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RELIEF SIZING

RELIEF SIZING. ERT 312 SAFETY & LOSS PREVENTION IN BIOPROCESS. OBJECTIVES. (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.

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RELIEF SIZING

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  1. RELIEF SIZING ERT 312SAFETY & LOSS PREVENTION IN BIOPROCESS

  2. OBJECTIVES • (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.

  3. Relief sizing calculations are performed to: Determine the vent area of the relief device The procedure involved: • To determine the rate of material release • Using an appropriate equation (hydrodynamic principles), to determine the relief device vent area

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

  5. 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

  6. Conventional Spring operated reliefs in liquid service P drop across the relief • 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), and Pb = the gauge backpressure (lb/in2).

  7. For most reliefs the Renolds no. is greater than 5000, and the correction is near 1.

  8. 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.

  9. 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.

  10. 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.

  11. Spring operated reliefs in gas service • Vapor discharge mass flow

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

  13. The constant X can be calculated using Equation 9-10. Refer Example 9-2

  14. Rupture disc reliefs in liquid service • 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).

  15. Rupture disc reliefs in gas service • Flow of vapor through rupture discs: • Where discharge coefficient C0 = 1.0

  16. 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.

  17. Substituting into Equation 9-3, we obtain

  18. ASSIGNMENT IN CLASS – RELIEF SIZING Estimate the diameter of spring-type liquid reliefs for the following conditions:

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