Functional characteristics of multifunctional water pump control valve
What is a multifunctional water pump control valve? What are the operating characteristics of the water pump that need to be controlled by a valve? Can the water pump control valve achieve these controls? And it and the traditional gate valve, butterfly valve, check valve, and hydraulic control check with constant speed and two-speed slow closing What are the qualitative differences in the principle and function of the valve? In this article, the piston-type multifunctional 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 principles, the above problems are raised Views for readers' reference.
Ⅰ. Control valve
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 a deflector is designed on the lower side of the valve disc, which minimizes the mechanical loss and the disc when the medium flows. Vortex loss in the lower dome. The piston in the cylinder is used as the driving element to drive the valve disc to move up and down under the pressure of the medium itself to realize the opening or closing of the valve. Pistons, opening and closing parts, and cylinders are arranged on the valve body. The streamlined, wide valve cavity of the valve body not only reduces the head loss by more than 30% than similar products, but also has good anti-cavitation performance. The servo system is composed of solenoid valve reversing mechanism (hereinafter referred to as solenoid valve) and pressure pipeline. The pressure water at both ends of the control valve is taken as the driving source. Through electrical signal instructions, the pressure water at either end can realize the setting of the pump control valve. The time and speed at which the pump is switched on or off.
Ⅱ. The operating characteristics of the pump and the functional characteristics and working principle of the control valve
1. Pump start characteristics and control
a) The zero-flow starting characteristics of the centrifugal pump and its control (ie closing the valve to start). The shaft power of the centrifugal pump is the smallest under the zero-flow condition, which is 30%-90% of the rated shaft power, so the starting characteristic of the centrifugal pump is zero flow Start (that is, start by closing the valve). After the pump reaches the rated speed, the control valve opens slowly at the set speed.
Working principle: When the pump is started (front) pressure water flows to the upper piston cavity in the cylinder through a solenoid valve with a delay, and the lower piston cavity is vented to the atmosphere through the lower end of the cylinder through the solenoid valve. At this time, the control valve is closed. After the motor compensation starts, the pump runs normally, and the solenoid valve executes the reversing command to cut off the pressure water source in the upper cavity of the piston, close the circuit from the lower end of the cylinder to the atmosphere, and at the same time inject pressure water into the lower cavity of the piston in the cylinder through the solenoid valve, and open the piston The upper cavity leads to the atmosphere. The water in the upper cavity of the piston is discharged out of the valve through the solenoid valve. The control valve opens slowly at the set speed. This completes and satisfies the centrifugal pump's minimum shaft power startup characteristics at zero flow, ensuring the pump unit Safe operation.
b) The high-flow startup characteristics of the axial flow pump and its control (ie, full-open valve start). The axial power of the axial flow pump is at the maximum when the flow is at zero flow, which is 140% to 200% of the rated shaft power, so the axial flow pump starts The characteristic should be a large flow start (that is, a fully open valve start). Working principle: The working principle of the control valve meeting the full-open valve start of the axial flow pump is the reverse operation of the centrifugal pump closing valve start, that is, the solenoid valve works first, and the pump starts again after the valve is fully opened. Please refer to Article a) without repeating it. 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 at the outlet end of the valve to open the valve, while the centrifugal pump uses the medium pressure at the inlet end of the valve to open the valve. No matter which end of the valve, the medium can realize the opening or opening of the control valve, which is one of the important features of the pump control valve function.
c) Control the starting characteristics of the mixed flow pump The shaft power of the mixed flow pump is between the above two pumps under the zero flow condition, which is 100% to 130% of the rated power, so the starting characteristics of the mixed flow pump should also be the above two pumps In between, the pump start and the slow opening of the control valve can generally be selected synchronously. Working principle: The working principle is the same as that of controlling the zero flow start characteristic of the centrifugal pump, as long as the delay setting of the solenoid valve is cancelled.
2. Stop the pump and its control
a) Control the zero flow of the centrifugal pump to stop the pump (that is, first close the valve and then stop the pump). For centrifugal pumps, the importance of slowly closing the control valve first, and then stopping the pump is that the flow rate of the medium in the pipe decreases from the rated flow rate to Zero, the speed change gradient is extremely small, which can effectively avoid the occurrence of pump stop water hammer and pump reversal; the shaft power of the pump is gradually reduced from the rated power to the minimum operating power, which is beneficial to the safety of the unit. Working principle: When the pump needs to be stopped, as long as the control power supply of the solenoid valve is cut off, the solenoid valve executes the reversing command to close the pressure water source of the lower piston cavity and the circuit of the upper piston cavity to the atmosphere, and at the same time open the pressure water source and the upper piston cavity The circuit where the lower chamber of the piston is open to atmosphere, that is, using imported pressure water as the driving source, slowly close the control valve, and after the control valve is fully closed, the pump is instructed to stop running through the limit switch, and the whole process is automatic control. The slow closing time of the control valve can be adjusted by the throttle valve in the range of 5-60s or selected according to user needs.
b) Controlling the stop function and working principle of the axial flow pump and the mixed flow pump. The stop function and working principle of the axial flow pump and the mixed flow pump are similar to the above content, and will not be redundant.
3. Change the device characteristics (that is, the function of stepless flow adjustment)
Changing the characteristics of the device belongs to the category of adjusting the operating conditions of the pump. The most commonly used method is the throttling method, also known as the valve adjustment method, which uses the opening degree of opening or closing the valve; changing the resistance coefficient ξ in the pump device to make the pipe The water supply characteristic of the road H=F(Q) changes to achieve the purpose of adjusting the flow. The opening degree of the control valve can be adjusted steplessly from fully closed to fully open. The opening degree is displayed by the opening degree display, which is memorized and stabilized. working principle:
a) The flow rate is increased by small adjustment of the pressure water injection pipeline of the lower piston cavity and the circuit connecting the upper piston cavity to the atmosphere. At the same time, the pressure water injection pipeline of the upper piston cavity and the circuit connecting the lower piston cavity to the atmosphere are closed. The flap opens slowly, and when the set flow rate is reached, the control circuit closes the circuit connecting the upper and lower chambers of the piston with the atmosphere according to the instruction, and at the same time injects the pressure water into the upper and lower chambers of the piston to open the pipeline. At this time, the upper and lower sides of the piston The total pressure is equal, the piston is at rest, and the entire cylinder body becomes a closed state, which stabilizes the opening of the valve, and the opening display shows the opening.
b) The flow rate is adjusted from large to small, which is the reverse operation of a), which is unnecessary. The instructions of the above control circuit can be executed by limit switches, or can be executed by a small electronic control box, both of which can realize automatic control.
4. The control of water hammer force when the pump is stopped unexpectedly. Sudden pump stoppage generally refers to the phenomenon of pump stoppage such as power supply network failure or strong lightning protective trip. The probability of such a pump stoppage is less than 1% of the total number of pump stoppages. The generated water hammer is destructive to the pipe network and water supply unit and should be taken seriously.
a) The physical phenomenon of pump-stop water hammer This article does not discuss the water hammer in the pipeline, but only to facilitate the description of the function and working principle of the control valve to reduce the hydraulic force. The Zhukovsky water hammer formula is used to stop the pumping water. The physical phenomenon of the hammer is briefly described. In the formula, H——water hammer force (m water column) when water hammer is generated; C——water hammer wave propagation velocity. If the elastic coefficient of the medium, pipe wall material and pipe wall thickness are not considered for the time being, approximately C=1000m/ s; g——acceleration of gravity, 9.81m/s; Vo——media velocity before water hammer (m/s); Vt——media velocity when water hammer occurs (m/s). It can be seen from Equation 4-1 that the necessary condition for water hammer is the change of fluid velocity, and the magnitude of the water hammer force depends on the magnitude of the change gradient. When the pump is stopped, the flow rate drops from V0 to zero. Due to the inertia of the fluid, the water layer is compressed and the pressure rises (ie water hammer pressure). Then due to the inertial force, one layer, one layer...stagnation, The pressure rises and is compressed, and microscopic high-pressure and low-pressure areas appear. Their interface is called water hammer wave (in this case, pressurized wave). The pressurized wave disappears until the end of the water pipe. Then it is reversed by one layer, one layer... to the first layer of liquid surface at the beginning of the water hammer (this time is a complex pressure wave), the time t required for this round-trip process can be expressed by the following formula: where L-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 indirect water hammer, and ΔH is less than direct water hammer. This is because in the case of indirect water hammer, the pressurization process that is developing in front of the valve will be interfered by the complex pressure wave and be weakened. The magnitude of ΔH is related to the degree of interference and the number of interferences. From the above brief description, it can be seen that the control valve cannot avoid the occurrence of water hammer when the pump is stopped unexpectedly, but it can be converted from direct water hammer to indirect water hammer to minimize ΔH according to the length of the water pipeline and by adjusting the valve closing time. Ensure the safety of water pipelines and pump units.
b) Adjustment of valve closing time At present, there are two commonly used closing time adjustment methods for water pump control valves. It should be pointed out that the adjustment method referred to in this article is one of the functions of the water pump control valve. Although there are various types of this function Check valves or other valves are not necessarily water pump control valves. Don't be misled by this. For details, see the "Conclusion" section in Chapter 3 of this article.
c) Two-stage valve closing (that is, the valve is closed quickly and then the valve is closed slowly) This type of valve closing can prevent the occurrence of water hammer when the pump is stopped. As for the fast closing time, it should also match the length of the pipeline, taking into account the pump The reversal speed and the amount of reverse water meet the requirements of eliminating water hammer when the pump is stopped.
d) Constant-speed closing valve The characteristic of this closing valve is that it can adjust different closing time according to various lengths of water pipelines and the number of round trips of the complex pressure wave, and always meet the condition of T off>L/500s. Significantly weaken the hydraulic hammer power of the water hammer when the pump is stopped suddenly, the adaptability is wider than the two-stage valve closing, and it is sufficient to meet the requirement that the reverse speed should not exceed 1.2 times the rated speed in GB/T50265-97 "Pumping Station Design Specification". The time should not exceed the 2min requirement. The above two methods of the water pump control valve are automatically controlled by solenoid valves.
5. The servo mechanism of the large maneuverable solenoid valve that selects the pressure source is designed to take pressure in both directions at the same time, and is distributed by the one-way valve, and the pressure source is injected into the solenoid valve from the water in a higher pressure infusion pipe. Whether it is the pressure water at the water inlet or the water outlet, the control valve can be opened or closed independently. It is precisely because of the existence of this function that the water pump control valve can meet the operating characteristics requirements of various water pumps 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 and an external pressure source is needed, all the functions of the control valve can be realized by only one servo pipeline.
6. Energy saving Controlling the energy consumption of the flow medium to a minimum is an important issue for water pump control valves. We sampled some control valves with diameters for experiments, and obtained the following experimental formulas for readers' reference: The resistance coefficient ξ is determined by the following formula: When DN≤250, ξ=14.68×V1.1339(7-1) When DN When >250, ξ=10.97×V1.1339(7-2) where ξ—— drag coefficient; V—— medium flow velocity (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—— medium flow velocity (m/s); g—— gravitational acceleration g=9.81 (m/s2). After testing, the head loss of this kind of water pump control valve is 30% less than that of the same type of hydraulic control valve.
Ⅲ. Conclusion
It is obvious that the valve with the function of controlling the operation characteristics of the water pump is called the water pump control valve. Articles a), b) and c) in Section 1 of "Pump Start Characteristics and Control" in Chapter II of this article; Articles a) and b) in Section 2 "Stopping the Pump and Its Control"; Section 3 Articles a) and b) in "Change Device Characteristics"; Article b) in Section 4 "Control of Water Hammer Force in Accidental Pump Stopping"; the eight functions described here belong to the technical requirements required for pump operating characteristics It is also the content that the pump itself requires the valve to control, and it is also the necessary function of the pump control valve. These essential functions distinguish the water pump control valve from various types of check valves or other valve qualities.
Why can't it be called a water pump control valve if it does not have the above necessary functions? This can be illustrated by the following example: If it does not have the function of first closing the valve and then stopping the pump in Section 2 a), for centrifugal pumps, all pumps are stopped All of them have become accidental and sudden pump stop, which accounts for more than 99% of the total number of pumps stopped. The safety of water hammer and pump reversal can be avoided. How dangerous.
If the functions of section 2 and section 3 are not available, only the start of the pump can realize the opening of the valve and the closing of the pump can realize the closing of the valve. Obviously, this is because the pump is working on the control valve and has lost the "control of the control valve". "Essentially, it is harder to understand it as a water pump control valve.
If it does not have the function of stepless flow adjustment in section 3, once the resistance of the pump is too small and the flow is too large in the operating conditions of the pump, not only will the pump deviate from the highest efficiency zone and consume energy, but also the unit will be damaged due to the sharp increase in shaft power and overload. So it is inevitable that the water pump control valve should have the above eight necessary functions.
