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There are several factors that must be considered to ensure the proper operation of a restrictor within a system. If these factors are not addressed, it can lead to reduced restrictor or system performance, damage to other components within the system, or to a total system failure. The following performance characteristics should be defined prior to the selection of a flow restrictor:
There are four pressure ratings that should be considered for any restrictor:
These four pressure ratings must be considered during design to ensure the restrictor is sufficiently durable for the application. Conventional hydrostatic and burst pressure testing applies equal pressure to all parts of the metering element of the restrictor, and it is therefore unaffected. Thus, maximum hydrostatic pressure is limited only by the strength of the restrictor housing.
A precision orifice must provide an optimal flow rate of fluid to an appropriate location. The target flow rate should be specified, and the following conditions identified: type of fluid, upstream pressure, downstream pressure, and temperature. This ensures the manufacturer and user have the same points of reference. A system without the proper flow rate may result in reduced system efficiency, damage to the system, or damage to other components.
It is also necessary to note the metered flow direction. Some flow restrictors flow equally in both directions, while others
are only intended for and tested to flow at a certain level in one direction.
It is important to understand the pressures at the inlet (directly upstream) and outlet (directly downstream) of the flow restrictor. As the differential pressure between the inlet and outlet of the orifice increases (or decreases), the flow rate through the flow restrictor will increase (or decrease). Too great a pressure differential may result in excess flow, while too little may result in too much pressure downstream. Performance requirements must take into account all potential pressure variations. In a closed system, the pressure may remain relatively constant. In an open system that consumes fluid, such as a fuel system, pressure may decrease over time. The flow restrictor needs to withstand and perform appropriately over the full range of pressures for the life of the system.
In a mass production environment, there will inevitably be some variation from orifice to orifice. A critical system requirement is an understanding of the way production tolerances of individual orifices influence the total tolerance.
The smaller an orifice, the more susceptible it becomes to clogging due to debris introduced during handling, manufacturing, or operation. It is important to know the minimum passage size of the metering device and whether any integral protection is required.
A restrictor may be comprised of several sub-components. The materials of each component and anything used to join the components—for example welds and brazes—must withstand the various forces applied during the restrictor’s operating life. This includes the pressures applied internally and externally, along with the associated pressure rise rates.
Materials must also be compatible with their environment, including external fluids, temperatures, and the system fluid that will flow through the restrictor. It is possible that a flow restrictor may be subject to extreme humidity or be incorporated in a system submerged in other liquids or gases. Failure to consider material compatibility may also create issues related to thermal expansion and corrosion.
Criteria for selecting a restrictor must include the envelope. A prime consideration is the location of the restrictor within the system and the desired flow path. The system may require that the restrictor be located within a specific area, limiting external dimensions or overall size. The envelope must also account for installation, retention, and maintenance requirements. For example, some envelopes incorporate threaded fitting ends, while others are installed into manifold housings. It is also important to determine whether the installation must be permanent or removable. Finally, there’s a need to evaluate if the restrictor may be used in a system in which weight is a factor, such as a portable system.
Once the required performance characteristics of a flow restrictor are determined, it is critical to identify other variables that will influence the restrictor’s performance. The internal and external environment of the system will affect the performance in a variety of ways—impacting every aspect of its functionality and limiting options for its construction. The following aspects of the system and environment must be considered for the design of a flow restrictor:
The performance of a flow restrictor is greatly affected by the operating fluid’s viscosity and specific gravity. Liquids and gases have different fluid properties that impact flow performance. The operating fluid also introduces other variables that must be considered, such as material compatibility. A fluid that is incompatible with the restrictor’s materials or coatings could cause damage to the valve, including corrosion or other harmful effects. Such damage will negatively affect the restrictor’s performance, and subsequently may harm the system. It is also possible that the restrictor’s materials may alter the fluid’s properties, negatively affecting system performance. For example, a system analyzing blood or chemicals must use components that are inert to the fluid being analyzed. Similarly, a system flowing a flammable gas may need to avoid
metals that may spark when making contact.
The fluid’s temperature and the ambient temperature can impact a restrictor’s performance. Changes to fluid temperature will alter its properties, including viscosity and specific gravity. Liquids will thicken and increase in viscosity with decreases in temperature and will impede fluid flow. Conversely, increased temperatures will cause the fluid to thin and lower its viscosity. These factors can influence the calibrated flow rate of a restrictor.
It is important to consider how long the flow restrictor must withstand the conditions to which it will be exposed during operation. This includes the maximum length of time the device will be in service and the number of impulses it must survive. A flow restrictor may experience wear or erosion due to exposure to its environment. Excessive erosion over an extended operating life could lead to performance or installation and retention issues.
A restrictor’s flow performance is typically based on changes in pressure and temperature within the system for which it was designed. However, the restrictor’s envelope may be subject to other environmental pressures, such as the high pressure found deep underwater or the vacuum of outer space. A restrictor must be capable of withstanding such external pressures.
A restrictor may have trace amounts of fluids, debris, or dust on or within the device. Contamination may occur during the manufacturing, transportation, or storage processes. If the end use cannot tolerate contamination, the restrictor may need to go through special cleaning and packaging procedures. An example of a low-tolerance application is a restrictor used in an oxygen system that provides breathable air to a person. Some industries, such as space and medical, have defined cleanliness levels that specify these requirements for components and systems.
Flow restrictors can be subject to forces external to the system in which they are installed, and to forces generated by the system’s operation; typically, they are vibration, shock, and g-forces. For example, some systems or vehicles that use flow restrictors generate levels of vibration during normal operation; a shock may occur if the system or vehicle in which the flow restrictor is installed suddenly encounters another object; or the system or vehicle may generate g-forces during operation due to a sudden forceful movement. The magnitude, frequency, and direction of the potential forces must be considered.
A flow restrictor typically does not include moving components; it can, however, be constructed of multiple components. Both the construction of the flow restrictor and its method of retention within the system must be able to perform as intended when subjected to these forces. An adjustable flow restrictor is more susceptible to shock and vibration due to the incorporation of moving components.
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Always verify flow calculations by experiment.
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