Important points for Chemical Engineers

Pump increases - 600# to 1500 #

Let down valve - 1500# to 600 #

Line # changes in a LDT changes when there is

branching or

PSV - Gas

PRV - Liquid

In a pipeline transporting crude oil or refined petroleum products, it is customary to talk about the “line fill volume” of the pipeline.

The volume of liquid contained between two valves in a pipeline can be calculated simply by knowing the internal diameter of the pipe and the length of pipe between the two valves.

The volume of liquid varies with temp., so the inlet volumetric flowrate may be different from the inlet or outlet depending on the temp. at which it is measured.

The difference between the temp. is because of the heat loss or gain between the pipeline liquid, surrounding soil or atmosphere.

The mass of a LIQUID DOES NOT change with temp. or pressure and it remains constant so the mass floret is constant.

Law of Continuity,

Mass = Area X velocity

Area x Velocity = constant = flowrate for liquids

Specific gravity = relative density = mass of liquid/ mass of water

Specific weight = weight / volume (N/m^3)

API gravity of 35 is always referred to at 60°F, not making 70°F.


Liquid - As the pressure increases, it gets more dense (earth’s core). As the temp. increases, it gets less dense (heating bitumin).

ST=S60-a(T-60) - Linear relationship

The above method of calculating the specific gravity of a mixture of two or more liquids cannot be directly applied when the gravities are expressed in °API values. If the component gravities of a mixture are given in °API we must first convert API values to specific gravities before applying Equation (2.4).

Sb = (Q1 XS1)+(Q2XS2)/(Q1+Q2)

where Sb=Specific gravity of the blended liquid

Q1, Q2, Q3, etc.=Volume of each component

S1, S2, S3, etc.=Specific gravity of each component

Gas - As the pressure increases, it gets more dense (Ideal gas law). ( Think about gas in a box with pressure increasing As the temp. increases, it gets less dense (Ideal gas law). (Think about warm gas rises and cold gas comes down)



As temperature increases, liquid viscosity decreases and vice versa. However, unlike specific gravity, viscosity versus temperature is not a linear relationship, its logarithmic.

Pressure - It does not vary much with pressure.


For gases, the viscosity increases with temperature.

The viscosity of a blend of two or more products can be estimated using the following equation:

This method is

sqrt (Vb) = Q1+Q2/(Q1/SQRT(V1))+( Q2/SQRT(V2))

where Vb=Viscosity of blend, SSU

Q1, Q2, Q3, etc.=Volumes of each component

V1, V2, V3, etc.=Viscosity of each component, SSU

Using this method, the blended viscosity of two products at a time is calculated and the process repeated for multiple products.

Thus if three products are blended in the ratios of 10%, 20%, and 70%, we would first calculate the viscosity of the blend using the first two liquids, considering 10 parts of liquid A mixed with 20 parts of liquid B.

This means that the blend would be calculated on the basis of one-third of liquid A and two- thirds of liquid B. Next this blended liquid will be mixed with liquid C in the proportions of 30% and 70% respectively.

Fore more read book Liquid Pipeline Hydraulics by Shahi Menon