The maximum operating pressure of a boiler is the highest pressure at which it is designed to operate safely under normal conditions. A switch on the boiler is configured to shut off fuel to the burner when the max operating pressure is reached.
Maximum operating pressure is often established by the installer or commissioning technician when configuring the boiler’s controls. This is the pressure where the boiler’s high limit switch is set, which will trip the burner requiring a local manual reset by an operator.
Maximum operating pressure is typically set at least 10% below the lowest safety valve set pressure. This is so the boiler will trip on high limit prior to the safety valves beginning to ‘weep’ or ‘simmer’. Safety valve simmer begins at the lower limit of the valves seat tightness rating. Typical boiler safety valves in industrial applications have a seat tightness of 10%. So a valve set at 150 psig would begin to simmer, leak and make noise at 135 psig. Setting the maximum operating pressure at 135 psig may allow the valve to simmer before the boiler trips. While this would not be a dangerous situation, it would lead to safety valves that do not shutoff tightly, wasting steam and leading to inefficiency. Valves with a seat tightness of 5% or 3% are available, but may be cost prohibitive for many applications.
It’s important to note that the maximum operating pressure is typically set below the maximum allowable working pressure (MAWP). The MAWP represents the maximum gage pressure the vessel is rated to handle safely during normal operation. However, it’s not an absolute limit—the code allows for a temporary exceedance when the safety relief valves are discharging. In practice, the maximum operating pressure is set below this threshold to ensure safe, reliable operation without triggering safety devices or causing premature wear.
What Is Valve Set Pressure?
The Valve Set Pressure refers to the specific pressure at which a valve, is designed to lift. This “set pressure” is a critical parameter that determines when the valve will be fully open and perform its function to protect the system. There is a tolerance from the rated set pressure to when the valve actually lifts that changes with the design pressure of the boiler. However, the safety valve manufacturer or repair shop ensures these rules are properly followed.
For safety or pressure relief valves, the set pressure is the point at which the valve is rated to fully release excess pressure from the system.
Importance of Adhering to Maximum Operating Pressure
Safety: Operating below the maximum operating pressure minimizes the risk of overpressure incidents, nuisance trips, and wear and tear on safety valves which can lead to equipment damage or catastrophic failures.
Efficiency: Maintaining the designated pressure range ensures the boiler operates at peak efficiency, reducing energy consumption and operational costs.
Regulatory Compliance: Adherence to maximum operating pressure is often mandated by industry standards and regulations. Commonly those established by the American Society of Mechanical Engineers (ASME). Namely ASME BPVC Section I and CSD-1.
Best Practices for Managing Boiler Pressure
- Regular Maintenance: Schedule routine inspections to identify and address potential issues such as corrosion, scaling, or component wear that can affect pressure control.
- Safety Valve Testing and Calibration: Test and calibrate safety valves periodically to lift within tolerance of desired set point pressure.
- Monitoring Systems: Use real-time pressure monitoring tools to track system performance and identify pressure fluctuations before they become problematic. If a system is regularly tripping on high pressure or lifting safety valves, system controls or operating practices should be examined.
- Operator Training: Equipment operators with the knowledge and skills to understand pressure parameters and respond appropriately to pressure anomalies.
What To Do If Max Operating Pressure Is Exceeded
Safety codes are there to ensure that over-pressurization is not a risk. The boiler should safely trip or relieve the pressure to a safe location when max operating pressure is exceeded. Operators should be vigilant for warning signs, such as unusual noises, leaks, or frequent activations of safety valves. If these occur:
- Change load more slowly: Allow the boiler firing rate controller to respond by bringing load online and offline slowly enough that the boiler can react with a fairly constant pressure.
- Inspect Components: Check for leaks, wear or damage in safety valves, piping, and other critical parts.
- Calibrate Safety Valves: Ensure relief valves are set correctly and functioning as intended.
- Consult Experts: Work with certified professionals to evaluate the system and identify any long-term solutions to prevent future incidents.
The Role of Material Strength and Safety Margins in Pressure Management
When determining the Maximum Allowable Working Pressure (MAWP) the strength of the materials used plays a critical role. The grade of steel used in the construction of pipelines and boilers significantly impacts their MAWP. Factors such as the chemical composition of the steel, including elements like carbon, manganese, and chromium, influence both ductility and toughness. To maintain adequate safety margins, industry standards require that the ratio of ultimate tensile strength (UTS) to yield strength must not exceed 0.93. This ensures the material can withstand service-induced stress without becoming brittle or prone to failure. For older or undocumented systems, accurate calculations and conservative safety margins are especially critical to avoid catastrophic failures. By adhering to these principles, operators can enhance both safety and efficiency in their pressure systems.
