Which high-pressure steam vent valve is the best? Please share your practical experience

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Which type of valve should be used for high-pressure steam vent valves? Valves used for high-pressure steam venting are prone to sticking or leaking after operation for some time, and these problems pose challenges to instrument technicians. In this article, CHANGHUI INSTRUMENTS shares its experience in selecting high-pressure steam valves through case studies. In a refinery’s 800,000 tons per year catalytic unit, the pressure in the medium-pressure steam main pipeline is not stable enough; therefore, a DN200 vent line was installed after the unit. Under normal conditions, the vent valve remains closed, but when the steam pressure rises above a set level, some of the pressure is released to keep the pressure within the pipeline relatively stable. The control valve is located on the ground surface. High-pressure steam relief valve yunrun.com.cn/tech/2765.html Description of fault symptoms: A hard-sealed ball valve was initially selected, and it functioned normally when first put into use. However, after some time, sticking problems often occurred, and both the leakage volume and the venting noise were high, severely affecting the normal operation of the device. Fault cause analysis: The operating conditions in this case place high demands on the valve. For a high-pressure steam relief valve to be usable, it must meet the following requirements: 1. The leakage rate must be low. Since a pressure of 2.5 MPa acts on the side of the valve before it, while the pressure on the other side is at atmospheric level, if the leakage level of the valve is low (meaning that a continuous stream of water can be observed coming out), it will continuously erode the sealing surfaces between the valve core and the seat, resulting in an effect similar to a dam breaking, which causes the gap to widen rapidly and eliminates the sealing function. Only when the leakage level of the control valve reaches grade V or above (with droplet-like leakage) can such phenomena be avoided and the sealing performance maintained over the long term. Both hard-sealed ball valves and triple-eccentric butterfly valves can meet the above requirements as well as withstand high-temperature conditions involving hot steam. It should be noted, however, that the material used for their components and the clearance between these components differ significantly from those in normal temperature conditions. In linear-motion valves, hard-sealed gate valves can also meet the sealing requirements. The leakage level of conventional sleeve control valves (cage double-seat design) is generally below grade IV (especially under high-temperature conditions). Single-seat valves can achieve Class VI for soft seals, while hard seals typically reach Class IV; special processing is required for Classes V and VI. Moreover, at a pressure difference of 2.5 MPa, it is difficult for the actuator thrust of a single-seat DN200 valve to meet the requirements. 2. It belongs to typical intermittent control, with severe fluctuations in the medium temperature. Based on the position of the valve on site, the height of the vertical pipe in front of the valve is approximately 15 meters. During normal operation, a certain amount of condensate water accumulates in front of the valve, with a temperature of only a few dozen degrees Celsius. However, during overpressure venting, this condensed water is blown away in a very short time, followed by superheated steam at 450°C. Such sudden changes in temperature can directly cause thermal expansion and contraction of the valve; therefore, the original hard-sealed ball valve cannot meet the requirements in this case. Because the contact area between the ball surface of the ball valve and the valve seat must remain tightly fitted at all times to ensure a good seal, under severe temperature changes, the thermal expansion coefficient of the spherical shape of the ball is much higher than that of the planar valve seat. The fit between the support shaft and the sleeve of the triple-eccentric butterfly valve is similar as well. By the same logic, rotary valves in which the valve core relies on clearance fit for support generally find it difficult to adapt to these operating conditions. Additionally, when selecting materials for valve components, it is necessary to take into account the thermal stresses resulting from sudden temperature changes. The valve body material should be molybdenum-containing castings or forgings such as WC6/WC9/CF8M, while the internal valve components should be forgings such as F11/F22/F316 in order to enhance their resistance to creep. 3. High requirements for adjustment performance. Since the opening degree of the relief valve is controlled based on the extent to which the pressure exceeds the normal level, in cases where the overpressure is not significant, there will be a short-period operation at a low opening degree. On the one hand, it increases the difficulty of control, and on the other hand, it exacerbates the erosion at the throttle orifice. The flow path of a gate valve is full-bore, and it has a large dead zone at the fully closed position; therefore, it is not suitable for regulation and is only appropriate for on/off operations. In this case, when operating at a low opening degree, the lower end of the gate seal surface is prone to being eroded locally by the medium, resulting in damage and a loss of sealing performance; therefore, it is also not suitable. 4. Low noise: Significant noise is generated when medium-pressure steam is released. Although a silencer is installed at the outlet, noise levels of over 85 dB still occur in the pipeline downstream of the valve; therefore, noise reduction measures must be taken. It is relatively easy to add noise-reduction measures to the sleeve of control valves; this is not as convenient for other types of valves. Solution: A multi-stage pressure-reducing sleeve valve is used as the high-pressure steam vent valve for this device. Control valves: yunrun.com.cn/product/list_88.html. In summary, it is difficult for conventional valves to meet all four of the above requirements simultaneously. The multi-stage noise-suppressing sleeve valve that utilizes BFS technology has successfully solved the problem of selecting a valve for high-pressure steam venting, thanks to the excellent shut-off performance provided by its linear sealing mechanism and the superior energy-absorbing and guiding properties of its elastic graphite sealing ring; it has continued to operate stably for many years.