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Oversizing – Pressure Regulator Turndown

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:

  • Evaluate how realistic the maximum inlet and minimum flowrate values are and the likelihood that they will simultaneously occur.
  • Consider how long the regulator might be expected to operate at this extreme turndown. Some applications exist where the extreme turndown requirement is real but brief and rare.
  • Consider using parallel runs

2 Replies

  • Hi Nathan,

    I want to understand that what would be the minimum % of opening of regulator is recommended and which International guideline need to follow for % of opening of regulator.
  • In reply to amitzala:

    Amitzala,

    The percentage open is a term more frequently used in a control valve application as the percentage open can be controlled via a valve controller. A pressure regulator is an analog device that is powered and controlled by the pressure in the system piping. Due to the self-contained nature of the pressure regulator the evaluation of minimum operating conditions uses percentage flow as opposed to percent open as the percent opening is not controlled directly in a regulator.

    Industry typical practice is to set a regulator at approximately 5% of the rated flow as this is a small enough flow to prevent a high lockup pressure. During a no demand scenario a pressure regulator will travel fully closed resulting in it being zero percent open. As such there is not a direct minimum percentage opening for a pressure regulator.

    As Nathan said in the original post the minimum flow that the regulator can control at depends on a multitude of factors and the properties of the system that the regulator will be installed into. For a self-operated regulator in general terms low flow conditions do not present as significant of an issue in most cases. For a pilot operated regulator, the application, piping conditions, sense line placement, and many other factors will affect the low flow performance of the regulator. As such for a pilot operated regulator the system should be evaluated on a case by case basis. Please do not hesitate to reach out to your local sales office or Impact partner for guidance on this that can be better tailored to your application.

    The second point of international regulations is best addressed by your local Emerson Sales office who have experience and expertise with local regulations and can ensure you comply with any concerns. Regarding North America the minimum flow conditions are governed by industry best practices as opposed to particular regulations.

    Best regards,

    Morgan Cummings | Applications Engineer | Pressure Management

    Emerson Automation Solutions