API RP 551:1993 pdf download

API RP 551:1993 pdf download.Process Measurement lnstrumentation.
2.3.1.1.6 Pitot Tubes and Pitot Venturis
Pitot tubes and pilot venturis are used where minimum pressure drop is required and accuracy is not of prime concern. Advantages of pitot tubes and pitot venturis include very low cost, availability of averaging types, use of one type of transmitter regardless of pipe size, and ability to be added on stream with a hot tap. Limitations of pilot tubes and pitot venturis include their requirement for a low-range differential transmitter and their dependence on flow profile for accuracy. A pitot venturi requires a larger tap size, and installation requires special attention to clearances. A straight run of upstream and downstream piping is required for a pitot tube or pitot venturi (see Figure 2).
2.3.1.2 Differential Measuring Devices
2.3.1.2.1 General
Several types of measuring devices are used to determine the differential produced by the primary element. Flow is proportional to the square root of the differential; therefore. to maintain accuracy at low flow readings, a range greater than 3: 1 is not recommended. Multiple transmitters or microprocessor-based transmitters may be used to increase range. To calibrate the flow measuring or differential device, a manometer or precision test gauge should be used to read the differential input. The calibration devices should be graduated in the same units as the meter range (for example. inches of water). Pneumatic outputs may be read on the same type of device. Electronic devices require a precision voltmeter or ammeter. Total flow may be obtained by manually integrating the flow chart with a plan imeter or by equipping the meter with an integrator. Corrections must be applied for changes in the condition of the flowing stream for deviation from original flow calculations. The most commonly used differential-pressure measurement devices are described in 2.3.1.2.2 and 2.3.1.2.3.
2.3.1.2.2 Diaphragm Transmitters
Differential-pressure transmitters of the diaphragm type are extensively used in refinery units. To provide overrange protection and dampening, the body or capsule of the transmitter is filled with liquid. The transmission signal may be either pneumatic or electronic. Because of their low displacement. these instruments can generally be used without a seal or condensate pot. Line mounting is preferred if the location is accessible and has minimum vibration. Gas meters are mounted above the line to allow any condensate to drain back. Liquid meters are mounted below the line to prevent gas or vapor from being trapped in the sensing lines, which could cause errors from unequal static heads.
2.3.1.2.3 Bellows Meters
In a bellows meter, the bellows is opposed by a calibrated spring system and is filled to prevent rupturing when the bellows is overpressured and to provide pulsation damping. Bellows meters can he line mounted or remotely mounted at grade or on platforms with adequate support. Seal chambers or condensate pots may be used; however, a 4-inch (20-millimeter) tee has sufficient volume to act as a liquid seal or condensate pot in steam or condensable-vapor service for instruments that displace less than I cubic inch (16.4 cubic centimeters) with full-scale deviation. If the displacement is greater than I cubic inch (16.4 cubic centimeters) or if the differential of the instrument is low compared with the column displacement, regular condensate pots should be used.
2.3.1.3 Installation
2.3.1.3.1 General
Installation of differential-pressure flow devices is generally the same regardless of the type of primary element and requires consideration of the factors described in 2.3.1.3.2 through 2.3.1.3.4.