Pressure regulator selection
Regulators are available in a variety of types, designs and materials of construction, and choices should be made with care.
Three main categories of pressure regulators include pressure reducing, back pressure and vaporising. Within each of these three classifications, one can choose between one- and two-stage regulators and piston and diaphragm regulators. Once the appropriate type of regulator has been identified based on the application, attention should be given to the materials of construction for critical components, such as diaphragms and poppet seats, to ensure safe and proper functioning of the regulator over time. Here we focus to some extent on high-purity applications, where even the smallest mix of corrosive gases and liquids or aggressive environmental conditions warrants the use of a stainless steel regulator.
Pressure-reducing and back-pressure regulators
Regulators are the pivotal control point between high and low pressure. It will always be the case that the pressure will be higher on one side of the regulator than on the other. 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 is 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 (37.5±2.5 bar) from the process line and reducing pressure to a stable supply pressure (2±0.025 bar, with a one-stage regulator) as the gas flows into the analyser. In this application, the analyser 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 analyser from the downstream pressure fluctuations.
A vaporising regulator is a pressure-reducing regulator used either to prevent condensation or to induce vaporisation. The reason for preventing condensation is to forestall rapid pressure drop that could result in the Joule-Thompson effect and cause a regulator to freeze up. The Joule-Thomson effect is caused by a gas losing heat as it undergoes a complete or partial condensation. A vaporising regulator applies heat via a steam or electric heating element at the point of the pressure drop, preventing the condensation from occurring.
In other cases, such as are typical for gas chromatograph applications, it may be desirable for a liquid to be vaporised. In this instance, the vaporising regulator applies heat to vaporise the liquid to a gas.
One- or two-stage regulators
The manner in which a regulator adjusts to variations in the high-pressure supply is known as supply pressure effect (SPE). In general, one-stage pressure-reducing regulators are suitable for most applications where the inlet pressure is relatively constant. In applications where the high pressure supply is subject to large variations, a two-stage regulator with a low SPE will provide the most stable, low-pressure delivery.
The degree of variation that can be expected in the outlet pressure differs between one- and two-stage regulators. A high-quality, one-stage regulator will deliver an outlet pressure range that may be estimated using the following formula:
In other words, the variability in outlet pressure is 1% of the inlet-pressure range. In Figure 1, the inlet pressure varies by 5 bar (35 to 40 bar), so 5 bar x 0.01 equals an outlet pressure variation 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 rise on the high-pressure inlet side 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 (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 stage reduces the intermediate pressure to the desired outlet pressure. To calculate the variability of outlet pressure for a high-quality, two-stage regulator, the inlet pressure difference is multiplied by 0.0001, or by 1% for each regulator (0.01 x 0.01 = 0.0001).
In a typical application for a two-stage regulator, a gas cylinder is emptied at a near-constant outlet pressure. As the cylinder empties, pressure at the regulator inlet will drop. If, for example, the pressure drops from 175 to 5 bar, the inlet pressure variation is 170 bar. If a two-stage regulator is used with a target outlet pressure of 2 bar, then the outlet pressure 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 increase from 2 to 3.7 bar.
While a two-stage regulator is handy, two one-stage regulators may work just as well, or better, in some applications. One example is a crossover arrangement, where two gas cylinders feed one point of entry (Figure 3). One cylinder is used until its pressure drops below a certain point. The second cylinder then goes into service. This specialised configuration places a one-stage regulator with each of the two cylinders. An additional regulator (often referred to as a line regulator) is located at the entry point to the system, so that at all times the gas is passing through two regulators.
Diaphragm regulators are generally the most sensitive in response to changes in pressure, especially in low-pressure applications. Depending on their rating, they may be used with inlet pressures up to 248 bar and controlled outlet pressures up to 35 bar. As inlet pressure rises, the thin metal diaphragm flexes up, allowing the poppet to rise into the regulator seat, reducing the effect of the increasing inlet pressure, thereby providing a constant outlet pressure.
As the inlet pressure drops, the diaphragm flexes down and pushes the poppet out of the seat. This action allows for a flow increase through the regulator, which in turn creates a stabilising pressure at the outlet. The flexibility of the diaphragm is vital to the long-term performance of the regulator.
Flexibility is attained either through perforation or convolution. The diaphragm can be perforated and then coated in PTFE or another flexible material. In this design, 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 provides a reliable seal and is less sensitive to changes in temperature than elastomeric or polymeric seals.
The use of a backing plate between the diaphragm and the cap assembly can guard against diaphragm rupture. The backing plate is a sturdy stainless steel disk that can also help to apply uniform pressure across the entire surface of the diaphragm.
The poppet is a critical piece in a diaphragm regulator. Generally made of a high-grade stainless steel, such as S17400, the poppet is electropolished to provide a high-tolerance seat seal. 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 and centres the poppet to reduce noise and vibration in high-flow conditions.
With stainless steel regulators for high-purity applications, particular attention should be paid to the materials of construction for the diaphragm and poppet seat. For the diaphragm, an alloy such as Inconel may be more appropriate than Type 316 stainless steel due to Inconel’s greater flexibility and high corrosion resistance. Likewise, the poppet seat is critical. A hard fluoropolymer is not as forgiving and will not seat as well as a softer material, but is more resistant to abrasion. The poppet seat should be modular, so an appropriate material (such as PEEK or PCTFE) may be chosen based on chemical compatibility, pressure requirements and temperature.
Piston regulators are generally used in applications with outlet pressures higher than 35 bar, although they may also be suitable for lower pressures. In a piston regulator, pressure is controlled by means of a spring-loaded piston, which is a stainless steel, inflexible disk that lies flat in the vertical cylinder of the regulator. The piston seals against the cylinder walls by means of an elastomeric O-ring seal. 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 when specifying piston regulators, particularly in high-purity service.
‘Droop’ and ‘creep’
‘Droop’ and ‘creep’ are two undesirable conditions. Droop affects the overall functionality of a regulator and occurs when more flow is required at the outlet than the regulator can provide. In other words, the throughput of the regulator (often given as flow capacity, or Cv) is not suited to the application. The way to avoid droop is by selecting a regulator with a flow capacity appropriate to the application.
Creep occurs when the poppet is in the closed position, yet the seat allows pressure to escape to the outlet side. It generally occurs because the seat may have been damaged or eroded. For instance, regulator seats can be compromised by particulates in the process stream, which can cause minor imperfections in the sealing surface. The high flow and small orifice that is created during the regulation of pressure combine to turn a very small particle into a very fast projectile. As such, these small particles can nick the surface of the seat and cause leakage of pressure from the high-pressure inlet to the low-pressure outlet.
In closed systems, this leakage can equalise the outlet and inlet pressures, which can result in an undesirable condition. When the system is opened via a control valve, a burst of high pressure could be the result. Creep is essentially a wear issue. If the seat of the regulator is damaged, the regulator needs to be serviced by replacing the seat material.
by Bill Menz, Manager, Field Engineering, joined Swagelok in 2002 and has worked closely with distributors and end users to define market trends, growth opportunities, new product needs and applications for Swagelok’s analytical and process instrumentation products. Menz has more than 13 years of experience gained through a variety of engineering and marketing roles with Rosemount Analytical. Menz graduated from the University of Cincinnati with a Bachelor of Science degree in chemical engineering.
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