Excess Flow Valves

What are excess flow valves?

Excess flow valves are spring loaded check valves. When the flow of the fluid exceeds the limit of the valve, it closes and shuts off flow.

What are they used for?

Excess flow valves are protective devices intended to prevent the uncontrolled release of hazardous materials from road, rail and marine transport vessels, stationary storage vessels and distribution networks. During downstream leak or rupture, the flow across the EFV will exceed its limit and closes the valve, preventing further release to atmosphere. Properly designed, installed, and maintained EFVs could be an effective accidental release prevention measure.

Incidents involving EFV failures

The following incidents have occurred because of inadequate protection involving EFVs in the past:

A hose rupture from a chlorine railcar released 48,000 pounds of chlorine in Missouri (2002). The incident involved evacuation of nearby residents, injuries needing hospitalization and damage to vegetation. Investigations found that EFV on the railcar did not function as needed, and there were no alternative means of stopping the chlorine release.

Rupture of offloading line on a railroad tank car released methyl mercaptan into the atmosphere. The incident cost the lives of 3 employees and resulted in evacuation of nearby residents. Similarly, it was found 2 EFV fitted onto the system did not function.

A hose coupling break from an LPG truck resulted in release of propane, and the EFVs fitted onto the system did not function to stop the release.

Lessons Learned

Flow restrictions in the affected lines (i.e pipe length, branches, reduction in pipe size, and partially closed shut-off valves). This is also true in lines fitted with pumps to transfer the material. If a release occurs downstream of the pump and the EFV activation point is greater than the pump capacity, the pump will function as a regulator limiting the flow below EFV activation limit.

An EFV is also has its limitations when the release involves small leaks. Upon closure of EFV, the pressure upstream will keep it closed until the downstream pressure equals the upstream pressure. When the pressure has equalized the valve will re-open. In the case of a hose rupture, line break situation the pressure will not equalize and the valve will stay closed. In the event of a bleed hole, the EFV does not provide 100% flow shutoff and will continue to bleed.

The installation of EFVs also play a role in its effectiveness. Some EFVs are designed for vertical mounting, and some for horizontal mounting. The mounting and positioning of the EFVs need careful attention to ensure they will perform adequately on demand.

Other factors that limit the functionality of an EFV are as follows:

Insufficient upstream pressure which limits the flowrate across the EFV
Buildup of contaminants in the EFV preventing its closure

Hazard Management and Control

The following is recommended in utilizing EFVs as hazard control measure in hazardous service:

Conduct adequate hazard identification and risk assessment (HAZOPs, HAZIDs) on the hazardous processes;
Analyze credible release scenarios and use these data to adequately design / select EFVs;
Consider the characteristics of the hazardous material in designing / selecting EFVs. (e,g two phase flows upstream of the release point upon flashing conditions);
Ensure correct installation of the EFVs (as per vendor instructions);
Adequate maintenance of the EFVs;
Applying proper change management procedures and operating procedures;
Additional protection systems, depending on the degree of hazard of the material handled. This may include

- Remote shut off systems;
- Emergency shut off valves; or
- Hoses with self-closing mechanism in case of detachment.

Guidance

Guidance is available from the following institution on the application of EFVs (list is not exhaustive):

Chlorine Institute (CI);
National Fire Protection Agency (NFPA); and
Compressed Gas Association (CGA).