Six types of internal measuring elements (Figure 500-23) are in general use. All PD meters…
PD Meter Design
• Service—The type of liquid(s) and the nature and approximate amount of abrasives and corrosives must be specified. The application (process, custody transfer, pipeline, or load rack) and service (continuous or intermittent) must be known. Use the space at the bottom of the form as needed.
• Temperature—Specify normal and maximum temperatures (in degrees F or C). These temperatures have direct impact on the range of viscosities at which the meter will be operating. Temperature and fluid type also affect selection of materials for the meter.
• Size—The capacity of a PD meter may vary by manufacturer and type. Obtain information from the manufacturer’s technical specification. Consider multiplemeter configuration for cost effectiveness and operating flexibility. It is usually better to over-range than to under-range a PD meter if a choice is necessary.
• Temperature and Pressure Rating—Specify normal and maximum temperatures and pressures so that proper material for and construction of the meter can be evaluated. Consider pressure drops across the strainer, deareator, meter, and other piping components to determine back pressure control.
• Enclosure Class—In most cases, housings for electrical outputs are required to be explosionproof. As a minimum, the housings should be suitable for NEC Class I, Group D, Division 2 areas and should meet the specifications of NEC Article 500, Hazardous (Classified) Locations.
• Materials of Construction—Consult the manufacturer concerning metallurgy. Smaller meters (e.g., under 4 inches) usually have iron (ductile or cast) or carbon steel housing. The larger sizes (6 inches and up) are usually carbon steel. A wide variety of trim materials is available for different types of internals. Trims are typically categorized as “standard trim,” “all iron trim,” or “LPG trim.” They are made of aluminum, bronze, iron, steel, stainless steel, or alloys. Seals are usually Viton or Teflon. Consult the manufacturer for recommended materials.
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