The key to control valve selection is to evaluate dynamic characteristics

The key to control valve selection is to evaluate dynamic characteristics

The water supply network distributes the tap water from the water works to thousands of households, so the water supply network is distributed in every corner of the city, and its length can be hundreds of kilometers or even thousands of kilometers.

The irreplaceability of water and the necessity of people’s survival determine the importance of safe operation of water supply network. However, due to subjective and objective reasons, the pipeline often has some faults, the pipe network always needs to be constantly updated, and the water users often have to increase or decrease, so the phenomenon of local pipe segment water cut off is difficult to avoid. In order to reduce the scope of water shutdown, it is necessary to install the control valve properly in the pipe network. Therefore, in a city’s water supply network, there are thousands of valves, which are distributed randomly under the city streets.

The valve operation of the water supply network is not frequent, long-term standby, once needed, the valve should be able to close quickly, reliable interception; Usually the valve should be opened in place to reduce the head loss of the pipe section, so the valve is a kind of control equipment that “keeps troops for a thousand days and uses troops for a while”; Valve integrity rate, related to the selection of valve, valve manufacturing, pipeline design, valve assembly, valve opening and closing and valve management, of course, the main reason is the quality of the valve.

Valves in the water supply network in large number, wide distribution, large role. Therefore, there are many problems in valve selection, inspection, performance and management.

First, valve selection

The valve has butterfly valve, gate valve, ball valve and plug valve and so on several kinds, in the water supply network use the range is different. In order to reduce the depth of soil covering, butterfly valve is optional for pipes with large diameter. For those with little influence on the depth of soil, try to choose the gate valve; Ball valve and plug valve casting and processing is difficult, the price is expensive, generally suitable for small and medium caliber pipeline. In recent years, due to the improvement of casting technology, the resin sand casting method can avoid or reduce the mechanical processing, thus reducing the cost, so the feasibility of ball valve for large diameter pipeline is worth exploring. As for the demarcation line of caliber size, each district should consider dividing by specific circumstances.

The main disadvantage of butterfly valve is that the butterfly plate occupies a certain water cross section, increasing a certain head loss; Although the gate valve has no such problem, the height of the large-caliber vertical gate valve affects the soil depth of the pipeline, and the length of the large-caliber horizontal gate valve increases the horizontal area occupied by the pipeline, which affects the arrangement of other pipelines. Ball valve and plug valve keep the gate valve single, small flow resistance, reliable sealing, flexible action, convenient operation and maintenance. The plug valve also has a similar advantage, but the water cross section is not circular.

In recent years, many domestic valve manufacturers have developed soft seal gate valves. Compared with traditional Qieer or parallel double gate valves, these gate valves have the following characteristics:

1. Soft-seal gate valve body, valve cover using precision casting casting, molding, no longer mechanical processing, do not use sealed copper ring, saving non-ferrous metals;

2. Soft seal gate valve bottom no pits, no residue accumulation, gate valve opening and closing failure rate is low;

3. Soft sealing rubber lining valve plate size uniform, strong interchangeability.

Therefore, the soft seal gate valve will be the development direction of the gate valve, but also the water supply industry is willing to use a valve. When opening and closing the soft seal gate valve, do not close too dead, as long as the sealing effect can be achieved, otherwise it is not easy to open or lining peel.

Most butterfly valves used in the water supply industry are soft-seal butterfly valves. For the butterfly valve, the rubber ring is easy to be damaged in the installation process, and the sealing property is affected. Many manufacturers have introduced metal seal butterfly valves instead of rubber ring seal butterfly valves. Metal seal butterfly valve because the elasticity of the seal is small, the general use of eccentric structure, especially with three dimensional eccentric structure is more reasonable.

Two, valve performance and testing

The particularity of the valve requires its reliable quality and excellent performance. When evaluating valve performance and performance testing, the following points should be noted:

1. The valve is flexible and light to open and close under the working water pressure. The opening torque is detected by torque wrench under the working water pressure.

2. The valve is tightly closed, under 1.1 times the working water pressure does not leak or leakage meets the standard requirements (metal sealed butterfly valve), which requires the two sides of the valve to take turns bearing pressure, respectively, and multiple opening and closing to achieve the same effect. Requirements of various diameters, different types of valves should be in the manufacturer and testing qualified units with load opening and closing life testing. This test also includes an evaluation of the effectiveness of the valve shaft seal.

3. Valve overflow capacity is stronger, especially butterfly valve, butterfly plate overflow resistance to be small, overflow effective area to be large. This requires a variety of diameters, different types of valves should be flow resistance coefficient determination.

4. The ABILITY OF THE VALVE BODY TO BEAR WATER PRESSURE SHOULD be consistent WITH the pipeline, that is, the valve can bear the requirements of the pipeline test pressure under the open state.

Three, the lining of the valve and external anticorrosion

The VALVE IS THE equipment FOR conveying drinking water. The lining of the valve body must be non-toxic, corrosion resistant, wear-resistant and clean, so that the flow resistance is as small as possible. Such as valve pressure plate, bolt and the butterfly material is different, it is easy to electrochemical corrosion, and corrosion rust generated extends to the sealing surface, influence of valve sealing effect, another valve installed in the borehole, soaking in the water, it is important to prevent corrosion, therefore improve the lining is intended to cover, to prevent rusting produce secondary pollution of water.

The EXTERNAL ANTI-CORROSION OF the valve can be taken after polishing and sand cleaning, and then electrostatic spraying non-toxic epoxy resin anti-corrosion, or you can first brush 1-2 times red lead paint, and then brush two times anti-rust paint.

Four, the operation of the valve management

Whether the valve can be opened and closed well, not to the valve selection is appropriate, good product quality, careful construction and installation, but also thoughtful management, in order to play the “thousands of days, a temporary” effect. Good operation management is reflected in the following three aspects:

1. Technical data are available

Valve TECHNICAL data including valve factory instruction, valve after purchase inspection sheet, valve assembly and position card, valve maintenance records. For the changes of streets, the valve card should be updated in time, and strive to establish a GIS management system.

2. Good management of valve operation

The quality requirements of valve operation management include the valve should be closed tightly, the valve shaft seal packing does not leak, the valve open and close light, good indication. The daily work of valve operation management includes the approval record of valve opening and closing operation list and the improvement of operation record, the opening and closing record of valve regular inspection, etc. For the valve that has not been operated for a long time, according to the size of the caliber, it is necessary to determine the different detection cycle. For the found fault should put forward the maintenance plan, timely treatment, especially after closing the valve can not be opened should be treated as emergency repair pipe burst.

3. Valve well in good condition

The CONDITION OF valve well INCLUDES THAT THE masonry OF valve well CONFORMS to industry standards and design specifications, the connection between well cover and road surface is intact, the hole position of the valve is accurate, there is no debris and sewage in the well, and there is no rust on the surface of the valve. If conditions permit, the technical measures of long-term air convection in large diameter valve Wells should be considered. The valve well should be regularly inspected, the loss and damage of the well cover should be dealt with in time.
The key to control valve selection is to evaluate dynamic characteristics
While some traditional factors are still important, they tend to focus on the “static” performance of the valve. They are actually measured on a “bench,” but such results are difficult to tell how the valve will perform under actual operating conditions. Conventional theory suggests that careful adjustment of the static factors will result in good performance of the valve (and thus the entire circuit). Now, however, we realize that this is not always the case.
Thousands of performance checks conducted by researchers and manufacturers have demonstrated that as many as 50 percent of the valves in use, many of which have been selected based on traditional considerations, are not effective in optimizing control loop performance. Subsequent studies show that the dynamic characteristics of the valve play an important role in reducing the flow variability. In many critical processes, even a 1% reduction in process variability with different valves can significantly improve productivity and reduce waste, resulting in economic benefits of more than $1 million. Obviously, such economic benefits make us completely able to deny the traditional practice, that is, only according to the initial purchase price of the valve to decide whether to buy.
Secondly, the conventional wisdom has always been that improvements in process optimization always come from the upgrading of control instruments in the control room. However, the test data show that the dynamic characteristics of the valve can have a significant impact on loop performance under the same control instrumentation. If the accuracy of the control valve is only 5%, then it does not help to spend a lot of money to configure an advanced control instrument system with a control accuracy of 0.5%.
The valve type
In THE SEARCH FOR A SUITABLE VALVE FOR USE, THE FOUR BASIC TYPES OF THROTTLING CONTROL VALVES SHOULD first be examined, namely, cage ball valve, rotary float ball valve, eccentric valve and butterfly valve.
Cage ball valves are available in a wide variety of disc forms and thus meet the needs of most applications, making them a priority among valves. There are many types of cage ball valve adjustment pieces, including balanced adjustment pieces, unbalanced adjustment pieces, elastic seat adjustment pieces, constrained adjustment pieces and full size adjustment pieces. In many cases, the various adjusting sheet configurations of a body are interchangeable.
Cage ball valves also have several disadvantages. The size of the valve is limited (typically 16 inches); Second, compared with the line of sight valve of the same specification (such as float ball valve or butterfly valve), its capacity is relatively low; Third, the price is higher, especially the large diameter cage ball valve. However, the excellent performance of cage ball valves in reducing process variability often more than compensates for these deficiencies.
The flow rate of rotary float ball valve is larger than that of cage ball valve of the same caliber. Although the range of control of rotary float ball valves is greater than that of cage ball valves, it is still superior to most other types of valves. The allowable pressure drop and temperature range of rotary float ball valve are smaller than those of cage ball valve. They usually have a maximum pressure drop of 7.0x105kg/m2 and are suitable for use at temperatures below 398 ° C. Float ball valves are not suitable for liquids that are prone to cavitation, and may often make a large noise when used for gases with a high pressure drop.
Eccentric valves have less friction and lower prices than float valves. The unique structure design makes it more accurate to control the flow variability. This is evident in Fisher’s new product, the BV500. In addition, the advantages and disadvantages of eccentric valve and float ball valve is not much different.
According to the performance of the valve to measure, butterfly valve belongs to the low – grade valve. Butterfly valves have high flow, low price, and are available in a variety of calibers. However, the butterfly valve has only one characteristic curve of equal proportion, which limits the butterfly valve’s ability to reduce process variability. For this reason, butterfly valves should only be used when the load is fixed. Although BUTTERFLY VALVES COME IN A VARIETY OF DIAMETERS AND CAN BE MANUFACTURED WITH MOST CAST ALLOYS, BUTTERFLY VALVES DO NOT MEET ANSI REQUIREMENTS FOR face TO FACE DIMENSIONS AND ARE NOT INTENDED FOR USE IN VACUOLATING FLUIDS OR IN NOISY SITUATIONS.