DESIGN & SAFETY HANDBOOK|
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PRESSURE REGULATORS: SELECTION/OPERATION
|The safest means to reduce cylinder pressure to a workable level for operating equipment and instruments is through a pressure reduction regulator. Scott offers over 40 regulator series with more than 120 different pressure ranges – all are intended for specific applications. Most gases listed on this website include Scott’s recommendation for pressure regulators that provide best service.|
Single-Stage vs Two-Stage - There are two basic types of regulators. Duration of gas usage helps to identify whether a single-stage or two-stage regulator provides the best service. A single-stage regulator is a good performer for short duration gas usage. It reduces the cylinder pressure to the delivery or outlet pressure in one step. This type of regulator is recommended when precise control of the delivery pressure is not required because delivery pressure variations will occur with decreasing cylinder pressure. A two-stage regulator provides better performance for long duration gas usage. It reduces the cylinder pressure to a working level in two steps. The cylinder pressure is reduced by the first stage to a preset intermediate level, which is then fed to the inlet of the second stage. Since the inlet pressure to the second stage is so regulated, the delivery pressure (manually set by means of the adjusting handle) is unaffected by changes in the cylinder pressure. Thus, the two-stage pressure regulators provide precise control of the gas being consumed. A two-stage regulator performs best when it is attached to the cylinder and adjusted to the desired reduced pressure, and then remains in service until the cylinder is ready for changeout.
Materials of Construction - A regulator must be constructed with materials compatible with the intended gas service and application. When selecting your regulator, you should first consider the wetted materials (those that will come in contact with the gas). Typical materials used for regulator construction are:
The ideal construction for high-purity gas service is a regulator that has a stainless steel diaphragm. They are noncontaminating and assure satisfactory use for all applications of noncorrosive and mildly corrosive gases.
|1. Noncorrosive:||Aluminum, Brass, Stainless Steel, Buna-N, PCTFE, Neoprene, TeflonŽ, VitonŽ, Nylon.|
|2. Corrosive:|| Aluminum, Stainless Steel, MonelŽ, Nickel, PCTFE, Teflon|
|For general use, brass regulators with Buna-N or Neoprene diaphragms will give good service in noncorrosive applications where slight contamination or diffusion from an elastomeric diaphragm is not important. Both Buna-N or Neoprene are permeable to oxygen. Therefore, regulators with these types of diaphragms are not suitable for GC analysis that can be affected by the diffusion of atmospheric oxygen through the elastomer diaphragm, or the outgassing of monomers and dimers from the elastomer. In fact, labs that perform temperature programmed analysis are faced with excessive baseline drift and large unresolved peaks due to this diffusion and outgassing.|
Brass regulators with stainless steel diaphragms have several advantages over the elastomeric type. Firstly, they prevent air diffusion and adsorption of gases on the diaphragm. This is important with low concentration mixtures of hydrocarbons where the trace components may be adsorbed on the elastomeric diaphragm. Secondly, these regulators do not outgas organic materials and prevent the diffusion of atmospheric oxygen in the carrier gas. The chemical potential of oxygen between the carrier gas and the atmosphere provides sufficient driving force for oxygen to intrude the carrier gas through a permeable diaphragm. Stainless steel diaphragms prevent this scenario from happening.
The two most important parameters to consider during regulator selection and operation are droop and supply pressure effect. Droop is the difference in delivery pressure between zero flow conditions and the regulator’s maximum flow capacity. Supply pressure effect is the variation in delivery pressure as supply pressure decreases while the cylinder empties. Single-stage and two-stage regulators have different droop characteristics and respond differently to changing supply pressure. The single-stage regulator shows little droop with varying flow rates but a relatively large supply pressure effect. Conversely, the two-stage regulator shows a steeper slope in droop but only small supply pressure effects.
- Droop: Regulator performance is characterized by droop – the change in delivery pressure as flow is initiated and increased through the regulator.
- Supply Pressure Effect: This effect is the change in delivery pressure as the inlet pressure changes. For most regulators, a decrease in inlet pressure causes the delivery pressure to increase.
- Repeatability: Refers to the change in delivery pressure after pressure has been set by turning gas flow on and off using an external valve.
- Delivery Pressure Creep: There are two types of creep. The first type is normal as a result of internal spring forces equalizing when the flow stops. The second type of creep is a result of contamination that, when left unchecked, can lead to regulator and/or supply line failure.
The effect of these differences on performance can be illustrated with some examples. For instance, when a centralized gas delivery system is supplying a number of different chromatographs, flow rates are apt to be fairly constant. Supply pressure variations, however, may be abrupt, especially when automatic changeover manifolds are used. In this scenario, a two-stage regulator with a narrow accuracy envelope (supply pressure effect) and a relatively steep droop should be used to avoid a baseline shift on the chromatographs. On the other hand, if gas is being used for a short-duration instrument calibration, a singlestage regulator with a wide accuracy envelope (supply pressure effect) but a comparatively flat droop should be chosen. This will eliminate the need to allow the gas to flow at a constant rate before the calibration can be done.
|Delivery Pressure Range - Determining an appropriate delivery pressure range for a regulator can be confusing but can be accomplished by following these steps:
Relieving/Non-Relieving - A relieving regulator has a hole in the center of the diaphragm. As long as the diaphragm is in contact with the poppet, the regulator does not relieve. When the pressure under the diaphragm increases as a result of back pressure from downstream, the diaphragm will rise, allowing the pressure to relieve through the opening in the diaphragm. While the internal gas is relieving through this opening, the surrounding atmosphere (i.e. air) is diffusing into the gas stream. Oxygen (a component of air) is a harmful contaminant, especially when a gas stream is intended to be oxygen-free. It is well documented that oxygen affects gas chromatographic results. Relieving regulators should not be used in specialty gas applications.Accuracy Envelopes for Single and Two-Stage Regulators at Two Supply Pressures - The envelopes are bounded by inlet pressure curves of 2000 psig (138 bar) and 500 psig (35 bar). Each regulator was set to the shown delivery pressure with 2000 psig (138 bar) inlet pressure and zero flow. Once set, this delivery pressure was not manually changed during the evaluation. The above curves generated are the result of increasing flow through the regulator to its capacity, decreasing the flow rate through the regulator to zero.
- Determine the gas pressure needed.
- Determine the maximum pressure the system might require (this pressure and the gas pressure are often the same)
- Select a delivery pressure range so that the required pressures are in the 25% to 90% range of the regulator’s delivery pressure (a regulator’s performance is at its best within this range).
Linked Poppet/Tied Diaphragm - The poppet and diaphragm are mechanically linked. An increase in pressure in the cavity below the diaphragm will cause the diaphragm to move upward, pulling the poppet to improve its seal against the seat. A tied diaphragm regulator is effective in corrosive gas service, especially in the event that corrosive particles form under the poppet or on the seat. Tied diaphragm or linked poppet are terms used by manufacturers to describe this regulator feature.
Gauges - Generally single and two-stage regulators are equipped with two gauges - a cylinder or inlet pressure gauge and a delivery or outlet pressure gauge. The cylinder pressure gauge has the higher pressure range and is located adjacent to the inlet port. The delivery pressure gauge of the lower pressure range is located adjacent to the outlet port. The actual pressure gauge range is usually greater than the pressure range for which the regulator is rated. For example, a regulator that has a delivery pressure range of 1-50 psig (0.1-3 bar) will typically be supplied with a 0-60 psig (0-4 bar) delivery pressure gauge. This ensures that the rise in delivery pressure as a result of the regulator’s supply pressure effect will not exceed the gauge pressure range. Not all cylinder regulators have two gauges. A line regulator is typically provided with a single gauge that monitors the outlet pressure or reduced pressure. This gauge is usually situated in the 12 o’clock position. Regulators designed for liquefied gases may not have a cylinder pressure gauge because the cylinder pressure varies only with temperature as long as liquid is present in the cylinder.
Regulator Placement - Specialty gas regulator applications are divided into two types. The first is when the regulator is fastened to a gas cylinder using a CGA, DIN or BS fitting. The second application is when a regulator is located in a gas line, providing a means to further reduce the line pressure. A line regulator is identified by having the inlet and outlet opposite of each other and by a single gauge as discussed above.