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What is a multi-function water pump control valve? What operating characteristics of the pump require a valve to control? Can the pump control valve achieve these controls? And it has a hydraulic control check with a conventional gate valve, butterfly valve, check valve, and constant speed, two-speed slow closing. The valve has different qualitative and functional aspects. In this paper, the piston multi-function water pump control valve (hereinafter referred to as the control valve) is taken as an example. Through the analysis of its structure, main functions and working principle, the above is proposed. The view of the problem is for the reader's reference.
First, the structure
Control valve structure
The main feature of the control valve structure is that the positioning mechanism in the middle of the valve seat and the spring on the upper side of the valve disc are eliminated, and the deflector is designed on the lower side of the valve flap to minimize the mechanical loss and valve flap when the medium is overcurrent. Vortex loss in the lower iliac cavity. The piston in the cylinder is used as the driving component to drive the valve flap up and down under the pressure of the medium itself to realize the opening or closing of the valve. The piston, the opening and closing member, and the cylinder are arranged on the valve body, and the valve body of the streamlined and wide valve chamber can not only reduce the head loss by more than 30%, but also has good cavitation resistance.
The solenoid valve reversing mechanism (hereinafter referred to as the solenoid valve) and the pressure line form a servo system, taking the pressure water at both ends of the control valve as the driving source, and the pressure water at either end can realize the setting of the pump control valve through the electric signal command. The time and speed of the pump is turned on or off.
Second, the pump's operating characteristics and control valve features, working principle
1. Pump starting characteristics and control
a) Zero flow starting characteristics of the centrifugal pump and its control (ie closing the valve)
The centrifugal pump has the lowest shaft power at zero flow conditions, which is 30%-90% of the rated shaft power. Therefore, the starting characteristic of the centrifugal pump is zero flow start (ie, the valve is closed). After the pump has been pumped to the rated speed, the control valve opens slowly at the set speed.
Working principle: When the pump starts (pre), the pressure water flows to the upper chamber of the piston through the solenoid valve with delay, and the lower chamber of the piston passes through the lower end of the cylinder to the atmosphere through the electromagnetic valve. At this time, the control valve is closed.
When the motor compensation starts, the pump runs normally. The solenoid valve executes the reversing command, cuts off the pressure source of the upper chamber of the piston, closes the circuit leading to the atmosphere at the lower end of the cylinder, and simultaneously injects the pressurized water into the lower chamber of the piston through the solenoid valve and opens the piston. The upper chamber leads to the atmosphere, the water in the upper chamber of the piston is discharged outside the valve through the solenoid valve, and the control valve is slowly opened at the set speed, which completes and satisfies the starting characteristic of the minimum power of the centrifugal pump at zero flow, and ensures the pump unit. Safe operation.
b) The large flow start characteristic of the axial flow pump and its control (ie full open valve start)
The axial flow pump has the largest shaft power in the zero flow condition, which is 140% to 200% of the rated shaft power. Therefore, the starting characteristic of the axial flow pump should be the high flow start (ie, full open valve start).
Working principle: The working principle of the control valve satisfying the full-opening of the axial-flow pump is the reverse operation of the centrifugal pump closing valve, that is, the solenoid valve works first, and after the valve is fully opened, the pump is restarted. See a) and do not repeat them.
Before the axial flow pump is started, the inlet pressure of the valve is zero at this time, the control valve uses the medium pressure of the outlet end of the valve to open the valve, and when the centrifugal pump starts, the valve is opened by the medium pressure of the inlet end of the valve. Regardless of which end of the valve the medium can open or open the control valve, this is one of the important features of the pump control valve function.
c) Control the starting characteristics of the mixed flow pump
When the mixed flow pump is in the zero flow condition, the shaft power is between the above two pumps, which is 100% to 130% of the rated power. Therefore, the starting characteristics of the mixed flow pump should also be between the above two pumps. It is synchronized with the slow opening of the control valve.
Working principle: The working principle is the same as the zero flow starting characteristic of the centrifugal pump. Just cancel the delay setting of the solenoid valve.
2. Stop the pump and its control
a) Control the zero flow of the centrifugal pump to stop the pump (ie, stop the pump after closing the valve first)
For the centrifugal pump, the importance of slowly closing the control valve and then stopping the pump is: at this time, the flow rate of the medium in the tube gradually decreases to zero from the rated flow rate, and the gradient of the speed change is extremely small, which can effectively avoid the occurrence of the pump water hammer. The reversal of the pump; the shaft power of the pump is gradually reduced from the rated power to the minimum operating power, which is conducive to the safety of the unit.
Working principle: When the pump needs to be stopped, as long as the control power of the solenoid valve is first cut off, the solenoid valve executes the reversing command, closing the pressure water source of the lower chamber of the piston and the loop of the upper chamber of the piston, and simultaneously opening the pressure water source of the upper chamber of the piston and The lower part of the piston passes through the atmospheric circuit, that is, the inlet pressure water is used as the driving source, and the control valve is slowly closed. When the control valve is completely closed, the pump is stopped by the limit switch, and the whole process is automatic control.
The control valve slow-closing time can be adjusted within the range of 5 to 60 s by the adjustment of the throttle valve or according to the needs of the user.
b) Control the pump stop of the axial flow pump and the mixed flow pump
The pump stop function and working principle of the axial flow pump and the mixed flow pump are similar to the above contents, and it is not awkward.
3. Change the characteristics of the device (ie, the flow stepless adjustment function)
Changing the characteristics of the device belongs to the regulation of the operating conditions of the pump. The most commonly used is the throttling method, also known as the valve adjustment method, which uses the opening degree of opening or closing a small valve; changing the resistance coefficient æ³µ in the pump device, thereby making the tube The water supply characteristic of the road H=F(Q) changes to achieve the purpose of regulating the flow.
The control valve can be adjusted steplessly from fully closed to fully open. The opening degree is displayed by the opening display and is memorized and stabilized.
working principle:
a) The flow is from small to large
The pressure water injection pipe in the lower chamber of the piston and the upper chamber of the piston open to the atmosphere, and the pressure water injection pipe in the upper chamber of the piston and the lower chamber of the piston are closed with the atmosphere. The valve flap is slowly opened to reach the setting. When the flow is constant, the control circuit closes the circuit connecting the upper and lower chambers of the piston to the atmosphere, and simultaneously injects the pressure water into the pipeline in the upper and lower chambers of the piston. At this time, the total pressure on the upper and lower sides of the piston is equal, and the piston is at At rest, the entire cylinder body is sealed, the valve opening degree is stabilized, and the opening degree is displayed by the opening degree display.
b) The flow is small
It is the reverse operation of a), not awkward.
The above control circuit commands can be executed by a limit switch or a small electronic control box, and both can be automatically controlled.
4. Accidental sudden stop pump water hammer control
Accidental sudden stop pump generally refers to the phenomenon of power supply network failure or strong lightning protection tripping, etc. The probability of such pump stop is less than 1% of the total number of pumps stopped, but the resulting water hammer has a pipe network and water supply unit. Destructive, should be taken seriously.
a) Physical phenomenon of pumping water hammer
This article is not to discuss the water hammer in the pipeline, just to facilitate the description of the function and working principle of the control valve to reduce the water hammer force, and to quote the Rukovsky water hammer formula to briefly describe the physical phenomenon of the pump water hammer. Where H is the water hammer force when the water hammer is generated (m water column);
C——the speed of water hammer wave propagation. If the elastic modulus of the medium and the wall material and the wall thickness of the pipe wall are not considered for the time being, C=1000m/s is approximated;
G——gravitational acceleration, 9.81 m/s;
Vo—the medium flow rate (m/s) before the water hammer occurs;
Vt - the medium flow rate (m/s) at which the water hammer occurs.
It can be seen from Equation 4-1 that the necessary condition for the water hammer to occur is the change in the velocity of the fluid, and the magnitude of the water hammer depends on the magnitude of the gradient of the change. When the pump is stopped, the flow rate is reduced to zero by V0. The first layer of liquid level is compressed due to the inertia of the fluid, the water layer is compressed, the pressure rises (ie, the water hammer pressure), and then one layer, one layer...stagnation due to the action of the inertial force. The pressure rises, is compressed, and there are micro-high pressure zones and low pressure zones. Their interface is called water hammer wave (in this case, booster wave), and the boosting wave disappears until the end of the pipe end of the water pipe. Then, the reverse layer, the first layer... propagates to the first liquid level at the beginning of the water hammer (in this case, the complex pressure wave). The time t required for this round trip process can be expressed by the following formula: where L is the length of the water pipe (m).
If the valve closing time is T off, the water hammer under the condition of T off L/500s is an indirect water hammer, and ΔH is smaller than the direct water hammer. This is because when the indirect water hammer is used, the supercharging process that is developing before the valve is weakened by the interference of the complex pressure wave. The magnitude of ΔH is related to the degree of interference and the number of disturbances.
It can be seen from the above brief that the control valve can not avoid the occurrence of water hammer when the pump is suddenly stopped accidentally, but the direct water hammer can be converted into the indirect water hammer to minimize ΔH according to the length of the water delivery pipe by adjusting the valve closing time. Ensure the safety of the water pipeline and pump unit.
b) adjustment of valve closing time
At present, there are two methods for adjusting the closing time of the pump control valve. It should be pointed out that the adjustment method referred to in this paper is one of the functions of the pump control valve. Although various types of check valves or other valves having this function are not Must be the pump control valve, do not mislead this, see the "Conclusions" section of Chapter 3 of this article.
c) Two-stage shut-off valve (that is, the valve is quickly closed after the valve is quickly closed)
This type of closing valve can prevent the occurrence of the water hammer of the pump stop. As for the time of quickly closing the valve, it should also match the length of the pipeline, taking into account the reverse speed of the pump and the amount of backflow water, to meet the requirements of eliminating the water hammer of the pump.
d) uniform speed closing valve
This kind of closing valve is characterized by the ability to adjust the different valve closing time according to the different lengths of the water pipeline and the number of round trips of the selected complex pressure wave, and the conditions of T off > L/500s are always met. The water hammer force of the sudden stop pump water hammer is greatly weakened, and the adaptability is wider than that of the two-stage shut-off valve, which is sufficient to meet the requirements of GB/T50265-97 "Pump Station Design Specification" that the reverse speed should not exceed 1.2 times the rated speed. The time should not exceed 2 minutes.
The above two methods of the water pump control valve are automatically controlled by the solenoid valve.
5. Select the pressure source for greater mobility
The servo mechanism of the solenoid valve is designed to take pressure in both directions at the same time. Through the one-way valve distribution, the water in a higher pressure infusion tube is injected into the solenoid valve as a pressure source. Whether the pressure water at the inlet end or the pressure water at the outlet end can be individually turned on or off.
Because of the existence of this function, the pump control valve can meet the requirements of various pump operating conditions such as starting and stopping the pump. The important thing is that when the conveying medium is turbid water and cannot be used as a driving source, an external pressure source is required, as long as one servo line can realize all functions of the control valve.
6. Energy saving
Controlling the energy consumption of the flow medium to a minimum is an important issue for the pump control valve. We have sampled some of the control valves of the path and made the following experimental formulas for the reader's reference:
The drag coefficient is determined by the following formula:
When DN≤250ξξ=14.68×V1.1339(7-1)
When DN>250, ξ=10.97×V1.1339(7-2)
In the formula - the drag coefficient;
V - medium flow rate (m / s).
The local head loss hf is determined by the following formula:
Hf=ξ×V2/2g (7-3)
Or: hf=0.749×V0.8661 (7-4)
Hf=0.56×V0.8661 (7-5)
Where hf - local head loss (m water column);
V——media flow rate (m/s);
G——gravity acceleration g=9.81 (m/s2).
After testing, the head loss of the pump control valve is reduced by more than 30% compared with the same type of hydraulic control valve.
Third, the conclusion
It is obvious that a valve having a function of controlling the running characteristics of a water pump is called a water pump control valve.
Sections a), b), and c) of Section 1 of Chapter 2, “Starting Characteristics and Control of Pumpsâ€, Sections a) and b) of Section 2, “Stop Pumps and Controlsâ€; Section 3 Sections b) and b) of “Changing Device Characteristicsâ€; Section b) of Section 4 “Control of Accidental Sudden Pump Water Hammer Forceâ€; the eight functions described here are technical conditions required for pump operating characteristics. It is also the content of the pump's own operating characteristics that requires the valve to control, and is also a necessary function of the pump control valve. These essential functions are defined by the difference between pump control valves and various types of check valves or other valve qualities.
Why can't we call the pump control valve if it doesn't have the necessary functions mentioned above? This can be illustrated by the following example:
If the function of stopping the pump after closing the valve in the second section a) is not provided, for the centrifugal pump, all the pumps are stopped and suddenly stop the pump, which takes up more than 99% of the total number of pumps and can stop the pump normally. The safety of avoiding the reversal of water hammer and pump is ruined and dangerous.
If the functions of Sections 2 and 3 are not available, only the start of the pump can realize the opening of the valve and the closing of the pump to achieve the closing of the valve. Obviously, this is the pump operating in the control valve and losing the control of the control valve. "Substance, it is more difficult to understand as a pump control valve.
If the function of stepless adjustment of flow in Section 3 is not available, once the resistance of the pump is too small due to the resistance, the flow rate is too large, which not only causes the pump to deviate from the energy consumption in the highest efficiency zone, but also damages the unit due to the sudden increase of the shaft power. . Therefore, it is inevitable that the pump control valve should have the above eight necessary functions.
Functional characteristics of multi-function water pump control valve