• Use full-port gate valves for block valves.
• Don’t install block valves in the relief valve system of any boiler or steam generator constructed to the requirements of the ASME Boiler and Pressure Vessel Code, Power Boilers, Section I. Some state boiler laws also prohibit this practice.
• Ensure the block valves are locked open when the relief valve is in service.
• Frequently check the status of the block valves.
Vents and bleeders. A common hazard is the leak of a hazardous chemical from a vent or bleeder to the atmosphere. Causes of the leak can include: the vent or bleeder inadvertently being left open (or not completely closed); erosion or corrosion of internals of the vent or bleeder valve (leakage then can’t be stopped); and a sample point not properly shut.
Another threat is someone working on a vent valve pulling it off the pipe to which it was connected (this usually requires corrosion to have weakened the joint). Someone standing on a vent/bleeder line or inadvertently kicking it also can cause this type of rupture. So, to minimize such risks, it’s important to ensure that bleeder and vent valves discharge to a safe location and are properly maintained.
Critical control valves in manual. The PHA team should list all control valves that might be operated in manual mode. For each valve, the team should ask three questions:
1. Why is the valve in manual?
2. What would happen if it were switched to automatic?
3. Is the valve part of a safety shutdown system?
The last question is key. Operators may say that in an emergency they’d move the valve to its fail-safe position. However, humans are unreliable during an emergency but you can trust high integrity instruments and valves to properly operate.
Ian Sutton is safety engineering manager for AMEC Paragon, Houston. E-mail him at Ian.Sutton@amec.com.