Feb-2008

Choosing the proper regulator for high-purity applications

Regulators are highly technical, highly specialised fluid-handling components. So, the next time you need one please do not just grab one out of the supply cabinet and install it with the vague sense that it will work.

Bill Menz, Swagelok Company

Article Summary

Even if it seems like it the regulator was properly selected, it may not be doing what you think. Contamination of your gas stream or inaccurate pressure are possible results that can lead to off-spec product. In any application with even the smallest mix of corrosive gases or liquids or aggressive environmental conditions, a stainless steel regulator should be considered.
  
There are several types of stainless steel regulators, including pressure-reducing, back-pressure and vapourising. Within each of these three classifications there are additional choices to be made between one-stage and two-stage regulators, and piston and diaphragm regulators.

In sum, regulators are available in a variety of types and designs, and choices between them should be deliberate, with specific consideration given to the gases, phases, pressures and temperatures in question.

Basic types of regulators
Regulators control pressure. They are the pivot point between high and low pressure. It will always be the case that on one side of the regulator there is higher pressure, and on the other there is lower pressure. On the high-pressure side, the regulator mechanically controls a pressure drop, so that on the low-pressure side pressure will remain relatively constant. Most common applications require a pressure-reducing regulator, which means the inlet pressure undergoes a mechanically controlled pressure drop, resulting in a relatively constant pressure at the outlet. In some cases, the reverse may be required. In such cases, a back-pressure regulator would be used to mechanically control the outlet pressure, so that a relatively constant pressure is maintained at the inlet.

Figure 1 shows an analyser system with
pressure-reducing and back-pressure regulators performing typical functions. Note that the pressure-reducing regulator is receiving high pressure (35 to 40 bar) from the process line and reducing pressure to a stable supply pressure (1.975 to 2.025 bar) as the gas flows into the analyser. In this application, the analyser system needs to maintain a pressure of 2 bar. Due to pressure fluctuations in the process stream where the sample is being returned, a back-pressure regulator is employed. It maintains a stable pressure on the inlet side and shields the analyser from the downstream pressure fluctuations.

A vapourising regulator is a pressure-reducing regulator used either to prevent a phase change or to induce one. A steam or electric heating element is part of the vapourising regulator. In some cases, a rapid pressure drop may result in the Joule-Thompson effect, where a gas loses heat as it undergoes a complete or partial phase change from a gas towards a liquid. In these cases, the regulator may freeze up. A vaporising regulator applies heat at the point of the pressure drop, preventing the phase change and freezing from occurring. In other cases, a liquid may need to be analysed in a vapour form, typically in gas chromatograph applications, in which case the regulator applies heat to vaporise the liquid to a gas.

One- or two-stage regulators?
One-stage pressure-reducing regulators are sufficient in most applications where the inlet pressure is relatively constant. While one-stage regulators are more susceptible to a phenomenon known as supply pressure effect (SPE) than two-stage regulators, the determining factor resides in the pressure variation of the high-pressure supply. SPE is the ability of a regulator to adjust to changes in the high-pressure supply to the regulator. In applications where the high-pressure supply is subject to large variations, a regulator with a low SPE will provide the most stable low-pressure delivery. Therefore, a one-stage will generally deliver a stable outlet pressure when the high-pressure supply is stable.

A high-quality, one-stage regulator will deliver an outlet pressure that may be estimated using the following formula: ∆P (outlet) = ∆P (inlet) x 0.01. In other words, outlet pressure is 1% of the difference in inlet pressure variability. In Figure 1, inlet pressure varies by 5 bar, so 5 bar x 0.01 equals an outlet pressure variability of 0.05 bar. If the outlet pressure is set for 2 bar, and the inlet pressure rises from 35 to 40 bar, the outlet pressure will drop from 2 to 1.95 bar. The inverse relationship between the high-pressure (inlet) rising and the low-pressure (outlet) dropping is typical of one-stage regulators. The high- pressure rise causes the valve seat to constrict slightly, reducing the regulator orifice size and the corresponding outlet pressure.

A two-stage regulator consists of two one-stage regulators in series and combined into one component (see Figure 2). The first regulator reduces the high-pressure supply to an intermediate point between the inlet pressure and the desired outlet pressure. The second regulator reduces the intermediate pressure to the desired outlet. To calculate the variability of outlet pressure for a high-quality, two-stage regulator, the variability in the inlet, high-pressure supply is multiplied by 0.0001 because each regulator reduces the variability by 1% (0.01 x 0.01 = 0.0001). In a typical application for a two-stage regulator, a cylinder gas is emptied at a near constant outlet pressure. As the cylinder empties, pressure at the regulator inlet will drop from 175 bar to 5 bar, for example, as the cylinder becomes depleted. In this example, the variability in inlet pressure is 170 bar. If the target outlet pressure is 2 bar, the outlet pressure with a two-stage regulator will drop from 2 to 1.983 bar. On the other hand, if the same gas cylinder were outfitted with a one-stage regulator, the pressure would drop from 2 bar to 0.3 bar.

While a two-stage regulator is handy, two one-stage regulators may work just as well or better in some applications, such as a cross-over arrangement, where two gas cylinders feed one point of entry (see Figure 3). One cylinder is used until its pressure drops below a certain point, then the other cylinder starts to be used. One-stage regulators are located off each cylinder. An additional regulator (often referred to as a line regulator) is located at the entry point to the system, so at all times the gas is passing through two regulators.