General Characteristics of Pipe Flow

Although not all conduits used to transport fluid from one location to another are round in cross section. These include typical water pipes, hydraulic hoses and other conduits that are designed to withstand a considerable pressure difference. There are two situations where one pipe is completely filled with the fluid being transported (pipe flow) and one through which rainwater flows without completely filling the pipe (open-channel flow). The difference between open-channel flow and pipe flow is in the fundamental mechanism that drives the flow. For open-channel flow, gravity alone is the driving force. For pipe flow, gravity may be important, but the main driving force is likely to be a pressure gradient along the pipe.

Fig. 2. Pipe Flow

Terms and Vocabulary

pressure difference	разность давлений
pipe flow	поток в трубопроводе (в полностью заполненной трубе)
open-channel flow	поток (в не полностью заполненной трубе)
driving force	движущая сила
pressure gradient	градиент давления

Laminar or Turbulent Flow

The flow of a fluid in a pipe may be laminar or turbulent. The distinction between laminar and turbulent pipe flow was first pointed out by Osborne Reynolds (1883). The actual transition or turbulent flow may take place at various Reynolds numbers, depending on how much the flow is disturbed by vibrations of the pipe, roughness of the entrance region, etc.

Fig. 3. Fluid Flow in a Pipe

The region of flow near where the fluid enters the pipe is the entrance region. A boundary layer is produced along the pipe wall so that the initial velocity profile changes with distance along the pipe, until the fluid reaches the end of the entrance length. The flow in long, straight constant diameter sections of a pipe becomes fully developed, i.e. the velocity profile is the same at any cross section of the pipe. The details of velocity profile are different for laminar and turbulent flows. The nature of the pipe flow is strongly dependent on whether the flow is laminar or turbulent. This is a direct consequence of the differences in the nature of shear stress in laminar and turbulent flows. The shear stress in laminar flow is a direct result of momentum transfer among the randomly moving molecules. The shear stress in turbulent flow is largely a result of momentum transfer among the randomly moving molecules, finite-sized bundles of fluid particles.

Fig.4. Entrance region, developing flow, and fully developed flow in a pipe system

Terms and Vocabulary

laminar flow	ламинарный поток
turbulent flow	турбулентный поток
transition(al) flow	переходный поток
roughness	шероховатость стенки трубы
entrance region	входной участок
boundary layer	граничный слой
cross section	поперечное сечение
velocity profile	скоростной профиль
fully developed flow	развитый поток
developing flow	развивающийся поток
shear stress	напряжение сдвига
momentum transfer	передача (перенос) импульса
pipe system	система трубопроводных линий, сеть трубопроводов

Valves

Valves are mechanical devices that are installed in pipelines to control flow or pressure. Valves are an important part of piping systems and if not properly selected and operated, they can cause operation problems. The primary valve types, classified by their function, are:

• control valves – used to control flow, pressure, liquid level, cavitation and pressure transients;

• isolation (block) valves – placed on each side of control valves and pumps, allowing them to be removed for repair or replacement;

• check valves – used to prevent reverse flow;

• relief valves – admit air to the pipe while the pipe is being drained to prevent excessive vacuum pressures and reduce the possibility of collapsing thin-walled pipes;

• air valves – designed to expel large amounts of air at low pressure during filling and release small amounts of pressurized air during operation.

(Kennedy, John L., Oil and Gas Pipeline Fundamentals, USA. 1999)