【2026 Control Valves】For control valves under the same operating conditions, whether to use a linear or equal percentage control strategy depends on the valve’s share of the total pressure drop
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This post was last edited by The one on 2026-7-25 at 13:53. For the same operating conditions, there are two valve selection options: one uses a linear approach and the other uses an equal percentage approach; each claims to be correct. Who is right after all? The answer lies not in the valve itself, but in what it becomes once it is installed in the pipeline. This is the difference between inherent characteristics and installation characteristics – the curves obtained when testing a valve on a test bench versus when it is installed in a pipeline are not at all the same. The most common mistake when making a selection is failing to consider these two curves separately.
「\"Factory setting\": Inherent flow characteristic. The inherent flow characteristic is the flow-vs-stroke relationship determined when the pressure drop across the valve remains constant. The pressure drop at the site is rarely constant, but a constant pressure drop eliminates the effects of the pipeline, leaving only the geometry of the valve element as a variable, which is ideal for comparing valves across different conditions. These three typical curves are obtained in this way.
Fast-opening characteristic: As soon as the stroke starts moving a little, the flow rate soars upward； The flow changes less as it moves further forward, and it hardly moves at almost full open. Use it for on/off valves. Linear characteristic: Flow rate is proportional to stroke, with a constant slope; at a constant pressure drop, the gain of the valve is the same at every flow rate point. Level control and flow control with stable pressure drop are commonly used. Equal percentage characteristic: For every equal increase in stroke, the flow rate increases by the same percentage – the change amount is always proportional to the current flow rate. For pressure control, and in situations where most of the pressure drop is absorbed by the system with only a small portion falling on the valve, this option should be preferred.
The slope of the curve represents the valve’s \"sensitivity\". The increase in flow rate divided by the increase in stroke gives the valve gain – which is the slope of the curve, and represents how \"sensitive\" the valve is to movements. The shape of the three curves corresponds to three sensitivity distributions: linear throughout the entire range ； Turn it on quickly and set the sensitivity at a low opening degree ； The highest sensitivity at equal percentages falls near full open.
Pressure control prefers equal percentages. The \"sensitivity\" of the controlled object itself often decreases as the flow rate increases (pressure vessels are a typical example). The equal percentage valve is exactly the opposite; its sensitivity increases as the flow rate rises. Only by offsetting one drop with one increase can the gain of the entire circuit remain roughly stable within the operating range. Choosing a characteristic is not about selecting a curve with an attractive appearance, but rather about using the valve’s sensitivity to compensate for the object’s sensitivity – the more the object drops, the more the valve pushes upward.
When it’s inserted into the pipeline, the curve becomes deformed. Once the valve is installed in the pipeline, the pressure drop across its ends is no longer constant; it varies with the flow rate. When the flow rate increases, more of the pressure drop is consumed by friction in the pipeline, leaving less for the valve. As a result, with the same proportional valve, as the opening increases, the flow rate is more quickly \"restricted\" by the pipeline, causing the inherent curve to be gradually flattened and raised. The less of the total pressure drop that falls on the valve, the more severely this curve becomes deformed.
The installation characteristic we truly want is a curve that is nearly linear with uniform sensitivity. When the valve pressure drop accounts for about half of the total pressure drop, the equal percentage valve is \"compressed\" by the pipeline, causing its installation characteristics to become nearly linear – which is exactly the origin of the common practice in the field that states \"the valve pressure drop should not be less than one-third of the total pressure drop\". The proportion is too low (for example, 10% in the figure); the middle part of the curve becomes too straight, while the ends distort, making it increasingly difficult to adjust.
Is choosing a valve one size larger really more secure? Let’s start with a criterion: include all components other than control valves, heat exchangers, containers, pumps, and transmitters in the calculation to determine the loop process gain. In engineering practice, it is generally accepted that this value ranges between 0.5 and 2.0, meaning the variation does not exceed 4:1. Beyond this range, the loop dynamics become problematic, with severe cases leading to direct oscillatory divergence.
Don’t go for too large a caliber – this is the most common mistake made when selecting a model. Many people think it’s safer to choose a valve one size larger, but the opposite is true. The valve is just one component in the loop gain ； If it contributes too much gain fluctuation on its own, the room left for controller tuning is reduced to nothing. It is good practice to ensure that the valve has as linear an installation characteristic as possible, leaving as much of the loop gain as possible to the controller. If the diameter is chosen too large, the valve operates for extended periods at a low opening degree – and it is precisely there that the inherent gain is highest and the sealing friction is greatest; as a result, process deviations are amplified twice, making the system difficult to regulate and unstable.
Under the same operating conditions, should linear or equal percentage be chosen? It depends on what proportion the valve accounts for in the total pressure drop. High proportion and stable voltage drop – linearity is sufficient ； When the proportion is low and the pressure drop changes significantly with flow rate, an equal percentage method is used to compensate for the \"flattening\" effect of the pipeline, bringing the installation characteristics back closer to a linear pattern. At the next higher level, maintain the 4:1 gain ratio for the loop, leaving as much adjustment margin as possible for the controller; also, avoid choosing too large a diameter. There is no hierarchy among the three curves; what matters is whether they are suitable or not. The sensitivity of the valve must exactly counteract that of the controlled element – this is the correct approach for selecting flow characteristics.