1. Leakage Classes and Corresponding Leakage Rates

1.1 Leakage Classes in Standards:

International Fluid Control Institute Standard: FCI 70-2-2021:

The International Electrotechnical Commission standard IEC 60534-4 is basically the same.

The formulas are too abstract. Let's look at the following (based on Globe valves):

1.2 Requirements from Compressor Manufacturers: Currently, the vast majority ofmanufacturers require Class IV for anti-surge valves. The reason is: if Class V is selected, valve manufacturers either cannot achieve it, or can achieve it but will charge extra.
1.3 Is Class IV Right Choice?

From the perspective of standard leakage rate: In the table above, for DN350 at 350kPa differential pressure, the leakage rate is 227l/min, less than 14 Nm³/h! For a DN350 globe valve that can typically handle 85,000 Nm³/h MAC at 5.6 bar(A), the leakage proportion is less than 0.02%! Even
if the actual pressure is higher than the 350kPa test pressure, it wouldn't exceed 0.03%. Is it worth worrying about?

FCI 70-2 has a statement: chaptaer 2.2 This standard isapplicable to production testing of control valve seat leakage and shall not be used as a basis for predicting leakage under conditions other than those specified in Section 5.

In other words, this production test cannot be used as a basis for assessment during field operation!

Why does the standard stipulate this? Is it to protectvalve manufacturers?

Look at a practical case: A DN500 anti-surge valve was disassembled after two years of operation:

It can be seen that there are obvious grooves on thesealing surfaces, caused by two factors: First, the medium cannot be absolutely clean. Second, since the valve leaves the factory with Class IV sealing, under a pressure difference of nearly 5 bar, the gas flow velocity is very high, like a "gas knife" scouring the sealing surface. The original leakage
points will become larger and larger - this is the same principle as water dripping through stone. For carbon steel valves with balanced trim, the standard material is martensitic stainless steel 410 or 420C quenched and tempered, with hardness reaching 28-32 HRC, but this is not as high as Stellite alloy, which also increases the
possibility of erosion.

Therefore, the only solution is: to make it as leak-tight as possible before leaving the factory.

2. Actual Field Conditions and Retrofit Results:

2.1 110,000 Nm³/h MAC, DN400 Globe valve, leakage on site:

If the temperature downstream of the valve is more than 10°C higher than the ambient temperature, then the anti-surge valve is definitely leaking. Because if it could shut off completely, the gas upstream would be dead gas, and the temperature difference caused by heat conduction would be very limited.

Then we replace the trim structure, and the flow direction was changed from FTO (Flow-to-open) to FTC (Flow-to-close) . Problem solved!

2.2 Another Case:
During operation of Units 7&8 in a 30,000 Nm³/h ASU Plant, the ASV valve (DN500 Globe) downstream pipe was hot when the valve was fully closed:

The retrofit solution was basically the same as above, with the actuator size also increased:

In other words: The original leakage was about 2,500Nm³/h, which is far beyond the Class IV sealing level.

There are series such cases on site. Readers with centrifugal compressors can test it themselves with an infrared thermometer.

3. What Should We Pay Attention To?

• First,when ordering a centrifugal compressor, require anti-surge valves to have Class V sealing.
• Second,specify the flow direction as FTC to reduce the possibility of trim eccentric wear.
• Duringinstallation, the piping from the compressor outlet to the vent valve must be clean and thoroughly blown out.

4. About BAC Return Valves:

It's not as easy to detect problems as with MAC anti-surge valves: the gas temperature after the aftercooler is low, and the temperature difference with the environment is small. However, internal leakage still has a relatively significant impact on energy consumption.

Conclusion: Bridging the Gap from "Paper Standards" to "Field Performance"

With these five articles, we have systematically examined the critical aspects of anti-surge valves—from selection and installation to sealing. The central theme throughout has been the undeniable gap that often exists between technical specifications and real-world performance.

The difference between Class IV and Class V sealing is merely a line in a procurement document, yet over the full lifecycle of an installation, that single line manifests as millions of cubic meters of leakage and hundreds of thousands of dollars in energy losses. While we invest heavily to improve compressor efficiency by 1%, a valve leaking at 2,500 Nm³/h can nullify all those efforts. Every overlooked detail ultimately converts into operating costs.

Only by confronting these gaps head-on can we ensure that every line of a technical specification truly delivers its intended value in the field.