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QUENCHING AND PREHEATING OF COKE
DRUMS: A DEEP LOOK
Francisco Fernández – PDVSA-INTEVEP
Infrastructure and Upgrading Department
Los Teques – Venezuela
Luz Mary Tenias – PDVSA-Petrocedeño
Process Engineering Department
Barcelona - Venezuela
QUENCHING AND PREHEATING
OF COKE DRUMS: A DEEP LOOK
• To present a case study of a drum that experienced a through-wall crack after
few years in operation.
• To describe the main deviations that were found to have the most impact on the
sudden temperature change of the drums.
• To propose a series of operational actions to improve the quenching and
preheating stages.
2
Objectives
QUENCHING AND PREHEATING
OF COKE DRUMS: A DEEP LOOK
• Introduction
• Background
• The findings
• The actions
• Discussion
• Conclusions
3
Presentation Outline
QUENCHING AND PREHEATING
OF COKE DRUMS: A DEEP LOOK
� The Orinoco Belt “Hugo Chávez”: The largest oil reserve of the world.
� Extra Heavy Oil (EHO): 8-10°API, 4-5 wt% S.
Orinoco Belt
259MMM Bbls*
* 298MMM Bbls total of the country
4
Introduction
QUENCHING AND PREHEATING
OF COKE DRUMS: A DEEP LOOK
C
R
U
D
E
U
N
I
T
V
A
C
U
U
M
DELAYED
COKING
HYDROCRACKING
NAPHTHA & DIESEL
HYDROTREATMENT
COKE
SULPHUR
HYDROGEN
RESIDUE BYPASS
UPGRADED CRUDE
16-32API
EXTRA HEAVY OIL
8API
Diluent
CRUDE
16API
RECOVERED
DILUENT
1. Physical 1. Physical 1. Physical 1. Physical
SeparationSeparationSeparationSeparation
2. Deep Conversion 2. Deep Conversion 2. Deep Conversion 2. Deep Conversion
of the residueof the residueof the residueof the residue
3. Secondary treatment3. Secondary treatment3. Secondary treatment3. Secondary treatment
4. Blending4. Blending4. Blending4. Blending
Typical Upgrading Scheme
55
Introduction
QUENCHING AND PREHEATING
OF COKE DRUMS: A DEEP LOOK
� “I have worked in coker units for 30 years Francisco, and I can tell you, these drums are
gonna crack anyway. Why do you wanna change that quenching ramp over and over again?”
Pedro M. , Field Operator
� “This bottom flange is a real pain in the neck. Besides, I have to wait 2 or 3 hours for the
maintenance people to come and help us close the drum. But I’m not putting this unit on
circulation because of that. Not even cutting a single barrel of charge. Not on my shift.”
José D. ,Shift Supervisor
� “After this crack, the only thing you can do is installing one of those strain monitoring
systems, that will predict when you have to repair or replace your drums.”
Benito A., Technical Advisor
A Common Belief…
6
Introduction
QUENCHING AND PREHEATING
OF COKE DRUMS: A DEEP LOOK
� Upgrader started operations in 2002
� 6-drum DCU(1) was revamped 3 years later. Capacity increased from 89 to 106
thousand barrels a day
� Coking cycles shortened from 19 to 16 hrs
� Unreliable semi-automatic unheading system caused frequent delays in the
decoking process
7(1): Delayed Coking Unit
Background
QUENCHING AND PREHEATING
OF COKE DRUMS: A DEEP LOOK
The first crack…
� A through-wall crack appeared in one of the drums after 9 years of operation and 3360 cycles
� Located at the weld seam C3, on the upper section of the vessel
Fig 1. External view Fig 2. Internal view
8
Background
QUENCHING AND PREHEATING
OF COKE DRUMS: A DEEP LOOK
Fig. 3. Bulged drum
Location of the crack
9
Background
QUENCHING AND PREHEATING
OF COKE DRUMS: A DEEP LOOK
As part of the study that was conducted to determine the root cause of the event,
the Quenching and Preheating stages were thoroughly examined.
A summary of the findings is described below:
� QUENCHING
When the unit was revamped, due to the reduction of the coking cycle, a new
quenching ramp was created (Short ramp).
� Quenching time reduced from 4hrs and 30min to 3hrs and 45min
� Total amount of water reduced in 20%
The original quenching ramp (Long ramp) was kept but it was also modified.
10
The Findings
QUENCHING AND PREHEATING
OF COKE DRUMS: A DEEP LOOK
11
The Findings
QUENCHING AND PREHEATING
OF COKE DRUMS: A DEEP LOOK
Although both ramps were available, it was found that almost always the Short
Ramp was used.
The temperature profile of the drum during quenching showed the following
behavior in almost all cycles studied:
� The Temperature Indicators (TI) located at the bottom of the drum started
to see a decrease in temperature after 20-25min of the initial water injection.
� The TIs of the cylindrical section started to see a slight decrease in
temperature after 20min of the beginning of the ramp, but then a sudden
decrease was perceived after 60min.
�The TIs of the skirt detected a reduction in temperature at the same time
that those of the cone, but the rate was much lower.
12
The Findings
QUENCHING AND PREHEATING
OF COKE DRUMS: A DEEP LOOK
13
The Findings
Typical behavior during quenching stage (using Short Ramp)Te
mp
era
ture
(°C
), q
ue
nch
flo
w (
m3/h
r)
Ad-Hoc Trend
06/10/2014 03:17:51 a.m. 06/10/2014 08:20:30 a.m.5,04 hours
50,
100,
150,
200,
250,
300,
350,
0,
400,
Upper Section
Lower section
Skirt
QUENCHING AND PREHEATING
OF COKE DRUMS: A DEEP LOOK
However, when the Long Ramp was used (which has a lower water flow throughout the
first hour and a half), the temperature decrease rate was less drastic.
14
Number of Cycles to have a through-wall crack = f (Initial Quench Flow Rate)(2)
(2) API Coke Drum Survey, 1996.23/01/2014 12:13:28 p.m. 23/01/2014 06:32:56 p.m.6,32 hours
50,
100,
150,
200,
250,
300,
350,
0,
400,
2. Still a sudden
temperature change
at the bottom
1. Smoother
temperature
change at the top
Typical behavior during quenching stage (using Long Ramp)
Tem
pe
ratu
re (
°C),
qu
en
ch f
low
(m
3/h
r)
The Findings
QUENCHING AND PREHEATING
OF COKE DRUMS: A DEEP LOOK
Sludge injection: It was not being considered in the quenching ramp, because the
water flow meter was located upstream of the connection of the sludge line.
Sludge is typically injected during the first hour of the quenching, and it accounts
for approx. 50% of additional water flow to the drum.
Sludge injection
downstream of quench
water controller
15
The Findings
QUENCHING AND PREHEATING
OF COKE DRUMS: A DEEP LOOK
� PREHEATING
Due to delays in other stages of the decoking cycle, the preheating stage was
often reduced or even completely skipped. Drum temperature when switching
was from as low as 120°C, to an average of 220°C, versus 315°C according to the
recommended procedure.
Dru
m T
em
pe
ratu
re (
⁰C)
Time
Skirt
Medium
Top
Design
Temperature of a drum when switching
16
The Findings
QUENCHING AND PREHEATING
OF COKE DRUMS: A DEEP LOOK
It was also noted that it was required at least 5 hours to get a drum temperature of around
260°C.
17
The Findings
Typical behavior of the Preheating stage
Ad-Hoc Trend
21/03/2014 02:01:54 p.m. 21/03/2014 11:42:11 p.m.9,67 hours
50,
100,
150,
200,
250,
300,
350,
0,
400,
0,
400,
0,
400,
0,
400,
0,
400,
0,
400,
0,
400,
0,
1,
1.-Hermeticity
test and steam
purge (45 min.)
2.- Preheating
(390 min.)
Preheating rate ≈
20ºC/hr
3.- Drum in service.
Temp. Increase rate
≈ 170ºC/hr66°C
132°C
259°C
387°C
Tem
pe
ratu
re (
°C)
QUENCHING AND PREHEATING
OF COKE DRUMS: A DEEP LOOK
In addition, the failure of the backpressure valve (SP3) in one of the trains allowed testing
the influence of that valve in the preheating performance. It was surprising to see that
throttling the SP3 seemed to have no effect at all on the temperature reached by the drum.
Drum A: SP3A 100% open
Time required to get 250⁰C≈ 6hrs
Drum E: SP3C fully operative
Time required to get 250⁰C ≈ 6hrs
Tem
pe
ratu
re (
⁰C)
Time (hrs)
Tem
pe
ratu
re (
⁰C)
Time (hrs) 18
The Findings
QUENCHING AND PREHEATING
OF COKE DRUMS: A DEEP LOOK
A review showed that the SP3 valve was usually closed up to 45-50%
19
The Findings
Va
lve
op
en
ing
(%
)
Ad-Hoc Trend
26/10/2011 06:28:31 a.m. 01/11/2011 01:41:52 p.m.6,30 days
50,
60,
70,
80,
90,
40,
100,
Opening percentage of backpressure valve SP3
QUENCHING AND PREHEATING
OF COKE DRUMS: A DEEP LOOK
Considering that the particular design of the backpressure valve (ring valve) prevents it from
going to full closure, it could be safely throttled even to 100%. An investigation of the
reasons why it was not applied indicated the following reasons:
� It was not clear in the procedure.
� The non-closure feature of the valve was not well known by everybody.
� The blowdown reprocessing line to the fractionator was plugged, and the condensate
vapors could not be recovered.
Example of a Ring Valve, similar to the installed
in the plant.
20
The Findings
QUENCHING AND PREHEATING
OF COKE DRUMS: A DEEP LOOK
�QUENCHING
A new quenching ramp was developed. The idea was to keep the water flow as
low as possible (max.60gpm) during the first 60-80minutes, and then increase it to
fill the drum within the same total time than the original one:
21
New quenching ramp
The Actions
QUENCHING AND PREHEATING
OF COKE DRUMS: A DEEP LOOK
QUENCH
WATER
FC
SLUDGE
THICKENER
The sludge injection was linked to the quench water flow. When sludge was
injected, a flow calculator included the amount of sludge into the total flow, and
the quench water was adjusted accordingly.
22
The Actions
QUENCHING AND PREHEATING
OF COKE DRUMS: A DEEP LOOK
23
� Problem identified: The quench water valve was too big to handle the small
flow desired, and behaved unstably at that flow. Steady operation of the valve
was achieved at 100-120gpm.
� Problem 2: Unreliable measurement of sludge flow has made it difficult to
link it to the quenching ramp.
� Problem 3: The quench valve…again. Sludge injection needs to be reduced
to avoid unstable behavior of the valve at low flows.
Only by shifting to the Long Ramp, the rate of decrease in temperature in
the upper section has been seen to be reduced in approx. 30%
When testing the new ramp…
The Actions
QUENCHING AND PREHEATING
OF COKE DRUMS: A DEEP LOOK
�PREHEATING
Operating procedure was updated, to allow the operators to close the SP3 as
required to get the desired temperature. Test were performed at 30% opening:
24
The Actions
Ad-Hoc Trend
50,
100,
150,
200,
250,
300,
350,
0,
400,
0,
400,
0,
400,
0,
400,
0,
400,
0,
400,
0,
1,
0,
100,
2.- Preheating rate:
61⁰C/hr
Tem
pe
ratu
re (
°C)
Preheating of a drum throttling more the SP3
- Time required to get 250⁰C:
2hrs
- Temperature reached with 3hrs
of preheating:
310⁰C
QUENCHING AND PREHEATING
OF COKE DRUMS: A DEEP LOOK
Placing the 4-way valve at mid-point proved to be another way to reduce the
sudden temperature change when switching. Tests performed showed that that
increase in temperature was 33% slower when keeping the valve at mid-point.
2524/01/2014 06:58:40 a.m. 24/01/2014 07:31:45 a.m.33,08 minutes
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
0,
1,
0,
1,
0,
1,
200,
270,
208°C
218°C
Mid-Point
5 min.
- Temp. Increase rate at
mid point:
120⁰C/hr
- Temp. increase rate
after switching:
180⁰C/hr
The ActionsTe
mp
era
ture
(⁰C
)
Preheating rate of a drum when in Mid-Point
QUENCHING AND PREHEATING
OF COKE DRUMS: A DEEP LOOK
26
On the other hand, test performed to try to preheat the drum when the unit was on
circulation (and there were no vapors from the other drum) were unsuccessful. Only by
using HCGO quench a slight warm up was achieved, but the amount of gasoil required was
so high that it was considered impractical.
18/02/2011 11:28:55 p.m. 19/02/2011 01:54:16 p.m.14,42 hours
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
0,
1,
0,
400,
0,
400,
0,
400,
0,
400,
-2,
18,
2/3 of normal HCGO
quench rate injected for
2hrs. Only 40⁰C gained.
Tem
pe
ratu
re (
⁰C),
qu
en
ch f
low
(t/
hr)
Preheating of a drum without vapors from the other drum
The Actions
QUENCHING AND PREHEATING
OF COKE DRUMS: A DEEP LOOK
Is it really worth it?
The extent to which the thermal stress of the coke drums can actually be reduced
is topic of discussion.
Reducing the drop temperature rate in 30% or preheating the drum 50⁰C higher
will certainly be beneficial for the drum…but how much?
27
Factors affecting drums life
Design Operation
Previous damage
Previous repairs
Fabrication
Discussion
QUENCHING AND PREHEATING
OF COKE DRUMS: A DEEP LOOK
How to measure the impact of the changes on drums life?
API Technical Report 934G(3) analyzes the effect of changes in operating practices
on drum reliability.
However, it only considers the impact of changing the coke cycle time on the drum
life (also taking into account a change in the amount of damage that occurs during
each cycle). It does not cover the scenario where cycle time is kept but damage
that occurs during each cycle is reduced.
Assuming (as a rough estimation) that an improvement in the operation of 30%
can be seen as a reduction of damage in 30% and also equivalent to an increase in
cycle time of 30%, multiplying factors of the API 934G can be used to estimate the
impact of the operational changes on the time between downtime repairs:
9 years * (1.20 to 1.53) = 10.8 to 13.77 years � 1.8 to 4.7 years more
(3) Design, Fabrication, Operational Effects, Inspection, Assessment and Repair of Coke Drums and
Peripheral Components in Delayed Coking Units. 2014 (DRAFT) 28
Discussion
QUENCHING AND PREHEATING
OF COKE DRUMS: A DEEP LOOK
More accurate estimations...
Strain gauges can be used to estimate the drum life and the amount of damage that
occurs during each coking cycle.
Utilizing those devices, a good comparison can be established between the damage
that occurs to the drum with different operating conditions or procedures.
This equipment has been installed recently in one of the drums of the Upgrader.
Data of the base case has been started to be collected.
Full profit of the strain gauges can be obtained if they are used not only to monitor
the remaining drum life, but to evaluate the impact of the changes in the
operating conditions.
29
Discussion
QUENCHING AND PREHEATING
OF COKE DRUMS: A DEEP LOOK
� Pedro was right. Coke drums will crack. But through a proper operation during
quenching and preheating, the occurrence of those cracks can be greatly retarded.
� The sudden temperature drop during quenching depends not only on the initial
quench flow rate, but on the flow rate used during all the first hour.
� Sludge injection must be considered in the quenching ramp as it can account for a
significant percentage of the total quench flow.
� It must be operationally determined how much the backpressure valve must be
throttled in order to speed up the preheating stage. Significant improvement was
found at 30% opening.
30
Conclusions
QUENCHING AND PREHEATING
OF COKE DRUMS: A DEEP LOOK
� José was also right. Partially. From the preheating point of view, it is worst to put
the unit in circulation. Any delay in the decoking process must be quickly identified
and rate must be reduced to allow enough time for preheating.
� Benito was right too. To some extent. Strain gauges will certainly provide better
information to monitor the condition of the drum. But they must be accompanied
with changes in the operating procedures.
31
Conclusions
QUENCHING AND PREHEATING
OF COKE DRUMS: A DEEP LOOK
This work would not have been possible without all the support of the field
operators, control panel technicians, supervisors and engineers of the plant that
worked together selflessly to improve the operation of the unit, only because they
love their coker unit as much as we do.
32
Acknowledgements
QUENCHING AND PREHEATING
OF COKE DRUMS: A DEEP LOOK
THANK YOU
33
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