ASME MFC-18M pdf free download
ASME MFC-18M-2001 pdf free download.MEASUREMENT OF FLUID FLOW USING VARIABLE AREA METERS.
7.5 Upper Body The upper body supports the top or outlet of the metering tube. It usually includes a packing or sealing device. It also provides the support for the flow outlet process conection. In the simplest meters, these func- tions are all a part of the top of the meter body.
7.6 Lower Body The lower body is at the bottom or inlet of the flow tube. It is similar in function and design to the upper body.
7.7 Process Connection The process connections are used to install the meter to the associated piping system. Standard connections include standard inch and millimeter piping threads and flanges.
7.8 Accessories Accessories include switches controlled by the float position; signal-transmiting devices, check valves to prevent reverse flow, needle valves to control flow, and constant differential relays to stabilize flow.
8 UNCERTAINTY In most catalog and technical literature, the uncer- tainty is given as a percent of full scale flow and is defined only between 10% and 100% of scale. The variable area meter is not sensitive to the pipe arrange- ment or the flow profile entering the meter. Uncertainties can be minimized with careful application and knowl- edgeable use. If the sizing is based on poorly defined or varying fAuid properties and operating conditions, then the accuracy ill be compromised. Poor installa- tions with high vibration or excessively non-vertical alignment will reduce accuracy. Calibration can reduce the uncertainty depending on the quality of the calibra- tion and care in meter transport, installation, and use. Effective accuracy is also a function of the instrument scale and indicator design. An unstable float position will interfere with accurate readings.
9 CLASSES Variable area flowmeters are of three general classes: purge or miniature meters, laboratory meters, and pro- cess flowmeters. This grouping scheme is only very general in nature.
9.1 Purge meter Purge meters (see Fig. 4) are small and typically have %4 NPT (6 mm) or smaller connections. Because the applications do not justify it, calibration is unlikely. Catalog claims of 2% repeatability and an uncertainty of 5% of flow rate may not always be realized in practice. Tubes vary considerably in design but are often between 50 mm and 100 mm (2 in. and 4 in.) in length.
9.2 Laboratory Meters The laboratory meters are usually longer [300 mm to 600 mm (12 in. to 24 in.)], have longer scales, and include more graduations than other meters of the same connection size and capacity. Repeatability is advertised as /2% and a standard accuracy of 1% is promised, which may be improved to /% with calibration.
9.3 Process Meter Process meters with 么in. (13 mm), or smaller connections typically have standard calibration uncer- tainties of 2%. Meters larger than % in. can often have certifed uncertainty of 1% at the specified conditions if they are calibrated. Tubes are typically between 150 mm and 250 mm long. 10 SAFETY Many users limit or prohibit the use of glass tubes n hazardous fuids in industrial service. Shields can be purchased with most glass tube meters. The user must determine if the level of protection provided by these shields is adequate for the application. The shield typically can deflect broken glass and flowing fluid, but is not designed to contain the fuid at the maximum pressure rating of the tube. Glass is brittle and damage to the tube can seriously weaken the tube. Ratings listed in the manufacturer’s catalog are for new and undamaged meters. Metal tube meters are available for services where the brittle nature of glass is a cause for concern.ASME MFC-18M pdf download.
