Low-flow (micro-flow) pneumatic flow control valve for small dosing and pilot-plant lines

01

Kvs steps from 0.0001 to 1.6 and the seat each one needs

SAMSON designed its Type 3510 for controlling low flow rates in pilot plants and technical research facilities. Its trims climb in standard steps from Kvs 0.00010 through 0.00016, 0.00025 and so on to 1.6, on only four seat sizes: [1]

All four groups use a 4 mm plug stem and 7.5 mm travel. Because of the two seat threads, trims can be swapped within Kvs 0.0001 to 0.4 or within 0.63 to 1.6, but not across that boundary. [1] Editorial note: if a pilot plant may later run at five or ten times its design flow, choose a body whose seat thread accepts both the start-up and the larger trim.

BCST's ZJHY small-flow valve covers the upper end of the same territory: seat diameters of 1.5 to 15 mm with rated Kv from 0.025 to 2.23, in G3/8, 1/2, G3/4 and G1 bodies, with a single-seat plunger plug in 304, 316 or 316L and linear or equal-percentage characteristics. [4]

Editorial calculation: Kv is the water flow in m³/h at 1 bar pressure drop, and Cv ≈ 1.156 × Kv. SAMSON's smallest trim, Kvs 0.0001, therefore passes about 0.1 L/h of water at 1 bar drop, under 2 mL per minute, and its 1.6 trim about 1.85 Cv.

Give the supplier the medium and its state, minimum, normal and maximum flow, inlet and outlet pressure at each, temperature, required characteristic, body connection and pressure rating, bonnet type (standard, extension, bellows), fail position, positioner signal, and any filter already upstream.

02

Below Kv 0.1 the turndown shrinks and dirt becomes the failure

SAMSON rates the Type 3510 at 50:1 rangeability, but less than 50:1 when Kvs is below 0.1 and less than 15:1 for the 0.0001–0.0063 trims, which are offered as linear or on/off trims only. [1]

Editorial calculation: at 50:1 a Kvs 0.4 trim controls down to Kv 0.008; a Kvs 0.0063 trim at the stated limit of below 15:1 controls no lower than about Kv 0.0004. When the minimum and maximum flows of a process are more than about 15× apart in the smallest range, plan for a trim change or a second valve rather than one micro-trim.

The small gaps that give these valves their resolution also trap particles. BCST states that when an unclean medium is regulated, blockage of the small throttling clearance has to be taken care of. [4] Badger Meter's Research Control Valve manual advises small micron filters, where the process permits, for valves with its Low Flow "P" series inner valves, and warns that over-torquing the seat on the smaller letter or "P" trims can shrink the orifice until plug and seat gall when the valve strokes. [3]

Badger Meter sets its standard air-to-open valves to lift off the seat at about 3.25 psig instrument signal and open fully at 15 psig, with 90 psig air at the inlet; process pressure can unbalance that and need the spring adjuster reset. [3]

03

PN 400 bodies, cryogenic to 450 °C bonnets and the leakage of a lapped micro-seat

Small bores let these valves reach pressure ratings far above their size class. The SAMSON Type 3510 is built from PN 40 to PN 400 in stainless steel, −10 to +220 °C as standard and −196 to +450 °C with an insulating section; a metal bellows seal up to PN 250 or PN 325 has an external tightness of ≤10⁻⁵ mbar·l/s. [1]

The two Badger Meter documents state different leakage figures, Class IV on the product page for size 0 and larger trims and Class III in the manual for installed replacement trims; the manual adds that bubble-tight shut-off can be reached by lapping, but not on any "P" series trim. [2] [3] Ask the supplier which class applies to the exact trim ordered, with the test.

Badger Meter lists the Type 807 in ¼ to 1 in line sizes with linear, equal-percentage, quick-opening or double-taper trims and states flow range as 0–6 Cv, and its manual says spare trims come in 39 Cv sizes. [2] [3] Editorial calculation: 6 Cv ÷ 1.156 = 5.2 Kv; the same product page also prints the range as 0–85.8 Kv, which does not match that conversion, so confirm the metric figure with the maker.

04

Send flows at each operating point, not a pipe size

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Medium: [liquid / gas / steam; cleanliness; filter fitted]
Flow: [minimum; normal; maximum]
Pressures: [p1 and p2 at each flow]
Temperature: [range]
Trim: [Kvs or Cv; characteristic; seat material]
Body: [connection; size; PN or class; material]
Bonnet: [standard; insulating section; bellows]
Actuator: [fail position; signal; positioner]
Shut-off: [leakage class and test]

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