Core Difference: Fluid Viscosity

Water: Its viscosity is very low (approximately 1 centipoise at room temperature). This low viscosity means that water generates very little internal frictional resistance when flowing through a pipe.

Oil (especially crude oil): Its viscosity is much higher than water (ranging from a few centipoise to several thousand centipoise at room temperature, with heavy crude oil being particularly high). This high viscosity means that oil generates significant internal frictional resistance when flowing.

The Impact of Viscosity on Pipeline Transportation: Overcoming Friction Losses

Fluid flow in a pipeline must overcome two main types of resistance:

Frictional resistance: Friction between fluid molecules and between the fluid and the pipe wall.

Altitude resistance: The energy required to lift the fluid to a higher altitude.

For long-distance pipelines, frictional resistance is often the dominant factor.

Water (low viscosity): Due to its low viscosity, water generates a relatively small frictional pressure drop when flowing through a pipe at the same flow rate and pipe diameter. Therefore, exceptionally high pumping pressures are not required to maintain the required flow rate and flow rate. Lower pressures (typically in the range of a few MPa to over ten MPa) are usually sufficient.

Oil (high viscosity): High viscosity results in a significant frictional pressure drop. If pressure is insufficient, oil flow can become very slow or even stagnate (especially at low temperatures or when transporting heavy oils). To maintain the necessary flow rate and flow rate, very high pressures (typically in the range of a few MPa to over ten MPa or even higher, with mains pressures of 4-10 MPa or more common) must be applied to overcome the significant frictional resistance. High pressure is essential for moving viscous oils over long distances.

Economic Considerations: Pipe Diameter and Pumping Stations

Ways to Reduce Friction Losses: There are two main approaches:

Increasing Pipe Diameter: A larger pipe diameter significantly reduces flow velocity, thereby significantly reducing friction losses (friction losses are roughly proportional to the square of the flow velocity). 

Increasing Pressure: This provides a greater driving force (pressure differential) to overcome friction losses. 

Choosing Water Pipelines: Water has low viscosity, so friction losses are inherently low. Even for long-distance transportation, increasing pipe diameter is generally a cost-effective option. While large-diameter water pipes have higher initial construction costs, they require lower operating pressures, resulting in relatively lower energy consumption and maintenance costs for pumping stations. Low-pressure operation is also safer and requires less material. Therefore, a common strategy is "low-pressure, large-diameter pipes." 

Choosing Oil Pipelines: Oil has high viscosity, resulting in significant friction losses. Simply increasing pipe diameter to reduce losses would require a very large pipe diameter, resulting in:

A sharp increase in pipeline material costs (steel).

Construction difficulty and costs (welding, laying, and corrosion protection) increase dramatically.

It takes up more space.

For long-distance, transnational/intercontinental pipelines, this is unacceptable.

Solution - High Pressure: Oil pipelines tend to utilize higher pressures combined with relatively smaller pipe diameters:

Small pipe diameters reduce material and construction costs.

High pressure provides sufficient driving force to overcome the significant friction caused by high viscosity.

Although high pressure requires thicker pipe walls (increasing material costs), more powerful pumps, and stricter sealing/safety measures, on balance, for long-distance transportation of high-viscosity fluids, a "high-pressure, small-diameter" solution is generally more economical than a "low-pressure, extra-large-diameter" solution. The investment and operating costs of high-pressure pumping stations are offset by significant savings in pipe and construction costs.

Density Difference (Minor Factor)

The density of water (~1000 kg/m³) is typically higher than that of crude oil (~800-900 kg/m³) and refined petroleum products.

When overcoming elevation changes (such as crossing mountains and hills), the higher the density, the higher the static head required to lift the fluid. This means that for the same amount of elevation gain, water pipelines must overcome a higher static head than oil pipelines. This is indeed a disadvantage for water pipelines.

However, in long pipelines, friction losses are often far greater than elevation losses. The significant friction losses caused by high viscosity are the primary driving force behind the high pressures required for oil pipelines, with the impact of density differences being relatively minor.

Other Factors

Transport Purpose: Municipal water supply typically has a maximum pressure limit (to prevent bursts and ensure appropriate water pressure at the customer end), while industrial oil transportation focuses primarily on efficient transportation.

Leakage Risk: The environmental and fire risks of leaks in high-pressure oil transportation are extremely high, but this does not prevent its use, as economic efficiency is the primary consideration. The risk of leaks in high-pressure water pipelines is relatively low (primarily due to flooding losses).

Fluid Compressibility: Although both oil and water have low compressibility, at extremely high pressures, this slight compressibility may have a subtle impact on system design (such as water hammer protection), but it is not the primary factor in determining pressure.

Heating to Reduce Viscosity: Some high-viscosity crude oil pipelines use heating to reduce viscosity, allowing operation at relatively low pressures. However, this requires additional heating stations and insulation measures, resulting in high energy consumption and costs. For most long-distance pipelines, high-pressure cold transmission remains the mainstream.

Summary:

The core reason for high-pressure operation in oil pipelines (compared to water pipelines) is the high viscosity of oil. This high viscosity creates significant frictional resistance, necessitating high pressure to drive the fluid to achieve the required flow rate. Adopting a "high-pressure, small-diameter" approach is more economical than adopting a "low-pressure, large-diameter" approach to reduce friction losses.

Because water has low viscosity and minimal frictional losses, a "low-pressure, large-diameter" approach often meets water pipeline transportation requirements. This approach offers advantages in terms of overall cost (construction and operation), safety, and end-use requirements. Density differences (water is heavier than oil) can slightly hinder water pipelines when overcoming elevation changes, but the impact is far less significant than differences in viscosity.

Thus, high pressure is not an inherent "property" of oil or water pipelines; rather, it is a design choice made by engineers based on the physical properties of the fluid (primarily viscosity) and economic efficiency to achieve their transportation goals.

For leaks in high-pressure water pipelines, high-pressure pipe repair clamps can be used.

For leaks in high-pressure oil pipelines, high-pressure pipe repair clamps can be used.