Why are there diameter changes at the connections of pipes and valves?

The diameter change at the connections of pipes and valves is determined based on actual conditions; in some cases, there is a requirement to reduce the diameter due to pressure drop； If the requirements can be met, the price for small diameters is also low.

The most common occurrence of diameter changes at valves is on both sides of the control valve, mainly to increase the pressure drop.

1. More accurate for fine adjustments of valves； 2. Small valves, low price ； 3. Design the diameter reduction based on the process pressure drop requirements!

To ensure the accuracy of flow rates, the diameter of the pipe downstream of control valves is usually made smaller! This also helps to save money!

Valves are generally specified based on their diameter, which helps to reduce pressure losses. Reducing the diameter is mainly applicable to control valves, while the connection diameter is determined based on process parameters. Typically, the size of a control valve is one size smaller than that of the pipeline; for example, if the pipeline is 6\" in diameter, the valve will be 4\" in diameter. The valve size should not be less than half of the pipeline diameter, as this ensures proper regulation accuracy and the correct opening degree of the valve at normal flow rates. Generally, the opening degree at normal flow rates should be between 50% and 70%. :P

The diameter change is based on the flow rate requirements； Diameter change is not inevitable; there are many cases where the diameter remains unchanged.

The diameter change usually occurs at the control valve, as the diameter of this valve needs to be calculated; variations may exist among different manufacturers. A larger valve certainly won’t cause any problems, but it is not economical.

According to Section 3-68 of the third edition of the Chemical Process Design Handbook, the control pressure drop across valves accounts for about 30% of the total control pressure drop in the pipeline system. Reducing the diameter of the pipes to increase the flow rate can raise the pressure drop at the valves, thereby making it easier to control the flow rate.

To save on investment, reducing the neck size by 1–2 levels is done as long as the pressure drop remains acceptable.

I don’t have any on-site experience, so I’m not sure, but I know that in general, control valves reduce the diameter of the valve stem and seat, not the diameter of the flange...

Generally, control valves or pressure relief valves have reduced-diameter connections at both the inlet and outlet; ordinary valves do not require such reduced diameters, as using pipes of the same diameter as the valve is the most economical approach.

It seems that what was said isn’t correct; having valves of too large a size, especially control valves, is also a problem. The diameter of control valves is determined through calculations, and those calculations are very precise. If the diameter is too small, it won’t be possible to meet the requirements for controlling flow and pressure. On the other hand, if the diameter is too large, the operating point of the control valve will be lower, forcing it to operate at a low opening degree, which is detrimental to both the control valve itself and the entire system. Therefore, whether the diameter changes or not is determined through rigorous calculations, and it is not set arbitrarily.

The diameter changes at the connections between pipes and valves; in pipeline installation diagrams, this is most common on both sides of control valves. Generally, the diameter of a control valve is at least one size smaller than that of the pipe, and therefore a diameter change is required to ensure proper installation.

It seems that the control valve needs to have its diameter changed; I’m not sure of the exact reason. Is this something that the instrumentation team is considering?

After the diameter is changed, the flow properties of the pipeline can be altered, thereby increasing the valve’s ability to regulate flow. In simple terms, when the pipe diameter decreases, the flow rate increases for the same volume of flow; this means that the movement of the valve disc has a greater impact on the flow rate, thus enhancing the valve’s regulatory capabilities

I agree with what was said on floor 14: a larger pipe diameter is used to reduce flow resistance, and control valves are designed through careful calculations.

The diameter changes at the connections of pipes and valves; in pipeline installation diagrams, this is most common on both sides of control valves. The diameter of the control valve must be determined through careful calculations. To ensure that the control valve can fulfill its regulatory functions, factors such as the process production capacity, variations in load, and potential future expansion of production must be taken into account when making these calculations. Choosing either an overly large or an overly small diameter is not appropriate. If the valve is too large, it not only leads to economic losses and high energy consumption in the system, but also results in operation at a small opening degree, which reduces the adjustable range and deteriorates the control performance; in severe cases, it can even cause oscillations that shorten the valve’s lifespan. If the valve is too small, it cannot meet the requirements for controlling flow rate and pressure.

Diameter reduction is generally used on both sides of control valves. During design, the pipe diameter is determined based on flow rate and pressure, and the control valve is selected by calculating values related to flow rate and pressure; as a result, it may happen that the diameters of the pipe and the valve do not match, in which case diameter reduction is the only solution.

Could you explain it in more detail? Specifically, explain the selection criteria and functions for large and small heads. Thank you.