PRDS Control valve at 25% opening generate lots of sounds & hunting and unable to control steam flow, moreover stem of valve vibrating up and down. Data for control valve (CV = 430, line size = 12 * 14,flow required max. 28 tons). Please, if any one has an idea about the above conditions let me know to troubleshoot.<firstname.lastname@example.org>
The valve is generating high noise and vibrating that definitely means that steam flow through the valve has reached at its choked flow condition.
At this stage the fluid velocity reached to its maximum limit of sound velocity ( 305 m/s). Normally this happens if the pressure drop across the valve is more than 50-60% of the upstream pressure. You have to reduce the pressure drop across the valve.
If you have a manual globe valve at upstream or downstream of the line, try to close them a bit so that the pressure drop across the control valve reduces. Then see if the condition improves.