The breakage of the valve stem in rotary and high-pressure differential water control valves both occur during the boiler startup process. This is because, during startup, the pressure on the feedwater pump side can reach as high as 18-20 MPa, while the pressure on the boiler side is very low. As a result, the pressure difference acting on the valve core can reach up to 18-19 MPa. Therefore, rotary control valves, such as the Dg300 and Dg250 models, are designed for a pressure difference of 1.5 MPa. If the pressure difference across the control valve is small, the leakage flow is minimal, and the regulation performance is good. Generally, it is preferable for the pressure difference across the control valve to account for about 30% of the pipeline resistance. When the load decreases, all resistances will drop. For example, at 70% load, the resistance of the superheater drops by approximately 50%. At this point, the differential pressure control valve must consume the excess pressure to ensure a stable pressure difference before and after the control valve, thereby ensuring the performance of the control valve. If the main steam pressure drops, the pressure difference that needs to be consumed by the differential pressure control valve increases, which raises the pressure difference before and after the differential pressure control valve, making it difficult to open and complicating the feedwater regulation. When the flow rate changes, it causes a change in the pressure difference before and after the differential pressure control valve, leading to another change in the flow rate. The main control valve must then regulate again to adapt to the flow rate changes. During startup, when the pressure difference consumed by the main control valve and the differential pressure control valve greatly exceeds the working pressure difference, there is a possibility of the valve stem breaking. Of course, due to manufacturing and processing errors, the gap between the valve core and the valve body may be too small, causing sticking, or due to poor pipe flushing protection, foreign objects may enter and also cause sticking. These are all overlapping factors contributing to the valve stem fracture, but they are certainly not the main factors.
Source: Beijing WeiMa Wall-Hung Boiler Repair Phone Number http://blog.tianya.cn/blogger/blog_main.asp?BlogID=3778832. Analysis of the causes of the valve stem fracture accident in the WeiMa wall-hung boiler water control valve.