Figure 3. External downstream feedback enables precise adjustments to control outlet pressure and minimize supply pressure effect when high flows are needed.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
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.FINAL THOUGHTS
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 [email protected] and [email protected].