A young engineer called us last month. He was writing a valve specification for a new project. He asked: “How many leakage classes are there? I see A, B, C, D, and some Class A, B, C from another standard. Are they the same?”
No. They are not the same.
At Fluid Tech Group, we get this question often. Industrial Valve leakage standards can be confusing because different standards cover different types of leakage. One standard covers seat leakage (through the closed valve). Another covers fugitive emissions (leakage to the atmosphere). They use similar letters but mean different things.
This article lists every leakage class you need to know. I will explain what each level means, where it applies, and how to choose the right one.
Part 1: Seat leakage classes (ISO 5208 / GB/T 13927)
This standard measures leakage through the valve seat when the valve is fully closed. It has four classes: A, B, C, and D.
Important rule: Classes B, C, and D only apply to metal hard-seated valves. Soft-seal valves (rubber, PTFE, EPDM) are always Class A – zero visible leakage.
| Class | Liquid leakage limit | Gas leakage limit | Application |
| Class A | Zero visible leakage | Zero visible leakage | Soft-seal valves, bellows seal valves |
| Class B | ≤ 0.01 × DN (mm³/s) | ≤ 0.3 × DN (mm³/s) | Precision metal hard-seated valves – high accuracy |
| Class C | ≤ 0.03 × DN (mm³/s) | ≤ 3 × DN (mm³/s) | High-temperature metal hard-seated valves – moderate accuracy |
| Class D | ≤ 0.1 × DN (mm³/s) | ≤ 30 × DN (mm³/s) | General purpose metal hard-seated valves – default class |
Which one should you choose?
| Class | When to use | Typical applications |
| Class A | When zero leakage is critical | Fire protection, drinking water, clean applications – any soft-seal valve |
| Class B | When leakage must be extremely low | High-pressure gas, precision control systems, expensive media |
| Class C | When leakage is acceptable but limited | High-temperature steam, thermal oil, hot flue gas, general industrial |
| Class D | When no special requirement exists | Standard water lines, low-pressure systems, general cast steel valves |
Part 2: Fugitive emission classes (ISO 15848)
This standard measures leakage from the valve stem packing to the atmosphere. It has three classes: A, B, and C.
| Class | Leakage limit (helium test) | Practical equivalent | Application |
| Class C | ≤ 10⁻² mg/(s·m) | Stem ≤ 500 ppmv; Body ≤ 10 ppmv | Ordinary steam, cooling water, non-toxic media |
| Class B | ≤ 10⁻³ mg/(s·m) | Tighter than Class C | Petrochemical, oil and gas, moderate VOC control |
| Class A | ≤ 10⁻⁴ mg/(s·m) | Tightest – near zero emission | Toxic gas, highly volatile media, strict environmental compliance |
What packing do you need?
| Class | Packing requirement |
| Class C | Standard flexible graphite packing |
| Class B | High-performance packing, advanced graphite or PTFE |
| Class A | Bellows seals or high-purity PTFE low-emission packing |
Summary table – all leakage classes at a glance
| Standard | Class | What it measures | Leakage level |
| ISO 5208 / GB/T 13927 | A | Seat leakage (soft seal) | Zero visible leakage |
| ISO 5208 / GB/T 13927 | B | Seat leakage (metal seat) | Very tight – precision grade |
| ISO 5208 / GB/T 13927 | C | Seat leakage (metal seat) | Moderate – industrial grade |
| ISO 5208 / GB/T 13927 | D | Seat leakage (metal seat) | Loose – general purpose |
| ISO 15848 | A | Fugitive emission (stem) | Near zero – strict environmental |
| ISO 15848 | B | Fugitive emission (stem) | Tight – moderate environmental |
| ISO 15848 | C | Fugitive emission (stem) | Basic – entry level low emission |
Three quick rules to avoid mistakes
Rule 1 – Soft-seal valves are always Class A in seat leakage. Do not specify B, C, or D. They do not apply. If your valve has a rubber, PTFE, or EPDM seat, it leaks zero through the seat.
Rule 2 – The two “Class C” are different. One is internal seat leakage. One is external stem leakage. They test different things, use different standards, and serve different applications.
Rule 3 – Default is not always good. Standard cast steel gate valves usually come as Class D seat leakage. If you need tighter sealing, you must specify Class C or B. The price goes up – but the performance improves.
What Fluid Tech Group does for you
We supply valves for fire protection and water systems. We know these standards because we work with them every day.
We supply soft-seal valves with ISO 5208 Class A – no visible leakage through the seat.
We supply metal-seated valves with Class B, C, or D based on your project requirements.
We supply ISO 15848 Class A, B, or C for fugitive emission control – with test reports.
We ask the right questions before we quote – so you get the right class the first time.
Your next step
Send us your valve requirements. Tell us your media, temperature, pressure, and any environmental regulations. We will recommend the correct leakage class and provide a quotation with test documentation.
Fluid Tech Group
Valve leakage classes – we speak all of them