When the temperature of hot water or oil does not stabilize as intended, one possible cause is that the selection of the temperature control valve does not match the operating conditions. To state the conclusion first, the key to selecting a temperature control valve is to organize five factors: the medium to be heated or cooled, the flow rate derived from the required heat load, the temperature setting range, the differential pressure across the valve, and the location where the sensing element is placed. If any one of these does not align, the temperature is prone to overshooting and undershooting (hunting) or to a delayed response. This article organizes the main causes of unstable temperature and the correct selection procedure from a practical standpoint.
Key points of this article
- The basics of temperature control valve selection are organizing the medium, flow rate, set temperature, differential pressure, and sensing element location
- Behind unstable temperature, oversizing, differential pressure mismatch, and poor installation of the sensing element tend to overlap
- Deriving the flow rate from the required heat load and matching the Cv and bore size to the differential pressure is a guideline for stable operation
Main causes of unstable temperature
A self-operated temperature control valve generally works by using the force of the fluid inside the sensing element (sensor) expanding and contracting with temperature to automatically adjust the valve opening without external power. Because of this characteristic, the temperature tends to fluctuate when the selection or installation does not match the conditions.
Mismatch between valve size and flow rate range
Selecting a bore larger than necessary means that even a slight change in opening moves the flow rate significantly, which tends to cause hunting, where the temperature repeatedly overshoots and undershoots. Conversely, if it is too small, the required heat load cannot be secured. Not only the maximum flow rate but also confirming whether the flow rate stays within the control range across the entire operating range, such as during start-up and steady-state operation, is essential.
Differential pressure and installation conditions of the sensing element
If the differential pressure across the valve differs greatly from what was assumed, the flow rate changes even at the same opening, making it difficult to stabilize at the set temperature. When the differential pressure is excessive, it can also be a cause of cavitation and noise. In addition, if the sensing element is installed in a position where it is difficult to capture the representative temperature on the heated side, a response delay (dead time) occurs, which tends to lead to temperature oscillation. As a guideline, the sensing element should be placed in a position where the flow does not stagnate and the actual controlled temperature is readily reflected.
Correct selection procedure
By organizing the conditions in order, you can approach a specification suited to temperature stability.
Steps 1 to 3: Determine the medium, flow rate, and set temperature
First, clarify the heating medium (steam, hot water, oil, etc.), the fluid to be heated, and the control method (whether to throttle the flow on the heating side or to control the cooling side). Next, calculate the flow rate from the required heat load and determine the temperature setting range. When the medium is oil, the viscosity and the operating temperature range are also items to be confirmed.
Steps 4 to 5: Select the Cv and bore size from the differential pressure, then the material and connection
Confirm the expected differential pressure across the valve, obtain the required Cv value together with the flow rate, and select the bore size. Since all of these values change depending on the operating conditions, please regard them only as a guideline. Finally, select the valve seat and seal material and the connection type (threaded, flanged, etc.) suited to the fluid, temperature, and pressure. Reviewing the installation location of the sensing element and the necessity of a protective well as part of the overall system at this stage also leads to stable operation.
Frequently asked questions
Q1. Why does the temperature rise and fall periodically?
A1. This is often seen when the valve size is too large and causes hunting. Excessive differential pressure and the installation location of the sensing element can also be factors. Reviewing the flow rate over the operating range and the installation conditions is a good guideline.
Q2. I only know the maximum flow rate. Can I still make a selection?
A2. The required flow rate often differs between start-up and steady-state operation, so understanding the minimum side is also important. Knowing the operating range makes it easier to narrow down a specification that suits the control range. Please consult us after organizing the conditions.
Q3. Where should the sensing element be installed?
A3. Basically, it should be in a position where the temperature of the target to be controlled can be captured representatively and where the flow does not stagnate. Responsiveness changes depending on the installation location. Using a protective well is also an item to consider as needed.
Q4. To what extent should the differential pressure be confirmed?
A4. Knowing the expected differential pressure across the valve allows the Cv and bore size to be selected appropriately together with the flow rate. When the differential pressure is excessive, it can also be a cause of noise and cavitation. We recommend organizing the conditions.
Q5. Does the selection method differ between steam heating and hot water heating?
A5. The required flow rate, differential pressure, and suitable material and specification differ depending on the medium. With steam, confirmation of drainage and pressure conditions is added; with hot water, circulation conditions and the like are added. Organizing the conditions according to the application is effective.
Q6. What should I do if I want to replace an existing temperature control valve with an equivalent product?
A6. Confirmation of specifications such as bore size, temperature setting range, flow rate, differential pressure, material, and connection type is required. Having a nameplate or drawing makes selection smoother. Consulting us after organizing the information makes it easier to provide a proposal.
Summary
Behind hot water or oil temperature that does not stabilize, multiple factors tend to overlap, such as a mismatch between valve size and flow rate range, a differential pressure mismatch, and the installation conditions of the sensing element. Organizing the medium, flow rate, set temperature, differential pressure, and sensing element location in order, and matching the Cv and bore size to the differential pressure, serves as a guideline for selection aimed at stable operation.
If you have concerns such as “I cannot judge whether the bore size is appropriate,” “I know the maximum flow rate but not the minimum flow rate,” “I am unsure where to install the sensing element,” or “I want to select an equivalent product for an existing valve,” please feel free to send your technical and selection inquiries to Yoshitake. After organizing your operating conditions, we will consider a suitable specification selection together with you.