If a boiler is fired to full capacity with the header valve closed, the safety valve should prevent steam pressure from rising above:

The safety valve on a boiler is a critical component designed to prevent dangerous pressure build-up within the system. When the boiler is fired to full capacity but the header valve is closed, there is a risk of pressure exceeding the safe working pressure (S.W.P.) because steam cannot escape.

The function of the safety valve is to open at a predetermined pressure to release excess steam and maintain safe operating conditions. The accepted industry standard specifies that the safety valve can allow for a rise in pressure of up to 6% above the S.W.P. or the valve setting before it opens. This limit is set to ensure that pressure does not rise to unsafe levels, while also accounting for minor variances in pressure that can occur during operation.

In this context, a 6% allowance above the S.W.P. is considered appropriate to ensure both safety and efficiency in the boiler operation. Thus, the correct answer reflects the regulatory guidelines and safety standards that govern boiler operations.