High-rise building water supply inverter failure negative pressure
Since the introduction of variable-frequency automatic water supply equipment in 2017, “negative pressure-free” technical devices and products have continued to emerge, and nearly a hundred patent applications have been filed. A seemingly simple application technology makes people feel profound and mysterious. But if we ignore those ingenious mechanisms, various patents and their respective technical secrets or know-how, we can only classify the frequency conversion automatic water supply equipment currently on the market according to the working principle of "no negative pressure", which can be summarized into the following types Methods to eliminate negative pressure:
1. Air supply method
1.1 Principle overview
If water enters at the upper end (connection network) and water flows out at the lower end (water pump) If one or a group of suction valves are installed on the top of a closed water tank, then the suction valve can be opened to inhale the atmosphere when the water pumping flow rate is greater than the water inlet flow rate of the pipe network and a vacuum is generated, making the closed water tank an opening under atmospheric pressure. container, thus eliminating the negative pressure and limiting the flow of the pipe network below the critical flow of negative pressure pumping. This commonly used suction and exhaust valve technology and device is obviously a mature and effective technology, so the "air supply method" has become the negative pressure elimination method used by most products of variable frequency automatic water supply equipment. Among the earlier patent applications of this type in my country, the "mechanical vacuum compensator" seen in 1997 is actually a float-type suction valve. When the water level drops, the float and valve core drop to connect to the atmosphere; it is called an "electric vacuum compensator". The patent application for "Type Vacuum Compensator" is a solenoid valve that is controlled to open by a liquid level contact point. Nowadays, many products are no longer kept secretly "confidential", but are openly equipped with a "suction and exhaust valve" on the top of the can. After the simple principles and common structures are clearly explained, the "variable frequency automatic water supply equipment" is easier to understand and accept.
1.2 Problems
1.2.1 The "gas supply method" is to inhale the atmosphere when the vacuum is generated. Naturally, there is a possibility that the air or inhaled matter will pollute the water quality. For this reason, the relevant documents of Beijing put forward the provisions of "preventing local pollution" and "preventing air pollution and water quality", which are aimed at this shortcoming of the "air supplement method". Products called "vacuum fully sealed" products have also appeared on the market, pointing directly at products using the suction valve air replenishment method.
1.2.2 The author once wrote an article raising doubts about the reliability of the action of the suction and exhaust valves (see note 1) for two reasons. The first is the float valve or solenoid valve as a mechanical structural device. There are bound to be problems with movement failure and mechanical life. Especially when the water pressure in the pipe network is sufficient and the water pump capacity is not large, there is no condition for negative pressure due to water shortage. Faced with the negative pressure generated by transient flow, the relatively slow-acting mechanical valve will not operate at all. Therefore, valve failure occurs. It is a phenomenon of being immobile for many years and being unable to move when it is time to move, thus becoming useless.
In addition, the large cross-sectional area of the water tank makes it take a long time for the water level in the tank to drop to the control height. During the transition period before the suction valve opens, negative pressure has already formed and may cause the water to vaporize and precipitate. The bubbles gather to form air pockets and air bags, which are filled at the top of the water tank, just at the water inlet connected to the pipe network, causing the hydraulic factors of unsteady flow motion of vapor-liquid two-phase flow to appear during the water inlet process. Can the intake valve be opened at this time? Can it be exhausted and replenished? These are difficult to grasp. If exhaust and suction cannot be carried out smoothly, the water resistance of the pipeline will increase, the pressure will increase, or the pressure will oscillate unstable, which may even cause leakage of pipe joints, fatigue of the pipe wall, reduced service life, or pipe explosion accidents. This is the reason for the pipe network more worrying things. As far as the author is aware, water pumps and electrical failures often occur due to failure of the suction valve or failure to open, and there are even vicious failures that collapse the water inlet tank.
1.2.3 Negative pressure elimination inspection standards, experimental methods and inspection means are too simple and vague. It is manifested by the lack of quantitative indicators and the lack of indicators of the dynamic transition process. The test method of "the negative pressure eliminator automatically opens when the inlet water flow is less than the outlet water flow" is adopted in the product standard, without specifying the numerical values of the inlet or outlet water flow. It is easy to meet the required low standards. For example, if the water inlet valve is closed, the water inlet volume must be less than the water outlet volume. The suction valve is easy to open, but this does not represent the flow situation under normal working conditions.
1.2.4 The volume of the water storage tank is difficult to select.
In the article, the author once proposed "expanding the scope of direct water supply under limited conditions that basically does not change the current situation and pattern of water supply", "prioritizing the value orientation of pipe network capacity and safety", "maintaining the secondary functions of some pools and water tanks" Views such as "sub-pressurized water supply method" (Note 1) do not support the use of water storage tanks to "storage water for peak adjustment" when the pipe network capacity cannot meet the larger water demand flow, that is, the water inlet flow of the pipe network will be less than the user's water flow It is not allowed to use "non-negative pressure" equipment when measuring. Therefore, the water inlet tank is only used to eliminate the need for buffering water during the negative pressure action, which can be very small. Of course, in order to expand the scope of use of non-negative pressure products, please users, and increase selling points, the volume of the water storage tank can be increased and named "steady flow compensator", and it can even be made as large as 100m3. Not to mention that there are many technical problems in the processing, installation, pressure, and water testing of such a huge thing. Just ask how much is it worth to buy such a huge pressure vessel? Besides, once the water storage capacity reaches 100 m3, it will be used to compensate for the water inflow and When the water consumption is different, the so-called vacuum suppressor or "suction valve" is in a state of being open to the atmosphere for a long time, and the sealed water tank is an open pool. There is no excess pressure in the pipe network that can be superimposed and utilized, and it is also necessary to consider the problem of water quality deterioration if the water storage time is too long. Based on this analysis, it is better to use the "pool" honestly. In addition, the calculation of the water tank volume is also problematic. For example, the formula is: V volume = (Q out - Q in) △t
Q in --- the amount of tap water incoming during the peak period of water use (m3 /h)
Qout---customer water consumption during peak water consumption period (m3/h)
△ t---duration of peak water consumption
In this formula, except for the Q item, which can be calculated by the designer according to the specifications, the water inflow volume Q of the tap water and the peak duration △t are almost impossible for designers to obtain reliable data, and it is difficult to obtain them. There are relatively accurate estimates because there are so many uncertain factors that affect the water output of the pipe network.
Based on the design idea of "water storage and peak shaving", "water tank-type non-negative pressure water supply equipment" has emerged. Some use switching water inlets to inlet water from the water tank or pipe network, and some use The two sets of equipment that draw water from the water tank and the pipe network switch to operate. This method almost loses all the advantages of "direct water supply from the pipe network". The pool still needs to be disinfected and cleaned, and in order to avoid stagnant water in the water tank, regardless of whether the pipe network pressure can ensure direct water supply, the pool needs to be emptied once a day, which adds a lot of trouble. As for the advantages of lamination and energy saving, it is necessary to calculate whether it is economically worth investing in "water tank type non-negative pressure" through the evaluation of investment recovery rate or investment benefit ratio.
1.3 Countermeasures for "inhalation pollution"
Inhaling air under vacuum, and the suction port is the drainage port, is also suspected of inhaling bugs and other dirt, which creates a problem for Criticism and rejection of the "Qi Supplementing Method". So there are several countermeasures as follows.
1.3.1 Add "filter". Installing air filters and other devices at the air inlet to prevent particulate matter from entering the water tank is of course an effective way to prevent and control pollution. But when you don't know the resistance loss of the filter, it is difficult to be sure that it can intake or exhaust air smoothly.
1.3.2 Rubber bladder or diaphragm tank. As a result, air and water are separated, and the water body does not contact the air and is called a fully sealed form. The mechanism to eliminate negative pressure is still to rely on inhaling atmospheric pressure outside the capsule to expand the capsule that is sucked in under the negative pressure. For this method, there are questions about the "capsule life" and "whether there are precipitates after rubber aging" and other material issues; during operation, the gas in the water cannot be discharged in the capsule or under the diaphragm, and can only be discharged through the water pump, which will cause cavitation of the water pump. As for the problem that affects the service life of the water pump, some people think that the capsule and diaphragm can operate normally under the condition of outward pressure, but it is difficult to ensure reliable operation under negative pressure suction. There are also accusations that "the suction force of the deflated capsule causes the machine to shut down without fully utilizing the water storage" (Note 2).
1.3.3 divided tank.
The tank is made into two chambers, which are divided into water chamber and air chamber. The tank operates in a fully sealed state. When negative pressure is generated in the water chamber, the suction valve opens and the gas is replenished from the air chamber, so as not to interfere with the tank. The outside air is connected. Due to the manufacturer's technical confidentiality, the author does not know enough about its performance. I only saw "whether the gas in the gas tank contains harmful substances", "the accumulation of gas in the tank causes the water tank to gradually decrease", "if the gas enters the water pump, it will There are different opinions such as affecting the performance of the water pump and causing certain damage to the water pump flow channel and impeller. (Note 3)
1.4 Summary
Suction (exhaust) valves are commonly used equipment in water transmission and distribution pipelines. The water supply and drainage design manual states, “At the bulge point and the straight line of the pipeline, An air exhaust (inlet) valve should be installed at the position of the section to remove the air in the pipe in time to prevent air blockage, and to introduce air when the pipeline is vented or water hammer occurs to prevent negative pressure in the pipeline." (Note 4) The suction valve method is a mature and simple technology with theoretical basis and practical proof. It is such a simple and clear working principle and a simple device that is feasible by installing a suction valve on the tank. It will evolve into a steady flow compensator, a vacuum suppressor, a negative pressure eliminator, a prepressure balancer, a pipe network ballast, and a pipe network protection. God and other new products from China and the world. As a result, "the concept of no negative pressure is flying all over the place, and the inventor is complaining endlessly", and "taking advantage of people's ignorance to confuse the public, it has tarnished the concept of no negative pressure" (Note 5). ---Someone said so.
2. Pre-pressure method
Pre-add a certain pressure in the air chamber of a rubber diaphragm pressure tank with water inlet from the top and water outlet from the bottom, using an air compressor or nitrogen bottle It can be achieved by applying pressure. Keeping the tap water inlet at a certain pressure limits the amount of water taken and does not cause negative pressure in the pipeline. This technology has been used in sealed liquid containers in the 1960s, and has recently been adopted by individual manufacturers in variable frequency automatic water supply equipment. Clearly a viable technology. However, its diaphragm tank must have various shortcomings described in Section 1.3.2.
3. Buffer tank supplementary flow method
When the pipe network capacity is sufficient and the user's water pump is small, there is no condition for generating water shortage negative pressure, that is, there will be no The outflow flow rate is greater than the inlet flow rate. At this time, the water pump can be connected in series to the pipe network to pump water directly, but the negative pressure generated by the transient flow will still affect the safety of the pipe network (Note 1), in many situations such as starting the pump, stopping the pump, oscillation, loss of control, breakage of the outlet pipe, etc. Transient flow negative pressure will occur under the condition. In order to avoid this phenomenon causing pulsation in the pipe network, an air tank can be connected in parallel to the water pump inlet pipe. The tank accumulates the potential energy of the pipe network pressure and the water level in the tank. When the water column suddenly separates due to transient flow conditions at the suction port of the water pump, the water column will suddenly separate. , the air tank supplies flow to the water pump port. Because the negative pressure in the supply pipe of the air tank is controlled above the vaporization pressure of the water, the negative pressure is eliminated, destroying the conditions for water column separation and re-bridging, and controlling the rise of water flow pulsation or water hammer pressure, which affects the operation of the pipeline. The safety, security and stability play an important role.
In some equipment, the water pump does not use a frequency converter to regulate the speed, but directly starts and stops 2-3 small pumps running at a fixed speed. The number of pumps is switched and controlled to adjust the flow. A small pump is set up next to each pump. Air tanks are used to buffer flow transients, and such equipment is common in Japan.
4. Intelligent control method
Based on the understanding of the pipeline negative pressure generation mechanism, advanced intelligent control equipment and technology are used to automatically control the working conditions of the water pump and the water body. The flow state fundamentally eliminates the negative pressure generation conditions, and the negative pressure will not be generated instead of being eliminated after it is generated. -----This product with a new design concept has attracted attention in the industry as soon as it was launched. Because theoretically, there are three points that can show that it is advanced. First, the dynamic process of generating negative pressure and then eliminating it is still an oscillatory process of sudden changes in flow, resulting in poor water safety, safety and adaptability. If negative pressure is not generated at all, water stability must be good. Secondly, the control system composed of electronic devices has high reliability and semi-permanent life span, and has the advantages of accurate action and fast response speed. This is unmatched by any mechanical device that eliminates negative pressure, so it has a more reliable and agile protection effect on the pipe network. Third, there is no suction and drainage process, no water storage tank, and the water pump can be directly connected to the pipe network, so it is truly fully sealed.
When the concept of automatic control can be understood in a broad sense, the very simple position control is also called automatic control.
For example, install an electric contact pressure gauge on the water inlet tank. When the pressure is lower than a certain value (for example, Beijing stipulates that the pump will stop when it is 0.2MPa), the power will be cut off and the pump will stop. Or install an electric contact vacuum gauge on the tank to control the suction valve when the vacuum reaches a certain value. In order to distinguish it from such simple control. The term “intelligent control method” is used in this article. In order to avoid generating negative pressure at all and complete pressurized water supply to meet users' water demand requirements, the intelligent control should be an automatic control system composed of the following functional modules:
4.1 Pressure or flow constraint condition control.
4.1.1 Monitor the pressure value of the pipe network at the water intake pipe port. When it is lower than the specified lower limit (for example, 0.2MPa), issue a control command to stop the operation of the water pump. When the pipeline network capacity is redundant and the pipeline network pressure is very high (for example, the pipeline network pressure in Beijing City can reach more than 0.5MPa), unless there is an accident in the pipeline network, it is difficult to generate a pressure lower than the lower limit due to excess water withdrawal. High pipe network pressure can also suppress the impact of water pump pulsation on the pipe network. Therefore, "lower pressure control" is a simple and feasible method under such pipe network conditions. Since transient flow negative pressure may cause an instant pressure drop, you can use a large damping device or set a long damping time to filter out the pressure fluctuations caused by the transient flow state, and pretend that this event has not occurred. Otherwise, it may cause problems such as sudden pump shutdown or multiple pump shutdowns that may affect normal water use.
If there is a buffer space before stopping the pump at the lower pressure limit, for example, when the pressure is 0.22Mpa, the water pump is restricted from increasing speed, so that the water pump can naturally change working conditions under the action of the pipe network characteristic curve. Pipe networks often regain pressure naturally. This reduces the number of sudden pump stops, which has a good effect when the pipe network pressure is relatively low and the pressure value drifts near the critical point of the constraint condition.
4.1.2 The flow constraint control method is suitable for conditions where the pipe network pressure is low and the pipe network capacity is weak. The pressure at the end of the main pipe in some urban pipeline networks is only a relatively low pressure of 0.12Mpa ~ 0.15Mpa, such as the pipeline network in Dezhou City. However, the water plant control system can appropriately dispatch to maintain the stability of the pipe network pressure. If more stringent pressure constraints are used, it is almost impossible to promote the use of non-negative pressure water supply equipment. The flow constraint control is used to limit the user's water intake flow. As long as the user does not exceed the allowed flow rate, it does not matter what the pressure of the pipe network is at the water intake. As long as there is no negative pressure, even if there is only 1m or 2m of water head left, it does not matter. This greatly expands the application scope of the non-negative pressure water supply method. The principles and calculation methods of flow restriction conditions have been discussed in (Note 1) and (Note 6).
Flow meter can be used to monitor the flow rate. However, flow meters are expensive and troublesome to maintain. They are generally only used at terminal pressurizing stations for large-flow water supply that is directly pressurized by the pipe network. Generally, non-negative pressure water supply equipment used in residential areas and buildings can use pressure instruments to detect the pressure. A mathematical model is made through the relationship between pressure and flow, and the pressure is calculated into flow for control. This is not difficult to achieve in a microcomputer control system, but the model The required data is often not available during design or the calculation is inaccurate, so on-site measurements are required, which increases the workload and time of on-site construction and increases the project cost. Therefore, most of them use estimated data and do not pursue the accuracy of the model. Therefore, leeway should be left when issuing the permitted flow indicators for pipe networks.
4.2 Suppression of transient flow negative pressure
To suppress transient flow is to avoid sudden changes in flow, including soft start and soft stop of water pumps, and smooth processing when the number of units increases or decreases. , flow (pressure) overshoot limitation, regulation oscillation prevention, control system failure and out-of-control processing methods, etc. The purpose of control is to maintain the continuous flow state of the unsteady flow, that is, to make the inlet water flow rate equal to the outlet water flow rate, so as not to cause the instantaneous flow rate of the outlet water to be greater than the inlet water flow and break the water column. For greater safety, a pressure buffer tank can be installed between the water inlet pipe and the pump suction port to prevent control system failure or lag and transient flow impact on the pipe network.
4.3. Constant pressure control of water outlet
When the water pump operates under superimposed pipe network pressure, many new problems (Note 7) arise. Among them, the outlet pressure is subject to two-way disturbances caused by changes in water flow and pipe network pressure, as well as problems such as the enlarged flow range of the water pump and changes in the characteristic curve after superimposed pressure, which all make it difficult to control the constant pressure of the outlet water.
The traditional PID adjustment and outlet pressure constant methods are difficult to achieve good control characteristics and power saving benefits. There is much to be done by using modern control theories and methods to form a control system with simple structure, stable control and good adaptability.
4.4 Diversified control functions
In addition to the basic functions of “suppressing negative pressure generation” and “constant water outlet pressure control”, the control functions of frequency conversion automatic water supply equipment also include It should have many practical additional functions, such as automatically starting after a water outage, restarting after a power outage, the backup pump automatically turning on when the water pump fails, regular switching between the working pump and the backup pump to prevent the water pump from being shut down and stagnant for too long, etc. Humanized accident self-handling and self-starting functions. There are also safety, full protective functions such as electrical switch spark-free, instrument disconnection or failure protection, water pump stall limit, and optimized control functions implemented by software such as maximum power saving and low-frequency operation control of water pumps. Diversified control functions promote the development of non-negative pressure water supply equipment to products that are unattended, maintenance-free, long-life, low-noise, maximized power saving, safe, full water supply without accidents, stable water supply and very comfortable.