Oversizing - Improving Regulator Turndown using Restricted Trim

Reduced orifice sizes and characterized cages are inexpensive ways of improving a regulator’s turndown capability. These restricted trims help low flow performance by increasing the travel at low flows, moving the valve plug further off the orifice.


Restricted trim types that improve low flow performance:

  • Changing to a smaller orifice (ex: numerous self-ops and a few pilot-ops)
    • On products without alternate orifices, use a smaller body size

  • Characterized trim (ex: 310A turbine start-up, 1098 linear and whisper cage, 133 restricted collar)


An additional attribute of these restricted trims is that in many cases the wide-open flow is also lowered which can reduce the size of a relief valve being used for overpressure protection. Confusion arises on some products with trims designed solely for reducing the wide-open flow with users expecting them to improve low flow performance. Because these Relief Size-reduction Trims don’t affect the flow area size when the valve plug is at low travel, they do not improve turndown.

Relief Size-reduction Trim examples:

  • Travel stops (1098)
  • Upstream orifice plates (EZR and EZH)

Because Relief Size-reduction Trims do not improve a regulator's turndown, use the 100% capacity when using turndown rules-of-thumb, not the reduced capacity.

1 Reply

  • Hello, Thanks for this post.
    For Series FL you used to have the restricted trim as well. I believe it was working same like in case of Type 310A by improving the stability at smaller flows/ delta P.
    Am I right?
    This FL restricted trim is not available since some years. It's a pity since Type 310A cannot be offered in some areas.
    Why don't you offer restricted trim for Type FL? B.Rgds, M. Ambroziewicz