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TitleLiquid Pipeline Hydraulics
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Page 2

Liquid Pipeline Hydraulics

E. Shashi Menon, P.E.



PDHengineer.com Online Course

Page 37

A graphical representation of the pressure variation along the pipeline from Point A to Point

B is depicted in Figure 3.2 and is known as the Hydraulic pressure gradient, or simply the

hydraulic gradient.



Terminus

Hydraulic Pressure Gradient

Elevation Profile

Peak

A B

PB

PA

D

E

P
re

ss
u
re

i
n
f

t
o
f

h
e
a
d

Flow

C

Pmin



Figure 3.3 Hydraulic Pressure Gradient with peak



Since the liquid pressure in the pipeline is shown along with the pipe elevation profile, it is

customary to plot the pressures in ft of liquid head instead of pressure in psi. At any point

along the pipeline, the liquid pressure is represented by the vertical intercept between the

hydraulic gradient and the pipeline elevation at that point. This is shown as ED in Figure

3.3. Of course, the pressure ED is in ft of liquid head and can be converted to psi, using the

specific gravity of the liquid.





SYSTEK Technologies, Inc November 24, 2005 36

Page 38

In addition to the elevation difference between the origin A and the terminus B, there may

be many elevation changes along the pipeline, with peaks and valleys. In this case, we must

also ensure that the liquid pressure in the pipeline at any location does not fall below zero

(or some minimum value) at the highest elevation points. This is illustrated in Figure 3.3

where the peak in pipeline elevation at C shows the minimum pressure Pmin to be

maintained.



The minimum pressure to be maintained, depends upon the vapor pressure of the liquid at

the flowing temperature. For water, crude oils and refined petroleum products, since vapor

pressures are fairly low and we are dealing with gauge pressures, zero gauge pressure

(14.7 psia) at the high points can be allowed. However, most companies prefer some non

zero gauge pressure at the high points such as 10 to 20 psig. For highly volatile liquids with

high vapor pressures such as LPG or propane, the minimum pressure along the pipeline

must be maintained at some value such as 200 to 250 psig to prevent vaporization and

consequent two-phase flow.

As the liquid flows through the pipeline its pressure decreases due to friction. The pressure

also increases or decreases depending upon the elevation change along the pipeline profile.

At some point such as C in Figure 3.3, the elevation is quite high and therefore the pressure

in the pipeline has dropped to a small value (Pmin) indicated by the vertical intercept

between the hydraulic gradient and the pipeline elevation at point C. If the pressure at C

drops below the specified minimum pressure for the liquid pumped, vaporization of the

liquid occurs and results in an undesirable situation in liquid flow. Two-phase flow damages

the pump impellers and must be avoided.







SYSTEK Technologies, Inc November 24, 2005 37

Page 73

The MAOP for a 16 inch pipeline, 0.250 inch wall, constructed of X-52 pipe material is

P = ( 2 x 0.250 x 52000 x 1.0 x 0.72 ) / 16 = 1,170 psig



The hydrotest pressure is therefore

1.25 x 1170 = 1,463 psig



SYSTEK Technologies, Inc November 24, 2005 72

Page 74

References

1. Liquid Pipeline Hydraulics, E. Shashi Menon, Marcel Dekker, Inc. 2005

2. Piping Calculations Manual, E. Shashi Menon, McGraw – Hill, 2005

3. Elementary Fluid Mechanics, Vennard & Street. Sixth Edition. John Wiley and Sons,

1982.

4. Handbook of Hydraulics, Brater & King. McGraw-Hill, 1982

5. Pipeline Design for Hydrocarbons, Gases and Liquids. American Society of Civil

Engineers, 1975.

6. Cameron Hydraulic Data. Ingersoll-Rand, 1981

7. Flow of Fluids through Valves, Fittings and Pipes. Crane Company, 1976

8. Centrifugal Pumps Design & Application, V.S. Lobanoff and R.R. Ross, Gulf Publishing,

1985

9. Hydraulic Institute Engineering Data Book - Hydraulic Institute, 1979











SYSTEK Technologies, Inc November 24, 2005 73

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