Pressure Relief Sizing on a Heated Solvent Tank
A 20 m³ vertical carbon-steel tank stores a flammable solvent with a design pressure of 0.5 barg and a design temperature of 60 °C. A steam coil inside the tank is intended to keep the contents at 35 °C. The relief valve currently fitted has a set pressure of 0.45 barg and a certified capacity of 900 kg/h of air equivalent. A fire case is being reviewed: the tank is in a bunded area and may be exposed to a pool fire. The solvent's latent heat is 310 kJ/kg and its molecular weight is 78. The engineer proposes to keep the existing valve because 'it has always been adequate'.
1. Explain why the existing relief valve cannot be justified on the basis of 'it has always been adequate', and outline the fire-case calculation that should be performed.
Read the worked answer
'Always adequate' is not a sizing basis. The valve must be shown to pass the governing relief load for every credible scenario. The fire case is usually the largest for a solvent tank.
Fire-case relief load (API 521 style, simplified): Q = F·A·q, where A is the wetted surface area, q the heat flux (commonly ~34,500 W/m² for a bare vessel with adequate drainage and fire-fighting, higher if not), and F an environment factor. The vapour rate is W = Q / latent heat. For a 20 m³ vertical tank, the wetted area to the relief level might be ~25 m². With q ≈ 34,500 W/m²: Q ≈ 862 kW, W ≈ 862/310 ≈ 2.78 kg/s ≈ 10,000 kg/h of solvent vapour.
That is an order of magnitude above the valve's 900 kg/h air-equivalent rating, and the rating must be corrected for the solvent's molecular weight and temperature (air-equivalent capacity must be converted to actual gas). The engineer must calculate the required orifice area for the solvent vapour and select a valve with certified capacity at the set pressure. The existing valve cannot be accepted without this calculation.
2. Identify the other scenarios that must be checked and the design features that affect the result.
Read the worked answer
Scenarios that must each be checked, with the largest governing the valve:
- Fire - as above, usually dominant.
- Steam coil failure - if the coil can overheat the contents, the vapour generation at the coil's maximum heat input must be calculated; a 35 °C control setpoint does not prevent a failed control valve from driving the tank to design temperature.
- Overfilling / overpressure from connected systems - check whether a transfer pump or nitrogen blanket can overpressure the tank.
- Thermal expansion of liquid in a full tank.
Design features that change the answer:
- Bunding and drainage affect the fire heat flux (drainage away from the tank lowers q).
- Insulation - if credit is taken, it must be fire-resistant and documented; otherwise use bare-vessel flux.
- Set pressure vs design pressure - the set pressure (0.45 barg) plus accumulation (typically 10% for fire) must not exceed the tank's design pressure (0.5 barg). 0.45 × 1.10 = 0.495 barg, which is just within 0.5 barg - acceptable only if the valve actually opens at 0.45 barg and the tank is rated for the accumulated pressure.
- Inlet and outlet line losses must be included in the rated capacity.
Conclusion: the fire case and coil failure must be calculated, the valve re-rated for the actual solvent, and the set/accumulation checked against the 0.5 barg design pressure before the tank is returned to service.