How to select control valves for high-temperature operating conditions

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Temperature regulation and control are challenges in automatic control implementation, and it is even more difficult to cut off high and ultra-high temperatures. The main manifestation of this problem is jamming and interruption caused by thermal expansion. Therefore, high-temperature and ultra-high-temperature control has always been a focus of attention for manufacturers and design institutes both at home and abroad. 9 \% s9 \! j) |$ Z% o4 P I. Several issues that need attention From the perspectives of design, selection, and use, the following issues should be taken into account: (1) The selection of materials for the valve body and its internal components. For control valves used in environments with temperatures above 450°C, it is necessary to consider the impact of temperature and pressure conditions on the mechanical strength of the materials. In high-temperature conditions found in boiler feedwater systems and superheater bypass systems, conventional materials for valve bodies and internal components are not suitable (such as O-rings, PTFE materials, elastic materials, and standard gaskets). Therefore, more durable materials must be selected. The maximum temperature that can be used for ordinary materials is around 500°C. For applications above 538°C, the valve body is usually made of chromium-molybdenum steel. For applications with maximum temperatures of around 1035°C, SUS310S stainless steel is typically used, and the carbon content in the material must be kept between 0.04% and 0.08%. For higher temperatures, it is recommended to use lining materials made of non-metallic heat-resistant materials (suitable for temperatures up to 1200°C) or special high-temperature resistant high-strength alloys (such as those used in engine combustion chambers, which can be used in environments with temperatures up to 1000°C). (2) Effects of thermal expansion and cold contraction + C5 w7 n# ]' Z$ J1 ^9 G There are significant differences in the structure and internal components of high-temperature valves and normal-temperature valves, such as guide clearance, valve disc rotation clearance, and bearing types. In addition to control from the aspects of design and manufacturing, it is more advisable to reduce the effects of thermal expansion and contraction by selecting an appropriate valve structure. Practice has shown that the baffle butterfly valve is an excellent high-temperature valve. The gap between the baffle and the interior cavity of the valve is 3–6 mm, which effectively solves the problem of sticking of the valve disc in the high-temperature environment inside the valve cavity, and enables high shut-off performance (5×10-4). # `0 f$ S# J( r3 v* A (3) Guiding bearing and valve plate positioning issues: For applications where the medium temperature is above 400°C, conventional positioning and guiding structures are not reliable. At this time, an external bearing structure should be used to ensure the positioning and support of the valve plate, thereby avoiding the impact of high internal temperatures on the guiding structure. At the same time, since the positioning system bears the weight of the valve disc and valve stem, it reduces the load on the actuator as well as the load on the external bearings, preventing the one-sided jamming that often occurs when conventional butterfly valves are installed horizontally; they can therefore be installed vertically. + @- J5 ?( s7 p9 M9 U* e5 R (4) The temperature resistance of the packing: 9 o/ `' h: o G6 d. Standard polytetrafluoroethylene packing can only be used in applications below 200°C; if it is to be used in high-temperature environments, an expanded valve cover must be employed to protect the packing from extremely high temperatures. However, longer and thinner valve stems have lower strength under high-temperature conditions and are prone to bending. Therefore, under high-temperature conditions, flexible graphite packing with excellent heat resistance (up to 600°C) should be used, which can also **reduce the height of the elongable valve cover. At the same time, the use of a \"rotating-type valve + thick valve stem\" helps to increase the overall strength of the valve, thereby addressing this issue more effectively. 1 B” T/ T1 Y( q6 H4 H’ w: I C; C (5) Selection of sealing methods 7 K3 O4 `8 a+ } It is difficult to achieve high cutting performance under high-temperature conditions, and many conventional high-performance sealing methods are not suitable (such as O-rings, PTFE materials, elastic metal materials, etc.). Below 500°C, a soft-sealing method using a special composite graphite valve seat can be employed. At temperatures above 500°C, only a metal-to-metal hard seal method can be used (typically a butterfly valve structure is employed). To prevent jamming caused by the expansion of materials due to high temperatures, the sealing gap is usually kept large. This results in a higher leakage rate. The high-temperature flap-type butterfly valve utilizes a flat flap (valve disc) that is placed on the protrusion of the valve body (in an integrated manner), thereby creating a circular sealing surface that ensures excellent sealing performance; the leakage rate is as low as 10-3 to 10-4. At the same time, the wear-resistant alloy welded on the sealing surface provides the valve with better sealing reliability, thereby extending its service life as well. ) q3 e5 A; I0 b1 v9 L ! @5 s9 Y9 B+ s4 Y/ M II. Typical high-temperature control valves* R3 G$ ^# i. ]: v (1) Ordinary high-temperature alloy butterfly valves: These are butterfly valves with a simple structure, the lightest weight for their diameter, and they are easy to forge, which helps to save a significant amount of expensive high-temperature resistant alloys. Ordinary high-temperature alloy butterfly valves can only be used in applications where low leakage requirements apply. It is also important to ensure that there is sufficient clearance between the valve body and the valve disc, in order to avoid jamming caused by thermal expansion of the materials under high temperatures. At the same time, appropriate positioning measures should be adopted to ensure the effective positioning of the valve plate, preventing it from sagging or shifting, which could lead to one-sided jamming or seizure. : `0 ~1 J" f. R+ p: r (2) Superalloy baffle butterfly valve: The baffle butterfly valve is a new type of high-temperature control valve developed to address the main problems that arise with control valves under high-temperature conditions. It features a simple structure, ease of forging, good sealing performance, and resistance to jamming; it also helps to save a significant amount of expensive high-temperature resistant alloys. Its sealing mechanism involves a flat baffle (valve plate) being placed on the protrusion of the valve body (as an integrated part), thereby creating a circular sealing surface that achieves an excellent sealing effect. (3) Ball valve dedicated for high-temperature heat transfer oil # A5 a- L1 t/ o r5 Q: This is a ball valve designed for high-temperature heat transfer oil, featuring an elastic laminated special composite graphite seat along with flexible graphite packing. It effectively meets the requirements of heat transfer oil processing, such as reliable shut-off and regulation, minimal impact of the heat transfer oil on the packing (since conventional extended valve covers and radiators are unnecessary), and excellent sealing performance. It is a high-performance medium-to-high temperature shut-off and control valve suitable for use in applications below 500°C. ; G2 k7 j/ \2 a# r) q （4）High-temperature resistant non-metallic lining ultra-high temperature butterfly valve 4 Y8 K5 j$ ^ The structure is made of super-high temperature resistant alloy materials, while a high-temperature resistant non-metallic material is used for the lining, thereby allowing for a separation between the interior and exterior of the valve body and ensuring its strength. Hollow or super-high temperature-resistant alloy valve plates, combined with circulation or cooling mechanisms, can ensure proper operation of the valve plates at around 1200°C.