Piping Systems: Properties of fluid, Fluid flows, Pressure losses in pipes & components

Piping Systems

Piping system selection

is an important aspect of system design in any energy consuming system. Selection issues such as pipe material, configuration, diameter, insulation, etc. have a specific impact on the overall energy consumption of the system. Piping is one of those rare systems where when you’re oversized, you’ll usually save energy, unlike a motor or a pump.

 The design of piping systems in large industrial complexes such as refineries, petrochemical plants, fertilizer plants, etc. is now done using design software, allowing us to try out many possibilities. It is relatively small and medium users who often do not have access to design tools that use different rules of thumb for pipe size selection in industrial installations. These pipeline design methods are based on “doing first” or “learning guess”. Since everything we do is based on sound economic principles to keep costs down, some common piping design rules may also be changed to accommodate current piping material costs and system energy costs. It is important to remember that there is no universal rule that will apply in all situations. They should be developed for different situations.

 For example, a 1km-long plumbing system that pumps water from the pumping station from the riverbed to the plant will have different regulations than a 5-meter-long plumbing system to supply water from the main collection tank to the reactor. The question of pipe size, i.e. diameter, therefore, must be based on reducing the overall cost of owning and operating the system.

This guide covers piping best practices with a primary focus on reducing energy costs, keeping in mind safety and reliability issues. The basic elements of best practices in piping are:

  • Analysis & optimum pipe size selection for water, compressed air and steam distribution systems
  • Good piping practices
  • Thermal insulation of piping system

Physical Properties of Fluids

 The characteristics related to the fluid flow are summarized below.

Density:

This is the mass per unit volume of a liquid and is usually measured in kg/m3. Another commonly used term is specific gravity. It is actually a relative density, comparing the density of a liquid at a given temperature with the density of water at the same temperature.

Viscosity:

This describes how easily a fluid flows. A substance like molasses has a high viscosity, while water has a much lower value. Gases, like air, have an even lower viscosity. The viscosity of a liquid can be described in two ways.

Absolute (or dynamic) viscosity:

This is a measure of a fluid’s resistance to internal deformation. It is expressed in Pascal seconds (Pa s) or Newton seconds per square metre (Ns/m2). [1Pas = 1 Ns/m2]

Kinematic viscosity:

This is the ratio of the absolute viscosity to the density and is measured in metres squared per second (m2/s).

Reynolds Number:

A useful factor in determining the type of flux involved is the Reynolds number. It is the ratio between the dynamic force of the mass flow and the shear resistance due to the fluid’s viscosity and is given by the following formula. Generally for liquids such as water when the Reynolds number is less than 2000 the flow is laminar. The turbulent flow for Reynolds numbers greater than 4000. Between these two values (2000 & lt; Re & lt; 4000) the flow is a mixture of the two and it is difficult to predict the behavior of the fluid

Types of Fluid Flow: 

 When a fluid moves through a pipe, there can be two distinct types of flow, laminar flow, and turbulent flow.

Laminar flow occurs in moving fluids with low mean velocities, and turbulent flows become apparent as the velocity increases above the critical velocity. In laminar flow, liquid particles move along the pipeline in a very orderly manner, with little or no horizontal movement along the width of the pipeline.

Turbulent flow is characterized by random and disorganized movement of particles, from side to side of the pipe as well as along its length. However, there will always be a laminar flow layer at the pipe wall – the so-called “boundary layer”. The two types of fluid flows are described by

different sets of equations. In general, for most practical situations, the flow will be turbulent.

Pressure Loss in Pipes

 Whenever fluid flows in a pipe there will be some loss of pressure due to several factors:

  • Friction: This is affected by the roughness of the inside surface of the pipe, the pipe diameter, and the physical properties of the fluid.
  • Changes in size and shape or direction of flow
  • Obstructions: For normal, cylindrical straight pipes the major cause of pressure loss will be friction. Pressure loss in a fitting or valve is greater than in a straight pipe. When fluid flows in a straight pipe the flow pattern will be the same through out the pipe. In a valve or fitting changes in the flow pattern due to factors (b) and (c) will cause extra pressure drops. Pressure drops can be measured in a number of ways. The SI unit of pressure is the Pascal. However, pressure is often measured in bar.

This is illustrated by the D’Arcy equation:

Before establishing pipeline losses, the coefficient of friction  must be calculated. The coefficient of friction  will depend on the size of the pipe, the internal roughness of the pipe, the flow velocity and the viscosity of the fluid. Flow conditions, whether “turbulent” or not, will determine the method used to calculate the coefficient of friction.

Figure 2.1 can be used to estimate the coefficient of friction. The roughness of the pipe is necessary to estimate the coefficient of friction. Diagram showing the relationship between Reynolds number and pipe friction. The calculation of coefficients of friction depends on the type of flow to be encountered. For the number Re  = 2320, the fluid flow is turbulent.

The following table gives typical values of pipe absolute roughness, k. The relative roughness k/d can be calculated from k and the inner diameter of the pipe.

Figure 2-1: Estimation of friction factor

The absolute roughness of pipes is given below.

Type of pipe k, mm
Plastic tubing 0.0015
Stainless steel 0.015
Rusted steel 0.1 to 1.0
Galvanized iron 0.15
Cast iron 0.26

A sample calculation of pressure drop is given below.

A pipe of 4” Dia carrying water flow of 50 m3/h through a distance of 100 metres. The pipe material is Cast Iron with absolute roughness of 0.26.

Standard Pipe dimensions

 There are a number of piping standards in existence around the world, but arguably the most global are those derived by the American Petroleum Institute (API), where pipes are categorised in schedule numbers. These schedule numbers bear a relation to the pressure rating of the piping. There are eleven Schedules ranging from the lowest at 5 through 10, 20, 30, 40, 60, 80, 100, 120, 140 to schedule No. 160. For nominal size piping 150 mm and smaller, Schedule 40 (sometimes called ‘standard weight’) is the lightest that would be specified for water, compressed air and steam applications. High-pressure compressed air will have schedule 80 piping.

Regardless of schedule number, pipes of a particular size all have the same outside diameter (not withstanding manufacturing tolerances). As the schedule number increases, the wall thickness increases, and the actual bore is reduced. For example:

  • A 100 mm Schedule 40 pipe has an outside diameter of 30 mm, a wall thickness of 6.02 mm, giving a bore of 102.26 mm.
  • A 100 mm Schedule 80 pipe has an outside diameter of 30 mm, a wall thickness of 8.56 mm, giving a bore of 97.18 mm.

Pressure drop in components in pipe systems

Minor head loss in pipe systems can be expressed as:

hminor_loss = k v2/ 2 g

where hminor_loss = minor head loss (m)

= minor loss coefficient

= flow velocity (m/s)

= acceleration of gravity (m/s2)

Minor loss coefficients for some of the most common used components in pipe and tube systems:

Table 2-1: Minor loss coefficients

Type of Component or FittingMinor Loss Coefficient, k
Flanged Tees, Line Flow0.2
Threaded Tees, Line Flow0.9
Flanged Tees, Branched Flow1.0
Threaded Tees, Branch Flow2.0
Threaded Union0.08
Flanged Regular 90o Elbows0.3
Threaded Regular 90o Elbows1.5
Threaded Regular 45o Elbows0.4
Flanged Long Radius 90o Elbows0.2
Threaded Long Radius 90o Elbows0.7
Flanged Long Radius 45o Elbows0.2
Flanged 180o Return Bends0.2
Threaded 180o Return Bends1.5
Fully Open Globe Valve10
Fully Open Angle Valve2
Fully Open Gate Valve0.15
1/4 Closed Gate Valve0.26
1/2 Closed Gate Valve2.1
3/4 Closed Gate Valve17
Forward Flow Swing Check Valve2
Fully Open Ball Valve0.05
1/3 Closed Ball Valve5.5
2/3 Closed Ball Valve200

The above equations and table can be used for calculating pressure drops and energy loss associated in pipes and fittings.

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