Dynamic simulation is becoming an important tool for
engineering design and plant operation.1,2 In this
case history, a first-principle dynamic model of a natural
gas/steam heat exchanger system is built using a
commercially available dynamic simulator. Four scenarios for
operability and safety are investigated to demonstrate how a
process and associated control system will respond to various
disturbances as a function of time.
Case history. Preheating of natural
gas (NG) is frequently used to prevent hydrate formation
due to the Joule-Thompson effect of the NG let-down stations.
The typical NG heater system consists of a shell-tube heat
exchanger, a condensate receiver and a steam trap, as shown in
Fig. 1. The steam control arrangement is also shown in Fig. 1.
This system includes:
One temperature control valve on the steam inlet
One level control valve on the condensate outlet
One pressure control valve on the vapor outlet
1. Simplified process scheme for the NG
As a part of the plant design, a steady-state simulation of the
system is done to check the heat-and-material balances and
equipment sizing. Table 1 lists the process conditions and
major equipment sizing data.
Scenario 1: Process upsets.
In this scenario, the impacts of both inlet NG temperature
changes from 0°C to 10°C and NG demand changes from
100,000 kg/h to 140,000 kg/h have been analyzed. The simulator
logic unit operationthe transfer function blockis
used to simulate sine wave changes of NG demand and inlet
Fig. 2 shows the process response to an inlet temperature
change of the NG from 0°C to 10°C. As shown in Fig. 2,
when the NG inlet temperature rises from 0°C to 10°C as
a result of falling heat load, the steam pressure in the heat
exchanger will drop about 200 kPa. Fig. 3 shows the process
response to a change in NG flow from 100,000 kg/h to 140,000
kg/h. Note: The steam pressure also drops over 250 kPa, while
the NG demand declines from 140,000 kg/h to 100,000 kg/h. These
dynamic simulation results confirm that steam pressure in the
condensate receiver cannot be maintained at stable ranges
during process upsets. If a steam trap is used, then the steam
control scheme will lead to reduced condensate flow from the
steam heater system, and it will form the so-called stall
2. Process responses to NG inlet
0°C to 10°C.
3. Process responses to NG demand
Scenario 2: Stall behavior.
This scenario discusses condensate removal from the heat
exchanger. As mentioned before, the temperature control valve
on the steam line maintains the NG outlet temperature by
opening or closing to adjust the steam flowrate, thereby
varying the steam space pressure. When the steam pressure in
the heat exchanger is equal to, or less than, the total
backpressure imposed on the steam trap, then the reduction or
cessation of condensate flow from the heat exchanger occurs.
The condensate will back up in the drain line and will flood
back into the exchanger. This condition can damage the control
valve and may cause corrosion of the exchanger. This symptom is
called the stall behavior.
Based on current heat exchanger sizing data, the dynamic
heat model for this steam heater system was built using
dynamics and spreadsheet tools. These conditions were assumed
for the model:
NG gas inlet temperature rising to 10°C
Low-pressure (LP) steam pressure of 442 kPa and
steam-trap backpressure of 338 kPa
NG consumption of 132,000 kg/h.
Fig. 4 illustrates the simulated stall behavior. Due to the
10% over design margin, the heat exchanger has more heating
area than required. So, the operating steam pressure will be
much lower than needed.
4. Simulated results of stall behavior for
the steam heater.
When the condensate is waterlogged in the heat exchanger, the
surface area available to condense steam is reduced. The heat
flow drops, and NG outgoing temperature begins to fall. While
the temperature sensor detects this change, the controller will
open the steam control valve. This raises the pressure in the
steam space to above the trap-back pressure and causes
condensate to pass through the trap. The condensate level
falls, and the NG temperature climbs. When the sensor detects
this, the controller closes the control valve. The steam
pressure falls, and then flooding begins again. The result is a
continual cycling of opening and closing the steam control
The side effects of stall include damaging the control valve
and water hammer along with corroding and leaking heat
exchangers. These operating conditions will increase maintenance incidents and reduce the
service life of the steam heater and associated equipment.
Scenario 3: Alternative control scheme.
There are different ways to prevent stall.3
Normally, we could use an alternative means to remove
condensate from the exchangers by installing a pumping trap,
instead of using steam traps if the pressure in steam space may
be less than the backpressure. We could also size the heat
exchangers and steam traps properly to ensure that the pressure
in steam space is stable and always higher than the
backpressure under all operating conditions. Or we should
reduce the backpressure of condensate discharge lines. In
reality, this cant always be done.
For the present NG steam heater, the most cost-effective
solution is to use an alternative control schemea bypass
control. This control approach bypasses a partial NG stream
around the exchanger and blends it with a fraction that has
passed through, as shown in Fig. 5. The temperature control
valve is relocated from the original steam line to the NG
5. Alternative control scheme for the NG
System dynamic responses to the process upsets over NG
demand and inlet temperature are illustrated in Figs. 6 and 7.
The results show that the maximum change of steam pressure is
much less, lower than 30 kPa. The stall behavior will not
happen, as the pressure in the steam space is always greater
than steam trap backpressure. Compared with the regular steam
control, the simple bypass control greatly improves the
operating performance of the steam heater.
6. Process responses to NG inlet
0°C to 15°C.
7. Process responses to NG demand
Scenario 4: Tube rupture contingency.
Pressure-relief systems are a critical part of any process
design. Proper design of these systems is required by
regulation and industrial codes. Due to the large operating
pressure difference between the exchanger tube and shell sides
(flange rating 900 lb at tube side vs. 150 lb at shell steam
side), the case of complete tube rupture is a valid case in the
steam heat exchanger.
Although the simulator cannot predict the instantaneous
pressure wave at the rupture site, it does provide important
insights on the dynamic system behavior under the tube-rupture
conditions. Normal operating data and pressure safety valve
(PSV) sizing results by the conventional method are listed in
Tables 1 and 2. These parameters were set to generate the
initial values of the dynamic model for the tube-rupture
UA value was set for the steam heat exchanger
Condensate receiver was set to real sizes to
simulate steam/liquid accumulation and liquid level
Normal valve with a customized spreadsheet was used
for constant NG rupture flow into the steam condensate
In general practice, to protect overpressure of steam system
from the high pressure of NG, a check valve should be installed
on the upstream steam line, and a PSV shall be provided on the
top of the vapor line in the condensate receiver.
The dynamic simulation with two different PSV sizes was
verified, Figs. 8 and 9 summarize the results. Some highlights
of the dynamic simulated results are discussed here:
Pressure in the condensate receiver begins to
build up immediately following the tube-rupture event. After
about 3 seconds, the receiver pressure reaches the set
pressure; then the PSV starts to relieve.
PSV would work fine if the normal PSV of 4M6
sized by a conventional method is installed on the top of shell
side in the steam heater. However if this PSV is relocated to
the top of the condensate receiver, a 40% overpressure in the
receiver would occur, as shown in Fig. 8. The major reason is
that, under the upset conditions of the tube rupture, the NG
has a strong stripping effect (due to vapor/liquid equilibrium)
that carries the steam out of the condensate phase. This causes
the PSV peak relief load (30,530 kg/h) from the condensate
receiver to be about 23% higher than the tube-rupture flow
(24,920 kg/h) estimated by API 521 method.
8. Tube rupture profiles for 4M6 PSV with 6
10 in. outlet piping.
As evaluated in Fig. 9, if installing a PSV on the
top of the condensate receiver, a larger sized 4P6 PSV and
associated larger inlet/outlet piping should be
9. Tube rupture profiles for 4P6 PSV with 8
12 in. outlet piping.
This example shows that, when upset conditions occur,
equilibrium conditions in vessels are changing, and the safety
system design must be adjusted to account for those
Options. This case study illustrates how critical it is to
consider vapor/liquid equilibrium changes and interaction of
process with controls in the system design, and how dynamic
simulation can improve plant performance, controllability and
safety in design and operation. HP
Special thanks to Alan Childs, manager of the process
department, for the valuable discussions, review and
1 Dissinger, G. R., Studying
simulation, Hydrocarbon Engineering, May
2 James, G. and J. Reeves, Dynamic Simulation
Across Project and Facility
Lifecycles, 6th World Congress of Chemical Engineering,
Melbourne, Australia, Sept. 2327, 2001.
3 www.spiraxsarco.com/Resources, Practical
Methods of Preventing Stall.
Hai-Ming Lai is a principal process
engineer in Jacobs Canada Inc., Calgary, Alberta, with
over 26 years of experience in process research and
development, design, and engineering of oil and gas, refining/upgrading, and petrochemical projects. His specialties
include simulation studies, conceptual and front-end
engineering design. He holds a PhD in chemical
engineering from Beijing University of Chemical Technology (BUCT), P.R. of
China., and is a registered professional engineer in
Alberta, Canada. Prior to joining Jacobs, Dr. Lai
worked for Aspen Technology, Calgary, Canada,
and Research Institute of Chemical Technology in BUCT, Beijing,
P. R. of China.