Control Valve Leakage: Often Used Upstream

Control Valve Leakage: What Process Engineers Should Know

In chemical engineering plants, control valves are essential for regulating flow, pressure, and temperature across a wide range of process equipment—including heat exchangers, reactors, vessels, condensers, and separation units.

Yet, even when correctly sized and installed, control valves may leak internally, allowing unwanted flow through the seat when the valve is in its “closed” position.

Typical leakage rates for Class IV–V valves may range from small measurable flow up to around 0.1% of rated capacity, depending on the valve size and shutoff class.

For process engineers, maintenance specialists, and design engineers, understanding the leakage behavior of modulating valves is crucial—not only during steady-state operation, but also in HAZOP studies, shutdown procedures, and the definition of safe isolation strategies during maintenance.

This article explains why control valves leak, how leakage can affect plant safety and operability, and why the placement of upstream block valves can be a decisive factor in preventing flow where it should not occur.

Evaluating Valve Arrangement in a Steam Supply Line

In the P&ID above, a portion of a steam supply line is shown, feeding the heat exchanger HE-101. The line includes two key valves before the exchanger:

• A green on-off valve, typically a fail-safe isolation valve
• A red modulating control valve, used to regulate steam flow during operation

This configuration—on-off valve before the control valve, follows a clear engineering logic. It’s a correct engineering choice, especially in systems where control valve leakage, thermal expansion, or maintenance operations must be considered.

Why On-Off Before Modulating?

Control valves, especially in steam service, are not perfectly tight. Even when fully closed, a modulating valve can leak a small amount of process fluid—sometimes up to 0.1% to 1% of the nominal flow, depending on its shutoff class and condition. This internal leakage may be negligible during operation but becomes critical during shutdowns or maintenance.

If the modulating valve were placed first, and a tight shut-off valve after it, any leakage would pressurize the segment between the two valves. This trapped volume—often overlooked—can lead to:

• Unexpected heating of downstream equipment
• Condensate accumulation in dead legs
• Pressure buildup that may be hazardous during maintenance or disassembly
• Water hammer risk, especially if trapped condensate is suddenly re-exposed to live steam

Instead, placing the on-off valve upstream ensures that when the valve is closed, no steam reaches the modulating valve. Any internal leakage in the modulating valve is then harmless, since it’s effectively isolated from the supply source.

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Design Implications for Safety and Maintenance

This simple example highlights a broader principle:

In process safety, a single valve in the wrong position can change everything.

Proper sequencing of block and control valves improves:

• Plant safety, by avoiding accumulation of hazardous energy
• Maintenance procedures, by ensuring safe depressurization
• HAZOP effectiveness, by identifying realistic leakage paths

Such configurations should always be evaluated during P&ID review sessions, especially for heat exchangers, reactors, and systems involving high-energy utilities like steam.

Why Do Control Valves Leak?

Control valves are not isolation valves. Their internal construction—especially the seat and plug design—is optimized for precise flow regulation, not for absolute tightness. Even brand-new valves, correctly installed, can allow a small flow through the closed seat. This is called internal leakage, and it is not necessarily a defect: it’s expected and measurable.

The leakage rate is typically defined by ANSI/FCI 70-2 standards, where valves are classified from Class I (least tight) to Class VI (most tight). Most modulating valves in chemical plants fall under Class IV or V, which allow small percentages of leakage relative to rated flow.

Here’s a simplified reference:

• Class IV: up to 0.01% of rated valve capacity
• Class V: tighter, but still not zero-leak
• Class VI: bubble-tight leakage, achievable only on small soft-seat valves because elastomeric or PTFE seating is required — and these materials cannot withstand steam or high-temperature services.

This means that when a Class IV valve is closed, a measurable amount of process fluid may still pass through—and in systems like steam, this can lead to heat transfer, pressure buildup, or unwanted energy release.

In other types of process lines, internal valve leakage may also cause product contamination or unwanted flow between systems.

Conclusion

Control valves regulate flow with precision, but they do not provide tight isolation.
This distinction becomes critical in shutdowns, maintenance, and HAZOP studies, where even small leakage paths can alter the behavior of a system.

Proper sequencing of block valves and modulating valves is not a detail—it is a design decision that directly affects plant safety, operability, and maintenance.
Understanding how a control valve leaks, and where that leakage can go, is part of thinking like a process engineer.

Ing. Ivet Miranda

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