Boiler Room Blog

20 Jun 2016

Common Boiler Problems, Part 5: Interlock Testing

Common Boiler Problems, Part 5: Interlock Testing

Author: David Fox  /  Categories: Boilers & Equipment  / 
Proper training, testing and documentation of interlocked safety devices can help ensure proper monitoring of combustion and fuel train equipment, and ensure the safety of operators and employees as well.

Burning fuel can be useful to mankind as long as it is done as a controlled process - meaning that combustion takes place where we want it to occur. Fuel trains are the complicated-looking series of valves, piping, wires and switches that provide this control.

Fuel trains regulate the amount and the pressure of gas to burners. They also keep gas out of the combustion chamber whenever equipment is shut off. This is accomplished with a series of regulators, flow control valves, and special shutoff valves. The special automatic shutoff valves are designed for low leakage and are spring loaded to close. These are called safety shutoff valves. Larger gas trains require dual safety shutoff valves in series. Some are also assembled into a piping arrangement that includes a vent between them for added safety. The vent and its piping are provided to allow any leakage past a shutoff valve to go outside the building when the equipment is off. The specific configuration that your equipment has depends on your insurer and local code requirements.

Fuel trains also have components and sequences programmed in to ensure that safe light-offs happen. Some devices also make sure fuel flow is immediately stopped if anything goes wrong during the operation of the equipment. Shut downs can occur from gas pressure switches, which attempt to make sure the gas pressures past the regulator are not too high or too low. Fuel trains also have air-flow proving switches to make sure the proper amount of air for purging fire boxes prior to light-off is happening. These air-flow purge switches then verify that air is also flowing while the burner is operating.

Flame sensing components also must exist to make sure flames are present whenever fuel valves are open. Other safety components include gas valve position switches for sensing that the fuel valve is at low fire prior to light-off. Your system could also include furnace pressure switches, high temperature limits, high steam pressure limits, and/or low water level cut-offs. 

All of these safety devices are logically linked or interlocked to a BMS (burner management system) safety controller. The BMS is the brain that supervises and sequences all of the light-off efforts, including the timing and adequacy of the purge prior to light-off and the time intervals allowed for getting pilots and main flames lit. The BMS then acts as your sentinel of safety and monitors all of the switches and safety conditions while waiting to direct the fuel valves to close if a problem occurs.

All of the safety interlocks and switches are supposed to be checked on a regular basis by law, but with maintenance budgets among the first to be cut, proper checkouts are seldom performed. Codes and manufacturers define what the testing frequencies should be for different types of components and safety systems. Frequencies of required inspection/testing range may be daily for some items like observing flames or annually for safety shutoff valve tightness testing.

From experience, it is common for a site where regular testing has not occurred to have at least one switch or device that is failed on each piece of equipment. This is like having the brakes out on one wheel of a car.

Or, for sites that are engaged in some type of regular service or testing, experience reflects that they usually are not doing everything – or at least not doing everything well. This situation varies depending on who is in charge and that person’s knowledge of the equipment or systems. And even if someone knowledgeable is doing the right thing, job rotations and turnover don’t guarantee this diligence and knowledge transfer will always continue.

Proper training, testing and documentation of interlocked safety devices can help ensure proper monitoring of combustion and fuel train equipment, and ensure the safety of operators and employees as well.


Number of views (14370)      Comments (6)


6 comments on article "Common Boiler Problems, Part 5: Interlock Testing"

Pedro Alcantara

3/15/2017 12:55 PM

Dear David,

do you know if there is any standard that specifies the test frequency we must have for a Burner Management System? NFPA 85, for example, does not specifies the minimum time for testing all boiler interlocks. You mentioned that "All of the safety interlocks and switches are supposed to be checked on a regular basis by law", were you talking about any particular law/standard?

Normally we perform a full interlock test every time before starting our boilers, and as we stop each one on a regular basis (2 years maximum), and our operations and control/SIS are very adherent to NFPA 85. However, I'd like to learn more about best practices for functional testing of BMS to keep improving our system and making sure we are as safe as practicable.

Best regards,


Duncan Lance

12/6/2018 4:46 PM

Wow, I had no idea that there were so many ways that a boiler could run into problems with interlocking parts. Although, it does make sense that the boiler would start to have trouble if any of them were to fail. In fact, you'd probably want to call for boiler repair as soon as you notice any of the parts having trouble just to be safe.


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