Flow Nozzle Flow Meter

A flow nozzle is a differential-pressure primary element with a smooth rounded inlet that contracts the flow into a short cylindrical throat. The constriction generates a square-law pressure drop that an external DP transmitter reads and square-root-extracts to volumetric or mass flow per ISO 5167-3 / GB/T 2624. You pick the nozzle over an orifice plate when the service erodes the orifice sharp edge within months; high-velocity gas, superheated steam, abrasive dust in flue gas, and power-station main-steam service are the classic cases. The rounded entry has no sharp edge to round off, so discharge coefficient drift is much slower over the install life. Mid-cost between orifice and venturi, permanent pressure loss roughly 30 to 40 percent of measured ΔP. The page covers DN50 to DN1200 on PN0.6 to PN42 MPa, −200 to +800 °C, ±0.5 % of flow per ISO 5167 wet-cal, repeatability ±0.1 %.

Both share the rounded-contraction principle and both resist erosion, but the venturi adds a long divergent recovery cone after the throat that recovers most of the kinetic-energy ΔP. The numbers are clean: classical venturi permanent loss is roughly 10 to 15 percent of measured ΔP, flow nozzle is roughly 30 to 40 percent, square-edge orifice is 50 to 70 percent. Pick the venturi when permanent pressure loss is the controlling cost; head-sensitive low-NPSH pumped lines, large-bore cooling-water headers on tight pump curves, mine dewatering. Pick the nozzle when erosion resistance matters but the head budget can absorb 30 to 40 percent; most superheated-steam mains, high-velocity gas headers, and power-station main-steam custody where the steam header is already at high ΔP and the recovery cone is not worth the cost or length. The nozzle is also shorter, cheaper, and ships faster than a fabricated classical venturi at the same DN.

Five out-routes. First, slurry, particulate-laden flow, or wet-gas with entrained liquid carryover plugs the impulse lines and shifts the discharge coefficient; go to electromagnetic for conductive slurry or to Coriolis for two-phase tolerance. Second, head-sensitive lines where the 30 to 40 percent permanent loss exceeds the pump head budget; go to a classical venturi (10 to 15 percent loss) or to an averaging pitot tube (under 5 percent loss). Third, very-high turndown above 1:10; DP physics caps the dynamic range; go to Coriolis (1:200), vortex (1:70), or ultrasonic (1:100). Fourth, very-large-bore service above DN1200; the nozzle range tops out at DN1200; the averaging pitot tube scales to DN9000 for power-station cooling-water headers and stack/duct gas. Fifth, high-viscosity liquid above 50 cSt; the discharge coefficient is Reynolds-sensitive and Reynolds correction grows; go to Coriolis or to oval-gear positive-displacement.

Lead time is 5 to 7 business days from the factory. The kit ships with the primary element (long-radius or ISA-1932 profile per quote), flanges or weld-prep ends per the connection code, an ISO 9001 wet-flow factory calibration certificate with discharge coefficient and beta ratio, and a sizing record traceable to the buyer-supplied operating P / T / fluid composition. The DP transmitter ships separately or as a pre-mated integrated build (go to the integrated DP element for factory zero/span). Install requires the longest upstream straight-pipe run on the site; typically 22 pipe diameters upstream of a single elbow at β = 0.5, scaling to 50 pipe diameters for two out-of-plane elbows. Five pipe diameters downstream is sufficient. Mount horizontally with impulse lines sloped downward for liquid service, upward for gas; freeze-trace impulse lines in cold ambient. After install, zero the DP transmitter at full pressure with flow stopped and the equalizing valve open.