JPH09158889A - Recirculating operation method for centrifugal pump - Google Patents

Abstract

PROBLEM TO BE SOLVED: To ensure a recirculating flow rate in a pump, without providing a complicated recirculation system and restraining energy loss to the minimum, in parallelly operating plural centrifugal pumps. SOLUTION: The discharge pipes 6 and 7 of plural pumps 4 and 5, operated at variable speeds, are communicated with each other. In parallelly operating these pumps 4 and 5, the rotation frequency of any one pump out of these pumps 4 and 5 is adjusted, so that the rotation frequency can be increased by a fixed frequency in a low flow rate region compared with the rotation frequency of the other pump. Such an operating method can ensure a recirculation flow rate in a pump required in a low flow rate region.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a centrifugal pump recirculation operation method used when a plurality of centrifugal pumps are operated in parallel.

[0002]

2. Description of the Related Art When a centrifugal pump is operated in a flow rate range lower than a design point flow rate, particularly in a flow rate range near and at a cutoff point, the temperature of the fluid in the pump rises. In a pump having a large lift or discharge flow rate, the temperature of the fluid increases greatly, which causes deformation of the pump body due to thermal expansion and the like, which makes it impossible to operate the pump in a stable manner due to increased vibration. Therefore, up to now, a system has been provided in which another pipe is branched from before the check valve provided in the discharge pipe of the pump and connected to a tank or the like on the suction side of the pump. Conventionally, this system is called a pump recirculation system, and a control valve for flow rate adjustment and an orifice for decompression are provided on this recirculation system so that a constant flow rate is always maintained. It will be secured through.

On the other hand, in order to meet demands from the pump system side such as expansion of operability, a method is used in which a plurality of centrifugal pumps are installed in parallel and operated. Even in such a case, the above-mentioned recirculation system must be provided.

An example of a conventional recirculation system when a plurality of conventional centrifugal pumps are installed in parallel will be described with reference to FIG. For simplicity, the number of pumps will be described as two. During normal operation, the fluid passes from the water storage tank 1 on the pump inlet side through the suction pipes 2 and 3 of the pumps 4 and 5, and after being pressure-increased by the pumps 4 and 5, is discharged to the discharge pipes 6 and 7. Come out and check valves 8, 9,
It flows into the tank 13 on the discharge side through the gate valves 10 and 11 and the flow rate control valve 12. On the other hand, during the recirculation operation, the fluid extracted from the discharge pipes 6 and 7 in front of the check valves 8 and 9 through the recirculation pipes 18 and 19 is the recirculation flow rate control valves 14 and 15, and the orifice 16 for decompression. , 17 to the water tank 1 on the pump inlet side for circulation. The pumps 4 and 5 are the driving machine 20 and
Driven by 21.

[0005]

In the conventional recirculation operation method, most of the energy of the fluid flowing through the recirculation system during the recirculation operation is lost as a loss. In addition, the high-pressure fluid that has been boosted by the pumps 4 and 5 must be decompressed by the recirculation flow rate control valves 14 and 15, the decompression orifices 16 and 17, and sometimes the recirculation pipes 18 and 19 and the recirculation flow rate control valves. Flash erosion, cavitation erosion, or the like occurs, and these may be damaged.

It is an object of the present invention to secure a recirculation flow rate in a pump when a plurality of centrifugal pumps are operated in parallel without providing a complicated recirculation system and minimizing energy loss. The present invention provides a method for recirculating a centrifugal pump as described above.

[0007]

In order to achieve the above-mentioned object, the invention according to claim 1 is such that when the discharge pipes of a plurality of centrifugal pumps that are operated at variable speeds are communicated with each other and these pumps are operated in parallel. By adjusting the rotation speed of one of the pumps to be higher than the other pump rotation speed by a certain amount in the low flow rate range, the recirculation flow rate required in the low flow rate range can be adjusted. It is characterized by being secured by a pump. In the invention according to claim 2, when the discharge pipes of a plurality of centrifugal pumps are communicated with each other and these pumps are operated in parallel, the discharge pressure of one of the plurality of pumps is adjusted to the other pump. It is characterized in that the recirculation flow rate required in the low flow rate range is ensured by each pump by installing an impeller that is increased by a certain amount in comparison with the discharge pressure.

In the invention of claim 1, the discharge pipes of the plurality of centrifugal pumps are communicated with each other, and the rotation speed of one of the variable speed pumps is constant by a certain amount as compared with the rotation speeds of the other pumps. The discharge pressure of this one pump can be made higher than the discharge pressures of the other pumps by a certain amount at a certain arbitrary flow rate by adjusting so as to be larger. For simplicity, the number of pumps is 2
The table will be described with reference to FIG. When two pumps are operated in parallel, the flow rate / lifting curve 30 of the pump has a high rotational speed, that is, the flow rate / lifting curve 31 of the pump with a high discharge pressure.
And the flow rate / lift curve 32 of the pump having a low rotation speed, that is, a low discharge pressure. Now, when the control valve or the like on the discharge path of the pump is closed and the flow rate of the system decreases, the flow rate inside the pump becomes zero first in the pump with the low discharge pressure. When the flow rate of the system further decreases, the operating point of the pump with a low discharge pressure enters the negative flow rate region on the flow rate / lift curve of the pump's complete characteristics, and the pump is rotating in the positive direction. , The fluid starts to flow backward from the discharge port of the pump toward the suction port. This backflow fluid passes through the suction pipe and returns to the water storage pump on the pump inlet side. At this time,
The operating point of a pump with a high discharge pressure is still in the positive flow range on the flow rate / lift curve with perfect characteristics, and a part of the flow rate discharged from this pump will flow backwards through the pump with a low discharge pressure. The flow rate is discharged to the pump discharge path. In this state, the fluid flows inside the two pumps, which prevents an excessive temperature rise.

In the invention of claim 2, the discharge pipes of the plurality of centrifugal pumps are communicated with each other so that the discharge pressure of one pump is increased by a certain amount as compared with the discharge pressure of the other pump. By installing the pump, the discharge pressure of this one pump can be made higher than the discharge pressure of the other pump by a certain amount at a certain arbitrary flow rate. As a result, the same action as described in the action of the invention of claim 1 is produced.

[0010]

Embodiments of the present invention will be described below with reference to the drawings. In FIG. 1, the suction pipes 2 and 3 of the pumps 4 and 5 are connected to the variable speed pumps 4 and 5 from the water tank 1 on the inlet side of the homp. The discharge pipes 6 and 7 of the pumps 4 and 5 are immediately merged into one,
It is connected to a discharge side tank 13 via a check valve 8, a gate valve 10 and a flow rate control valve 12. Pumps 4 and 5 are drive units 20,
It is driven by 21, but these driving machines 20, 21 are variable speed, the rotation speed is detected by the speed detectors 22, 23 installed on the pump shaft, and this speed signal is input to the controller 24, and after calculation, , And transmits a rotation speed control signal to the respective driving machines 20, 21. Reference numeral 25 is a flow rate switch that outputs a signal for shifting to the recirculation mode to the controller 24.

Next, the operation of the above configuration will be described. The two variable speed pumps 4 and 5 operate at the same rotation speed during normal operation, and function as exactly the same pumps. Now, when the flow rate control valve 12 in the discharge path of the pumps 4 and 5 is closed and the flow rate in the system decreases, the flow rate switch
By 25, shift to a preset recirculation operation mode. That is, the rotational speed of one of the two variable speed pumps 4 and 5 is
One pump 4 raised by a predetermined amount
The discharge pressure of is kept higher than the discharge pressure of the other pumps 5. As a result, when the flow rate in the system further decreases, the pump 5 with the lower discharge pressure will
Input into the negative flow area on the lift curve, and start backflow from the normal discharge port to the suction port. This backflow fluid passes through the suction pipe 3 and returns to the water storage tank 1 on the pump inlet side. At this time, the operating point of the pump 4 with a high discharge pressure is still in the positive flow rate region on the flow rate / lift curve of the perfect characteristic, and a part of the flow rate discharged from this pump flows backward to the pump with a low discharge pressure. The remaining flow rate is discharged from the pump discharge line to the discharge side tank 13. In this state, the fluid flows inside the two pumps 4 and 5, and an excessive temperature rise is prevented.

Further, another embodiment of the present invention will be described with reference to FIG. In FIG. 3, the suction pipes 2 of the respective pumps 4, 5 from the water tank 1 on the pump inlet side,
3 is connected to variable speed pumps 4, 5. The discharge pipes 6 and 7 of each pump are immediately merged into one, and the check valve 8 and
It is connected to a tank 13 on the discharge side through a gate valve 10 and a flow rate control valve 12. The pumps 4 and 5 are driven by driving machines 20 and 21, respectively, and these driving machines 20 and 21 rotate at a constant speed.
The impeller of one pump 4 is designed to increase its discharge pressure by a certain amount as compared with the discharge pressures of the other pumps 5.

In the present embodiment, the pump driving machine 2
0 and 21 rotate at a constant speed. By installing an impeller in one pump 4 that increases its discharge pressure by a certain amount as compared with the discharge pressures of the other pumps 5,
The discharge pressure of each pump can be made higher than the discharge pressures of other pumps by a certain amount at any given flow rate. As a result, the same operation as described in the operation of the above-described embodiment occurs.

Up to this point, the description has been made assuming that the number of pumps is two. However, in the case of two or more pumps, one of them, or more pumps, or the impeller of the remaining pumps may be changed. Causes exactly the same action.

In the conventional pump recirculation operation method, most of the energy of the fluid flowing through the recirculation system during the recirculation operation is lost as a loss, and the fluid whose pressure has been raised by the pump does nothing. Instead, waste energy will be wasted over a long period of time.
In the present invention, the pressure difference existing in the discharge pressure of the plurality of pumps can be utilized to perform the recirculation operation, and the energy loss can be minimized.

Further, in the conventional recirculation operation method, the high pressure fluid pressurized by the pumps 4 and 5 is recirculated to the recirculation flow control valve 14,
It is necessary to reduce the pressure between the recirculation pipes 18, 19 and the recirculation flow rate control valves 14, 15 due to the need to reduce the pressure between the recirculation pipes 18, 19 and the recirculation flow control valves 14, 15 and to damage them. However, by applying the present invention, these recirculation flow rate control valves 14, 15 and the pressure reducing orifices 16, 17 can be eliminated, and the maintainability can be greatly improved.

[0017]

As described above, according to the present invention, when a plurality of centrifugal pumps are operated in parallel, there is no need to provide a recirculation system which causes damage such as flash erosion or cavitation erosion, and Energy losses can be minimized to ensure recirculation flow rates in the pump.

[Brief description of the drawings]

FIG. 1 is a system diagram showing an embodiment of the present invention.

FIG. 2 is a characteristic diagram showing a flow rate and a head when two pumps according to the present invention are operated in parallel.

FIG. 3 is a system diagram showing another embodiment of the present invention.

FIG. 4 is a system diagram showing a recirculation system of a centrifugal pump according to a conventional technique.

[Explanation of symbols]

 1 Water storage tank 4, 5 Pumps 6, 7 Discharge pipe 12 Flow rate control valve 20, 21 Driver 22, 23 Speed detector 24 Controller

Claims (2)

[Claims] 1. When the discharge pipes of a plurality of centrifugal pumps operating at variable speeds are made to communicate with each other and these pumps are operated in parallel, the rotational speed of at least one of the plurality of pumps is set to another value. The centrifugal pump is characterized in that the recirculation flow rate required in the low flow rate range is ensured in each pump by adjusting it so that it is increased by a fixed amount compared to the rotation speed of the pump. Recirculation operation method. 2. The discharge pressure of at least one of the plurality of pumps when the discharge pipes of the plurality of centrifugal pumps operating at a constant rotation speed are connected to each other and the pumps are operated in parallel. It is characterized in that each pump can ensure the recirculation flow rate required in the low flow rate range by installing an impeller that increases the discharge pressure of other pumps by a certain amount. , Recirculation operation method of centrifugal pump.