How much do you know about the practical knowledge of valves?


Valve Basics
1. The basic parameters of the valve are: nominal pressure PN, nominal diameter DN
2. The basic functions of the valve: cut off the medium, adjust the flow, and change the flow direction
3. The main methods of valve connection are: flange, thread, welding, clip
4. The pressure of the valve – the temperature level indicates: different materials and different working temperatures, the maximum allowable non-impact working pressure is different
5. There are two main systems for pipe flange standards: the European system and the American system.
The pipe flange connection dimensions of the two systems are completely different and cannot be matched with each other;
It is more appropriate to distinguish by pressure level:
The European system is PN0.25, 0.6, 1.0, 1.6, 2.5, 4.0, 6.3, 10.0, 16.0, 25.0, 32.0, 40.0MPa;
The 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 flat welding (PL), neck flat welding (SO), neck butt welding (WN), socket welding (SW), screw (Th), butt welding ring Loose sleeve (PJ/SE)/(LF/SE), flat welding ring loose sleeve (PJ/RJ) and flange cover (BL), etc.
Flange sealing surface types mainly include: full plane (FF), raised surface (RF), concave (FM) convex (M) surface, tenon (T) groove (G) surface, ring connection surface (RJ), etc.
Common (Universal) Valves
1. The Z, J, L, Q, D, G, X, H, A, Y, and S of the valve type code respectively represent: gate valve, globe valve, throttle valve, ball valve, butterfly valve, diaphragm valve, plug valve, stop valve Return valve, safety valve, pressure reducing valve, steam trap
2. The connection codes 1, 2, 4, 6, and 7 of the valve respectively indicate: 1—internal thread, 2—external thread, 4—flange, 6—welding, 7—wafer
3. The transmission mode codes 9, 6, and 3 of the valve respectively indicate: 9-electric, 6-pneumatic, 3-turbine worm
4. The valve body material codes Z, K, Q, T, C, P, R, V respectively represent: gray cast iron, malleable cast iron, ductile iron, copper and alloys, carbon steel, chrome-nickel stainless steel, chrome-nickel-molybdenum stainless steel, Chrome Molybdenum Vanadium Steel
5. Valve seat seal or lining code R, T, X, S, N, F, H, Y, J, M, W respectively indicate: austenitic stainless steel, copper alloy, rubber, plastic, nylon plastic, fluoroplastic, Cr series stainless steel, cemented carbide, rubber lining, Monel alloy, valve body material
6. The cast iron valve body is not suitable for the following occasions:
1) Water vapor or wet gas with high water content;
2) Flammable and explosive fluids;
3) The environment temperature is lower than -20℃;
4) Compressed gas
Control valve
1. The control valve consists of the valve body, the actuator and its accessories.
2. The pneumatic membrane actuator has two forms of positive action and reverse action; with the increase of the signal pressure, the push rod moves down for a positive action, otherwise the push rod moves up for a reaction; usually the standard signal pressure is 20~100KPa; with a positioner The higher pressure is 250KPa. There are six basic strokes (mm): 10; 16; 25; 40; 60; 100.
3. Compared with pneumatic actuators, what are the characteristics of electric actuators, and what are the output forms?
The driving source is electric power, which is simple and convenient, with large thrust, torque and rigidity. But the structure is complex and the reliability is poor. It is more expensive than pneumatic ones in small and medium size. It is often used in occasions without air source or without strict explosion-proof and flame-proof.
There are three output forms of angular stroke, straight stroke, and multi-turn.
4. What are the characteristics of the straight-through single-seat control valve? Where is it applied?
1) The leakage flow is small, because only one valve core is easy to ensure the sealing. The standard discharge flow is 0.01%KV, and further design can be used as a shut-off valve.
2) The allowable pressure difference is small, and the thrust is large due to the unbalanced force. The valve △P of DN100 is only 120KPa.
3) The circulation capacity is small. The KV of DN100 is only 120. It should be used in occasions with small leakage and small pressure difference.
5. What are the characteristics of the straight-through double-seat regulating valve? Where is it applied?
1) The allowable pressure difference is large, because it can offset many unbalanced forces. The valve ΔP of DN100 is 280KPa.
2) Large circulation capacity. The KV of DN100 is 160.
3) The leakage is large, because the two valve cores cannot be sealed at the same time. The standard discharge flow is 0.1%KV, which is 10 times that of the single-seat valve.
It is mainly used in occasions with high pressure difference and low leakage requirements.
6. What are the main advantages of sleeve regulating valve?
It has the advantages of both single and double seat valves. There are:
1) Good stability. Because valve plug throttling is used instead of valve core valve seat throttling, and the valve plug is provided with a balance hole, which can reduce the unbalanced force of the medium acting on the valve plug. At the same time, the guide surface between the sleeve and the valve plug is large, and the change of the unbalanced force is small, so it is not easy to cause the valve core to vibrate.
2) Strong interchangeability and versatility. As long as the sleeve is replaced, different flow coefficients and different flow characteristics can be obtained.
3) The allowable pressure difference is large, and the thermal expansion effect is small. The balance principle of the sleeve valve with balance hole is the same as that of the double seat valve, so the allowable pressure difference is large. And because the sleeve and the valve plug are made of the same material, the expansion caused by the temperature change is basically the same.
4) The throttling window provided by the sleeve has two types: large opening and small hole (jet type). The latter has the functions of noise reduction and vibration reduction, and further improvement becomes a special 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 the single, double seat valve and sleeve valve, what other valves have adjustment functions?
Diaphragm valve, butterfly valve, O-type ball valve (mainly cut off), V-type ball valve (large adjustment ratio, shearing effect), eccentric rotary valve.
8. What is the adjustable ratio R, the ideal adjustable ratio and the actual adjustable ratio of the control valve?
The ratio of the larger flow and the smaller flow that the regulating valve can control is called the adjustable ratio R.
When the pressure difference across the valve remains constant, the ratio of the larger flow to the smaller flow is called the ideal adjustable ratio.
In actual use, the pressure difference between the two ends of the valve changes, and the adjustable ratio at this time is called the actual adjustable ratio.
9. What is the flow coefficient C, Cv and KV of the regulating valve?
The flow capacity of the regulating valve is expressed by the flow coefficient.
1) Definition of Cv in engineering unit system: when the regulating valve is fully opened, the pressure difference between the front and rear of the valve is 1kgf/cm2, and the number of cubic meters per hour of water at a temperature of 5~40℃.
2) Definition of imperial unit system C: When the regulating valve is fully opened, the pressure difference between the front and rear of the valve is 1bf/in2 (1 degree 60. The number of US gallons of water passing through per minute.
3) SI KV: When the regulating valve is fully opened, the pressure difference between the front and rear of the valve is 100kPa, and the number of cubic meters per hour of water with a temperature of 5~40℃.
Cv=1.17 KV
KV=1.01C
10. Which parts of the force required by the regulating valve should the output force of the actuator meet?
1) Overcome the static unbalanced force on the valve core.
2) Provide the tight pressure of the seat load.
3) Overcome packing friction.
4) Additional forces required for a specific application or structure (eg bellows, soft seals, etc.).
11. What does the flow opening and flow closing of the regulating valve refer to?
For the flow direction of the medium, it has nothing to do with the air-opening or air-closing of the regulating valve. The importance of flow direction is that it affects stability, leakage and noise, etc.
Definition: At the throttling port, the flow direction of the medium is the same as the opening direction of the valve, which is called flow opening; otherwise, it is called flow closing.
12. Which valves need flow direction selection? how to choose?
Flow direction selection is required for single-sealed control valves such as single-seat valves, high-pressure valves, and single-sealed sleeve valves without balance holes.
There are pros and cons to opening and closing. The flow-open type valve is relatively stable, but has poor self-cleaning performance and sealing performance, and has a short service life; the flow-closed type valve has a long service life, good self-cleaning performance and sealing performance, but when the diameter of the valve stem is smaller than the diameter of the valve core, the stability is poor. .
Single-seat valve, small flow valve, and single-sealing sleeve valve are usually selected to flow open, and can be selected to flow closed when flushing is severe or self-cleaning is required. The two-position quick-opening characteristic regulating valve selects the flow-closing type.
13. What are the three main factors that should be considered when selecting an implementing agency?
1) The output of the actuator should be greater than the load of the regulating valve, and should be reasonably matched.
2) When checking the standard combination, whether the allowable differential pressure specified by the regulating valve meets the process requirements. When the pressure difference is large, the unbalanced 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. What are the seven steps to determine the diameter of the regulating valve?
1) Determine the calculation flow – Qmax, Qmin
2) Determine the calculated pressure difference – select the resistance ratio S value according to the characteristics of the system, and then determine the calculated pressure difference (when the valve is fully opened);
3) Calculate the flow coefficient – select the appropriate calculation formula chart or software to find the max and min of KV;
4) Selection of KV value – according to the max value of KV, the KV of the selected product series is closer to the first grade, and the primary selection caliber is obtained;
5) Check calculation of opening degree – ≯90% valve opening degree when Qmax is required; ≮10% valve opening degree when Qmin is required;
6) Actual adjustable ratio check calculation – general requirements should be ≮10; R actual > R requirements
7) Determination of caliber – if unqualified, re-select KV value and re-verify.
15. What are the auxiliary devices (accessories) of the pneumatic control valve? What role does each play?
1) Valve positioner – used to improve the working characteristics of the regulating valve and achieve correct positioning;
2) Valve position (travel) switch—displays the stroke working position of the upper and lower limits of the regulating valve;
3) Pneumatic retaining valve – maintain the current position of the valve when the air source fails;
4) Solenoid valve – realizes the automatic switching of the gas circuit. Two-position three-way for single air control; two-position five-way for double air control;
5) Manual mechanism – manual operation can be switched when the system fails;
6) Pneumatic relay – to speed up the action of the pneumatic membrane actuator and reduce the transfer time;
7) Air filter pressure reducer – for air source purification and pressure regulation;
8) Air storage tank – When the air source fails, the valve can continue to work for a period of time, and it is generally required for three-stage protection.
16. Under what circumstances should a valve positioner be used?
1) When the friction force is large and precise positioning is required. Such as high temperature, low temperature control valve or control valve with flexible graphite packing;
2) The occasions where the slow process needs to improve the response speed of the control valve. For example, adjustment systems for parameters such as temperature, level, analysis, etc.
3) When it is necessary to increase the output force and cutting force of the actuator. For example, single seat valve with DN≥25, double seat valve with DN>100. When the pressure drop across the valve ΔP>1MPa or the inlet pressure P1>10MPa.
4) Sometimes it is necessary to change the air-on and air-off forms during the operation of the split-range regulating system and regulating valve.
5) When it is necessary to change the flow characteristics of the regulating valve.
17. What are the characteristics of the self-operated control valve? What do the K and B in the model of the self-operated pressure regulating valve stand for?
Self-operated control valves are also known as direct acting control valves. It does not need any external energy, and unifies the three functions of measurement, adjustment and execution, and uses the energy of the object to be adjusted to drive its action. It has the characteristics of simple structure, low price and reliable action.
It is suitable for occasions where the flow change is small, the adjustment accuracy is not high, or the supply of instrument gas source is difficult.
Self-operated control valves can be divided into: pressure, liquid level, temperature and flow control valves according to their uses. At present, most of the production is pressure regulating valve and nitrogen sealing valve.
K——Press-to-open type, used for pressure relief, stable in front of the valve;
B——Pressure closed type, used for voltage stabilization, stable after valve.

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