The flat part in the middle is not perfectly flat. Usually, it slopes downward. This is called droop. As flow increases, outlet pressure will drop some—or a lot, depending on the regulator
design. While droop is relatively modest along the flat part of the curve, it is quite steep at the far ends of the curve.
Supplying a regulator with pressures substantially lower than the inlet pressure rating results in a flow curve with more droop than flow curves for regulators whose inlet pressure rating closely matches actual system pressure. In addition, selecting a regulator that closely matches inlet pressure requirements provides the best handle resolution (smaller amount of pressure change per turn of the handle) and control, enabling a broader ideal operating range.
Seat-load drop occurs on the far left of the regulator curve, where there is initially a steep drop in pressure. If reading the curve from left to right, imagine that the system is in a no-flow state. The regulator is set to a certain pressure, but there is no flow. Then, imagine that an operator slowly opens a downstream valve to initiate flow. Immediately, there is a sharp drop in pressure because it is difficult for a regulator to maintain pressure at this location. A regulator operating along this steep drop in the curve may emit chattering or pulsating sounds as it fluctuates between flow and no-flow conditions.
Now read the curve from right to left. Imagine that the system is operating along the flat part of the curve. Then, imagine that an operator slowly closes a downstream valve, reducing flow to near zero. We are moving up the curve. As the no-flow state nears, the regulator has difficulty maintaining the set pressure. Again, the regulator may emit a chattering sound. Eventually, the regulator snaps shut, stopping flow. This is called lockup. The terms seat-load drop and lockup are essentially interchangeable. Sometimes, lockup is used to describe both conditions. It is not advisable to operate a regulator under these conditions.
On January 17th, in a live technical webinar, customers of Swagelok Northern California will receive a 30 minute briefing from Swagelok field engineer and regulator expert Eric Kayla. Kayla will discuss how to properly select and size a pressure reducing regulator and cover:
In a hurry or have a question? Please call our office at 510-933-6200.
Additional resources:
Related articles:
Webinar Replay: The Theory and Operation of Pressure Reducing Regulators
Related downloads:
Technical Bulletin and Article on Pressure Regulator Flow Curves
Slide Deck: The Theory and Operation of Pressure Reducing Regulators
Q&A from The Theory and Operation of Pressure Reducing Regulators
Swagelok Pressure Regulators Catalog