Choose the proper regulator for high-purity applications

Regulators are available in a variety of types, designs and materials of construction and must be selected carefully. In any application with corrosive gases or liquids, or aggressive environmental conditions, consider a stainless steel regulator. Types of stainless steel regulators include pressure reducing, back pressure and vaporizing. Within each classification, choose between one and two-stage regulators, and piston and diaphragm regulators. Once the appropriate type of regulator has been identified, think about the materials of construction for critical components.

Fig 1. Process analyzer with pressure-reducing and back-pressure regulators.

 Figure 1 shows an analyzer system with pressure-reducing and back-pressure regulators performing typical functions. 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 analyzer. In this application, the analyzer system needs to maintain a pressure of 2 bar. Because of 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 analyzer from downstream pressure fluctuations.

A vaporizing regulator is a pressure-reducing regulator that incorporates a steam or electric heating element and is used either to prevent or induce a phase change. 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 toward a liquid. In these cases, the regulator may freeze-up. A vaporizing 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 analyzed in a vapor form, typically in gas chromatograph applications, in which case the regulator applies heat to vaporize the liquid to a gas.

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 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 regulator will generally deliver a stable outlet pressure when the high-pressure supply is stable.

Fig 2. Two-stage regulator.

A two-stage regulator consists of two one-stage regulators in series and combined into one component (Fig. 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 two-stage regulator application, 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 crossover arrangement, where two gas cylinders feed one point of entry (Fig. 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.

Diaphragm regulators generally are the most sensitive to pressure changes – especially in low-pressure applications. Depending on their rating, they may be used in pressures up to 248 bar. In a diaphragm regulator, a thin metal diaphragm flexes as the high-pressure inlet varies. This flexure causes the regulator poppet to move in and out of the regulator seat. This compensating action causes the downstream pressure to remain constant.

As inlet pressure rises, the diaphragm flexes up so that the poppet rises into the seat and reduces the effect of the increasing inlet pressure to provide a constant outlet pressure. As inlet pressure drops, the force is reduced so that the diaphragm flexes down and pushes the poppet out of the seat. This action allows for an increase in flow to pass through the regulator, which in turn stabilizes pressure at the outlet.

The flexibility of the diaphragm is vital to the long-term performance of the regulator and can be attained in one of two ways. The diaphragm could be perforated and then coated in PTFE or another flexible material. However the PTFE may erode, in which case a leak can occur since the diaphragm is designed with holes in it. An alternative design is to use a solid, convoluted diaphragm with a fluted configuration around its perimeter to enhance flexibility.

Perhaps the best seal for a diaphragm regulator is a metal-to-metal seal, which is less sensitive to temperature changes. In this design, the diaphragm sits in the regulator body and is held in place by the cap assembly. A backing plate between the diaphragm and cap assembly can guard against diaphragm rupture and help apply uniform pressure across the entire surface of the diaphragm.

Made of a high-grade stainless steel, such as S17400, and electropolished to provide a high-tolerance seat seal, the poppet is a critical piece in a diaphragm regulator. In a pressure-reducing regulator, the poppet is spring-loaded and held vertically in the inlet channel, with the tip in constant contact with the diaphragm.

With the poppet pushing up and the diaphragm pushing down, the two work together toward the desired balance. The poppet closes or opens the regulator inlet as its conical shape fits against a precision machined seat. A damper fitted to the bottom of the poppet supports the poppet to reduce noise and vibration in high-flow conditions.

In high-purity applications, it’s important to consider the materials of construction for the diaphragm and poppet seat. For example, a diaphragm made from a flexible, corrosion-resistant alloy may be more appropriate than one made from 316 stainless steel. Likewise, the poppet seat is critical and should be modular so that you can choose an appropriate one based on chemical compatibility, pressure requirements and temperature.

Fig 3. Crossover arrangement with one-stage regulators acting together for a result equivalent to a two-stage regulator.

The thickness of the piston, along with the O-ring seal, allows a piston regulator to achieve higher working pressures than diaphragm regulators.

Compatibility of the O-ring material with the regulated process stream is an important consideration. Similarly, the surface finish of the inside chamber is critical so that the O-ring seal between the piston and the cylinder wall can move freely up and down, thereby increasing the sensitivity of the regulator.