Page 291 - International safety guide for oil tankers and terminals

TERMINAL OPERATIONS

16.8 Generation of Pressure Surges in Pipelines
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16.8.1 Introduction

A pressure surge is generated in a pipeline system when there is an
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
abrupt change in the rate of flow of liquid in the line. In tanker loading
operations, it is most likely to occur as a result of one of the following:
Closure of an automatic shutdown valve.

Slamming shut of a shore non-return valve.
Slamming shut of a butterfly type valve.
Rapid closure of a power operated valve.

If the pressure surge in the pipeline results in pressure stresses or
displacement stresses in excess of the strength of the piping or its
components, there may be a rupture, leading to an extensive spill of oil.

16.8.2 Generation of a Pressure Surge
When a pump is used to convey liquid from a feed tank down a pipeline
and through a valve into a receiving tank, the pressure at any point in the
system while the liquid is flowing has three components:
ึ
Pressure on the surface of the liquid in the feed tank. In a tank with its
ullage space open to atmosphere, this pressure is that of the
atmosphere.

Hydrostatic pressure at the point in the system in question.
Pressure generated by the pump. This is highest at the pump outlet,
decreasing commensurately with friction along the line downstream of
the pump and through the valve to the receiving tank.

Of these three components, the first two can be considered constant
during pressure surge and need not be considered in the following
description, although they are always present and have a contributory
effect on the total pressure.

Rapid closure of the valve superimposes a transient pressure upon all
three components, owing to the sudden conversion of the kinetic energy
of the moving liquid into strain energy, by compression of the fluid and
expansion of the pipe wall. To illustrate the sequence of events, the
simplest hypothetical case will be considered, i.e. when the valve closure
is instantaneous, there is no expansion of the pipe wall, and dissipation
due to friction between the fluid and the pipe wall is ignored. This case
gives rise to the highest pressures in the system.

When the valve closes, the liquid immediately upstream of the valve is
brought to rest instantaneously.

This causes its pressure to rise by an amount P. In any consistent set of
units:
P = wav
where: w is the mass density of the liquid
a is the velocity of sound in the liquid
v is the change in linear velocity of the liquid, i.e. from its linear
flow rate before closure.

© ICS/OCIMF/IAPH 2006 257