Pressure Reducing Valves Part Two: Factors that Influence Offset (droop)June 24, 2016 by admin
In our last blog, we discussed what offset (droop) is and why it occurs. This week, we will discuss what factors influence offset, and the amount of that offset. Some of these factors are intrinsic to all regulator designs, and cannot be controlled by the user. However, other factors can be controlled either by regulator selection or option specification.
Factors influencing offset that are intrinsic to regulator design:
Some amount of droop is inherent in all regulator designs.
There are several physical forces acting on the Diaphragm, Spring and Plug as flow changes that cause offset. These forces can never be completely eliminated from any manufacturers design.
But….they can be influenced by a regulator manufacturer’s basic design, and are therefore under the control of the person that selects a particular regulator design, and it’s specific features or options. For example; regulators with large diaphragm areas, a small amount of total stroke (distance the stem moves from full open to full close), and a properly selected spring relative to the set point, generally have less offset than those that don’t have those features.
- Diaphragm area: The lines in the test chart (Exhibit 1 below) vividly illustrate the difference in offset of two different regulators on the same application. The red line is the offset curve for a small, clean gas and air regulator’s diaphragm with about the same total stroke length and spring rate as the large diaphragm regulator that produced the blue offset curve. In this case, the difference in offset performance primarily has to do with the diaphragm area difference. The regulator photographs below (Figure 1 and 2) give you a relative idea of the diaphragm area difference. It really makes a difference.
Stroke Length: A regulator with a shorter total stroke will outperform a regulator with a longer stroke if all other factors are held constant.
- Spring Rate, or compression at set point: Selecting the right spring range for your specific regulator set point influences offset.
- Generally speaking the more compressed the spring is at your set point pressure, the less offset exhibited. In other words, when selecting a spring range for your regulator, select a spring range that puts your set point closer to the upper end of the spring range. Look at the droop figures circled in the comparative graphs below (exhibit 2) for an example. A spring range of 5 to 25 psi is clearly a better spring choice for a 15 psi set point than the 15 to 50 psi range spring.
Forces Acting on the Plug: Regulators with larger diaphragms are much less susceptible to the hydraulic and velocity forces acting on the plug.
- The force acting on the small plug of a large diaphragm regulator (Figure 2) is insignificant relative to the force acting on that regulators diaphragm because of the relative area those forces are acting on. (Force = Pressure x Area). By contrast, Small clean air and gas style regulators (Figure 1 below) have a much smaller diaphragm area relative to the plug.They are therefore more susceptible to forces acting on the plug.As a result they exhibit more offset.
In our next blog post, we will discuss factors that a user can control during commissioning to lower offset.
Karl Lutkewitte, Steriflow Valve Sales and Product Manager