ASME MFC-8M pdf free download

ASME MFC-8M pdf free download FLUID FLOW IN CLOSED CONDUITS: CONNECTIONS FOR PRESSURE SIGNAL TRANSMISSIONS BETWEEN PRIMARY AND SECONDARY DEVICES
The detail design for the installation of the flowmetersecondary system should consider instrument trouble-shooting and calibration. Access to the impulse lines, thevalves，manifold and the secondary device is required.Installations providing this access must not compromisethe measurement accuracy by adding excessive pressuresensing lines and fittings. Longer and more complexpipingmay increase uncertainties and provide moreopportunity for plugging. Plugged lines lead to loss ofcontrol and may create hazardous situations.
Any difference in elevation between the primarydevice pressure taps and the secondary will result ina pressure difference between the two ends. If thefluids in the two lines are not identical in density，adifference in pressure is generated. Density differenceswill arise if there is a temperature difference betweenthe fluids in the two impulse lines. (See Appendix Bfor an example of a typical calculation.) It is recom-
mended that the two impulse lines are fastened togetherand,if insulated,they are insulated together. Non-
identical fuids in the two impulse lines can also giverise to density differences.
lt is also recommendled that，where allowable，thesecondary be“bled”or “vented”after installation toclear the impulse lines of fluids left during the construc-tion or after hydrostatic testing or system cleaning.
Bleed valves may be included in manifolds or in thesecondary device body，or installed as needed.
Periodic bleeding may be required if the characteristicsof the fluids in the impulse lines change over time
with fluid aging and with diffusion or leakage into orout of the impulse lines.
The general experience in industry is that dirt is
everywhere,and that liquids will have entrained orcondensed liquids. It is good practice to design theinstallation to allow for natural draining of liquids or
venting of gases.
3.10 Valve Arrangement
Where the primary device uses flange taps in thesmaller size pipes，it is likely that block valves andflanges will physically interfere with each other if theyare mounted directly in line with the primary devicepressure taps (see Fig.C2). In vertical lines,alternateflange taps can be used to avoid mechanical interference;but this practice is not encouraged. Vertical flow installa-tion of head type meters in vapor service is discouraged.
4 HORIZONTAL PIPING INSTALLATIONS
4.1 Gas Pressure taps on the primary element shall be on the horizontal centerline or up to the top of the pipe unless the measured fluid is a vapor which is intended to condense in the secondary system (see para. 4.3). Liquids or condensate must be free to flow down and out of the measurement system (see Fig. C3). The recommended slope for self draining is a minimum of 1 in 12.
4.2 Liquid Pressure taps shall be on the horizontal centerline. Taps below the centerline may accumulate solids; taps above the centerline will accumulate air or non-condens- able gases. In liquid service, the connecting lines from the primary device shall slope downward to the second- ary with no up turns or pockets (see Fig. C4). Gas bubbles must be free to flow up and out of the measurement system. The recommended slope for self venting is a minimum of 1 in 12.
4.3 Condensing Vapor The pressure taps shall be on the horizontal centerline of the primary device. In condensing hot vapor service, such as steam, the fluid in the impulse lines is liquid condensed from the vapor. Follow the arrangement requirements for liquids with the secondary device below the primary (see Fig. C4). Cryogenic (very low temperatures) systems may require special designs. The liquid in the lines will isolate the secondary device from the temperatures of the primary flowing fluid. The temperature difference may be considerable over a short distance of 100 mm to 200 mm (4 in. to 8 in.). There is a concern that at startup the secondary device could be exposed to the vapor temperature before the lines fill with condensate and cool. A plugged tee fitting in the impulse line will permit filling the impulse tubing and secondary with water (for steam service) before startup (see Fig. C5). Where permitted, this problem may be mitigated by a careful commissioning procedure slowly filling the system and allowing sufficient time for pressure trans- mitting lines to condense vapors.