ERT 312 SAFETY & LOSS PREVENTION IN BIOPROCESS RELIEF SIZING Prepared by: Pn. Hairul Nazirah Abdul Halim

OBJECTIVES Calculate the size of various relief apparatus: a) Spring operated reliefs in liquid & gas service b) Rupture disc reliefs in liquid and gas service

Relief vent area calculation depends on: a) type of flow (liquid, vapor or two-phase) b) type of relief device (spring or rupture disc)

Conventional Spring operated reliefs in liquid service Liquid velocity through the spring relief: Relief area: A = the computed relief area (in 2 ). Q v = the volumetric flow through the relief (gpm), C 0 = the discharge coefficient (unitless), (ρ / ρ ref ) = the specific gravity of the liquid (unitless), ΔP = the pressure drop across the spring relief (lb/in 2 ). K v = the viscosity correction (unitless), K p = the overpressure correction (unitless), K b = the backpressure correction (unitless), P s = the gauge set pressure (lb/in 2 ), and P b = the gauge backpressure (lb/in 2 ).

Definitions Set pressure – the gauge pressure at which the relief begins to activate. Maximum allowable working pressure (MAWP) – the maximum gauge pressure permissible at the top of a vessel for a designated temperature. Operating pressure – the gauge pressure during normal service, usually 10% below the MAWP.

Definitions Overpressure – the pressure increase in the vessel over the set pressure during the relieving process. Backpressure – the pressure at the outlet of the relief device process resulting from pressure in the discharge system. Refer to Figure 8-3.

Example 9-1 A positive displacement pump pumps water at 200 gpm at a pressure of 200 psig. Because a dead- headed pump can be easily damaged, compute the area required to relieve the pump, assuming a backpressure of 20 psig and a 10% overpressure. Solution a. The set pressure is 200 psig. The backpressure is specified as 20 psig and the overpressure is 10% of the set pressure, or 20 psig.

Spring operated reliefs in gas service Vapor discharge mass flow

Relief vent area for Ideal gas Relief vent area for Non-ideal gas

The constant X can be calculated using Equation Do Example 9-2

Rupture disc reliefs in gas service Flow of vapor through rupture discs: Where discharge coefficient C 0 = 1.0

Rupture disc reliefs in liquid service Relief area: A = the computed relief area (in 2 ). Q v = the volumetric flow through the relief (gpm), C 0 = is the discharge coefficient (unitless), (ρ / ρ ref ) is the specific gravity of the liquid (unitless), ΔP = the pressure drop across the spring relief (lb/in 2 ).

Example 9-3 Determine the diameter of a rupture disc required to relieve the pump of Example 9-1, part a. Solution The pressure drop across the rupture disc is The specific gravity of the water (ρ/ρ ref ) is 1.0. A conservative discharge coefficient of 0.61 is assumed.

Substituting into Equation 9-3, we obtain