Pressure regulator turndown (i.e. rangeability or minimum flow) values are a common inquiry from customers who are sizing and selecting a regulator. It is a question that I’ve always dreaded answering. Why? Because the regulator’s capability is only part of the equation… process conditions and installation also impact stability and their impact is difficult to predict.
Oversizing (i.e. exceeding the regulator’s turndown) is the most common product selection issue. When a regulator is oversized, the valve plug is forced to operate very close to the orifice. At this low travel, the forces from the fluid flowing past the valve plug battles the regulator diaphragm trying to position the valve plug, causing the regulator to cycle: open, closed, open, closed… This high frequency cycling, known as instability, causes wear on the moving parts which can lead to component failure and loss of pressure control. This cycling can also produce significant noise and inaccurate pressure regulation.
Think of the turndown of a specific regulator type as a bell curve distribution. Construction options such as body size, orifice size, spring selections create variability. Add to that installation variability and you have a distribution of outcomes. Some variables combine to have excellent turndown and others combine to have poor turndown.
Due to this variability and the complex multi-factors, the best advice for turndown typically comes in the form of rules of thumb, handed down like an old family recipe from one generation to the next. Often, these rules of thumb are conservative but provide guidelines for when additional consideration is needed. When the calculated turndown is approaching the rule-of-thumb threshold: