wbd-rd.nl · R&D lab · supplier information
An example table pins us to your chosen pressure and temperature. A resistance coefficient ζ per stroke lets us regenerate this entire figure at any condition — move the sliders above: that recalculation is exactly what we must be able to do ourselves, for every valve in a comparison, on identical terms.
A bench curve is the inherent characteristic: flow against stroke at a constant pressure loss across the valve. In a plant that pressure loss is not constant. As the valve opens, flow rises and the rest of the system — pipework, diffusers, the header controller — takes a larger share of the available pressure, so the valve keeps less of it. The same valve therefore shows an installed characteristic flatter than its bench curve: a strongly equal-percentage valve can end up close to linear. The two are linked by valve authority a, the share of the flow-dependent pressure loss sitting in the valve at that duty:
the link
| d(ln Q)/d(stroke) = a × d(ln Kv)/d(stroke) | the right-hand factor is the valve's, the authority is the system's |
That split is the reason we ask for ζ and Kv instead of a flow table, and the reason we compute installed behaviour ourselves against our own measured duty: your dataset stays site-independent, and every supplier is compared on identical terms. Send the inherent curve — the installed one is ours to calculate.
From the same ζ/Kv table we derive the figures below for every submission — nothing to fill in, and nothing we ask you to declare. They are dimensionless, so a DN125 and a DN500 can be judged against each other: nominal size is a design variable, not a category. Everything is read at 90 % stroke, because a control valve is not sized to sit at full travel, and because close to 100 % most constructions stop gaining area — the last two rows say whether yours does, straight from your own numbers. You can watch them being computed in the submission form.
| metric | definition | what it tells us |
|---|---|---|
| r | d(ln Kv)/d(stroke) at 90 % stroke, % per % stroke | the exchange rate between dosing resolution and throttling loss |
| stroke for a 10 % capacity change | ln(1.1) / r | how much travel a 10 % flow correction costs |
| capacity step per 0.5 % of stroke | 0.5 × r | what one positioner step does at the reference stroke |
| controllable stroke range | strokes where 0.5 % of stroke moves capacity ≤ 5 % | the travel that is actually usable for control |
| capacity range covered | Kv ratio across that range | the turndown reachable inside the controllable travel |
| gain uniformity | rmax / rmin across that range | 1 is perfectly uniform; a large value means the loop is retuned by the duty |
| capacity reached at 90 % | Kv(90 %) / Kv(full travel) | what the valve still has in reserve above the sizing point |
| tail gain ratio | last stroke step's ΔKv ÷ the largest step's ΔKv | 1 = the valve keeps gaining area to the end; well below 1 = the final travel is geometry, not capacity, and reading anything at 100 % stroke is misleading |
None of these score a valve on its own. A low r is fine resolution; a high r buys range in less travel. What decides the choice is where those numbers land at the duty we actually have, together with the pressure the valve costs there — and that calculation is ours to make.
Tier 1 · required
Resistance coefficient at ≥ 10 points from 10 % to 100 % stroke, with the reference area named (nominal bore or seat area).
Kv in m³/h at the same stroke points. Deliberately redundant with ζ — the redundancy is our cross-check.
DN / flow area, face-to-face length, and any installation assumptions (upstream straight length, orientation).
Flow versus pressure loss per stroke at fully stated conditions: medium, absolute pressure, temperature, humidity, and the Nm³ reference basis — or simply mass flow in kg/h, which needs no basis at all.
Which sizing relation produced the example — IEC 60534-2-1, a ζ-based relation, or a house formula written out. Named, not implied.
The uncertainty band on ζ/Kv (± %). A coefficient without a tolerance cannot carry a guarantee.
Who produced this dataset and from what: a supplier submission, our own digitisation of your published sheet, or a third-party source — plus the document it traces back to. A number without a origin cannot be defended three years later.
Confirm the curve is the inherent one, measured at constant Δp. An installed curve belongs to somebody else's pipework and cannot be recomputed for our site.
Tier 2 · welcome, not blocking
Step 1 — pick the medium class. The coefficients are the same either way (ζ is dimensionless); what changes is which conventions are mandatory.
Step 2 — fill the template below (it is pre-filled with the fictional DN200 valve so the checker demonstrably passes; replace every value with your own). Flows in kg/h are always unambiguous; Nm³/h is accepted only together with the declared basis. Step 3 — run the self-check: it is the same set of checks we run on receipt, so anything red here becomes a clarification round later. Step 4 — return the JSON with your quotation, through the inquiry that pointed you to this page.
Before a valve enters evaluation we rebuild your worked example from your own coefficients and conventions. The example is the checksum: if we cannot reproduce it, we cannot reproduce anything else either.
Three consistency checks sit inside that recomputation:
These are the ambiguities we most often have to write back about. Stating them costs one line each; leaving them out changes results by more than typical differences between competing valves.
| convention | please state | effect when unstated |
|---|---|---|
| Nm³ reference basis | 0 °C / 1.01325 bar, or 20 °C, or other | ≈ 7 % on density and flow |
| ζ reference area | nominal bore or seat area | up to several % near full stroke |
| pressure basis of the example | absolute upstream pressure | Qₙ ∝ √p — 1.49 vs 1.7 bar(a) ≈ 6.8 % |
| gas-sizing formula variant | the exact relation and constants | 1 – 3 % between common variants |
| humidity basis | dry air or a stated RH | up to ≈ 1 % on density |