When a Vendor's Own Software Says 'Pass' and CONVAL Says 'Retire

TL;DR: Yes, two valve-sizing tools can produce different verdicts on the same valve in the same duty. Vendor sizing software is typically designed to select and size valves from that manufacturer's own catalogue; it is commercially incentivised to return a "pass" for whatever it has to sell. An independent tool such as CONVAL 12 by F.I.R.S.T. GmbH applies the same calculation methodology regardless of who made the valve: IEC 60534-2-1 for flow capacity, IEC 60534-8-3 and 8-4 for noise, ISA-RP75.23 and the CONVAL Reliability Index for cavitation and choked-flow prediction. When a vendor tool and an independent tool disagree, the burden of investigation sits with the vendor, not with the engineer who ran the independent check.

Why Two Tools Can Reach Different Answers

Control valve sizing looks, from the outside, like a solved problem. You know your upstream pressure, downstream pressure, temperature, flow rate, and fluid properties. You run them through the equations in IEC 60534-2-1, and the standard tells you the required flow coefficient (Cv or Kv) and whether the valve is at risk of choked flow, cavitation, flashing, or unacceptable noise. The physics are not in dispute.

The disagreement arises in the layer between the physics and the tool.

A valve manufacturer's proprietary sizing software typically does several things at once. It calculates the required Cv for your duty. It selects a trim from the manufacturer's catalogue. It checks the selection against the same IEC 60534 equations. And then, critically, it presents the result in the most favourable light for the product being offered. That is not a conspiracy; it is engineering software written by people who work for a valve company. The goal is to help their sales engineers specify their products correctly.

The pressure to report a "pass" is structural. Reporting a borderline selection as a "fail" means proposing a larger, more complex, or more expensive valve. That is a harder sell, and the software is rarely designed to push that conversation automatically.

An independent tool has no catalogue to defend. CONVAL 12 carries a comprehensive database of third-party valve coefficients, but the calculation engine applies the same criteria to every entry. If a valve's published FL factor (the liquid pressure recovery factor, which governs how severely pressure drops below vapour pressure at the vena contracta) is too low for the duty, CONVAL flags the risk whether the valve comes from a major US manufacturer or a European niche supplier.

The Specific Parameters That Create the Gap

Understanding where the disagreement most often surfaces requires looking at the valve factors that matter most for reliability prediction.

FL: the liquid pressure recovery factor

FL is a dimensionless coefficient that describes how well pressure recovers downstream of the vena contracta, the point of minimum cross-section inside the valve where velocity is highest and local pressure is lowest. A high FL (close to 1.0) means that pressure recovers strongly; a low FL means the fluid spends longer at low pressure and is more likely to begin forming vapour bubbles.

IEC 60534-2-1 uses FL explicitly in the choked-flow condition for liquids. The maximum allowable pressure drop before choked flow occurs is proportional to FL squared:

ΔP_choked = FL² × (P₁ - 0.96 × Pv)

where P₁ is the upstream pressure and Pv is the fluid vapour pressure. A globe valve with FL around 0.90 gives a substantially different choked-flow threshold than a butterfly valve with FL around 0.60, even if both valves have an apparently adequate Cv for the rated flow.

Here is where vendor tools can diverge from independent calculations. A vendor's published FL for a given trim at a given opening may reflect a best-case or nominal test result rather than the lower bound across the operating range. CONVAL can model FL as a function of valve travel, so the calculation is not just correct at the rated duty point but across the full working range, including startup, minimum stable flow, and turndown.

xT: the critical pressure drop ratio for gases and steam

For compressible fluids, the equivalent parameter is xT (sometimes written x_T). It defines the critical ratio of pressure drop to upstream pressure at which flow chokes:

x_choked = Fk × xT

where Fk is the isentropic exponent ratio for the gas. Both xT and Fk are factors defined within IEC 60534 itself and applied inside the calculation, rather than values entered on screen. Once choked flow is reached, adding more pressure drop does not increase flow; it adds turbulence, noise, and mechanical stress. A valve selected purely on rated Cv for gas service may be operating in choked conditions at part-load, generating far higher noise levels than the vendor tool predicts if the tool uses a fixed or simplified xT.

IEC 60534-8-3 (aerodynamic noise for compressible fluids) and IEC 60534-8-4 (hydrodynamic noise for liquids) both depend on accurate xT and FL values, respectively, to produce meaningful noise predictions. Vendor tools that use proprietary rather than standard-derived noise algorithms can disagree with a CONVAL calculation by 5 to 15 dB(A) on the same duty. That is not a cosmetic difference: levels above approximately 100 dB(A) typically require source treatment (multi-stage trim, multi-stage restriction structures) rather than path treatment (lagging, thicker pipe walls), and the correct advice cannot emerge from a tool that underestimates the noise at source (F.I.R.S.T. GmbH / Vogt and Hinssen, 2016).

The Reliability Index: a verdict across the whole operating range

CONVAL introduces a Reliability Index (Ri), a single key performance indicator that aggregates the reliability-relevant parameters for every operating point across the full valve travel range. An Ri of 0 indicates no reliability concerns. Between 0.1 and 0.5 suggests possible reliability problems requiring review. Above 1 indicates a risk of mechanical damage.

The Ri accounts for pressure drop magnitude, energy dissipation (power loss at the valve), outlet velocity, noise level, and flow phenomena including cavitation, flashing, and choked flow, weighted and combined in an algorithm validated against hundreds of real plant cases documented by end users and valve manufacturers over more than two decades (Vogt, 2016; Hinssen, 2017).

Vendor sizing tools do not, in general, produce an equivalent rating. They may show a green-amber-red flag for some parameters, but those flags are typically assessed at the rated duty point only, not across the full working range. A valve that passes at rated flow but cavitates badly during low-load operation will show a green flag from a tool that only evaluates the single design case.

An Illustrative Scenario (not a documented real case)

The following is a simplified illustrative scenario to show how the divergence plays out in practice. Numbers are chosen to be physically plausible; they do not represent a specific documented project.

Consider a liquid let-down duty: cooling water at 25°C, inlet pressure 50 bar(a), outlet pressure 10 bar(a), maximum flow 120 m³/h through a 4-inch line. The vapour pressure of water at 25°C is approximately 0.032 bar(a).

A vendor's sizing tool selects a 4-inch globe valve with a rated Cv of 95 GPM (US), FL listed as 0.82 in the catalogue. The tool calculates the required Cv at maximum flow as approximately 72, gives a sizing ratio of 76% of rated, and reports the selection as acceptable.

Running the same duty in CONVAL against the IEC 60534-2-1 equations produces a comparable Cv requirement. But CONVAL also calculates the choked-flow limit:

ΔP_choked = 0.82² × (50 - 0.96 × 0.032) ≈ 33.6 bar

The actual pressure drop is 40 bar, which exceeds this threshold. The valve is operating in choked-flow conditions. CONVAL additionally checks the operating point against the xFz cavitation damage index and the outlet velocity against recommended limits, producing an Ri in the "possible reliability problems" band at maximum flow and a more severe Ri at turndown.

The vendor's tool did not surface this because it evaluated Cv adequacy only and used a simplified cavitation check that compared ΔP to a single published sigma value rather than tracking FL across travel. CONVAL's recommendation: increase the pressure drop in stages using a resistance structure downstream, or specify a valve class rated for cavitation service. The vendor's recommendation: the 4-inch globe valve is fine.

Both results followed directly from the data provided. The difference was entirely in what each tool chose to check, and how thoroughly it checked it.

Why This Matters More for Critical Valves Than for Standard Ones

Not every valve in a plant needs this level of scrutiny. The overwhelming majority of control valves in a typical facility handle moderate pressure drops, are in no danger of cavitation or choked flow, and will perform reliably from any reasonably competent sizing. For those valves, vendor tools and independent calculations will agree, because both are straightforward applications of a shared Cv equation.

The divergence matters for the three to six percent of valves in a typical process plant that operate in genuinely difficult conditions: high pressure drops, liquids close to their vapour pressure, compressible flow at or near choked conditions, severe noise, anti-surge service, or duties where multiple phenomena overlap. Experience from large capital projects shows that these "high performance" valves, though a small proportion by number, can account for up to 40% of the total valve purchase cost and are disproportionately represented in startup delays, unscheduled shutdowns, and early trim failures (Vogt, 2014; Vogt, 2016).

For those valves, getting an independent second opinion is not belt-and-braces over-engineering. It is prudent risk management. A six-figure cost consequence from a cavitating valve that needed a different trim is not recovered by the modest cost of an independent sizing check during the design phase.

What "Independent" Actually Means in This Context

It is worth being precise about the term. Independent does not mean that vendor-supplied data is ignored; FL, xT, and xFz factors still come from the manufacturer's published valve coefficients, and a good sizing exercise requires those numbers to be current and complete. Getting up-to-date coefficient data from manufacturers is, in practice, one of the genuine challenges in large project valve sizing (Vogt, 2016). Without accurate coefficients, no calculation, independent or otherwise, is reliable.

What independence means is that the calculation methodology is not modified to favour any particular product. The IEC 60534-2-1 sizing equations, the IEC 60534-8-3 and 8-4 noise predictions, the ISA-RP75.23 cavitation guidance, and the VDMA 24422 recommendations are applied consistently, regardless of which manufacturer's valve is being evaluated. CONVAL 12 carries formal documentation of every standard it uses and the edition it implements, updated with each software release (F.I.R.S.T. GmbH, 2023).

It also means that the result of the calculation is a Reliability Index, not a pass/fail against a threshold set by the same company that made the valve.

The Fair Point About Vendor Tools

None of this is an argument that vendor sizing tools are dishonest or poorly engineered. Manufacturers' own sizing tools are sophisticated pieces of software maintained by teams of experienced engineers. For the valves in their own catalogues, with the trim coefficients they know best, they often produce highly accurate results.

The structural limitation is simply this: a tool built by a manufacturer to size that manufacturer's products is, by design, optimised to find a product that fits the duty from within that catalogue. When the duty is borderline or genuinely difficult, the incentive is to find a way through rather than to raise a flag that might result in the customer going elsewhere. An independent tool has no equivalent incentive. Its only job is to tell you what the standards say about the valve in front of it.

For straightforward duties, both approaches converge on the same answer. For the critical minority of valves where they diverge, the independent answer is the one you want to know about before you commit to purchase, not during commissioning.

How DP-Flow Approaches This

DP-Flow offers access to CONVAL 12 by F.I.R.S.T. GmbH as the calculation backbone for control valve sizing and selection work across the UK and Ireland. CONVAL 12 implements IEC 60534-2-1 (2011 edition), IEC 60534-8-3 (2010), IEC 60534-8-4 (2015), ISA 75.01 (2012), ISA-RP75.23 (1995), and VDMA 24422 for control valve work, as well as a broad suite of standards for safety valves, orifice plates, thermowells, and pipe wall thicknesses.

If you are working with a control valve specification where the vendor has returned an acceptable sizing and you would like an independent check to IEC 60534, particularly for duties involving high pressure drops, liquid service close to vapour pressure, or compressible flow approaching choked conditions, contact the DP-Flow team.

References

Evans, B. and Ritter, R.L. (2015) 'Flashing and cavitation', Valve Magazine, Summer 2015, pp. 1-8. Available from the publisher.

F.I.R.S.T. GmbH (2023) Standards used in CONVAL 12. Wermelskirchen: F.I.R.S.T. Gesellschaft für technisch-wissenschaftliche Softwareanwendungen mbH. [Internal document supplied by F.I.R.S.T. GmbH / DP-Flow, November 2023.]

Hinssen, H. (2017) 'Catching unfit control valves', Valve World, April 2017, pp. 1-4. [Print publication, copy held by DP-Flow.]

Hinssen, H., Siemers, H. and Vogt, A. (2012) 'Key performance indicators for spotting the best fit for purpose control valve', Proceedings of the Texas A&M Instrumentation Symposium, 26 January 2012. [Conference paper, copy held by DP-Flow.]

IEC (2011) IEC 60534-2-1:2011 - Industrial-process control valves - Part 2-1: Flow capacity - Sizing equations for fluid flow under installed conditions. Edition 2.0. Geneva: International Electrotechnical Commission. Available at: https://webstore.iec.ch/en/publication/2461 (Accessed: 16 June 2026).

IEC (2010) IEC 60534-8-3:2010 - Industrial-process control valves - Part 8-3: Noise considerations - Control valve aerodynamic noise prediction method. Geneva: International Electrotechnical Commission.

IEC (2015) IEC 60534-8-4:2015 - Industrial-process control valves - Part 8-4: Noise considerations - Prediction of noise generated by hydrodynamic flow. Geneva: International Electrotechnical Commission.

Vogt, A. (2014) 'Reliable valve sizing in large-scale projects', Industrial Valves 2014/2015, pp. 43-46. [Print publication, copy held by DP-Flow.]

Vogt, A. (2016) 'Applying KPIs for valve reliability in projects with valve manufacturers, EPCs and end-users', Industrial Valves 2016/2017, pp. 37-40. [Print publication, copy held by DP-Flow.]