Operating controls for boilers could be broadly defined to include burner management controls; however, this inspector guide will be limited to the pressure and temperature operating controls required by the ASME BPV Code for steam and hot water boilers.
Steam boilers require a device which senses steam pressure and cycles the burner or other source of heat in order to maintain a consistent, predetermined operating pressure. A second device is used to prevent the boiler from exceeding the maximum allowable working pressure (MAWP) indicated on the boiler nameplate.
Hot-water boilers require a device which senses water temperature and cycles the burner or other source of heat in order to maintain a consistent, predetermined operating temperature. A second device is used to prevent the boiler from exceeding the design temperature of an ASME BPV Code Section I boiler or the maximum water temperature indicated on an ASME BPV Code Section IV boiler nameplate.
The secondary device referenced above for both steam and hot-water boilers is referred to as a high-limit control and under normal conditions, would never be called upon to operate. However, if the primary, or operating, control should fail, the high-limit control must operate, stopping the burner or other source of heat. Some high-limit controls incorporate a manual reset. The purpose of this is to alert the operator that the high-limit control has been activated. The operator should then look for the problem which caused the high-limit control to activate before resetting the device and restarting the boiler.
ASME BPV Code Section I does not specifically mandate pressure or temperature controls but they will be found, in one form or another, on almost all ASME BPV Code Section I boilers. In addition, ASME Standard CSD-1 (if applicable) requires the controls on boilers with input ratings up to 12,500,000 Btu/hr. Jurisdictional regulations will specify the use of ASME Standard CSD-1 if it is mandated.
All operating controls must operate as designed in order to perform their required tasks.
- A pressure control must be installed so as to always sense pressure from the steam space of the boiler. The ASME BPV Code requirements and device manufacturer's instructions should be followed for any installation details. When a siphon or pigtail is used to prevent live steam from entering and damaging the device, the orientation of the siphon loop is critical to the proper operation of a device containing a mercury switch. If the siphon loop is installed in the incorrect orientation shown in Figure 1, movement of the loop caused by heat and/or pressure can cause the mercury switch in the device to activate or deactivate at other than the set pressure.
- A temperature control must be installed as required by the ASME BPV Code and the manufacturer's instructions. Temperature controls are designed and manufactured in different configurations to sense temperature on the surface of a pipe (sometimes called a surface mount device), in a thermo-well, or directly in the water.
- Either type of operating control can be bypassed electrically with jumper wires. Jumper wires can be used legitimately by qualified service personnel during maintenance and testing, but must be removed before returning the boiler to normal operation. Jumper wires could be used inappropriately in an attempt to permanently bypass a control which has malfunctioned and will not allow the boiler to operate.
- Some jurisdictional authorities require the high-limit control be equipped with a manual reset.
While inspecting low-pressure or high-pressure boilers, the inspector will be observing operating controls. The inspector must take the time necessary to completely evaluate the condition and operational effectiveness of these controls. The inspector should:
- Compare the pressure gage reading on a steam boiler with the set pressure of the primary operating control. If the pressure gage reading is higher than the set pressure of the control, request the installation of a second, reliable pressure gage in order to determine the accuracy of the first pressure gage. If the second pressure gage reading agrees with the operating control set pressure, the first pressure gage must be recalibrated or replaced. If, however, the second pressure gage reading agrees with the first pressure gage, the boiler should be removed from service until the primary operating control can be repaired or replaced.
- Compare the thermometer reading on a hot-water boiler with the set temperature of the primary operating control. If the thermometer reading is higher than the set temperature of the control, request the installation of a second, reliable thermometer in order to determine the accuracy of the first thermometer. If the second thermometer reading agrees with the operating control set temperature, the first thermometer must be recalibrated or replaced. If, however, the second thermometer reading agrees with the first thermometer, the boiler should be removed from service until the primary operating control can be repaired or replaced
- Observe the boiler while it cycles off and on, if possible, to determine if the primary operating control is functioning reliably.
- Request the owner or owner's representative test the high-limit control in accordance with the control manufacturer's instructions. This test may involve disabling the primary operating control or setting the primary control's pressure or temperature, as applicable, higher than the setting of the high-limit control. Since each installation can be unique, the inspector should rely on the control manufacturer's instructions for guidance. Before returning the boiler to its normal operating condition, ensure all operating controls are enabled and set to the proper pressure or temperature.
Additional information to aid inspections of operating controls, including installation requirements, can be found in the following publications and sources:
- National Board Inspection Code
- ASME BPV Code Section IV
- ASME BPV Code Section VI
- ASME BPV Code Section VII
- ASME Standard CSD-1
- Manufacturer's Installation, Operation, and Maintenance Documentation
- Jurisdictional Laws, Rules, and Directives