(40d) When a Control Valve Phase Change Results in Unfavorable Control Response

When a relief system is designed per API STD 521, favorable instrument response from basic process
controls should not be taken, but unfavorable response should be assumed to occur. In most cases, the
response from the control valve can be predicted without much effort. For example, if an upstream
vessel loses liquid feed, vapor can blow through into a downstream vessel through the upstream level
control valve (LCV), and the predicted response from the control valve is to move towards the closed
position as the liquid level drops. In this case, the control valve would be assumed to remain stationary.
However, if the level control cascades with flow control, the control valve may move towards the open
position, resulting in a larger relief requirement. This special case of vapor blow-by will be discussed
with cautions for properly sizing pressure relief devices based on conservative assumptions.