Technology for Energy Corporation (TEC) - provides (amongst others) products for monitoring valves.
www.tecnuclear.com/1414.htm. Acoustic monitoring of safety and relief valves in nuclear power plants provides reliable indication of flow through the valves so that the operator knows when there is a path open for loss of coolant. This method of valve surveillance satisfies the requirements of NRC Regulatory Guide 1.97 (Dec. 1980) and can be easily implemented using the TEC Acoustic Valve Flow Indication System.
The National Board of Boiler and Pressure Vessel Inspectors (http://www.nationalboard.org). The Board was created in 1919 to promote greater safety to life and property through uniformity in the construction, installation, repair, maintenance and inspection of boilers and pressure vessels. The National Board membership oversees adherence to codes involving the construction and repair of boilers and pressure vessels. Some very interesting incident reports concerning safety valves can be viewed on their site.
The Department of Industrial Relations (www.dir.ca.gov) was established to improve working conditions for California's wage earners, and to advance opportunities for profitable employment in California. Site contains information about how safety valves and pressure relieving devices are to be used in various industries.
The US Nuclear Regulatory Commission (www.nrc.gov). Searching this site for 'safety valve' reveals several interesting 'Information Notices', such as No. 83-26: Failure of safety/relief valva discharge line vacuum breakers and No. 89-90, Supplement 1: Pressurizer safety valve lift setpoint shift.
The ISA (www.isa.org) offers a course on sizing, selecting & applying safety relief devices (Course E135).