Title:
Safety in Water System Valve Vault Design
Hazard Category:
- Safety
Hazard(s):
- Confined spaces
Problem:
This case-study is from a fatality investigation conducted by NIOSH (National Institute for Occupational Safety and Health), the federal agency responsible for occupational safety and health research.
A 35-year old male water system operator (victim) was asphyxiated after entering a valve vault at a municipal water system plant. The victim was assigned to turn on a water line valve serving a nearby tree farm. The valve was located at the water treatment plant inside an underground valve vault that “always had normal air.” The victim entered the valve vault through a ground-level manhole without testing or ventilating the vault atmosphere. A co-worker, who had last seen the victim 1 hour earlier, checked the manhole and saw the victim lying on his back at the bottom. The victim did not respond to any calls. Other workers summoned from the plant building and local fire department personnel ventilated the valve vault and removed the victim. The vault atmosphere was subsequently found to be oxygen deficient. There were no witnesses to the incident, but evidence suggests that the victim lost consciousness and fell from the ladder railings to the bottom of the vault.
NIOSH investigators determined that, in order to prevent future similar occurrences, employers should:
- Recognize that confined space atmospheres are dynamic environments subject to unexpected changes, and address those dynamics in all written and practiced safe work procedures and subsequent worker training.
- Develop and implement a comprehensive confined space entry program.
In addition, municipalities should ensure that:
- Police, as well as fire and rescue personnel, are trained in confined space entry and rescue procedures.
Risk Description:
Water transmission systems frequently have below-grade vaults that contain large system valves. Utility workers enter the valve vaults to operate valves, inspect or repair equipment, or during construction. Valve vaults are confined spaces when the entry is through a hatch or manhole and down a ladder. When there is possibility of a hazardous atmosphere (oxygen deficiency, flammable or toxic atmosphere), they become permit-required confined spaces.
All permit-required confined space entries require atmospheric testing, control of engulfment, ventilation, a written permit, retrieval equipment, standby attendant, communications plan, and emergency plan.
PtD Strategy
Prevention through Design is the effort by design engineers to make safer workplaces through implementing safety at the project design stage. Valve vaults can be designed so that they are not confined spaces by improving access (through doors or large hatches and stairways to lower levels), and by providing adequate mechanical ventilation.
When there is not enough room to include stairways, the valve vault remains classified as a confined space, however, there are strategies that improve safety for workers, such as:
- Eliminate the need for workers to enter the vault for valve operation by moving the valve operator above grade.
- Locate gages and controls above grade.
- Install a mounting sleeve for a retrieval device next to the hatch, to facilitate lifting out workers in an emergency.
