Regarding the pressure relationship between the safety valve and the bubble safety valve of the boiler air collection box.
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As stated in the title, the specific issues are as follows: The safety valve of the boiler’s gas collection box operates at a pressure of 3.53 Mpa, with a set pressure of 3.67 Mpa. One of the bubble safety valves operates at a pressure of 4.0 Mpa, with a set pressure of 4.25 Mpa; the other one also operates at 4.0 Mpa, but its set pressure is 4.32 Mpa. The problem we’re facing is that the safety valve of the gas collection box trips prematurely, starting to activate at a pressure as low as 3.44 Mpa. Therefore, the client wants the set pressure of this safety valve to be increased to 3.92 Mpa. However, the design team is concerned that increasing this value might cause the bubble safety valves to trip first, as there is theoretically a resistance at the superheater outlet of around 0.4 Mpa that needs to be taken into account. I would like to ask the experts here: Is the reasoning provided by the technical team valid? Will increasing the set pressure indeed cause the bubble safety valves to activate first?
There is evidence for it. GB/T 16507.4-2013, Page P78, Section 7.2: The operating pressure is the sum of the boiler’s rated pressure, the flow resistance of the working fluid, and the static pressure of the liquid column. Generally, the total resistance of the superheating system should not exceed 10% of the operating pressure of the superheater; for a specific boiler, it is necessary to calculate the magnitude of the flow resistance.
The operating pressure of the drum is the sum of the rated pressure of the boiler at its outlet and the flow resistance from the drum to the boiler outlet; the superheating system generally does not contribute to the hydrostatic pressure.
It should be determined by the manufacturing and design unit, right? It should be determined at the time of installation; it’s not up to the owner to decide how it should be, and it must meet the standards. Either there’s an issue with the pressure loss calculations you’ve done; how could the safety valve of the collection tank then activate?
It’s either a problem with the safety valve or an issue with the pressure measurement display. Regardless of whether the overheat system pressure drop calculation is accurate or not, the boiler control system uses the boiler outlet pressure as the control point, rather than the drum operating pressure.