Understanding Vapourisation in Pipeline Systems

When designing pipeline systems, especially those carrying liquid over varying terrain, it's crucial to prevent vapourisation of the liquid inside the pipe. Vapourisation, also known as cavitation, occurs when the absolute pressure of the liquid drops below its vapour pressure. This is particularly important when a pipeline rises over a high point or a ridge.

Why Vapourisation (Cavitation) is a Problem

Vapourisation leads to the formation of vapour bubbles within the liquid. When these bubbles collapse as they move into regions of higher pressure, they create shock waves that can cause significant damage to the pipe walls, fittings, and pumps over time. It can also disrupt flow and reduce the efficiency of the pipeline.

The Role of the Hydraulic Gradient Line (HGL)

The hydraulic gradient line (HGL) represents the sum of the pressure head and the elevation head along a pipeline. In simple terms, it shows the level to which the liquid would rise if a piezometer tube were inserted into the pipe at any point. If the pipeline runs above the HGL, it indicates that the pressure inside the pipe is below atmospheric pressure (gauge pressure is negative). While negative gauge pressure is possible, there is a limit to how low the absolute pressure can drop before vapourisation occurs.

Preventing Pipeline Vapourisation Over Ridges

To avoid vapourisation, pipelines laid over ridges or high points must be kept at or below a certain height relative to the hydraulic gradient line. The pressure inside the pipe decreases as the pipe rises above the HGL. If the pipe is too high, the absolute pressure can fall to the liquid's vapour pressure, causing it to vapourise.

The maximum allowable height is determined by considering atmospheric pressure, the vapour pressure of the liquid, and a safety margin. For water at standard conditions, the atmospheric pressure head is approximately 10.3 meters of water. The vapour pressure head is relatively small but not zero. To prevent vapourisation, the absolute pressure must remain above the vapour pressure. This typically means the pressure head (gauge pressure) must not drop too far below atmospheric pressure.

Based on common engineering practice and to maintain a sufficient absolute pressure margin above the vapour pressure, pipelines are typically designed so that they are not more than a specific distance above the hydraulic gradient line.

Maximum Allowable Height Above HGL

To avoid vapourisation in pipelines, they should not be laid more than a certain height above the hydraulic gradient line. This height is often limited to approximately:

• 6.4 meters

Keeping the pipeline at or below this height ensures that the absolute pressure inside the pipe remains high enough to prevent the formation of vapour bubbles, thereby avoiding cavitation and its damaging effects.

Understanding Pressure in Pipelines

The pressure at any point in a fluid system can be described in terms of absolute pressure or gauge pressure.

• Gauge Pressure: Pressure measured relative to atmospheric pressure. Positive gauge pressure means pressure is above atmospheric; negative gauge pressure means pressure is below atmospheric (vacuum).
• Absolute Pressure: Pressure measured relative to absolute zero pressure (perfect vacuum). Absolute pressure = Gauge Pressure + Atmospheric Pressure.

Vapourisation occurs when the absolute pressure drops to the liquid's vapour pressure. This corresponds to the gauge pressure dropping significantly below atmospheric pressure. Limiting the height of the pipe above the HGL directly limits how low the gauge pressure can become.

Revision Table: Pipeline Vapourisation Limits

Additional Information on Pipeline Design

Designing pipelines to avoid negative pressures is essential for long-term performance and structural integrity. Beyond preventing vapourisation, excessively low pressures can also lead to:

• Increased risk of leakage at joints or fittings.
• Potential for air to enter the pipeline through faulty seals.
• Collapse of the pipe if it is not designed to withstand external pressure greater than internal pressure.

Therefore, limiting the pipeline's elevation relative to the HGL, especially at high points like ridges, is a fundamental principle in hydraulic engineering design to ensure smooth and safe operation.