Valve Basics
1. The basic parameters of a valve are: nominal pressure PN, nominal diameter DN.
2. The basic functions of a valve: shut off and connect the medium, adjust flow, change flow direction.
3. The main connection methods for valves are: flange, thread, welding, and wafer.
4. Valve pressure-temperature rating: under different materials and working temperatures, the maximum allowable working pressure varies.
5. The standards for pipe flanges mainly include two systems: European system and American system.
The connection dimensions of the two systems are completely different and cannot be interchanged.
For pressure classification, the most suitable distinction is:
• European system: 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: PN1.0 (Class 75), 2.0 (Class 150), 5.0 (Class 300), 11.0 (Class 600), 15.0 (Class 900), 26.0 (Class 1500), 42.0 (Class 2500) MPa.
• Flange types mainly include: integral (IF), plate welding (PL), neck butt weld (SO), neck butt welding (WN), socket weld (SW), threaded (Th), butt weld ring loose flange (PJ/SE)/(LF/SE), flat weld ring loose flange (PJ/RJ), and flange cover (BL).
• Flange sealing face types include: flat face (FF), raised face (RF), concave (FM) convex (M), tongue (T) groove (G), ring joint (RJ), etc.
Common (General) Valves
1. The valve type codes Z, J, L, Q, D, G, X, H, A, Y, S represent: gate valve, globe valve, throttle valve, ball valve, butterfly valve, diaphragm valve, plug valve, check valve, safety valve, pressure reducing valve, and steam trap.
2. Valve connection type codes 1, 2, 4, 6, 7 represent: 1—internal thread, 2—external thread, 4—flange, 6—welded, 7—wafer.
3. Valve drive type codes 9, 6, 3 represent: 9—electric, 6—pneumatic, 3—worm gear.
4. Valve body material codes Z, K, Q, T, C, P, R, V represent: gray cast iron, ductile cast iron, nodular cast iron, copper and alloys, carbon steel, chromium-nickel stainless steel, chromium-nickel-molybdenum stainless steel, chromium-molybdenum-vanadium steel.
5. Valve seat sealing or lining material codes R, T, X, S, N, F, H, Y, J, M, W represent: austenitic stainless steel, copper alloy, rubber, plastic, nylon plastic, fluoroplastics, Cr stainless steel, hard alloy, rubber lining, Monel alloy, valve body material.
6. Cast iron valve bodies are not suitable for the following conditions:
• Water vapor or wet gases with high moisture content;
• Flammable or explosive fluids;
• Environments where the temperature is below -20°C;
• Compressed gases.
Control Valves
1. A control valve consists of a valve body, actuator, and its accessories.
2. Pneumatic diaphragm actuators have two types: direct action and reverse action. With increasing signal pressure, the push rod moves down in direct action, and moves up in reverse action. The standard signal pressure is 20-100 kPa; the highest pressure with a positioner is 250 kPa. The basic stroke sizes are: 10, 16, 25, 40, 60, 100 mm.
What are the characteristics of electric actuators compared to pneumatic actuators, and what are the different output types?
• The power source is electricity, which is simple and convenient, with high thrust and torque, and greater rigidity. However, the structure is more complex, and reliability is lower. It is more expensive in small to medium sizes compared to pneumatic actuators. It is often used in situations without an air source or where explosion/fire prevention is not strict.
• There are three output types: angular stroke, linear stroke, and multi-turn.
What are the characteristics of a direct-acting single-seated control valve, and where is it applied?
• Low leakage, as only one valve core ensures better sealing. Standard leakage is 0.01%KV, and further design can make it a shut-off valve.
• Small allowable pressure differential due to high unbalanced thrust force. For DN100, the pressure differential is only 120 kPa.
• Small flow capacity. The KV for DN100 is only 120. It should be used in cases with small leakage and low pressure differential.
What are the characteristics of a direct-acting double-seated control valve, and where is it applied?
• Large allowable pressure differential, as it can offset many unbalanced forces. For DN100, the pressure differential is 280 kPa.
• Large flow capacity. The KV for DN100 is 160.
• High leakage, as the two valve cores cannot seal at the same time. The standard leakage is 0.1%KV, which is 10 times that of a single-seated valve.
It is mainly used in high-pressure differential situations where leakage is not a strict requirement.
What are the main advantages of sleeve control valves?
Combines the advantages of both single and double-seated valves. Key advantages:
1. Good stability. Instead of using the valve core and valve seat for throttling, the valve plug is used, which has a balance hole to reduce unbalanced force on the valve plug. The large guide surface between the sleeve and the plug, along with a small change in unbalanced force, makes it less prone to vibration.
2. Strong interchangeability and versatility. By replacing the sleeve, different flow coefficients and flow characteristics can be achieved.
3. Large allowable pressure differential, with minimal thermal expansion effects. The balance principle of the sleeve valve with the balance hole is similar to that of a double-seated valve, allowing for a large pressure differential. Since the sleeve and plug are made of the same material, thermal expansion is consistent.
4. The throttling window provided by the sleeve can have large openings or small holes (jet-type). The latter has noise reduction and vibration reduction effects, and further improvement can make it a low-noise valve.
It is suitable for situations with large pressure differential and low noise requirements.
What other valves with regulating functions exist besides single, double-seated, and sleeve valves?
Diaphragm valves, butterfly valves, O-ball valves (mainly for shut-off), V-ball valves (large control range, shear action), and eccentric rotary valves.
What is the adjustable ratio R, ideal adjustable ratio, and actual adjustable ratio of a control valve?
The adjustable ratio R is the ratio between the maximum and minimum flow a valve can control.
When the pressure differential across the valve remains constant, the ratio of maximum to minimum flow is called the ideal adjustable ratio.
In practice, the pressure differential changes, so the ratio is called the actual adjustable ratio.
What is the flow coefficient C, Cv, and KV value of a control valve?
The flow capacity of a control valve is represented by the flow coefficient.
1. Cv in engineering units: the amount of water passing through the valve per hour when fully open, with a pressure differential of 1 kgf/cm² and a temperature of 5-40°C.
2. C in imperial units: the number of gallons per minute of water passing through the valve when fully open, with a pressure differential of 1 psi.
3. KV in international units: the volume of water passing through the valve per hour when fully open, with a pressure differential of 100 kPa and a temperature of 5-40°C.
Cv = 1.17 KV KV = 1.01 C
What forces must the actuator output satisfy for the control valve?
1. Overcome the static unbalanced force on the valve core.
2. Provide the closing pressure to support the valve seat.
3. Overcome the friction of the packing.
4. Additional forces required by specific applications or structures (e.g., bellows, soft seals, etc.).
What do flow opening and flow closing mean in control valves?
It refers to the direction of the medium flow, and is not related to the valve's function (air-open, air-close). Flow direction is important because it affects stability, leakage, and noise.
Definition: If the flow direction at the throttling opening is the same as the valve opening direction, it is called flow opening; otherwise, it is called flow closing.
Which valves require flow direction selection, and how is it chosen?
• Single-seal control valves, such as single-seated valves, high-pressure valves, and single-sealed sleeve valves without balance holes, need flow direction selection.
• Flow opening and flow closing each have their pros and cons. Flow opening valves are more stable, but they have poorer self-cleaning and sealing performance, leading to a shorter lifespan. Flow closing valves have a longer lifespan, better self-cleaning, and sealing performance, but stability is poor when the stem diameter is smaller than the valve core diameter.
• Single-seated valves, small flow valves, and single-sealed sleeve valves usually choose flow opening. If there is severe scouring or a requirement for self-cleaning, flow closing can be selected. Two-position fast-opening control valves typically choose flow closing.
What three main factors should be considered when selecting an actuator?
• The output force of the actuator must be greater than the load of the control valve and should match reasonably.
• Check whether the allowable pressure differential specified by the control valve matches the process requirements. For large pressure differentials, calculate the unbalanced force on the valve core.
• The response speed of the actuator must meet the process operation requirements, especially for electric actuators.
What are the seven steps to determine the size of a control valve?
1. Determine the calculated flow rate — Qmax, Qmin.
2. Determine the calculated pressure differential — select the resistance ratio S based on system characteristics, then calculate the pressure differential (when the valve is fully open).
3. Calculate the flow coefficient — use appropriate formulas, charts, or software to determine KVmax and KVmin.
4. Select the KV value — based on KVmax, choose the closest available KV value from the selected product series.
5. Verify the opening — when Qmax is required, the valve opening should be less than 90%; for Qmin, it should be greater than 10%.
6. Verify the actual adjustable ratio — it is generally required that the actual adjustable ratio should be greater than the required adjustable ratio.
7. Determine the valve size — if it does not meet the requirements, re-select the KV value and re-verify.
What are the auxiliary devices (accessories) for pneumatic control valves, and what are their functions?
1. Valve positioner — used to improve the control valve's performance and achieve correct positioning.
2. Valve position (stroke) switch — shows the upper and lower stroke limits of the control valve.
3. Pneumatic hold-up valve — holds the valve in its current position when the air source fails.
4. Solenoid valve — automatically switches the air path. For single-air control, use a 2-position 3-way valve; for double-air control, use a 2-position 5-way valve.
5. Manual mechanism — allows manual operation in the event of a system failure.
6. Pneumatic booster — speeds up the movement of the pneumatic diaphragm actuator and reduces transmission time.
7. Air filter regulator — used for air purification and pressure regulation.
8. Air reservoir — provides air for continued valve operation during air source failure, generally requiring three-stage protection.
Under what conditions is a valve positioner required?
1. In situations with high friction and where precise positioning is required. For example, high-temperature, low-temperature control valves, or control valves using flexible graphite packing.
2. In slow processes where the response speed of the control valve needs to be improved. For example, systems controlling temperature, liquid level, analysis, etc.
3. In cases where the actuator's output force and shut-off force need to be increased. For example, single-seated valves with DN ≥ 25, double-seated valves with DN > 100, or valves with a pressure differential (△P) > 1 MPa or inlet pressure P1 > 10 MPa.
4. In segmental control systems, or when the control valve's air-open or air-close form needs to be changed during operation.
5. In situations where the control valve’s flow characteristics need to be changed.
