Page 293 - International safety guide for oil tankers and terminals

TERMINAL OPERATIONS

16.9 Assessment of Pressure Surges
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16.9.1 Effective Valve Closure Time

In order to determine whether a serious pressure surge is likely to occur
pressures are usually known.ย์ฝกพาณิชย์นาวี
in a pipeline system, the first step is to compare the time taken by the
valve to close with the pipeline period.

The effective closure time, i.e. the period during which the rate of flow is
in fact decreasing rapidly, is usually significantly less than the total time of
movement of the valve spindle. It depends upon the design of the valve,
which determines the relationship between valve port area and spindle
position. Substantial flow reduction is usually achieved only during the
closure of the last quarter or less of the valve port area.

If the effective valve closure time is less than, or equal to, the pipeline
period, the system is liable to serious pressure surges. Surges of
reduced, but still significant, magnitude can be expected when the
effective valve closure time is greater than the pipeline period, but they
become negligible when the effective valve closure period is several times
greater than the pipeline period.

16.9.2 Derivation of Total Pressure in the System
ึ
In the normal type of ship/shore system handling petroleum liquids,
where the shore tank communicates to the atmosphere, the maximum
pressure applied across the pipe wall at any point during a pressure
งานห้องสมุด ศูน
surge is the sum of the hydrostatic pressure, the output pressure of the
pump at zero throughput and the surge pressure. The first two of these

If the effective valve closure time is less than or equal to the pipeline
period, the value of the surge pressure used in determining the total
pressure during the surge should be P, derived as indicated above in
Section 16.8.2. If it is somewhat greater than the pipeline period, a
smaller value can be used in place of P and, as already indicated, the
surge pressure becomes negligible if the effective valve closure time is
several times greater than the pipeline period.

16.9.3 Overall System Design

In practice, the design of a more complex system may need to be taken
into account. In this Section, the simple case of a single pipeline has
been considered. For example, the combined effects of valves in parallel
or in series may have to be examined. In some cases, the surge effect
may be increased. This can occur with two lines in parallel if closure of
the valve in one line increases the flow in the other line before this line, in
its turn, is shut down. On the other hand, correct operation of valves in
series in a line can minimise surge pressure.

Transient pressures produce forces in the piping system which can result
in large piping displacements, pipe rupture, support failure, and damage
to machinery and other connected equipment. Therefore, the structural
response of the piping system to fluid induced loads resulting from fluid
pressures and momentum must be considered in the design. In addition,
restraints are usually required to avoid damage ensuing from large
movements of the piping itself. An important consideration in the selection
of the restraints is the fact that the piping often consists of long runs of

© ICS/OCIMF/IAPH 2006 259