The single biggest cause of unstable control when sizing a control valve is deciding the valve size to match the pipe size. A control valve should not be selected as the “same size as the pipe.” Instead, you calculate the required Cv value (flow coefficient) from the actual maximum and minimum flow rates you need to pass and the differential pressure across the valve at those conditions, and then choose a size that keeps the opening within an appropriate range (generally around 20 to 80%). Getting this wrong leads to operating almost fully closed at all times, or conversely running near fully open with insufficient flow, resulting in hunting or loss of control. This article explains the judgment criteria and points to watch for preventing sizing mistakes, focusing specifically on control valve selection.
Key points of this article
- Determine control valve size from the “required Cv value,” not the “pipe size.”
- Check not only the maximum flow but also the minimum flow, differential pressure variation, and rangeability.
- A selection where the opening falls outside 20 to 80% tends to produce unstable control.
Do not size a control valve by “pipe size”
The first thing to keep in mind when sizing a control valve is not to carelessly match the valve size to the pipe size. Many control problems arise from a reasoning like “the pipe is 50A, so the valve is 50A too.” Because a control valve is a device that controls flow by throttling it, the required size is often smaller than the pipe.
Determine the size from the required Cv value
The capacity of a control valve is expressed by its Cv value (flow coefficient), and the required Cv value is calculated from the fluid, the flow rate, and the differential pressure across the valve. For a liquid, it is generally obtained from the flow rate, the differential pressure, and the specific gravity. If the rated Cv value of the selected valve is too large relative to this required Cv value, the required flow is reached at only a slight opening, and the valve ends up operating only in a low-opening region (for example, around 10%). The low-opening region is where flow changes sensitively, making it prone to unstable control.
Conversely, if the rated Cv value is too small, the flow is insufficient even near fully open, leaving no control margin. As a general rule of thumb, the rated Cv value is selected so that the opening during normal operation falls within a range of about 20 to 80%.
Do not misjudge the differential pressure estimate
In calculating the required Cv value, the estimate of the differential pressure across the valve (the pressure consumed by the valve) largely determines the result. If you calculate using only the rated differential pressure without grasping how much of the pump head or the total system pressure loss the control valve is meant to absorb, the opening in actual operation deviates from what was assumed. In particular, in a system where pipe losses increase as the flow rises and the differential pressure across the valve falls, there can be insufficient differential pressure at the maximum flow, so the flow does not increase. It is important to treat differential pressure not as a “single point” but as something that changes across the entire operating range.
Check the minimum flow and the rangeability
Sizing mistakes also occur when you focus only on the maximum flow and neglect the minimum flow. In practice, the flow varies greatly at startup, at low load, and with seasonal changes, so the valve must be a size that can control across that entire range.
Check the opening at the minimum-flow side
Even if the opening stays within 80% at the maximum flow, if it drops to just a few percent at the minimum flow, the flow changes sensitively in response to plug movement in that region and becomes hard to stabilize. Rangeability (the ratio of the maximum to the minimum controllable flow) is determined by the valve’s characteristics; for a typical equal-percentage control valve, a rule of thumb is on the order of several tens to one. If the anticipated flow ratio exceeds this range, consider a configuration that does not cover the entire range with a single valve, such as splitting it into two valves of different sizes (split range, etc.).
Match the flow characteristic to the application
Control valves have flow characteristics such as linear, equal-percentage, and quick-opening. The suitable characteristic changes with the pressure characteristics of the system; for example, equal-percentage is said to be easier to handle in a system where pipe losses are dominant and the valve differential pressure fluctuates. Not only the size but also the selection of the flow characteristic is a factor affecting control stability.
Frequently asked questions
Q1. Is it forbidden to select a control valve the same size as the pipe?
A1. It is not forbidden, but it tends to be oversized relative to the required Cv value, leading to low-opening operation. First calculate the required Cv value and confirm that the opening falls within the appropriate range.
Q2. What opening percentage should I aim for?
A2. In general, about 20 to 80% during normal operation is the rule of thumb. Because the optimal range changes with operating conditions, check it at both the maximum and minimum flows.
Q3. I only know the maximum flow and not the minimum flow.
A3. It can sometimes be estimated by identifying the operating conditions at startup and at low load. If it is unknown, we can examine it together based on your operating pattern, so please feel free to consult us.
Q4. Hunting is occurring. Is the size the cause?
A4. Low-opening operation due to an oversized valve can be one contributing factor, but multiple factors are involved, including control parameters and actuator response. Isolating the cause requires operating data.
Q5. How should I select the flow characteristic?
A5. It depends on the tendency of the system’s pressure loss. In a system where the valve differential pressure fluctuates, equal-percentage is said to be easier to handle, but since it varies with conditions, select it according to the application.
Q6. My existing control valve is old and I want to replace it with an equivalent product.
A6. If the nameplate information, size, Cv value, and connection specifications are available, and ideally the operating flow and differential pressure as well, it becomes easier to examine a compatible model.
Summary
The key points to prevent control valve sizing mistakes are: determine the size from the required Cv value rather than the pipe size; confirm that the opening falls within the appropriate range (a rule of thumb of 20 to 80%) at both the maximum and minimum flows; and treat the differential pressure as a change across the entire operating range. When these are in order, it becomes easier to avoid the sensitive behavior caused by low-opening operation and the loss of control margin caused by insufficient capacity.
In situations such as “I only know the pipe size and cannot judge whether the valve size is appropriate,” “I know the maximum flow but not the minimum flow,” or “I want to select an equivalent product for an existing valve,” examining the selection based on operating conditions is effective. All figures are rules of thumb, and the appropriate selection changes with the actual system and fluid conditions. If you have any questions about valve sizing or Cv calculation, please make use of Yoshitake’s technical and selection consultation. Based on the operating flow, differential pressure, fluid conditions, and other factors, we will examine a suitable model selection together with you.