When selecting a control valve, first of all, collect the physical parameters of the complete process fluid and the operating conditions of the control valve. The composition, temperature, density, viscosity, normal flow rate, maximum flow rate, minimum flow rate, maximum flow rate and minimum flow rate of the main fluid Import and export pressure, the maximum pressure and so on. In the technical aspects of the main control and determine the structure of the valve itself, flow characteristics, rated flow coefficient Kv value, caliber size, the process allows the calculation of pressure and actuator selection, materials and installation aspects. The following principles should be followed when choosing a regulating valve. A control valve structure type: Should be able to meet the media temperature, pressure, mobility, flow direction, the adjustment range and tightness requirements. Two regulating valve flow characteristics: should be able to meet the system characteristics of a reasonable compensation. The flow characteristics of the control valve refer to the relationship between the relative flow of the medium through the valve and the relative displacement of the valve stem. The mathematical expression is as follows: Q / Qmax = f (l / L), where Q / Qmax is the relative flow, Valve at a certain opening when the flow Q and Qmax ratio of full-open flow; l / L is the relative displacement of the valve in a certain degree of opening valve spool displacement l and full-open displacement L ratio. The general rule of thumb is that the flow characteristics of the regulating valve should be opposite to the characteristics of the regulating object and the characteristics of the regulator so that the overall characteristics of the regulating system approach to linearity. The selection of flow characteristics is usually carried out under the requirements of the process system, but the following actual situation is also to be taken into account. 1, the linear flow characteristics of the scope of application: ①differential pressure changes small, almost constant; ② process parameters of the main changes in the linear; ③ most of the system pressure loss distribution in the control valve (to change the opening, the valve differential pressure changes Relatively small); ④ small external interference, a small change in the given value, the adjustable range of small requirements. 2, the percentage of the characteristics of the scope of application: ①actual adjustable range; ② opening changes, valve pressure changes relatively large; ③ piping system pressure loss; ④ process system load fluctuations; ⑤ regulating valve often small Opened under the opening. 3, in addition to the above two commonly used flow characteristics, there are other flow characteristics of the parabolic characteristics and quick-opening characteristics of the valve. Three regulating valve caliber: should be able to meet the process on the flow requirements. According to the known fluid conditions, calculate the necessary Kv value, select the appropriate control valve diameter. Fourth, the control valve material: should be able to meet the media temperature, pressure, pressure, corrosion requirements. 1, the impact of fluid pressure and temperature on the material. 2, fluid corrosive material impact. 3, the phenomenon of fluid cavitation or mud fluid on the material. 4, from the structural considerations, the material group with any problems. Fifth, the regulating valve actuator output force: Should be able to meet the requirements of the field use of pressure, the stiffness should meet the requirements of system stability; switch type should meet the requirements of safe operation of the system. Six, the control valve in the choice of, at the same time to consider under special circumstances, through the valve positioner, solenoid valves, limit switches and other auxiliary devices so that the valve should meet the operating hours, split-step control and other requirements. In the initial installation of the control valve, pay attention to purging the process of pipe to take isolation or removal measures to prevent the pipe due to slag, rust and other restrictions in the throttle, guide the cause of the blockage of the media poor circulation, or control valve overhaul Filler is too tight, resulting in increased friction, resulting in small signals do not move too much action large signal phenomenon. In the installation of regulating valve to pay attention to the regulating valve of the gas and gas off, to prevent the valve stem discomfort, resulting in valve leakage. Gas valve stem is too long and gas off the valve stem is too short, likely to cause a gap between the valve core and valve seat, can not fully contact, leading to lax closure and internal leakage. In order to prevent cavitation, pay attention to the following points in the original selection and installation: 1, try to install the control valve in the lowest position of the system, so you can relatively increase the pressure at the control valve inlet P1 and outlet P2. 2. Install a stop valve or orifice on the upstream or downstream of the control valve to change the original installation pressure drop characteristics of the control valve (this method is generally effective for small flow conditions). 3, with a special anti-cavitation inner parts can also be effective in preventing flash or cavitation, it can change the fluid flow in the control valve changes, thereby increasing the internal pressure. 4, try to use the harder material control valve, because in the event of cavitation, for such a control valve, it has a certain degree of erosion and wear resistance, cavitation can exist under certain conditions, and not Will damage the control valve trim. On the contrary, for the soft material control valve, due to its poor erosion resistance and wear resistance, when the cavitation occurs, the internal components of the control valve will soon be worn, which can not be in the case of cavitation Working normally. Control valve should be installed upright, if tilt should be supported. The media flow must be the same as the body arrow. Control valve should be installed in a convenient location for observation, operation and maintenance. Enforcement agency's signal management operations have sufficient redundancy to not impede the operation of the implementing agencies. In addition, the operation process should also pay attention to the valve leakage, wear and tear, oscillation, valve positioner failure, the regulator valve stuck and the emergence of the phenomenon of action, in a timely manner to be addressed.
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