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Design Considerations for Silencers Vesting Flamable Gas

IP.com Disclosure Number: IPCOM000219907D
Publication Date: 2012-Jul-17
Document File: 4 page(s) / 206K

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This text was extracted from a PDF file.
This is the abbreviated version, containing approximately 23% of the total text.

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1984 Industrial Pollution Control Symposium ASME Publication No. 100175
American Society of Mechanical Engineers

DESIGN CONSIDERATIONS FOR SILENCERS VENTING FLAMMABLE GASES

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ABSTRACT

   This paper examines the design considerations
for silencers which vent flammable gases. The proper selection of these silencers is a compromise between conflicting objectives. On one hand, the silencer outlet size should be large to reduce the exit velocity of the gas in order to prevent regenerated noise. On the other hand, the possible ignition of the vent stream demands a high exit velocity So as to maintain a properly shaped flame in crosswinds. This paper presents a procedure for dealing with these cross-purposes, along with a description of design features recontnended for silencers in this application.

NOMENCLATURE

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INTRODUCTION

    In the energy industry it is often necessary to vent gases to the atmosphere. When the nature of the relieved gas does not present a safety hazard, the design of these vents is a relatively

velocity of sound in atmosphere (m/s)
dianmter of vent stack (m)

distance from vent to receiver (m)
peak frequency (Hz) logarithm to the base 10 overall sound pressure
level of combustion noise component at 300 m from
the flare (dB re: 2 x 10-Spa)

overall sound pressure level of regenerated n~ise component (dB re: 2 x lO-~Pa)

Reynolds number

Strouhal number
exit velocity of vented gas (m/s)
flow rate of the flammable components in the vent stream (kg/hr)

density of the vented gas at atmospheric pressure (kg/m3)

density of air a~ ambient conditions (kg/mj)

DESIGN BASIS: PRESUMPTION OF IGNITION

    The ignition of a flammable vent stream may seem improbable, but there are actually several possible sources of ignition:

open flames hot surfaces

electrical equipment lightning
static electricity exothermic chemical reaction autoignition

    Because the mortal engineer does not have absolute control over all of these possible sources, he must assume the released gas will ignite and design the vent system accordingly,

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Page 02 of 4

DESIGN CONSIDERATION #I: HEAT RADIATION

    The most important consideration in the design of flan~nable gas vents is the analysis of the potential heat release. The thermal radiation from a vent will depend on the heat liberatlon, the emissivity, the size of the flame, and the distance between the flame and the point of interest.

   Several methods for calculating thermal radiation levels are publlshed in the literature.

The industry stanqard on the subject, the APl Recon~ended Practice 521 "Guide for Pressure- Relieving and Depressuring Systems" (I) o contains two methods for performing this calculation: the sooceI1ed "Simple Approach" and the method of Brzustowski and Summer (.~_). The choice of which method to use is a matter of reader preference.

DESIGN CONSIDERATION #2: DILUTION OF THE VENT GAS

    In order to confine the flammable zone to elevations above the level of release, it is necessary to discharge t...