Page 292 - International safety guide for oil tankers and terminals

INTERNATIONAL SAFETY GUIDE FOR OIL TANKERS AND TERMINALS

The cessation of flow of liquid is propagated back up the pipeline at the speed
of sound in the fluid and, as each part of the liquid comes to rest, its pressure
---ใช้เพื่อการศึกษาเท่านั้น---
is increased by the amount P. Therefore, a steep pressure front of height P
travels up the pipeline at the speed of sound, a disturbance known as a
pressure surge.
pressure P and the output pressure of the pump at zero throughput (assumingย์นาวี
Upstream of the surge, the liquid is still moving forward and still has the
pressure distribution applied to it by the pump. Behind it, the liquid is stationary
and its pressure has been increased at all points by the constant amount P.
There is still a pressure gradient downstream of the surge, but a continuous
series of pressure adjustments takes place in this part of the pipeline which
ultimately results in a uniform pressure throughout the stationary liquid. These
pressure adjustments also travel through the liquid at the speed of sound.
งานห้องสมุด ศูนย์ฝกพาณิช
When the surge reaches the pump, the pressure at the pump outlet (ignoring
the atmospheric and hydrostatic components) becomes the sum of the surge

no reversal of flow), since flow through the pump has ceased. The process of
pressure equalisation continues downstream of the pump.

Again taking the hypothetical worst case, if the pressure is not relieved in any
way, the final result is a pressure wave that oscillates throughout the length of
the piping system. The maximum magnitude of the pressure wave is the sum
of P and the pump outlet pressure at zero throughput. The final pressure
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adjustment to achieve this condition leaves the pump as soon as the original
surge arrives at the pump and travels down to the valve at the speed of sound.
One pressure wave cycle therefore takes a time 2L/a from the instant of valve
closure, where L is the length of the line and a is the speed of sound in the
liquid. This time interval is known as the pipeline period.

In this simplified description, therefore, the liquid at any point in the line
experiences an abrupt increase in pressure by an amount P followed by a
slower, but still rapid, further increase until the pressure reaches the sum of P
and the pump outlet pressure at zero throughput.

In practical circumstances, the valve closure is not instantaneous and there is
then some relief of the surge pressure through the valve while it is closing.
The results are that the magnitude of the pressure surge is less than in the
hypothetical case and the pressure front is less steep.

At the upstream end of the line, some pressure relief may occur through the
pump and this would also serve to lessen the maximum pressure reached. If
the effective closure time of the valve is several times greater than the pipeline
period, pressure relief through the valve and the pump is extensive and a
hazardous situation is unlikely to arise.

Downstream of the valve, an analogous process is initiated when the valve
closes, except that, as the liquid is brought to rest, there is a fall of pressure
which travels downstream at the velocity of sound. However, the pressure drop
is often relieved by gas evolution from the liquid so that serious results may not
occur immediately, although the subsequent collapse of the gas bubbles may
generate shock waves similar to those upstream of the valve.

258 © ICS/OCIMF/IAPH 2006