1, method of adding air
1. 1 principle overview
If one or a group of suction valves are installed at the top of the closed water tank with water inlet at the upper end (connected to the pipe network) and water outlet at the lower end (connected to the pump), when the pumping flow of the pump is greater than that of the pipe network, the suction valve can be opened to suck in the atmosphere, so that the closed water tank becomes an open container at atmospheric pressure, thus eliminating negative pressure and limiting the pipe network flow below the critical flow of negative pressure pumping. This kind of commonly used suction and exhaust valve technology and device obviously belong to mature and effective technology, so "air supplement method" has become the method to eliminate negative pressure adopted by most products of frequency conversion automatic water supply equipment. In the earlier patent application of this kind in China, it was found that the "mechanical vacuum compensator" in 1997 is actually a float suction valve. When the water level drops, the float and valve core drop and communicate with the atmosphere. The patent application is called "electric vacuum compensator", which is a kind of electromagnetic valve controlled by liquid level contact. At present, many products are not treated as "secret" in secret, but an "intake and exhaust valve" is installed on the top of the tank publicly. After the simple principle and common structure are explained clearly, "frequency conversion automatic water supply equipment" is easier to understand and accept.
Problems in 1.2
1.2. 1 When the "air-supplementing method" generates vacuum, the air is inhaled, and there is naturally the possibility that air or inhalants will pollute the water quality. To this end, the relevant documents in Beijing put forward the clauses of "preventing local pollution" and "preventing air from polluting water quality", which is aimed at this shortcoming of "air supplement method". There are also products called "vacuum full seal" on the market, which point to the products of air supplement method with intake valve.
1.2.2 the author once wrote an article (see note 1) questioning the reliability of the intake and exhaust valves, which is caused by two reasons. The first is the inevitable failure and mechanical life of the floating valve or solenoid valve as a mechanical structure device. Especially when the water pressure in the pipe network is sufficient and the capacity of the pump is not large, there is no condition of water shortage and negative pressure. Faced with the negative pressure generated by transient flow, the mechanical valve with relatively slow action will not act at all. Therefore, there has been a phenomenon that the valve has not moved for many years, and it has not moved, but it is useless.
In addition, due to the large cross-sectional area of the water tank, it takes a long time for the water level in the water tank to drop to the control height. In the transitional period before the suction valve is opened, negative pressure has been formed and may vaporize the water body, precipitate bubbles, gather into air pockets and airbags, and fill the top of the water tank, just at the water inlet connected by the pipe network, which makes the water intake process show the hydraulic factor of unsteady flow of vapor-liquid two-phase flow. Can the intake valve be opened at this time? Can you exhaust and replenish qi? It's harder to understand. If you can't exhaust and inhale smoothly, the water resistance of the pipeline will increase, the pressure will increase or the pressure oscillation will be unstable, even the pipe joint will leak, and the pipe wall will be fatigued, which will reduce the service life or cause a pipe explosion accident. This is something that the pipe network is worried about. As far as the author knows, the failure or failure of the suction valve leads to the occurrence of water pump and electrical failures from time to time, and even the malignant failure of sucking the intake tank flat.
1.2.3 The inspection standard, experimental method and inspection means of negative pressure elimination are too simple and vague. It is characterized by the lack of quantitative indicators and dynamic transition process indicators. It is easy to meet the required low standard by adopting the test method of "negative pressure eliminator automatically turns on when the inflow is less than the outflow" in the product standard, without specifying the inflow or outflow value. For example, when the water inlet valve is closed, the water inlet must be smaller than the water outlet, and the suction valve is easy to open, but this does not represent the situation under normal working conditions.
1.2.4 It is difficult to select the volume of water storage tank. In this paper, the author puts forward some viewpoints, such as "expanding the scope of direct water supply without basically changing the current situation and pattern of water supply", "giving priority to the value orientation of pipe network capacity and safety and integrity" and "maintaining the secondary pressurized water supply mode of some pools and water tanks" (note 1). In the case that the capacity of the pipe network can not meet the large water demand, it is not in favor of "storing water through the storage tank for peak shaving". Therefore, the negative pressure can be eliminated by only a small amount of buffer water in the water inlet tank. Of course, in order to expand the application range of non-negative pressure products, please users and increase selling points, the volume of water storage tank can be increased, named as "steady flow compensator", or even made as big as 100m3. Let's not say that there are many technical problems in the processing, installation, suppression and water testing of such a behemoth. Just ask how much it costs to buy such a huge pressure vessel. In addition, once the water storage capacity needs to reach 100 m3, the difference between water intake and water consumption can be compensated. The so-called vacuum suppressor or "suction valve" is open to the atmosphere for a long time, and a closed water tank is an open pool. There is no residual pressure in the pipe network to be superimposed and utilized, and the problem of water quality deterioration due to long storage time needs to be considered. In this analysis, it is better to use the "pool" honestly. In addition, the calculation of water tank volume is also problematic, such as the formula: V volume = (Q out-Q in) △ T.
Q Peak water inflow of tap water (m3/h)
Q—— Water consumption of customers during the peak water consumption period (m3/h)
△t—— duration of peak water consumption
In this formula, it is almost impossible for designers to obtain reliable data and estimate the water inflow Q and peak duration △t, except for the Q term, which can be calculated by designers according to the specifications, because there are too many uncertain factors affecting the water output of the pipe network.
Based on the design idea of "storing water and peaking", "water tank type non-negative pressure water supply equipment" appeared. Some of them use two sets of equipment to switch the inlet end, and some take water from the water tank and pipe network respectively. In this way, the advantages of "direct supply through pipe network" are almost completely lost. The pool still needs disinfection and cleaning, and in order to avoid water accumulation in the water tank, the pool needs to be emptied once a day regardless of whether the pipe network pressure can ensure direct water supply, which adds a lot of trouble. As for the advantages of superimposed energy saving, it is necessary to calculate whether the investment of "non-negative pressure water tank type" is economically worthwhile through the evaluation of investment recovery rate or investment benefit ratio.
1.3 Countermeasures for "Inhalation Pollution"
Inhaling air in a vacuum state, the suction port is a drain port, and it is also suspected of inhaling dirt such as insects, which leads to criticism and rejection of the "invigorating qi method". So there are the following countermeasures.
1.3. 1 Add "filter". Installing an air filter at the air inlet to prevent particles from entering the water tank is of course an effective way to prevent pollution. However, when the resistance loss of the filter is unknown, it is difficult to determine whether it can smoothly intake or exhaust.
1.3.2 rubber airbag or diaphragm oil tank. In this way, the gas-water separation is formed, and the water body will not contact with the air, so it is called a fully enclosed form. The mechanism of eliminating negative pressure is still that the atmospheric pressure outside the capsule is inhaled to make the capsule concave under negative pressure expand. There are some questions about this method, such as "capsule life" and "whether there is precipitation after rubber aging". Some people think that the air in the water can not be discharged from the air bag or diaphragm during operation, but only through the pump, which will cause cavitation of the pump and affect its service life. Some people think that capsules and diaphragms can work normally under external pressure, but it is difficult to ensure reliable work under negative pressure. And "the deflated capsule has suction, which leads to the shutdown without making full use of the stored water" (Note 2) and so on.
1.3.3 subdivision. The tank body is made into two silos, which are divided into a water silo and a gas silo. The tank runs in a completely closed state. When negative pressure is generated in the water tank, the suction valve is opened, and gas is added from the gas tank, which is not communicated with the outside air. Because the manufacturer's technology is confidential, the author doesn't know enough about its performance, and only sees "whether harmful substances are stored in the gas bin", "the water bin is gradually shrinking due to gas accumulation in the box" and "if gas enters the water pump, it will affect the performance of the water pump, which will affect the performance (Note 3).
1.4 summary
Suction (exhaust) valve is a common equipment for water transmission and distribution pipeline. "Design Manual for Water Supply and Drainage" states that "exhaust (intake) valves should be installed at the positions of pipeline uplift and straight pipe section, so as to eliminate air in the pipeline in time, avoid air resistance, introduce air when emptying the pipeline or generating water hammer, and prevent the pipeline from generating negative pressure". (Note 4) Suction valve mode is a mature and simple technology with theoretical basis and practice. It is such a simple and clear working principle and a simple and feasible device. By installing an air suction valve on the tank, it will evolve into new products in China and even the world, such as steady flow compensator, vacuum suppressor, negative pressure eliminator, preloading balancer, pipe network ballast and pipe network protector. It makes "the concept of no negative pressure fly all over the sky, and the inventors complain bitterly", and "uses people's ignorance to confuse people and defile the concept of no negative pressure" (Note 5). -Some people say that.
2. Preloading method
Preloading a predetermined pressure in the air chamber of a rubber diaphragm pressure tank with a water inlet and a water outlet can be achieved by using an air compressor or a nitrogen cylinder. Maintaining a certain pressure at the water inlet of tap water will limit water inflow and will not cause negative pressure in the pipeline. This technology was used in closed liquid containers in 1960s, and recently it was used in frequency conversion automatic water supply equipment by individual manufacturers, which is obviously feasible. However, its diaphragm groove must have various shortcomings described in section 1.3.2.
3. Surge tank replenishment method
When the pipe network capacity is abundant and the user pump is small, there is no condition of water shortage and negative pressure at all, that is, there will be no disconnection phenomenon in which the outlet flow is greater than the inlet flow. At this time, pumps can be connected in series to the pipe network to directly pump water, but the negative pressure generated by transient flow will still affect the safety and security of the pipe network (note 1), and transient flow negative pressure will be generated in various situations such as pump start-up, pump stop, oscillation, out of control, and water outlet pipe rupture. In order to avoid the pulsation of pipe network caused by this phenomenon, an air tank can be connected in parallel to the inlet pipe of the water pump to accumulate the potential energy of pipe network pressure and water level in the tank. When the water column at the suction inlet of the pump suddenly separates due to the transient flow pattern, the gas storage tank provides flow to the pump inlet. Because the negative pressure in the air tank air supply pipe is controlled above the water gasification pressure, the negative pressure is eliminated, the conditions of water column separation and reconnection are destroyed, and the fluctuation of water flow or the rise of water hammer pressure are controlled, which plays an important role in the safety, guarantee and stability of pipeline operation.
In some equipment, the water pump does not use frequency converter to adjust the speed, but switches 2-3 small pumps that start and stop directly at a constant speed to control the flow, and a small gas storage tank is set next to each pump to buffer the flow transient. Such equipment is quite common in Japan.
4. Intelligent control method
On the basis of understanding the mechanism of pipeline negative pressure, advanced intelligent control equipment and technology are adopted to automatically control the working condition of water pump and water flow pattern, and the conditions for negative pressure generation are fundamentally eliminated, so that negative pressure will not be generated after it is generated. -once the product with this brand-new design concept was launched, it attracted the attention of the industry. Because there are three points in theory that can explain that it is advanced. First, the dynamic process of negative pressure generation and then elimination is still the oscillation process of sudden flow change, which leads to poor safety, safety and adaptability of water use. If there is no negative pressure at all, the stability of water use will be good. Secondly, the control system composed of electronic devices has the advantages of high reliability, half life, accurate action and rapid response. This is unmatched by any mechanical device to eliminate negative pressure, so it has a more reliable and agile protection effect on the pipe network. Third, there is no need for the process of suction and discharge, and there is no need for a water storage tank. The water pump can be directly connected to the pipe network, so it is completely closed.
When the concept of automatic control can be widely understood, 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, stop the pump at 0.2MPa in Beijing), cut off the power supply and stop the pump. Or install an electric contact vacuum gauge on the tank, and control the suction valve when the vacuum degree reaches a certain value. To distinguish it from this simple control. This paper adopts the formulation of "intelligent control method". In order to completely avoid negative pressure, complete pressurized water supply and meet the water demand of users, intelligent control should be an automatic control system composed of the following functional modules:
4. 1 Pressure or flow constraint control.
4. 1.65438+ When the pipe network capacity is redundant and the pipe network pressure is high (for example, the pipe network pressure in Beijing can reach above 0.5MPa), unless there is a pipe network accident, it is difficult to generate pressure below the lower limit due to excessive water intake. High pipe network pressure can also restrain the impact of pump pulsation on pipe network. Therefore, under this pipe network condition, "low pressure control" is a simple and easy method. Because the negative pressure of transient flow may produce a sudden pressure drop, it can be filtered by using a larger damping device or setting a longer damping time, assuming that this event does not occur. Otherwise, it may cause problems that affect normal water use, such as sudden pump stop and repeated pump stop.
If there is a buffer space before stopping the pump at the lower pressure limit, for example, when the pressure is 0.22Mpa, the rotation speed of the pump will not increase, so the pump will naturally change its working condition under the action of the pipe network characteristic curve. Pipe networks tend to restore pressure naturally. So as to reduce the number of sudden pump stop, and it will have a good effect when the pipe network pressure is low and the pressure value drifts near the critical point of the constraint condition.
4. The control method of1.2 flow constraint is suitable for the situation of low pressure and weak capacity of the pipeline network. In some cities, the terminal pressure of the main pipe is only 0. 12 MPa ~ 0. 15 MPa, such as the pipeline network in Texas. However, the control system of waterworks can be properly dispatched to maintain the stability of pipe network pressure. If stricter pressure constraints are adopted, it is almost impossible to popularize the use of non-negative pressure water supply equipment. Flow restriction control is used to limit the water inflow of users. As long as the user does not exceed this allowable flow, it doesn't matter what the pipe network pressure of the water intake is, as long as it is not negative pressure, even if there is only a head of 1m or 2m. This greatly expands the application range of non-negative pressure water supply mode. The principle and calculation method of flow restriction conditions have been discussed in (Note 1) and (Note 6).
The flow can be monitored by a flowmeter. However, flowmeters are expensive and troublesome to maintain. Generally, it will only be used in the terminal pressurization station of large-flow water supply directly pressurized by the pipe network. The non-negative pressure water supply equipment used in general residential quarters and buildings can detect the pressure with pressure instruments and make a mathematical model through the relationship between pressure and flow, which is not difficult to realize in microcomputer control system. However, the data required by the model are often not available in the design or the calculation is inaccurate, so it is necessary to run field measurement, which increases the workload and time of field fortifications and increases the project cost. Therefore, most of them are estimated data, and the accuracy of the model is not pursued. Therefore, there should be room when the pipe network flow index is issued.
4.2 Suppression of Transient Flow Negative Pressure
Suppressing transient flow is to avoid sudden changes in flow, including soft start and soft stop of pumps, smooth handling when the number of pumps increases or decreases, limiting flow (pressure) overshoot, preventing regulation oscillation, and handling control system failures out of control. The purpose of control is to maintain the continuous flow pattern of unsteady flow, even if the inflow and outflow are equal, there will be no phenomenon that the instantaneous outflow is greater than the inflow and the water column is broken. In order to be safer, a pressure buffer tank can be installed between the water inlet pipeline and the pump suction port to prevent the control system from failing or lagging and the transient flow from impacting the pipe network.
4.3. Constant pressure control of effluent
When the pump runs under the pressure of superimposed pipe network, many new problems appear (Note 7). Among them, the outlet pressure is disturbed by the two-way change of water flow and pipe network pressure, and the problems such as the increase of pump flow range and the change of characteristic curve after superposition make it difficult to control the outlet constant pressure. Traditional PID regulation and constant outlet pressure hardly have good control characteristics and energy-saving benefits. Using modern control theory and methods to form a control system with simple structure, stable control and strong adaptability is a subject worth studying.
4.4 Diversified control functions
In addition to the basic functions of "suppressing negative pressure" and "controlling constant outlet pressure", the control function of frequency conversion automatic water supply equipment should also have many practical additional functions, such as automatic start when water is stopped, restart after power failure, automatic input of standby pump when water pump fails, and regular switching between working pump and standby pump when water pump stops to avoid prolonged water accumulation time. There are also safety full protection functions such as spark-free electrical switch, instrument disconnection or fault protection, pump stall limitation, and optimization control functions realized by software such as power saving maximization and low-frequency operation control of water pump. Diversified control functions promote the development of non-negative pressure water supply equipment to unattended, maintenance-free, long life, low noise, more electricity-saving, safe, accident-free water supply, stable and comfortable water use.