Pharmaceutical Solvent Storage Tank Relief

Case Study 2: Pharmaceutical Solvent Storage Tank Relief

Scenario

A cGMP-compliant pharmaceutical manufacturing facility in Dublin, Ireland, installed a new 15,000-L stainless steel tank for storing anhydrous ethanol (USP grade). The tank operates at atmospheric pressure but requires emergency relief for fire exposure per IEC 61511 and EU GMP Annex 1. Constraints included zero product contamination risk (no spring-loaded valve internal parts contacting liquid), strict material compatibility (316L SS only), and verification traceability for regulatory audit. A conventional thermal expansion relief was insufficient due to low vapor pressure; fire-case sizing governed.

Given Data

• Fluid Type: liquid
• Required Relieving Capacity: 1,840 lb/h (calculated per API RP 520 Eq. 3-21 for 15,000-L tank exposed to 100% pool fire: $W = 0.000167 \cdot A^{0.82} \cdot H_v$; $A = 32.5\ \text{m}^2$, $H_v = 350\ \text{Btu/lb}$ → $W \approx 835\ \text{kg/h} = 1,840\ \text{lb/h}$)
• Set Pressure: 3.5 psi (0.24 barg — just above max operating pressure to prevent nuisance lifting)
• Overpressure: 0.7 psi (20% of set pressure, per EN ISO 4126-1 for low-pressure systems)
• Back Pressure: 0.2 psi (friction loss in short vent line to roof-mounted atmospheric vent)
• Coefficient of Discharge: 0.62 (conservative value for balanced bellows relief valve with metal-to-metal seat, per ISO 4126-7 Annex B)

Calculation

For liquids, the tool uses API RP 520 Eq. 3-19:

$$ A = \frac{Q}{C_d \cdot K_v \cdot \sqrt{\frac{\Delta P}{\rho}}} $$

Where:

• $Q = 1,840\ \text{lb/h} = 0.511\ \text{lb/s}$
• $\Delta P = P_{set} + P_{overpressure} - P_{back} = 3.5 + 0.7 - 0.2 = 4.0\ \text{psi} = 576\ \text{psf}$
• $\rho = 49.2\ \text{lb/ft}^3$ (ethanol at 20°C)
• $K_v = 1.0$ (viscosity correction negligible; $\mu < 1\ \text{cP}$)

Tool computes:

• Orifice Area = 0.2163 in²
• Valve Size = 1.00 in (NPS — corresponds to standard ISO 4126-1 DN25 valve with minimum orifice diameter ~0.52 in; 1-inch NPS inlet provides adequate flow path and meets minimum mechanical strength requirements)

Result and Decision

A DN25 (1-inch) balanced bellows relief valve (316L SS body, Hastelloy C-276 bellows, PTFE-free metal seat) was procured and installed. The valve was certified to ISO 4126-1 with full test reports and material traceability. Fire-case capacity was verified via third-party calculation review and accepted by the Irish Medicines Board (IMB) during pre-commissioning audit.

Lesson

For low-set-pressure liquid relief applications, back pressure—even if small—must be subtracted before calculating net differential pressure ($\Delta P$); misapplying it as additive (like in gas cases) leads to severe undersizing. Here, omitting the 0.2 psi back pressure would have inflated $\Delta P$ by 5%, reducing calculated area by ~2.5%—potentially pushing the selection into an unqualified orifice class.