Control Valve Leakage Class IV/V/VI: Four Criteria for the Right Selection

Leakage class is the most commonly over-specified parameter in control valve selection. Many engineers default to Class VI without fully understanding what each class means in terms of allowable leakage, service suitability and cost. This article starts from the standard definitions and gives four practical criteria for selecting the right leakage class.

1. What the standards actually say

Two core standards govern control valve leakage classes: the Chinese GB/T 4213-2008 (Pneumatic Control Valves) and the American ANSI B16.104 (aligned with FCI 70-2). Both use Roman numerals from Class I to Class VI — the higher the number, the tighter the shutoff.

Key figures (based on rated Cv):

Class III: allowable leakage ≤ 0.1% × Cv. For high differential pressure, high flow, leakage not critical.

Class IV: allowable leakage ≤ 0.01% × Cv. Typical for metal-seated single-seat and eccentric rotary valves; for tight leakage but moderate differential pressure.

Class V: tighter than IV but not yet bubble-tight.

Class VI: allowable leakage < 10⁻⁷ × Cv — effectively “bubble-tight.” Tested at 0.35 MPa air, measured in bubbles per minute by size: DN50 allows 3 bubbles/min, DN100 allows 11.

2. Four criteria for selecting leakage class

Criterion 1: Medium temperature

This is the hardest constraint. Soft seats (PTFE/RTFE) easily reach Class VI but are temperature-limited — PTFE is typically good to about 200 °C, with long-term service recommended below 150 °C. Above that, metal seating is required and the leakage class drops to Class IV or even III.

Sizing tip: when service temperature exceeds 200 °C, do not default to Class VI — confirm whether metal seating can meet the requirement.

Criterion 2: Differential pressure

In high-ΔP service, soft seat materials can be extruded into clearance gaps even at moderate temperatures; metal seating may be necessary. The higher the differential pressure, the more demanding the sealing structure.

Sizing tip: for ΔP above 4.0 MPa, confirm soft-seat feasibility with the manufacturer.

Criterion 3: Process safety requirements

For certain media — toxic, flammable/explosive, strongly corrosive — process safety codes may directly mandate a leakage class. In such cases the class is not “selected” but “mandated.” Toxic media typically require Class VI; environmentally sensitive areas may also specify a class for discharge valves.

Sizing tip: check the process safety documentation (HAZOP, SIL requirements) first, then see what the valve can deliver.

Criterion 4: Cost and lead time

The cost jump from Class IV to Class VI is significant. Soft seats reaching Class VI is relatively easy and cost-effective; metal seats reaching Class V or VI require precision machining and cost considerably more. Not every service needs the highest class — rational selection can significantly reduce procurement cost.

3. Leakage classes by valve type

Per product catalog data:

Single-seat valve (ZJHP series): metal seat Class IV, soft seat Class VI (temperature ≤ 200 °C).

Double-seat valve (ZH(A/B)N series): structurally limited to Class III.

Eccentric rotary valve (ZSPF/ZSNF series): metal seat 1×10⁻⁴ × Cv (~Class IV), soft seat Class VI per GB/T 4213-2008; spherical seating is insensitive to particles.

Cage-plug valve (ZJHM series): metal seat Class IV, soft seat Class VI.

Diaphragm valve (ZH(A/B)T series): selectable Class IV–VI depending on lining and diaphragm material; suited for corrosive media.

Sizing tip: a double-seat valve is structurally limited to Class III; if the process requires Class IV or above, do not select a double-seat valve.

4. Common selection mistakes

Mistake 1: always specifying Class VI. Soft seats cannot achieve it in high-temperature service; metal seats reaching Class V/VI are expensive. The correct approach is temperature-based: soft-seat Class VI for ambient-temperature shutoff, metal-seat Class IV for high-temperature duties covers most needs.

Mistake 2: ignoring valve-type limitations. A double-seat valve can only reach Class III — this is a physical limitation of the design, not a machining precision issue. Select a single-seat or eccentric rotary valve instead.

Mistake 3: insufficient temperature margin for soft seats. A catalog rating of 200 °C is the极限; long-term working temperature should be kept below 150 °C. Services with temperature swings need even more margin.

Summary

Selecting a leakage class is fundamentally about balance: finding the optimum among sealing performance, temperature capability and cost — not simply chasing the highest number. Understand the standard definitions, master the service conditions and recognize valve-type limitations, and the selection will be both rational and economical.

If you are unsure about the selection, send your medium, temperature, pressure and leakage requirements to the WeChat mini program “问阀” (search 问阀 in WeChat) for item-by-item engineering evaluation.

Dacheng is a Shanghai brand specialized in special-service control valves, with a long commitment to the eccentric rotary technology route (ZSPF/ZSNF series). Engineers are welcome to discuss sizing details.

(This article is technical exchange; final selection shall be based on formal service-condition calculation, and technical parameters shall follow the latest product catalog.)