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.
An additional benefit of balanced poppet regulators is their ability to reduce lockup — the tendency for the poppet to snap shut as downstream flow is decreased to zero. Excessive lockup is undesirable because it may cause a brief spike in outlet pressure when the poppet rapidly closes.
Two-stage regulation. A balanced poppet regulator is suitable for lowering SPE in many applications, especially high-flow ones. For lower-flow and some other applications, using two-stage pressure reduction may provide a good alternative. This method involves installing two single-stage regulators in series or combining the two regulators into one assembly. Each regulator controls pressure variation to some degree; together, they bring outlet pressure very close to the target.
To calculate the variability of outlet pressure for a two-stage regulator setup, the inlet pressure difference is multiplied by the SPE of each regulator:
ΔP (outlet) = ΔP (inlet) × SPE1 × SPE2
Remember that SPE is an inverse relationship. As a gas cylinder empties and inlet pressure decreases, the first-stage regulator will encounter an increase in outlet pressure. That increase will result in a subsequent decrease on the outlet side of the second-stage regulator. However, because the first-stage regulator experiences the majority of SPE, the relative pressure decrease after the second-stage regulator is minimal.
Let's return to our example of a cylinder emptying from 2,200 psig to 500 psig and assume that each regulator has a 1% SPE. With a 1,700-psig (118-bar) inlet pressure drop, the first-stage regulator will experience a 17-psig rise in outlet pressure. As a result of that increase, the second-stage regulator will experience a 0.17-psig (0.012-bar) decrease in outlet pressure. The net result is a minimal pressure decrease — only 0.01% of the initial cylinder pressure drop. Once the cylinder pressure drops below the first-stage setting, only the SPE of the second-stage regulator applies.
In terms of controlling SPE, a two-stage regulator setup typically will achieve a better outcome than a single pressure-reducing regulator with a balanced poppet design. In a plant using one gas cylinder source to feed multiple operations that all use the same outlet pressure, either option may be feasible. However, if the application requires the gas cylinder to service multiple operations with at least one of these calling for a different pressure, you will need to use two-stage regulation. In this case, locate the first-stage regulator near the gas source and a second-stage regulator on each of the process lines.
A common mistake is using a two-stage regulator at the gas supply source and a single-stage regulator at the point of use. This setup is overkill, as it amounts to three-stage regulation.
Dome-loaded regulators. A single dome-loaded regulator also can serve to control SPE. This is most practical when regulating pressures from large-volume gas cylinders where high flows are required. A dome-loaded regulator operates in much the same way as a spring-loaded one, except a pressurized dome, instead of a spring, exerts force down on the diaphragm and poppet.
Figure 3 shows a setup for controlling SPE with a dome-loaded regulator. Besides the dome-loaded regulator, this setup requires a pilot regulator and three tubing loops. The first loop connects the pilot regulator to the dome of the dome-loaded regulator so it can make adjustments in dome pressure in response to system pressure. The second tubing loop allows excess dome pressure to bleed back into the downstream system media. The third loop is for external feedback — it enables the pilot regulator to accurately read downstream gas pressure to make quick adjustments in dome pressure. Because this system is making adjustments based on actual downstream pressure readings, it can effectively minimize SPE.
In the case of a pilot-operated dome-loaded regulator, the SPE of the pilot and the dome are added together to provide the SPE of the system.
Manual adjustments. It's also possible to manage SPE by manually adjusting a regulator based on the reading of a downstream pressure gauge. However, this method is impractical in most situations. If a cylinder is servicing an application that requires a continuous supply of gas, the outlet pressure always will be changing. This means someone will have to check the downstream pressure gauge frequently — the cost of labor may far surpass the cost of introducing one of the system configurations described above.
One of the few applications where manual adjustments may make sense is a laboratory system in which the demand for cylinder gas is limited to short intermittent intervals. On occasions when gas is needed, the lab technician can adjust the regulator's set pressure.
With a regulator controlling outlet pressure from a gas cylinder, SPE is a phenomenon that's always at play. Whenever inlet pressure changes, outlet pressure also will change. It's only noticeable — or only becomes an issue — in certain situations, for example, when regulation of the outlet pressure must be very precise or when inlet pressure has changed significantly, such as when a gas cylinder is emptying. You can minimize the effects of SPE for many applications by using a single regulator with a balanced poppet design or a two-stage regulator. However, if your gas source is servicing multiple operations with different pressure requirements, you may need multiple single-stage regulators — one near the gas source and another on each process line — to enable two-stage regulation at each point of use.
Also consider a dome-loaded regulator with feedback to a pilot regulator for high-flow large-volume gas cylinder applications. In addition, managing SPE manually may suffice in certain applications.
Regardless of its style, the regulator you select should closely match the particular range of pressures in your system. As a rule of thumb, regulators with broader pressure ranges have higher SPEs than those with lower ranges. You should choose the regulator configuration with the inlet pressure and control range as near as possible to the application parameters.
MICHAEL D. ADKINS is manager, field engineering and pressure regulators, for Swagelok Company, Solon, Ohio. WOUTER PRONK is a senior field engineer, pressure regulators, for Swagelok in Nieuw-Vennep, The Netherlands. E-mail them at Michael.Adkins@swagelok.com and Wouter.Pronk@swagelok.com.