Valves are essential equipment in industrial production, including chemical production. From selection to operation and maintenance, you may encounter numerous questions. Today, we've compiled some common questions to help you.

First, what are the three main factors to consider when selecting an actuator?

1) The actuator's output must be greater than the valve load and properly matched.

2) When checking standard combinations, consider whether the valve's specified allowable pressure differential meets process requirements. For large pressure differentials, calculate the unbalanced force on the valve core.

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

Second, what are the advantages of electric actuators compared to pneumatic actuators, and what output types are available?

Electric actuators use electricity as a simple and convenient drive source, offering high thrust, torque, and stiffness. However, they are complex in structure and lack reliability. They are more expensive than pneumatic actuators in small and medium sizes. They are often used in applications where an air source is unavailable or where strict explosion and flameproofing are not required.

Electric actuators are available in three output types: quarter-turn, linear, and multi-turn. 3. Why do quarter-turn valves have a larger cutoff pressure differential?

Quick-turn valves have a larger cutoff pressure differential because the combined force of the medium on the valve core or valve disc creates a very small torque on the rotating shaft. Therefore, they can withstand larger pressure differentials.

Butterfly valves and ball valves are the most common quarter-turn valves.

4. Which valves require flow direction selection? How should they be selected?

Single-seal control valves, such as single-seat valves, high-pressure valves, and single-seal sleeve valves without balancing holes, require flow direction selection.

Flow-open and flow-close valves each have their advantages and disadvantages. Flow-open valves operate more stably, but have poor self-cleaning and sealing properties, resulting in a shorter lifespan. Flow-close valves offer a longer lifespan, better self-cleaning and sealing properties, but suffer from reduced stability when the stem diameter is smaller than the valve core diameter.

Single-seat valves, low-flow valves, and single-seal sleeve valves are generally flow-open. Flow-close valves can be selected when erosion is severe or self-cleaning is required. Two-position, quick-opening control valves are flow-closed. 5. Besides single- and double-seat valves and sleeve valves, what other valves have regulating functions?

Diaphragm valves, butterfly valves, O-type ball valves (primarily for shutoff), V-type ball valves (large turndown ratio, shearing function), and eccentric rotary valves are all valves with regulating functions.

6. Why is selection more important than calculation? Compared to calculation, selection is far more important and complex. Calculation is simply a simple formula. Its importance lies not in the accuracy of the formula itself but in the accuracy of the given process parameters.

Selection involves many aspects, and carelessness can lead to inappropriate selection, resulting in not only a waste of manpower, material, and financial resources, but also unsatisfactory performance and numerous operational issues, such as reliability, lifespan, and operational quality.

7. Why can't double-seat valves be used as shut-off valves?
The advantage of a double-seat valve's spool is its force-balanced structure, which allows for large pressure differentials. However, its significant disadvantage is that both sealing surfaces don't maintain good contact at the same time, resulting in significant leakage.
If it is artificially and forcibly used for shut-off applications, the effect will obviously be ineffective. Even with numerous improvements (such as the double-seat sleeve valve), this is undesirable.

8. Why are double-seat valves prone to oscillation when operating at small openings?
For single-seat valves, valve stability is good when the medium is flow-opening, but poor when the medium is flow-closing. Double-seat valves have two spools: the lower spool is in the flow-closing position, and the upper spool is in the flow-opening position.

As a result, the flow-closing spool is prone to causing valve vibration when operating at small openings, which is why double-seat valves cannot be used for small openings.

9. What are the characteristics of a straight-through single-seat regulating valve? In what applications is it used?
1) The allowable pressure differential is small, due to the large thrust of the unbalanced force. The ΔP of a DN100 valve is only 120 kPa.
2) Low flow capacity. The KV of a DN100 valve is only 120. It is often used in applications with low leakage and small pressure differentials.

10. What are the characteristics of a straight-through double-seat control valve? In what applications are they used?
1) Large allowable pressure differential, because it can offset many unbalanced forces. The ΔP of a DN100 valve is 280 kPa.
2) High flow capacity. The KV of a DN100 valve is 160.
3) High leakage, because the two valve cores cannot seal simultaneously. The standard leakage rate is 0.1% KV, which is 10 times that of a single-seat valve.
Straight-through double-seat control valves are mainly used in applications with high differential pressures and less stringent leakage requirements.

11. Why do linear control valves have poor anti-clogging performance, while angular-turn valves have better anti-clogging performance?
The spool of a linear valve throttles vertically, while the medium flows in and out horizontally. The flow path within the valve cavity inevitably twists and turns, making the valve's flow path quite complex (e.g., an inverted "S" shape). This creates numerous dead zones, which provide space for the medium to settle. Over time, this can lead to blockage.
The throttling direction of an angle-turn valve is horizontal, with the medium flowing in and out horizontally, making it easier to remove impure media. Furthermore, the flow path is simpler, leaving less room for medium to settle. Therefore, angle-turn valves offer excellent anti-clogging performance.

12. When is a valve positioner necessary?
1) Where high friction forces require precise positioning. Examples include high-temperature and low-temperature control valves or control valves with flexible graphite packing.
2) Where slow processes require faster control valve response. Examples include temperature, liquid level, and analytical control systems.
3) Where increased actuator output and shutoff force are required. Examples include single-seat valves with a DN of 25 or larger and double-seat valves with a DN of 100 or larger. Where the pressure drop across the valve, ΔP, is greater than 1 MPa or the inlet pressure, P1, is greater than 10 MPa.
4) Where the air-to-open and air-to-close modes of the range control system and control valve are sometimes required during operation.
5) Where the flow characteristics of the control valve need to be changed.

13. What are the seven steps for determining the caliber of a control valve?
1) Determine the calculated flow rate—Qmax and Qmin.
2) Determine the calculated differential pressure—Select the resistance ratio S value based on system characteristics, then determine the calculated differential pressure (when the valve is fully open).
3) Calculate the flow coefficient—Use an appropriate calculation formula chart or software to calculate the maximum and minimum KV values.
4) Select the KV value—Use the KV value closest to the first-stage KV in the selected product series to determine the initial caliber.
5) Verify the opening—Require that Qmax ≤ 90% valve opening; Qmin ≤ 10% valve opening.
6) Verify the actual adjustable ratio—Generally, it should be ≤ 10; Ractual > Rrequired.
7) Determine the caliber—If unsatisfactory, reselect the KV value and re-verify.

14. Why has the sleeve valve not succeeded in replacing single- and double-seat valves?
Sleeve valves, introduced in the 1960s, saw widespread use both domestically and internationally in the 1970s. Sleeve valves accounted for a significant portion of petrochemical plants introduced in the 1980s. At that time, many believed that sleeve valves could replace single- and double-seat valves, becoming a second-generation product.
Today, this is no longer the case. Single-, double-seat, and sleeve valves are all equally used. This is because sleeve valves offer improved throttling, stability, and maintenance compared to single-seat valves. However, their weight, blockage resistance, and leakage performance are comparable to those of single- and double-seat valves. How could they replace them? Therefore, they can only be used in conjunction with each other.

15. Why should hard seals be used for shut-off valves whenever possible?
Shut-off valves require the lowest possible leakage. Soft-seal valves offer the lowest leakage and, while they offer superior shut-off performance, suffer from wear resistance and poor reliability. Considering the dual criteria of low leakage and reliable sealing, soft-seal shut-off valves are not as effective as hard-seal ones. For example, full-function, ultra-lightweight control valves are sealed and protected by wear-resistant alloys, offering high reliability and a leakage rate of 10-7, which already meets the requirements of shut-off valves.

16. Why are linear control valve stems thinner?
This involves a simple mechanical principle: high sliding friction and low rolling friction. As the stem of a linear valve moves up and down, even slightly tightening the packing compresses it tightly, resulting in a large backlash. To address this, the valve stem is designed to be extremely thin, and the packing is often made of PTFE, a low-friction packing, to reduce backlash. However, this results in a thin stem that bends easily, shortening the packing life.
The best solution to this problem is to use a rotary valve stem, also known as a quarter-turn control valve. These stems are two to three times thicker than linear valve stems and are made with long-lasting graphite packing. This results in excellent stem rigidity, long packing life, and, conversely, low friction torque and backlash.