Flare Header Sizing Design Guide

PROCEDURE NO.

PTD-DGS-105 PREPARED BY

PROCESS TECHNOLOGY PROCEDURES

Edited from existing doc by J.R.Barnhart

PAGE

OF

1

3

DATE

April 30, 2000

APPROVED BY DEPARTMENT: SUBJECT:

PROCESS ENGINEERING

FLARE HEADER SIZING DESIGN GUIDE

JRB REVISION DATE

REV.

11/01/2002

1.0

1

SCOPE

This design guide 1 establishes procedures for designing a flare system using a flare header design computer program. 2.0

RESPONSIBILITIES

The process engineer is responsible for the design and sizing of flare headers. 3.0

DEFINITIONS

The following definitions pertain to flare header sizing: 1. Destination pressure, expressed in pounds per square inch absolute (psia), is the terminal pressure from where the program works back. For flare stack, use 2 pounds per square inch gage (psig), unless the actual pressure drop across the stack is known. 2. Minimum inside pipe diameter fixes the minimum inside pipe diameter and is used only when a minimum size is required; otherwise, leave the input blank. 3. Fixed inside pipe diameter fixes the pipe inside diameter and is used when pressure drops are being calculated or when the pipe diameter is predetermined; 1 otherwise, leave the input blank. 4. Average gas temperature is the average gas temperature of the streams entering a pipe. Average gas temperature is calculated using the following equation: T =

W1 W2 Wh T1 + T2 + ...... Tn ∑W ∑W ∑ Wn

(Eq. 3-1)

5. Mach number limit is the largest ratio of fluid velocity to the speed of sound in the fluid. Use a mach number limit of 0.5 for header and 0.5 for branches. 6. Maximum percent backpressure is the maximum pressure that develops at the outlet of a relief valve. Use 10 percent of set pressure for conventional valves and 30 percent of set pressure for balanced valves. 7. Gas temperature is the temperature downstream of the relief valve, which must be calculated assuming adiabatic expansion. 8. Viscosity (optional) is the property of gas or liquids as expressed in centipoise at the flowing temperature. The computer uses 0.02 if this value is omitted. 1

Rev. 0 of the Design Guide was adapted with minimal changes from a Pasadena document, DSG-MP12C, 10/92

9. Specific heat ratio (optional) is the ratio of the specific heat at constant pressure to the specific heat at constant volume for any gas. The computer uses 1.4 if this value is omitted. 10. Critical case is the determining case for the pipe diameter, as indicated on the computer printout. 4.0

PROCEDURE 1. Review the program manual and follow the examples, paying attention to: a. The line identification method b. Case studies and critical cases 2. Study the flare system to determine: a. Flare system layout (see Figure 1) b. The different cases to be simulated c. Line identification method (allow for the addition of lines later) d. Relief valve operating conditions, including allowable percent backpressure e. Pipeline lengths and routing. Disregard the knockout drums; add their entrance loss to the inlet piping and their exit loss to the discharge piping. 3. Lay out the system so that lines and relief valves can be clearly numbered and identified. 4. Run the flare sizing program. 5. The simulation should be evaluated for the following considerations: a. Some pipeways have limits on the maximum pipe diameter, and two headers may be necessary. b. Since this represents cost, the ideal design would be of minimum weight. c. Each branch line needs to be checked by using backpressures calculated by this program at the junction of each branch with the header. To confirm that calculated size does not exceed the maximum allowable backpressure, use relief valve capacity, a Mach number of 0.7, and run line sizing. Increase branch line size, if necessary, to meet the criteria of a Mach number of 0.7 and maximum allowable backpressure.