Valve leakage is a common occurrence in industrial settings. Even valves labeled as “zero leakage,” or “bubble tight,” are not necessarily completely leakage-free. This article explores the types and common causes of valve leakage, and then provides information about industry standards governing leakage rates for different types of valves.
Types of valve leakage
There are two main types of valve leakage:
- Fugitive emissions leak from the valve into the atmosphere, posing potential environmental and safety hazards. More than half of a plant’s fugitive emissions are due to valves.
- Leakage that remains in the piping system poses potential safety risks to the system itself.
Common causes of valve leakage
Valves can leak for a variety of reasons, including:
- The valve is not fully closed (e.g., due to dirt, debris, or some other obstruction).
- The valve is damaged. Damage to either the seat or the seal can cause leakage.
- The valve is not designed to close 100%. Valves that are designed for precise control during throttling may not have excellent on/off capabilities.
- The valve is the wrong size for the project.
Valve leakage industry standards
Leakage standards for different types of valves are defined by the American Petroleum Institute (API), the American National Standards Institute (ANSI), the Fluid Controls Institute (FCI), and the Manufacturers Standardization Society (MSS).
Pressure relief valves
It is normal for pressure relief valves to exhibit leakage (aka simmer or warn) as the system operating pressure approaches 80% to 90% of the nameplate set pressure. API Standard 527 specifies methods for determining the seat tightness of metal- and soft-seated pressure relief valves and defines the maximum acceptable leakage rates.
Ball, butterfly, check, gate, globe, and plug valves
API Standard 598 provides the baseline for industrial valve seat leakage. This standard covers leakage requirements for ball, butterfly, check, gate, globe, and plug valves, both metal and soft seated, for both liquid and gas testing.
Check, isolation, and stop valves
MSS Standard SP-61 establishes leakage allowances for both metal and soft-seated valves. This standard provides guidance for testing valves typically used in “full open” and “full closed” service, such as check, isolation, and stop valves, as opposed to throttling and control valves.
ANSI/FCI Standard 70-2 defines six different leakage classifications specifically for control valves, ranging from Class 1 (no testing required) to Class IV (0.01% of rated capacity). Class V represents “effectively zero leakage,” tested with water. Class IV is a soft seat classification, specifying the allowable leakage based on valve size, tested with air or nitrogen.
How can I protect against damage caused by valve leakage?
Valve leakage will happen, but that doesn’t mean it needs to cause problems for your project. Here are three things you can do to protect against damage caused by valve leakage.
- Specify the amount of leakage tolerance for the project. Leakage tolerance can vary widely from project to project. “Zero leakage” is not always necessary or desirable.
- Select the appropriate valve size. Oversizing is one of the most common causes of valve leakage, particularly for control valves. Fortunately, it is also the most preventable through correct valve sizing and selection.
- Perform regular testing and inspection. The best way to ensure that valve leakage doesn’t damage a system or cause undue harm to the environment is to perform regular valve testing and inspection. Early detection will allow you to fix minor problems before they become major ones.