ERT 322SAFETY & LOSS PREVENTION

(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.

Calculate the size of various relief apparatus used in different situation.

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

b) type of relief device (spring or rupture disc)

Relief vent area calculation depends on:

a) type of flow (liquid, vapor or two-phase)

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:

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), andPb = the gauge backpressure (lb/in2).

P drop across the relief

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 9-1A 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.

Solutiona. 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 10-2

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 9-3Determine the diameter of a rupture disc required to relieve the pump of Example 9-1, part a.

SolutionThe 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