Valve basic knowledge
1, the basic parameters of the valve: nominal pressure PN, nominal diameter DN
2, the basic function of the valve: cut off the medium, adjust the flow, change the direction of flow.
3, the valve connection methods are mainly flange, threaded, welded, clamps
4, the valve’s pressure-temperature level: different materials, different operating temperatures, allowing no impact on the working pressure is different.
5、Pipe flange standards are mainly two systems: the European system and the U.S. system.
Two systems of pipe flange connection size is completely different, not interchangeable.
According to the pressure level to distinguish between the more appropriate:
European system for PN0.25, 0.6, 1.0, 1.6, 2.5, 4.0, 6.3, 10.0, 16.0, 25.0, 32.0, 40.0 MPa.
American system is PN1.0 (CIass75), 2.0 (CIass150), 5.0 (CIass300), 11.0 (CIass600), 15.0 (CIass900), 26.0 (CIass1500), 42.0 (CIass2500) MPa.
The main types of pipe flanges are: Integral (IF), Plate Plain Welding (PL), Necked Plain Welding (SO), Necked Butt Welding (WN), Socket Welding (SW), Screw (Th), Butt Welding Ring Loose Sleeve (PJ/SE)/(LF/SE), Flat Welding Ring Loose Sleeve (PJ/RJ), Flange Cover (BL).
Types of flange sealing surfaces are full-flat (FF), raised face (RF), concave face (FM) convex face (M), tongue-and-groove (T) groove (G) surface, ring bonding surface (RJ) and so on.
6、Cast iron valve body is not suitable for use in the following occasions
1）Water vapor or wet gas with high water content;
2）Flammable and explosive liquids;
3) Ambient temperature below -20℃;
4) Compressed gas
Control valve
1、Control valve consists of valve body and actuator and its accessories.
2、Pneumatic thin-film actuator has two forms of positive and negative action; with the increase of signal pressure, the actuator downward for positive action, and vice versa, the actuator upward for negative action; usually accepts the standard signal pressure of 20-100KPa; with the locator when the pressure is higher than 250KPa. six basic strokes (mm): 10; 16; 25; 40; 60; 100.
3、What are the characteristics of electric actuator compared with pneumatic actuator, and what kinds of output forms are there?
The driving source is electric power, simple and convenient, large thrust and torque, high rigidity. However, the structure is complex, poor reliability. In small and medium-sized specifications, pneumatic expensive. Commonly used in applications where there is no air source or where strict explosion-proof or explosion-proof is not required.
Provide three types of output: angle stroke, straight stroke and multi-turn.
4、What are the characteristics of straight-through single-seat adjustment valve? Applicable to what occasions?
(1) As there is only one spool, the drain flow is small and easy to seal. The standard drain flow is 0.01% KV, and further design can be used as a globe valve.
(2) Due to the large unbalanced force thrust, the permissible differential pressure is small. DN100 valve △ P is only 120KPa.
(3) small flow capacity. DN100 KV is only 120. suitable for use in small leakage, differential pressure occasions.
5、What are the characteristics of straight-through double-seat adjustment valve? Applicable to what occasions?
(1) the allowable differential pressure is large, because it can offset many unbalanced forces. DN100 valve △ P is 280KPa.
(2) The flow capacity is large. DN100 is 160 KV.
(3) The two spools cannot be sealed at the same time, and the leakage is large. The standard leakage rate is 0.1% KV, which is 10 times that of single-seat valve.
Mainly used in high differential pressure, leakage requirements are not strict occasions
6、What are the main advantages of sleeve adjustment valve?
Combines the advantages of single-seated and double-seated valves. The main ones are:
(1) good stability. As the spool throttling instead of the slide valve seat throttling, and the spool is equipped with a balance hole, which can reduce the unbalanced force of the medium acting on the spool. At the same time, the guiding surface of the sleeve and the spool, coupled with small changes in the unbalance force, so it is not easy to cause spool vibration.
(2) Interchangeability, versatility. Only need to replace the sleeve to obtain different flow coefficients and different flow characteristics.
3）Large permissible differential pressure and small influence of thermal expansion. The balancing principle of sleeve valve with balancing hole is the same as that of double-seat valve, so the allowable differential pressure is larger. And because the sleeve and valve plug are made of the same material, the expansion caused by temperature change is basically the same.
(4) The sleeve provides two kinds of throttling windows: large hole and small hole (injection type). The latter has the effect of reducing noise and vibration, and is further improved to be a specialized low-noise valve.
It is suitable for occasions where the pressure difference between the front and rear of the valve is large and the noise requirement is low.
7, in addition to single and double seat valves and sleeve valves, what other valves have regulatory functions?
Diaphragm valves, butterfly valves, O-type ball valves (mainly cut-off), V-type ball valves (regulation ratio, with shear effect), eccentric rotary valves.
8, the adjustment ratio of the control valve R, the ideal adjustment ratio, the actual adjustment ratio is how much?
Control valve can control the ratio of large flow and small flow is called the adjustment ratio R.
When the pressure difference between the two ends of the valve to maintain a constant, the larger flow rate and the smaller flow rate of the ratio known as the ideal adjustment ratio.
In practice, the pressure difference between the two ends of the valve is variable, the adjustable ratio is called the actual adjustable ratio.
9, what is the flow coefficient of the control valve C, Cv, KV value?
Control valve flow capacity of the size of the flow coefficient to indicate.
(1) engineering unit system Cv definition: when the control valve is fully open, the pressure difference between the valve before and after 1kgf/cm2, the number of cubic meters of water per hour through the temperature of 5 ~ 40 ℃.
(2) British system of units C definition: when the regulator valve is fully open, the valve before and after the pressure difference of 1bf/in2 (1 degree 60.F of water per minute through the US gallons of water.
(3) International System of Units KV: when the control valve is fully open, before and after the valve pressure difference of 100kPa, the temperature is 5 ~ 40 ℃, the number of cubic meters of water through the hour.
CV = 1.17KV
Voltage = 1.01°C
1) To overcome static unbalance forces on the spool.
2) Provide tightening pressure for seat loads.
3) Overcome packing friction.
4) Additional forces required for specific applications or configurations (e.g. bellows, soft seals, etc.).
11、What does flow open and flow closed of a control valve refer to?
It is for the medium flow direction, and has nothing to do with the mode of action of the regulating valve for air open, air closed. The importance of flow direction is that it affects the stability, leakage and noise.
Definition: In the throttle port, the direction of media flow and the valve opening direction is called flow open: Conversely, known as flow closed.
12, which valves need to choose the flow direction? How to choose?
Single-seat valves, high-pressure valves, single-sealed sleeve valves without balancing holes, such as single-sealed regulating valves need to choose the flow direction.
Flow-open and flow-closed type have advantages and disadvantages. Flow-open type valves work more stable, but the self-cleaning performance and sealing is poor, short life; flow-closed type valves long life, self-cleaning performance and sealing is good, but when the stem diameter is smaller than the spool diameter stability is poor.
When scouring is serious or self-cleaning is required, usually choose single-seat valves, low-flow valves and single-sealed sleeve valves, flow open and flow closed. Select two-position quick-open characteristic regulating valve for flow closed.
(1) The output of the actuator should be greater than the load of the regulating valve and reasonably matched.
2) Check whether the permissible differential pressure specified when the regulating valve adopts the standard combination meets the process requirements. For large differential pressures, the unbalance force on the spool should be calculated.
(3) Whether the response speed of the actuator meets the process operation requirements, especially the electric actuator.
14, to determine the caliber of the control valve seven steps are?
1) Determine the calculated flow rate - Qmax, Qmin
2) Determine the calculated differential pressure - according to the selected resistance ratio S value of the system characteristics, and then determine the calculated differential pressure (valve fully open);
3) Calculation of flow coefficients - select the appropriate calculation formula chart or software to find the maximum and minimum values of KV;
(4) KV value selection - according to the selected product line in the closest KV level of KV maximum value, to get the initial selection of the caliber;
(5) Opening check - valve opening Qmax ≯ 90% of the requirements; valve opening Qmin ≮ 10%;
6) actual adjustable ratio check - general requirements should be ≮10; R actual > R requirements
7) Orifice determination - if unqualified then re-select the selected KV value, and then verify.
15、What are the auxiliary devices (accessories) of pneumatic control valve? What is the role of each?
(1) valve positioner - used to improve the operating characteristics of the control valve to achieve the correct positioning;
2) Valve position (travel) switch - to display the upper and lower limit travel working position of the control valve;
3) Pneumatic hold position valve - to keep the valve in the current position in case of air source failure;
4) solenoid valve - to realize the automatic switching of the gas circuit. Single gas control has two three,; double gas control two five;
5) manual mechanism - the system can be switched to manual operation in case of failure;
6) Pneumatic relays - to enable faster movement of pneumatic thin-film actuators and reduce transmission time;
7) air filter pressure reducer - for air purification and pressure regulation;
(8) air storage tank - gas source failure, so that the valve can continue to work for a period of time, generally need to be equipped with three levels of protection.
16, what need to use the valve positioner?
(1) larger friction, the need for precise positioning occasions. For example, high temperature, low temperature control valves or control valves with flexible graphite packing;
(2) slow process, the need to improve the response speed of the control valve. For example, regulation systems for parameters such as temperature, level, and analysis.
3) Occasions that require increased actuator output force and cut-off force. For example, DN ≥ 25 single-seated valves and DN > 100 double-seated valves. Where the pressure drop between the two ends of the valve △ P > 1MPa or inlet pressure P1 > 10MPa.
(4) The operation of the split-range adjustment system and adjustment valve sometimes need to change the gas open, gas closed form of the occasion.
(5) Need to change the flow characteristics of the control valve.