Performance Testing and Selection of Control Valves

In the field of industrial automation, control valves play a crucial role in regulating fluid flow, and their performance directly impacts the stability and efficiency of the entire production process. However, for a long time, the performance testing of control valves has primarily focused on static characteristics, neglecting the importance of dynamic characteristics. This article will delve into the testing of both static and dynamic characteristics of control valves and analyze the pros and cons of different types of control valves to help readers better understand how to select control valves suitable for specific field conditions.

The factory testing of control valves mainly focuses on static characteristics, which include basic error, rated travel deviation, hysteresis, start and end point deviations, dead band, repeatability error, sealability, and leakage. These tests are typically conducted under no-load conditions, that is, by measuring the control valves on a test bench. Although these static characteristics can provide performance indicators of control valves under ideal conditions, they hardly reflect the actual performance of control valves under real operating conditions comprehensively.

Basic error refers to the difference between the actual flow rate of a control valve and the set flow rate. During factory testing, precise flow measurement equipment is used to compare the actual flow rate of the control valve at different openings with the theoretical flow rate, thereby calculating the basic error. Generally, the basic error should be controlled within the allowable range, for example, ±5%.

Rated travel deviation is the difference between the actual travel of a control valve and the rated travel. In practical applications, the travel accuracy of a control valve directly affects the precision of flow control. By using precise displacement sensors to measure the travel of the control valve, the rated travel deviation is calculated. Usually, the rated travel deviation should be less than ±1% of the rated travel.

Hysteresis refers to the output difference of a control valve between the forward and reverse strokes under the same input signal. The presence of hysteresis can cause a lag in the control valve when switching directions, affecting control accuracy. By conducting forward and reverse stroke tests on the control valve on a test bench, hysteresis is measured and calculated. Generally, hysteresis should be less than ±1% of the rated travel.

Start and end point deviations refer to the differences between the actual flow rate and the theoretical flow rate of a control valve at the fully closed and fully open positions. These deviations can affect the control range and accuracy of the control valve. By conducting fully closed and fully open tests on the control valve on a test bench, start and end point deviations are measured and calculated. Usually, start and end point deviations should be less than ±5% of the rated flow rate.

Dead band refers to the range within which the output flow rate of a control valve remains unchanged when the input signal varies. The presence of dead band can cause the control valve to fail to respond promptly to small signal changes, affecting control sensitivity. By conducting small signal variation tests on the control valve on a test bench, the dead band is measured and calculated. Generally, the dead band should be less than ±1% of the rated travel.

Repeatability error refers to the difference in output flow rate of a control valve under multiple identical input signals. The presence of repeatability error can cause inconsistent flow control when the control valve is operated repeatedly, affecting control stability. By conducting multiple identical input signal tests on the control valve on a test bench, repeatability error is measured and calculated. Usually, repeatability error should be less than ±5% of the rated flow rate.

Sealability and leakage refer to the ability of a control valve to prevent fluid leakage when in the closed state. By conducting sealability tests on the control valve on a test bench, leakage is measured and calculated. Usually, the leakage should be less than ±0.1% of the rated flow rate.

Although static performance testing can provide performance indicators of control valves under ideal conditions, the dynamic characteristics under actual operating conditions are equally important. Dynamic characteristics refer to the response capability of a control valve to changes in input signals during actual operation. Research has shown that dynamic characteristics play a key role in reducing process variability, and may even be more important than static characteristics.

Tens of thousands of performance checks conducted by researchers and manufacturers have proven that as many as 50% of control valves (many of which were selected based on traditional factors) have not had much effect on optimizing control loop performance. Subsequent studies have shown that the dynamic characteristics of valves play a very important role in reducing process variability. In many key processes, even a 1% difference in the ability of different valves to reduce process variability can significantly increase production efficiency and reduce waste, thereby achieving great economic benefits. Clearly, such economic benefits fully justify the rejection of the traditional practice of deciding whether to purchase a valve based solely on its initial purchase price.

The traditional view has always been that improvements in process optimization always come from the upgrading of control instruments in the control room. However, test data shows that under the same control instruments, the dynamic characteristics of valves can have a significant impact on loop performance. If the accuracy of a control valve can only reach 5%, then spending a lot of money to configure a high-level control instrument system with a control accuracy of 0.5% will not be of much use. Therefore, selecting control valves with excellent dynamic characteristics is crucial for improving the performance of the entire control loop.

When selecting control valves, it is necessary to consider both the static and dynamic characteristics of the valves based on the requirements of the actual application. Below is an analysis of the pros and cons of four basic types of throttling control valves: cage-guided globe valves, rotary plug valves, eccentric plug valves, and butterfly valves.

The variety of trim configurations for cage-guided globe valves is very extensive, which can meet the needs of most application scenarios, making them the first choice among various types of valves. There are many types of trims for cage-guided globe valves, including balanced trims, unbalanced trims, elastomer seated trims, restrained trims, and full-size trims, etc. In many cases, the trim configurations of a single valve body are interchangeable.

However, cage-guided globe valves also have several disadvantages. First, the size of this type of valve is limited (usually up to 16 inches); second, compared with valves of the same specification (such as plug valves or butterfly valves), its capacity is relatively low; third, the price is relatively high, especially for large-diameter cage-guided globe valves. Despite these drawbacks, cage-guided globe valves have excellent performance in reducing process variability, which often makes up for these shortcomings.

The flow capacity of rotary plug valves is greater than that of cage-guided globe valves of the same size. Although the control range of rotary plug valves is larger than that of cage-guided globe valves, it is still better than most other types of valves. The allowable pressure drop and temperature range of rotary plug valves are smaller than those of cage-guided globe valves. Typically, their upper limit of pressure drop is 7.0×10⁵ kg/m², and they are suitable for use in environments with temperatures below 398℃. Plug valves are not suitable for liquids that are prone to cavitation, and when used in gases with high pressure drops, they often produce significant noise.

Eccentric plug valves have less friction and are cheaper than plug valves. Their unique structural design makes them more precise in controlling process variability. In addition, the pros and cons of eccentric plug valves are not much different from those of plug valves.

In terms of valve performance, butterfly valves are considered low-end valves. Butterfly valves have a large flow capacity, are the cheapest, and come in a variety of different sizes. However, the characteristic curve of butterfly valves only has one type, which is the equal percentage characteristic curve, greatly limiting the performance of butterfly valves in reducing process variability. For this reason, butterfly valves can only be used in scenarios with fixed loads. Although butterfly valves come in a variety of sizes and can be made from most cast alloys, they do not conform to ANSI requirements for face-to-face dimensions and are not suitable for fluids prone to cavitation or applications with high noise levels.

When selecting control valves, it is necessary to consider both static and dynamic characteristics to ensure that the control valves can meet the requirements of the field conditions. The following are several key factors to consider when selecting control valves.

Different application scenarios have different performance requirements for control valves. For example, in high-precision control scenarios, control valves with high precision and low dead band should be selected; in high-temperature and high-pressure scenarios, control valves capable of withstanding high temperatures and pressures should be chosen.

Dynamic characteristics play a key role in reducing process variability. When selecting control valves, it is important to focus on the dynamic response capability of the valves to ensure that they can respond quickly and accurately to changes in input signals.

Static characteristics are the basic performance indicators of control valves, and it is necessary to ensure that the basic error, rated travel deviation, hysteresis, start and end point deviations, dead band, repeatability error, sealability, and leakage of the control valves meet the required standards.

Although the importance of dynamic characteristics is increasingly recognized, economic considerations remain an important factor when selecting control valves. Under the premise of meeting application requirements, choosing cost-effective control valves can reduce production costs.

The performance testing of control valves should not only focus on static characteristics but also pay attention to dynamic characteristics. Dynamic characteristics play a key role in reducing process variability and can significantly increase production efficiency and reduce waste. When selecting control valves, it is necessary to consider the requirements of the application scenario, the dynamic and static characteristics of the control valves, as well as economic considerations. Through close communication between manufacturers and users, selecting control valves suitable for field conditions can bring significant economic benefits to industrial production. In summary, the performance testing and selection of control valves is a complex process that requires consideration of multiple factors. It is hoped that the analysis in this article can help readers better understand the performance testing and selection of control valves, thereby providing references for practical applications.