Valve safety trains regulate the flow and pressure of fuel. In doing so, they keep the fuel delivery consistent with the requirements of the process. Importantly, they also keep gas out of the combustion chamber when equipment is cycled or shut off and ensure emergency shutdowns take place if a problem occurs.
Valve safety-train systems typically handle natural gas, but they can also transport propane, landfill, methane, sewer gas, oil, air, nitrogen or other types of bio-gas products. Regardless of the media handled, the maintenance, cleaning and repair of valve safety trains are mission-critical to employee safety and process control. When a valve safety train is not inspected and maintained on at least an annual basis, the results can be catastrophic. Unfortunately, action is often taken only after a tragic event.
What is a valve safety train?
Because of their complexity, employees tasked with servicing valve safety trains often misunderstand how they operate. To better appreciate what a valve safety train is and isn’t, a brief overview of their engineering is in order.
Valve safety trains are a coordinated group of mechanical and electrical devices to manage and monitor the fuel or other media. At the heart of every valve safety train is a safety shut-off valve (SSOV). The SSOV is the key safety component that ensures full flow isolation. Larger trains require dual SSOVs in series. Preceding the SSOV is various fuel-conditioning devices to clean the fuel and manage the pressure. After the SSOV, there are often flow-control valves and other devices to support the process needs.
Augmenting these devices are various manual shut-off valves (MSOV) that are used to isolate the process for maintenance purposes. Gas pressure switches are incorporated at key locations to monitor the fuel pressures and check against abnormal conditions. Additional ancillary devices such as pilot lines, vents, proving systems, instrumentation or flow meters may be incorporated to provide specific process-control needs.
Aside from the valve safety train, a burner management system (BMS) safety controller supervises and sequences light-offs and monitors safety. The BMS will direct fuel valves to close if a problem occurs. The safety premise of a BMS is to assume an unsafe condition exists unless each individual interlock has been proven and maintained in a safe condition. Only then will the BMS permit the combustion system to operate.
All components that make up a valve safety train must be listed or approved for their intended service by a nationally recognized testing agency, such as Underwriters Laboratories (UL), Factory Mutual (FM) or American Gas Association (AGA). Every component must be selected and properly rated not only for normal operating conditions but also for abnormal and failure modes.
Additionally, a valve safety train must be approved for use by an appropriate authority, which can vary depending on the locale and application of the larger system. On some applications, an expert from the underwriting authority or local government will inspect the valve safety train before it is put into service.
What could go wrong?
Preventing a valve safety-train incident requires diligence in both maintenance and inspection. Valve safety trains, like all industrial equipment, wear with use. For instance:
- Fuel flowing to a system can convey condensate, piping scale or other foreign materials that will either damage or block a safety valve from fully closing.
- Diaphragm valves are vulnerable to embrittlement, aging and rupture.
- Outdoor trains endure seasonal extremes, adverse weather and UV exposure.
- Control valves and linkages may loosen with cycling.
- Debris accumulates in fans, burners and air piping, which alters air-to-fuel ratios and reduces energy efficiency.
- Incorrectly vented valves, relief devices, regulators and pressure switches can fail to properly respond upon use or not react at all.
- Gas pressure switches should be wired with a means that will prevent fuel from a failed switch flowing through a conduit or wiring to an electrical enclosure. Such a failure could cause an explosion if the fuel were to reach an arcing contact inside the electrical system.
- If plug valves are installed, they must be properly serviced with the correct sealant. Plug valves that are low on sealant may leak externally or permit fuel to bypass a closed valve. Excessive use of sealant can build up in the piping, which may subsequently flow into other components.
- Worse yet, an employee may defeat, bypass or jumper-out safety controls, creating unknown hazards.
Add up all these dangers and the need for valve safety-train inspection and maintenance by trained technicians becomes readily apparent.
Different components of the valve safety train may be on their own unique inspection frequencies set forth by the manufacturer or codes. It is strongly recommended, however, that the entire combustion system be inspected at least annually, both internally and externally while out of service, to ensure compliance.
Consult with NFPA 86 (Ovens and Furnaces) as a starting point. It provides guidelines to establish these measures, clearly stating, “The user has the responsibility for establishing a program of inspection, testing and maintenance with documentation performed at least annually.”
NFPA 86 applies to both new installations and modifications to existing equipment. However, because NFPA 86 is only a minimum standard, the authority having jurisdiction (AHJ) has the final say. The AHJ could be an insurer or the local fire marshal, for instance. Whoever it is, the AHJ is the point of definitive reference for all compliance matters.
Does this mean that you are in the clear if the AHJ gives your valve safety train the OK? The short answer is “no.” Annual testing and preventive maintenance are typically requirements of insurance agencies, but other (often overlapping) codes and standards may need to be adhered to besides NFPA. These may include ANSI, ASME, NEC and the EPA. Oil-fired burners must comply with UL-296 Standard for Oil Burners, UL-726 Standard for Oil-Fired Boiler Assemblies or UL-2096 Standard for Commercial/Industrial Gas and/or Oil-Burning Assemblies with Emission Reduction Equipment.
A final word of caution: If you are in a location that does not mandate ASME CSD-1 (Controls & Safety Devices for Automatically Fired Boilers) or NFPA 85 compliance, the AHJ will rarely address fuel-system issues. Interlock and gas-train testing are typically assumed to be a responsibility of the owner. Therefore, an AHJ saying your boiler or dryer “passed” will bring no assurances that the valve safety train is compliant.
Leave it to the Experts
Given the potential danger of valve safety-train accidents, surprisingly few organizations have staff on-hand with the skills required to perform inspections or even basic maintenance, such as tuning a burner or changing a control valve. This absence of skilled staff comes despite statistics citing human error or lack of knowledge as the cause behind approximately 83% of boiler accidents, according to the National Board of Boiler and Pressure Vessel Inspectors.
Valve safety-train inspections should only be performed by highly trained, experienced flame-safeguard-control technicians. This person will inspect every part of the valve safety train, including interlocks, and replace necessary components. Many forward-thinking companies wisely contract a third party to inspect and maintain their burner equipment. An analysis of the plant’s code compliance or deficiencies will be conducted. The inspector will also review the plant’s maintenance practices regarding fuel delivery.
Upon conclusion of the inspection, a written report with data and photographic documentation will be presented to the plant manager. When done regularly, this written report allows the manager to compare and contrast the valve safety-train’s condition with that of previous inspections.
If annual inspections are not completed within the allotted time or if they are ignored, the plant owner may lose insurance coverage. Without insurance, a catastrophic explosion resulting from an unmaintained valve safety train represents a massive financial liability as well as a life-threatening hazard to employees.
Third-party contractors can provide both inspections and maintenance. If you are planning to do your own preventive maintenance, please begin by reviewing NFPA 54: The National Fuel Gas Code, which details safe gas-piping repair practices. Like NFPA 86, it is required reading. Valve safety-train manufacturers can provide training for more basic maintenance procedures.
Basic maintenance and housekeeping procedures that an in-house staff can perform include:
- SSOV leak testing – If a gas train has a double SSOV and vent line, you will need to perform a monthly test on each valve to ensure gas is not leaking into the combustion chamber or up the vent. At the very least, a leak will cost money in lost fuel and reduce the efficiency of the burner.
- Flame color and shape – You can learn a lot from a burner flame. Check it daily. If it has been over a year since the last air-to-fuel ratio tuning, hire a technician to adjust it. Your flame should produce a clean burn with no soot or smoke and exhibit a uniform flame envelope. A natural gas flame that is blue or violet means you have a lean fire with excessive amounts of air. Unchecked, an excessively lean fire will create high levels of CO and white smoke. If your flame burns yellow, however, you have a fuel-rich fire. An orange flame or one that produces dark smoke is dangerous. Shut off the fuel immediately.
- Vent-line inspection – Many vent lines are old and not maintained. When a venting device fails, it stays partially open and flows fuel to the vent line. Some failed devices will result in a loss of control, but not always. Regardless of the cause, raw gas pours out of the vent whenever equipment is pressurized. Failure is often uncovered only when someone reports the smell of gas. NFPA does state that all safety devices shall be tested on a regular basis. Some manufacturers offer “ventless” solutions as an option. This reduces the labor and material cost for installing proper venting.
- Don’t adjust the burner purge cycle – One of the best maintenance steps you can take is to not take it at all: Do not adjust the burner purge cycle. This feature prevents explosions caused by a buildup of unburned fuel in the chamber. Ensure the timers work and are properly set and maintained. Certainly, it is frustrating when a boiler fails to ignite, which requires one to wait for another complete purge cycle. But don’t use that frustration to justify doing something rash like shortening or bypassing the purge cycle. Doing so greatly increases the chances of a serious explosion.
Failure to inspect and maintain valve safety trains in proper working order will result in higher fuel costs, reduced heat transfer and the potential for a catastrophic explosion. Today’s valve safety trains are complex assemblies consisting of both electronic and mechanical components that need professionals to keep them in top shape.
For more information: Contact Robert Sanderson, director of business development; Rockford Systems, 5795 Logistics Parkway, Rockford, Ill. 61109; tel: 800-922-7533; e-mail: email@example.com; web: www.rockfordsystems.com.