1. What are the three main factors to consider when selecting an actuator?

1) The actuator’s output must exceed the valve's load and be appropriately matched.

2) When checking standard combinations, ensure that the allowable pressure differential specified for the valve meets process requirements. For high pressure differentials, calculate the unbalanced forces on the valve core.

3) Consider whether the actuator’s response speed meets the process operation requirements, especially for electric actuators.

2. What are the characteristics of electric actuators compared to pneumatic actuators, and what are the different output forms?

Electric actuators are powered by electricity, making them simple and convenient, with high thrust, torque, and rigidity. However, they have a more complex structure and lower reliability. They are typically more expensive than pneumatic actuators for medium and small sizes. Electric actuators are commonly used in situations where there is no air source or where stringent explosion-proof or fire-proof requirements are not necessary. Electric actuators have three output forms: rotary, linear, and multi-turn.

3. Why do rotary valves have a higher shut-off pressure differential?

Rotary valves have a higher shut-off pressure differential because the resultant force of the medium acting on the valve disc or plug generates a very small moment on the rotating shaft. As a result, the valve can withstand a higher pressure differential.

4. Which valves require flow direction selection and how should it be chosen?

Valves such as single-seal control valves, including single-seated valves, high-pressure valves, and single-seal sleeve valves without balance holes, require flow direction selection. Each flow direction has its advantages and disadvantages.

Flow opening type: These valves generally operate more stably but have poorer self-cleaning performance and sealing, leading to a shorter lifespan.

Flow closing type: These valves have a longer lifespan, better self-cleaning performance, and sealing, but can exhibit poor stability when the valve stem diameter is smaller than the valve core diameter.

Typically, single-seated valves, small-flow valves, and single-seal sleeve valves are chosen with flow opening. For applications with severe scouring or self-cleaning requirements, flow closing may be selected. For two-position quick-opening characteristic control valves, the flow closing type is preferred.

5. What other valves, apart from single-seated, double-seated, and sleeve valves, have regulating functions?

Valves with regulating functions, besides single-seated, double-seated, and sleeve valves, include:

- Diaphragm valves

- Butterfly valves

- O-type ball valves (primarily for on/off control)

- V-type ball valves (offering a large range of regulation and cutting action)

- Eccentric rotary valves

6. Why is selecting the right valve more important than calculations?

Compared to calculations, valve selection is far more critical and complex. Calculations are simply based on straightforward formulas, where the focus is not on the precision of the formula itself but on the accuracy of the given process parameters. Valve selection, however, involves numerous factors and requires careful consideration. A minor oversight can lead to improper selection, resulting in wasted human, material, and financial resources. Additionally, improper selection can negatively impact performance, reliability, lifespan, and operational quality, leading to various operational issues.

7. Why can't double-seal valves be used as shut-off valves?

Double-seal valves, such as double-seat valves, have the advantage of a force-balanced structure that allows for high pressure differentials. However, their major drawback is that the two sealing surfaces cannot make simultaneous and effective contact, leading to significant leakage. Even with various improvements, such as double-seal sleeve valves, forcing these valves to be used in shut-off applications is generally ineffective and undesirable due to their inherent limitations.

8. Why do double-seat valves tend to oscillate when operating at small openings?

For single-seat valves, stability is generally good when the medium is flow-open type, and poor when the medium is flow-closed type. Double-seat valves have two valve cores: the lower valve core is flow-closed and the upper valve core is flow-open. As a result, when operating at small openings, the flow-closed valve core can cause the valve to oscillate. This is why double-seat valves are not suitable for use at small openings.

9. What are the characteristics of straight-through single-seat control valves, and in what applications are they used?

1) Allowable pressure differential is small due to the large unbalanced force.

For a DN100 valve, the pressure differential (ΔP) is only 120 kPa.

2) Flow capacity is low.

For a DN100 valve, the Kv value is only 120. They are commonly used in applications with low leakage rates and small pressure differentials.

10. What are the characteristics of straight-through double-seat control valves, and in what applications are they used?

1) High allowable pressure differential, as they can counteract many unbalanced forces.

For a DN100 valve, the pressure differential (ΔP) is 280 kPa.

2) High flow capacity.

For a DN100 valve, the Kv value is 160.

3) Large leakage rate, because the two valve seats cannot seal simultaneously.

The standard leakage rate is 0.1% Kv, which is ten times greater than that of single-seat valves.

Straight-through double-seat control valves are primarily used in high-pressure differential applications where leakage requirements are not stringent.

11. Why do straight-through control valves have poor anti-clogging performance compared to angle-seat valves?

Straight-through control valves have a vertical throttling direction while the medium flows horizontally in and out, which creates a complex flow path inside the valve body (shaped like an inverted "S"). This complexity leads to several dead zones where sediment can accumulate over time, causing blockages.

In contrast, angle-seat valves have a throttling direction that is horizontal, with the medium flowing horizontally in and out. This design facilitates the removal of contaminants and sediment from the flow path. The simpler flow path and reduced sedimentation space contribute to better anti-clogging performance in angle-seat valves.

12. In what situations is it necessary to use a valve positioner?

1) Situations requiring precise positioning due to high friction: For example, high-temperature or low-temperature control valves, or control valves using flexible graphite packing.

2) Processes needing improved response speed of the control valve: For instance, control systems for temperature, level, or analysis parameters.

3) Situations requiring increased actuator output force and shutoff capability: For example, single-seated valves with DN ≥ 25, double-seated valves with DN > 100, or scenarios where the pressure drop across the valve ΔP > 1 MPa or the inlet pressure P1 > 10 MPa.

4) Applications where it is necessary to change the form of air-to-open or air-to-close: In segmented control systems and during the operation of control valves.

5) When altering the flow characteristic of the control valve is needed.

13. What are the seven steps to determine the size of a control valve?

1) Determine the Design Flow Rates — Calculate the maximum (Qmax) and minimum (Qmin) flow rates.

2) Determine the Design Pressure Drop — Select the resistance ratio (S) based on the system characteristics, then calculate the pressure drop when the valve is fully open.

3) Calculate the Flow Coefficients — Use appropriate formulas, charts, or software to determine the maximum (KVmax) and minimum (KVmin) flow coefficients.

4) Select the KV Value — Choose a KV value closest to KVmax within the selected product range to obtain the initial valve size.

5) Verify Valve Stroke — Ensure that the valve opens to 90% at Qmax and does not open more than 10% at Qmin.

6) Verify the Actual Rangeability — Generally, the required rangeability should be greater than 10, with the actual rangeability (R_actual) exceeding the required rangeability (R_required).

7) Confirm the Valve Size — If the valve size does not meet requirements, reselect the KV value and revalidate.

14. Why did the sleeve valve fail to completely replace single and double-seat valves?

The sleeve valve, introduced in the 1960s and widely used in the 1970s, was initially believed to potentially replace single and double-seat valves and become a second-generation product, especially since it accounted for a significant portion of the valve types used in petrochemical plants in the 1980s. However, this has not been the case. Today, single-seat, double-seat, and sleeve valves are all used equally. The reason is that while the sleeve valve improved throttling methods, stability, and maintenance compared to single-seat valves, it did not surpass single and double-seat valves in terms of weight, clogging resistance, or leakage performance. Thus, it cannot fully replace single and double-seat valves, and all three types continue to be used together.

15. Why Should Isolation Valves Preferably Use Hard Seals?

Isolation valves require the lowest possible leakage. While soft-seal valves offer the lowest leakage and therefore good shut-off performance, they are not as wear-resistant and have lower reliability. Considering both low leakage and reliable sealing, soft-seal shut-off is not as effective as hard-seal shut-off. For example, advanced lightweight control valves with hard sealing and wear-resistant alloy protection offer high reliability with leakage rates as low as 10⁻⁷, meeting the requirements for isolation valves.

16. Why Are Straight Stroke Control Valve Shafts Relatively Thin?

This involves a simple mechanical principle: sliding friction is greater than rolling friction. In straight stroke valves, the shaft moves up and down, and if the packing is slightly compressed, it can tightly wrap around the shaft, causing significant backlash. To mitigate this, the shaft is designed to be very thin, and packing with a low friction coefficient, such as PTFE, is used to reduce backlash. However, a thinner shaft can easily bend and may have a shorter packing lifespan. The best solution to this issue is to use a rotary valve shaft, such as in angle-stroke control valves, which have shafts 2 to 3 times thicker than those in straight stroke valves. These valves also use long-lasting graphite packing, offering better shaft rigidity, longer packing life, reduced friction torque, and minimal backlash.