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Noise Level Calculations: Example #1

The document describes formulas for calculating noise levels from pressure relief valves discharging gases, vapors, and steam. The key formulas calculate the sound level at 100 feet (L100) based on factors like the mass flow rate, temperature, and molecular weight. A second formula calculates the sound level (Lp) at any distance by adjusting the L100 based on the distance. An example calculation is shown for a natural gas relief valve, finding the noise level would be around 89.9 decibels at a distance of 500 feet.

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Susheel Wankhede
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175 views2 pages

Noise Level Calculations: Example #1

The document describes formulas for calculating noise levels from pressure relief valves discharging gases, vapors, and steam. The key formulas calculate the sound level at 100 feet (L100) based on factors like the mass flow rate, temperature, and molecular weight. A second formula calculates the sound level (Lp) at any distance by adjusting the L100 based on the distance. An example calculation is shown for a natural gas relief valve, finding the noise level would be around 89.9 decibels at a distance of 500 feet.

Uploaded by

Susheel Wankhede
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as DOCX, PDF, TXT or read online on Scribd
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Noise Level Calculations

The following formulae are used for calculating noise level


of gases, vapors and steam as a result of the discharge of
a pressure relief valve. The expressed formulae are
derived from API Recommended Practice 521. Table T7-4
on page 7-11 lists relative noise levels.
L100 = L + 10 LOG10 (0.29354 W k T/M)
Where:
L100 = Sound level at 100 feet from the point of
discharge in decibels.
L = Noise intensity measured as the sound pressure
level at 100 feet from the discharge.
Reference Figure F7-7 on page 7-11.
W = Maximum relieving capacity, pounds per hour.
k = Ratio of specific heats of the fluid. Reference
Table T7-7 on page 7-26.
(For steam, k = 1.3 if unknown.)
T = Absolute temperature of the fluid at the valve
inlet, degrees Rankine (°F + 460).
M= Molecular weight of the gas or vapor obtained
from standard tables or Table T7-7 on page 7-26.
(For steam, M = 18)
When the noise level is required at a distance of other
than 100 feet, the following equation shall be used:
Lp = L100 - 20 LOG10 (r/100)
Where:
Lp = Sound level at a distance, r, from the point of
discharge in decibels.
r = Distance from the point of discharge, feet.
Example #1
Gas/Vapor Mass Flow (lb/hr)
Fluid: Natural Gas
Set Pressure: 210 psig
Overpressure: 10%
Back Pressure: 50 psig
Inlet Relieving Temperature: 120F
Molecular Weight: 19.0 (page 7-26)
Compressibility: 1
Selected Area: 0.503 square inches
Noise to be calculated at: 500 feet
Although the required capacity has been given, the noise
level of the valve should be calculated on the total flow
through the selected valve at the specified overpressure.
Therefore the rated flow must first be calculated. The
following formula is a rearrangement of the area calculation
formula for gas and vapor in mass flow units (lb/hr).
Reference page 5-3.
P1 = Absolute relieving pressure
210 + 21 + 14.7 = 245.7 psia
Kb = 1.0
C = 344 from Table T7-7 on page 7-26.
K = 0.975
W = A P1 C K Kb M / TZ
W = (0.503)(245.7)(344)(0.975)(1)( 19)/ (580)(1)
W = 7502 lb/hr
T = 120F + 46F = 580R
Pb = 50 psig
Continuing with the noise level calculation:
L100 = L + 10 LOG10 (0.29354 W k T/M)
Where:
PR = Absolute relieving pressure/absolute back
pressure
= P1 (Pb + 14.7) = 245.7/(50 + 14.7) = 3.8
L = 54.5 (Figure F7-7, page 7-11.)
k = 1.27 (Table T7-7, page 7-26.)
W = 7502 lb/hr
M = 19
T = 120F + 460F = 580R
L100=54.5+ 10 LOG10 [(0.29354)(7502)(1.27)(580)/(19)]
L100=103.8 decibels
At a distance of 500 feet:
Lp = L100 - 20 LOG10 (r/100)
Where:
r = 500 feet
Lp = 103.8 - 20 LOG10 (500/100)
Lp = 89.9 decibels

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