ERT 312
SAFETY & LOSS PREVENTION IN
BIOPROCESS
 Calculate the size of various relief apparatus
used in different situation.
 (A) Spring operated reliefs in liquid and gas
service.
 (B)Rupture disc reliefs in liquid and gas service.
 (C)Vents for low pressure and high pressure.
Determine the vent area of the
relief device
1) To determine the rate of material release
2) Using an appropriate equation
(hydrodynamic principles), to determine the
relief device vent area
 Relief vent area calculation
depends on:
a) type of flow (liquid, vapor or twophase)
b) type of relief device (spring or rupture
disc)
A relief pressure is designed to maintain the pressure at the set
pressure.
Normally, relief devices are specified for overpressures from
10 to 25%
Figure 10-1
 Liquid velocity through the spring relief:
P drop across the relief
 Relief area:
A = the computed relief area (in2).
Qv = the volumetric flow through the relief (gpm),
C0 = the discharge coefficient (unitless),
(ρ / ρref) = the specific gravity of the liquid (unitless),
ΔP = the pressure drop across the spring relief (lb/in2).
Kv = the viscosity correction (unitless),
Kp = the overpressure correction (unitless),
Kb = the backpressure correction (unitless),
Ps = the gauge set pressure (lb/in2), and
Pb = the gauge backpressure (lb/in2).
For most reliefs the Renolds no. is greater than 5000, and the correction is
near 1.
 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.
 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 deadheaded 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.
 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 9-10.
Refer Example 9-2
 Relief area:
A = the computed relief area (in2).
Qv = the volumetric flow through the relief (gpm),
C0 = is the discharge coefficient (unitless),
(ρ / ρref) is the specific gravity of the liquid (unitless),
ΔP = the pressure drop across the spring relief (lb/in2).
 Flow of vapor through rupture discs:
 Where discharge coefficient C0 = 1.0
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/ρref) is 1.0.
 A conservative discharge coefficient of 0.61 is assumed.
 Substituting into Equation 9-3, we obtain
ASSIGNMENT IN CLASS – RELIEF SIZING
Estimate the diameter of spring-type liquid
reliefs for the following conditions: