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Thursday, January 7, 2010

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Earlier post "Assess Potential Piping Failure Due to CV, BDV & PRV Quick Opening with Vapor Discharge" discussed about  severe vibration and peak force act on the piping due to quick opening of valve. An assessment method to assess the potential failure of piping due to quick opening of valve has been presented. The equation in determining peak for is mainly for dry gas and/or vapor. This post will focus on two phase vapor liquid at the outlet of valve.

Peak force induced by sudden two-phase vapor liquid release
When a valve quick open from FULL Close to FULL Open, instantaneously release two-phase vapor liquid at downstream piping results peak force (FMax, kN) considering two-phase vapor liquid is in homogenous flow or frozen flow with no-slip.

W = Vapor mass flow (kg/s)
Di = Pipe internal diameter (m)
x = Vapor mass fraction
ρV = Vapor density (kg/m3)
ρL = Liquid density (kg/m3)

Piping Limiting Force
A steel piping with Piping limiting force (FLimit, kN),

with piping wall thickness correction factor,

Do = Pipe external diameter (m)
Di = Pipe internal diameter (m)
Wt = Pipe wall thickness (mm)
WtSch_40 = Schedule 40 pipe wall thickness (mm)
C =  Pipe support correction factor

Pipe support stiffness level and correction factor
Pipe support stiffness level and correction factor subjects to support span length (LS) and pipe external diameter (Do) which can be determined from below chart.

Read more in "Quick Determination Pipe Support Stiffness Level and Correction Factor" for equations for differentiating support type.

Assessment Criteria
To ensure piping will not failed on sudden opening of valve, the following shall be met :

FMax < 0.3 FLimit

In  the event FMax is more than 0.3 but less than 0.5 of FLimit, a detail small bore connection checking shall be conducted.

Ref :
1. "Guideline for avoidance of vibration induced fatigue in process work"

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posted by Webworm, 4:06 PM


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