The problem of valves sticking in steam pipelines

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Dear all**, I have a question: the valves on the steam pipelines in our factory often get stuck, meaning they cannot be opened after being closed. The valve is a pneumatic ball valve; the air supply pressure is within the required range. Usually, it cannot be opened even manually, though occasionally it can be opened manually. We know that globe valves are commonly used in steam pipelines, but in this case, steam and water need to be heated and cooled in the same coil; therefore, high sealing performance of the valve is required, which is why globe valves were chosen. The valve core is made of 304 material, the valve seat is made of WCB for a metal hard seal, and the packing is made of flexible graphite. The steam temperature is generally between 180°C and 130°C, while the pressure is between 1.5 MPa and 2.0 MPa. Is it the deformation of the valve core and seat due to thermal expansion and contraction that causes the valve to fail to open? (There are almost no cases where it can’t be closed after being opened.) Usually, we can open the valve by loosening the screws on the valve seat. Could impurities in the steam cause problems with the smooth operation of the valves inside? I can’t figure out the reason either; I’m an expert in instrumentation, and I really don’t know much about the mechanical aspects of valves. I hope everyone can give me some guidance.