Hydrocarbon Processing - May 2021 - 45

Special Focus

Maintenance and Reliability
E. PARVIN and A. ROBINSON,
Parv Process Consulting and Engineering, Denver, Colorado

Isentropic effects: Application and simulation
A process safety risk in industry is having unknown problems that are not accounted for. Although the cause of many
issues can be determined, others cannot
be quantified without awareness. This
article reviews an issue with high-velocity gases in piping systems, otherwise
known as isentropic or kinetic effects.
This issue can be difficult to detect and
is often missed in most process engineering evaluations. It can result in temperature variances that metals, soft goods or
internals in a system are not designed to
tolerate. Most process simulations are
not suited for identifying isentropic effects or require the implementation of
a non-default setting. To avoid " traps "
from simulations and an increased risk of
damage to a system, identifying the characteristics of isentropic effects and how
to physically address them is vital. The
following are several case studies to demonstrate this effect.
Case study 1: Discovery. Hydrogen

experiences a flat or reverse adiabatic
effect with pressure drop. During a site
visit, a 99.9% hydrogen stream was venting to flare. The temperature on the pipe
indicated the occurrence of the reverseadiabatic effect. Upon further examination, cooler than expected temperatures
were detected at the valve outlet. A forward-looking infrared (FLIR) camera
was utilized to measure the temperatures
at and around the hydrogen valve. The
images collected by the FLIR camera are
shown in FIG. 1.
The FLIR images show that while the
inlet and outlet temperatures from the hydrogen valve are relatively equal as expected, the temperature at the 1-in. valve connection was approximately 16°F lower.
Since the fluid is hydrogen, the adiabatic
or Joule-Thompson ( JT) effect can be
neglected. This temperature variance was

not immediately explained by common
knowledge or past simulation results.
It should also be noted that the temperature reversed over 3-5 pipe diameters
out of the expander as the velocity decreased (FIG. 2). By investigating the principles that govern the kinetic effects, the
cause of the velocity impacts on the temperature of the fluid can be determined.
Simply put, when gas is flowing at a high
velocity, it robs the temperature of the
fluid to achieve that velocity. Through energy balances, the relationships between
temperature, velocity and other variables
demonstrate how isentropic effects impact a system.
Isentropic effect fluid correlations.
During this evaluation, an isentropic model is assumed. No heat is added to the system; therefore, being adiabatic, and reversibility is achieved since viscosity effects,
thermal conductivity and mass diffusion
are absent. Application of the second law
of thermodynamics captures any potential
irreversibility in the process and accounts
for the grade of conversion between work,
heat and system energy change. Combining the second law and the first law of thermodynamics (Eq. 1) with no reactions
while ignoring intermolecular forces, results in a thermally perfect gas.
T
S2 S1 c p ln 2
T1
P
R
ln 2
MW
P1

where:
S1 and S2 = the initial and final
entropy; [Btu/(lb × R)]
( J / kg × K)
cp = heat capacity; [Btu/(lb × R)]
( J/(kg × K))
T1 and T2 = the initial and final
temperatures; °R (K)

R = the universal gas constant;
1.985 (Btu/mol × R)
[8.314 ( J/(mol × K)]
P1 and P2 = the initial and final
pressure; psia (Pa)
MW = molecular weight; lb/lbmol
(kg/kmol)
To isolate and explain the temperature
variance, a steady-state energy balance is
created from inlet and outlet conditions
to derive Eq. 2 for the isentropic assumptions (left hand side of Eq. 1 is set to zero)
by algebraic rearrangement and with the
utilization of the Mach number expression in Eq. 3. Via Eq. 4, the speed of sound
for a given fluid can be calculated with the
fluid properties.

(1)

FIG. 1. FLIR camera imaging of inlet
temperature, outlet temperature, and 1-in. valve
connection temperature for pure hydrogen.
Hydrocarbon Processing | MAY 2021

Hydrocarbon Processing - May 2021

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