationless control method characterized in that an increase in pump flow rate is prevented by maintaining the frequency of the inverter so that the above equation is satisfied.

Images