Home >> Science & Nature >> Science

Factors That Control Flow in Pipes

Many industries constantly measure the fluid flow of various liquids. It can make a major difference in the profit/loss reports. The flow rate is determined by the liquid's velocity. That, in turn, depends on the pressure differential that forces the liquid through a pipe and the diameter of the pipe itself. The viscosity, drag, friction and density are all factors that exert an influence on the flow rate.

Factors Affecting Flow Rate
- Engineers calculate the flow rate by multiplying the velocity of the liquid by the cross-sectional area of the pipe. Several factors influence the flow rate or the velocity of a liquid through a pipe system. The factors inherent in the nature of the fluid itself include viscosity, temperature and density. Pipe characteristics such as length, inner diameter, internal roughness, elevation changes, bends, valves and the entrance and exit conditions of the pipe network all play a part in determining the flow rate of a liquid.
Viscosity and Drag
- Viscosity measures a liquid's internal resistance to flow. It is a drag force called fluid resistance. In pipe flow, drag is increased by the internal roughness or friction of the pipe conducting the liquid. Internal bumps and unevenness in a pipe produce a friction force that acts in a perpendicular direction to the flow. Here it opposes the liquid flow and slows down its velocity. If the viscosity is high, the velocity of the liquid is low, and a laminar flow results in the liquid flowing gently in layers through the pipe. A liquid with a low viscosity flows at a higher velocity, and turbulent flow results as the liquid breaks up into eddies.
Friction
- Other than PVC, many pipes are composed of various metals that eventually corrode internally from exposure to the liquid flow. Corrosion increases the internal roughness or friction within the pipe. Roughness is a calculated value that represents the average size of the irregularities or bumps on the internal pipe wall. Engineers calculate that value by comparing the size of the bumps to the internal diameter of the pipe. Smooth plastic pipes like PVC often have a roughness factor of zero, and corrosion-resistant pipes of copper exhibit a small roughness factor.
Liquid Denstity
- Density is a factor in the fluid flow of compressible materials such as gases. When pressure is applied, the gas will decrease in volume and increase in density. If the temperature is also lowered in the process, the gas will increase its density further by changing its state to liquid. In that liquids are incompressible, density is a factor in the mass flow rate. This is the amount or mass of a substance that can pass through a pipe during a given time at a defined flow rate. According to the laws of fluid dynamics, a liquid will increase its velocity when it passes through a pipe constriction, so the amount of mass that enters the pipe is the same amount that exits the pipe, and its density and flow rate do not change.

Science