Hydrocarbon Processing - December 2020 - GP-33

HEAT EXCHANGE

TABLE 3. Case study for two BAHEs comparing the rate of temperature change (external ﬁre case)

Depressurizing
section

Design pressure,
barg

Depressurization from design pressure
to 6.9 barg

Depressurization from design pressure
to 50% of design pressure

dT/dt, °C/min

Time to reach 6.9 barg, min

dT/dt, °C/min

Time to reach 6.9 barg, min

Seven-pass gas/gas BAHE
Pass A

75

14

15

5

51

Pass B

75

14

15

5

51

Pass C

92

28

15

5

49

Pass D

75

10

15

5

54

Pass E

38

3

15

3

40

Pass F

30

5

15

2

15

Pass G

75

14

15

1

51

Pass A

75

14

15

5

51

Pass B

38

3

15

1

40

Pass C

38

3

15

2

40

Pass D

38

3

15

1

40

Four-pass reboiler BAHE

BAHE pass so that the cold stream can be
further cooled with respect to pressure
reduction, which means the rate of temperature change across the BAHE will
be lower. Conversely, if a depressurizing
valve is provided at the hot side, then
opening of the valve will push cold liquid
toward it through the cold box pass, resulting in high temperature change across
the cold box pass.
The location of the depressurizing
valve can allow the use of carbon steel
material instead of stainless steel, resulting in considerable cost reduction in piping, valves and fittings. For example, the
piping at the cold side of the BAHE pass
is stainless steel, and the warm side of
the same pass is carbon steel. If tapping
of the depressurizing valve happens at
the cold, stainless steel side, then the low
temperature during depressurizing may
not require a change from carbon steel to
stainless steel, as the warm fluid will migrate through the blowdown valve during
depressurizing and the temperature at
the warm side can remain within the carbon steel temperature limit.
In another case, if the tapping of the depressurizing valve is placed on the warm
carbon steel side, then the temperature of
this side will be reduced during depressurizing. If the depressurizing temperature is
less than the carbon steel limit, then the
stainless steel material should be used.
The settle-out temperature of the entire section at the upstream and down-

TABLE 4. Rates of temperature change for two BAHEs
Spurious depressurizing
Depressurizing
section

Design pressure,
barg

Depressurization from
design pressure to 6.9
barg, dT/dt, °C/min

Depressurization from design
pressure to 50% of design
pressure, dT/dt, °C/min

Seven-pass gas/gas BAHE
Pass A

75

18

5

Pass B

75

8

5

Pass C

92

17

5

Pass D

75

14

5

Pass E

38

7

5

Pass F

30

4

4

Pass G

75

28

5

Pass A

75

18

4

Pass B

38

2

1

Pass C

38

1

1

Pass D

38

1

1

Four-pass reboiler BAHE

stream of the BAHE pass should also be
checked for pressurized shutdown (without depressurizing) to ensure the use of
the carbon steel material.
Depressurize slowly. Equipment designed as per ASME Section 8 are depressurized in case of external fire. Internal
pressure is quickly reduced to keep the
stresses below the rupture stress.
These equipment, including BAHE
passes, are depressurized within 15 min to
either 6.9 barg or to 50% of design pressure.

For a high-design-pressure BAHE, it can
be challenging to depressurize the passes
to 6.9 barg while keeping a maximum temperature change criterion of 5°C/min.
TABLE 3 show data for a case study for
the depressurization of two BAHEs comprising high and low design pressure passes. It compares the rate of temperature
change (dT/dt) with regard to the design
pressure and whether the pass should be
depressurized to 6.9 barg or to 50% of design pressure.
Once the restriction orifice is sized for

Gas Processing & LNG | NOVEMBER/DECEMBER 2020

33
Hydrocarbon Processing - December 2020

Table of Contents for the Digital Edition of Hydrocarbon Processing - December 2020
