Relief Valve Sizing After a Fuel Gas Header Change
A plant fires a boiler on natural gas from a header regulated to 250 kPa gauge. During a project the gas supply pressure upstream of the regulator is raised, and a new larger gas train is installed. The existing gas pressure relief valve on the downstream header is a spring-loaded valve set at 350 kPa gauge, sized years ago for the old maximum flow. The regulator now has a larger orifice and can pass considerably more gas if it fails open. The relief valve discharges to a vent line that terminates 2.5 m above the roof, close to an air intake. A review asks whether the relief valve still protects the downstream gas train and piping, which are rated for 400 kPa gauge.
1. Explain what determines the required relief capacity and why the existing valve may no longer be adequate even though its set pressure is below the piping rating.
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Relief capacity must be based on the worst credible overpressure scenario, which here is the regulator failing wide open with the new, larger orifice and the higher upstream pressure. The required relief flow is the maximum flow the failed regulator can pass at the upstream pressure, not the boiler's normal gas demand. If the new regulator can pass more gas than the old one, the required relieving capacity rises, and the existing relief valve may be undersized even though its set pressure (350 kPa) is below the piping rating (400 kPa).
Set pressure protects against overpressure only if the valve can pass the required flow at an accumulation within the allowable limit. A valve that is too small will let pressure rise past its set point toward the piping rating while it is fully open. Also confirm the set pressure plus allowable accumulation (typically 10% for this service, per the applicable code) stays at or below the piping rating: 350 kPa × 1.10 = 385 kPa, which is under 400 kPa, so the set point is acceptable, but capacity is the open question.
2. Identify the additional safety deficiencies in the relief discharge arrangement and the documentation steps needed.
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The vent terminates only 2.5 m above the roof and near an air intake, so relieved flammable gas could be drawn into the building or reach an ignition source; the vent should discharge to a safe location, well above roof level and away from intakes and ignition sources, per the applicable gas code and site standards. Confirm the vent line and its supports can take the relief reaction force and that the line is not blocked or fitted with a valve.
Documentation and action: obtain the new regulator's failed-open capacity, calculate required relief capacity, and select/resize the relief valve and vent accordingly. Verify the relief valve is rated for gas service and has the correct orifice. Update the relief valve datasheet, sizing calculation, and register; record the set pressure and test date; and complete a management-of-change review covering the higher supply pressure, new gas train, and vent location. Do not return the system to service on the assumption that the old valve is adequate.