BATANGASSTATEUNIVERSITY
College of Engineering, Architecture&Fine Arts
Gov. Pablo Borbon Campus II, Alangilan, Batangas City, Philippines 4200
www.batstate-u.edu.ph Telefax: (043) 300-4404 locs. 106-118
CHEMICAL AND FOOD ENGINEERING DEPARTMENT
PIPE SIZE SELECTION
INTRODUCTION
The chemical process industry is involved in many operations, for different types of
fluids, with different applications. Though in principle, various guidelines and formulae are
available for pipeline sizing for different services. Hence it becomes critical at times
conceptualization is necessary before deciding design parameters.
When fluids are to be carried from one place to another in household piping to cross country
pipeline, piping and fitting constitutes a high cost. The size of piping plays an important role in
the pumping cost. Hence the selection of the line size becomes important. Though in principle,
various formulae are available for sizing for different services, conceptualization is necessary
before deciding parameters.
In any chemical process industry, various types of fluids are being used in different forms
like liquid, gaseous, slurry, etc. Raw material, intermediate product or finished product produced
through various unit operations require connectivity of all the units with pipelines and fittings
due to the following reasons:
Ease of operation
Safe handling of materials
Avoiding loss of material
Hygienic conditions of the plant
DISCUSSION
If the motive power to drive the fluid through the pipe is available free, for instance when
pressure is let down from one vessel to another or if there is sufficient head for gravity
flow, the smallest pipe diameter that gives the required flow-rate would normally be used.
If the fluid has to be pumped through the pipe, the size should be selected to give the
least annual operating cost.
Typical pipe velocities and allowable pressure drops, which can be used to estimate
pipe sizes, are given below:
Rase (1953) gives expressions for design velocities in terms of the pipe diameter. His
expressions, converted to SI units, are:
where d is the internal diameter in mm.
Simpson (1968) gives values for the optimum velocity in terms of the fluid density.
His values, converted to SI units and rounded, are:
The maximum velocity should be kept below that at which erosion is likely to occur.
For gases and vapours the velocity cannot exceed the critical velocity (sonic velocity)
(see Volume 1, Chapter 4) and would normally be limited to 30 per cent of the critical
velocity.
ECONOMIC PIPE DIAMETER
The capital cost of a pipe run increases with diameter, whereas the pumping costs
decrease with increasing diameter. The most economic pipe diameter will be the one which gives
the lowest annual operating cost. Several authors have published formulae and nomographs for
the estimation of the economic pipe diameter, Genereaux (1937), Peters and Timmerhaus (1968)
(1991), Nolte (1978) and Capps (1995). Most apply to American practice and costs, but the
method used by Peters and Timmerhaus has been modified to take account of UK prices (Anon,
1971).
The formulae developed in this section are presented as an illustration of a simple
optimisation problem in design, and to provide an estimate of economic pipe diameter that is
based on UK costs and in SI units. The method used is essentially that first published by
Genereaux (1937).
The cost equations can be developed by considering a 1 metre length of pipe.
The purchase cost will be roughly proportional to the diameter raised to some power.
The value of the constant B and the index n depend on the pipe material and schedule.
The installed cost can be calculated by using the factorial method of costing discussed
in Chapter 6.
where the factor F includes the cost of valves, fittings and erection, for a typical run of
the pipe.
The capital cost can be included in the operating cost as an annual capital charge. There
will also be an annual charge for maintenance, based on the capital cost.
Only the friction pressure drop need be considered, as any static head is not a function
of the pipe diameter
To calculate the pressure drop the pipe friction factor needs to be known. This is a
function of Reynolds number, which is in turn a function of the pipe diameter. Several
expressions have been proposed for relating friction factor to Reynolds number. For simplicity
the relationship proposed by Genereaux (1937) for turbulent flow in clean commercial steel
pipes will be used.
Equations 5.14 and 5.15 can be used to make an approximate estimate of the economic
pipe diameter for normal pipe runs. For a more accurate estimate, or if the fluid or pipe run is
unusual, the method used to develop equation 5.13 can be used, taking into account the special
features of the particular pipe run.
The optimum diameter obtained from equations 5.14 and 5.15 should remain valid with
time. The cost of piping depends on the cost power and the two costs appear in the equation as a
ratio raised to a small fractional exponent.
SAMPLE PROBLEM
REFERENCE
COULSON AND RICHARDSON’S CHEMICAL ENGINEERING VOLUME 6 FOURTH
EDITION: CHEMICAL ENGINEERING DESIGN. R.K SINNOTT. PAGE 218
