System operators running a process from a gas cylinder source sometimes may observe a puzzling phenomenon — on the downstream side of a pressure-reducing regulator the outlet pressure increases for no apparent reason. As the cylinder drains, inlet pressure to the regulator will decrease. Simple logic suggests the outlet pressure also should decrease. However, in fact, the outlet pressure rises, leading many an operator to think the regulator is malfunctioning.
This phenomenon may have a number of possible causes. The most likely one isn't a malfunctioning regulator but is the supply pressure effect (SPE), which sometimes is referred to as "dependency." It's best to minimize SPE. If a regulator's SPE is too high, the pressure change may impact the effectiveness of the system.
Fortunately, you can manage SPE by selecting the right regulator or combination of regulators for a given application. In rare cases, you may be able to reduce SPE by manually adjusting the regulator's set pressure. More likely, you'll need to make some changes in your system configuration. It's preferable to make these changes at the outset, when you are designing your system, rather than in an existing system, where you may encounter challenges such as space limitations and extra costs that may limit your options.
The phenomenon is defined as the change in outlet pressure due to a change in inlet (or supply) pressure. Inlet and outlet pressure changes are inversely proportional. An inlet pressure decrease leads to a corresponding outlet pressure increase, and an inlet pressure rise reduces outlet pressure.
SPE isn't a periodic phenomenon in gas cylinder applications. It occurs whenever inlet pressure alters. However, you only may notice it in situations where inlet pressure has changed significantly, such as when your gas cylinder has drained and pressure has dropped, say, from 2,200 psig (153 bar) to 500 psig (35 bar).
You can estimate the degree of outlet pressure variation for a regulator via: ∆P (outlet) = ∆P (inlet) × SPE
In other words, if the regulator's SPE is 1%, the variability in outlet pressure is 1% of the change in inlet pressure. For example, as a cylinder empties from 2,200 psig to 500 psig, the regulator's outlet pressure will increase by 17 psig (1.2 bar). Remember, SPE is an inverse relationship — if inlet pressure goes down, outlet pressure goes up. SPE values vary among regulator models and typically are provided by the manufacturer either as a percentage or as a ratio.
To understand why SPE occurs, you must look inside a regulator. Figures 1a and 1b show the internals of a basic spring-loaded pressure-reducing regulator with an unbalanced poppet. The system media pressure — both at the inlet (FI), acting on the poppet over the entire seat area (A1), and outlet (FO), acting on the underside of the diaphragm — as well as the poppet spring force (FS2) keep the poppet shut when downstream valves are closed (Figure 1a). When the valves are opened to initiate flow downstream, the outlet pressure drops. This causes the set spring force (FS) — whose value the operator dials in — to flex the diaphragm downward, opening the poppet to enable flow through the regulator (Figure 1b). A regulator works by balancing forces. So, a subsequent decrease in inlet pressure will increase downstream pressure. However, the rise in outlet pressure isn't strong enough to counterbalance the set spring force; closing the poppet will require a higher pressure. This is what SPE looks like inside a regulator.
Several options exist for decreasing SPE.
Balanced poppet design. For many applications, a common method for reducing SPE is to use a regulator with a balanced poppet design (Figure 2). The objective of this design is to allow the inlet and outlet pressures to control the poppet more evenly.
An O-ring around the lower stem of the poppet prevents the inlet pressure (FI) from acting on the bottom of the poppet (B2O), which is much lower than the inlet pressure, to act on the bottom of the poppet. The inlet pressure acts only on a small area (A2 - B2), minimizing SPE.
In this design, both inlet and outlet pressure act to close the poppet when system pressure is too high. The inlet pressure can push up on the unbalanced area while the outlet pressure can push up on the bottom of the poppet.
Now, let's return to our SPE scenario and again imagine that inlet pressure has decreased as a gas cylinder is depleting. Inlet pressure will have less effect on the poppet because it acts on a smaller area. With this balanced poppet design, the outlet pressure has sufficient leverage to close the poppet and restrict pressure, correcting the rise in pressure downstream. Outlet pressure pushes up on the diaphragm and spring force (FS) and, at the same time, up on the bottom of the poppet.