Life Cycle Costing of Stainless Steel in the Petrochemical Industry
ByD. L. Bagnoli and D. J. Tillack
Consultants for the Nickel Development Institute
Reprinted Paper
Presented at the Stainless Steel World Amercias Conference in Houston, Texas, U.S.A., February
14, 2002.
Decision-making based on life cycle costing (LCC) is seeing increased use in the present economic climate.
Some of the earliest applications have been in the defence and aircraft industries where assessed mean time
between failure has been used as a basis for life expectation.
More recently, a joint industry project was started in 1994 in the oil and gas industry which set out to
establish standards for LCC implementation. An outgrowth of this activity has been a new ISO standard
(IOS/TC67/WG96).
LCC is of particular importance for applications comparing stainless steels with other materials of construction. A basic attribute of stainless steels is their ability to provide long term performance with a minimum of down time in aggressive environments.
An LCC application usually starts with the determination of the key cost drivers for a given application. For equipment/materials applications the key cost drivers are closely related to significant damage mechanisms, or failure modes, in a given service. In some cases, data are available that permit an assessment in terms of remaining life vs. materials and fabrication costs.
In other cases where quantitative methods can not easily be applied a probability of failure can be assigned based on engineering judgement. After combining probability of failure with consequence a cost of risk can be assigned [as described by Sims and Becht (1)].
Two examples have been selected for refinery/petrochemical elevated temperature services that demonstrate the advantages offered through selection of a given alloy. In each of these cases the major damage mechanisms will be defined and where possible estimates of remaining life will be used to evaluate various materials related options. Inspection methods that can be used to assess remaining life and improve reliability will also be discussed.
Case 1: Catalytic Steam Methane Reformer Heater
Tubes
Catalytic steam reforming finds extensive world-wide applications for the production of several
products including: ammonia, methanol and hydrogen
Many refineries require hydrogen for hydrotreating and hydrocracking processes and as a result have hydrogen plants with steam methane reformer heaters. In most plants methane is a feedstock which is combined with steam in catalyst filled tubes to produce the "water gas" reaction.
The steam and methane reform into hydrogen, carbon monoxide and carbon dioxide. The reaction takes place in the heater tubes and is endothermic. Thermodynamic calculations show that the typical catalytic steam methane reformer heater tubes operate in an oxidizing atmosphere. Operating pressures are usually in the 1.75-5 Mpa (250 - 450 psi) range and tube metal temperatures are usually between 900-1010C (1650 and 1850F).
Typical diameters for steam methane reformer heater tubes are usually in the range of 10-13 cm (4 to 5 inches). Wall thickness will depend on pressure, temperature and diameter and of course tube material creep rupture properties. In nearly all cases centrifugally cast alloys are used.
Failure modes
Based on operating conditions and operating environments various failure modes can be considered for
steam methane reformer tubes such as creep rupture and high temperature corrosion, i.e.,
oxidation/carburization. Actually typical steam/hydrocarbon ratios for steam methane reformers are much
higher than for steam cracking where carburization is the major damage mechanism.
Thermodynamic studies have shown that under normal operating conditions only oxidation is possible in
steam methane reformers and there have been very few tube failures attributed to oxidation. This leaves creep
rupture as a significant failure mode. The cast steam methane reformer heater tube shown in Figure 1 has
failed by leakage due to creep rupture.
Figure 1
Years of experience with a great many tube failures have verified that the dominant failure mode for steam
methane reformer tubes is creep rupture. The creep rupture failure mechanism was elucidated in a Joint
Industry Program conducted many years ago at Battelle Memorial Institute (2). The Battelle model considered
the typical cast steam reformer tube as a "heavy wall" cylinder from a stress analysis standpoint.
Accordingly, the tube cross section was broken up into a series of finite elements to portray the damage
sequence in each element, Figure 2.
Figure 2
In the failure model shown in the above figure initial creep damage is shown to occur near midwall and to
progress primarily in an inwards direction. The creep fissures found initially in midwall area will also
preferentially move towards the ID. Interestingly, the Battelle model showed that atleast 10,000 hours of
life remain in a typical cast steam reformer tube when fissuring encompasses approximately 1/3 the wall
thickness. More will be said concerning the importance of this observation as related to tube inspection.
Materials selection for various operating
conditions
Many years ago the favoured cast alloy was HK 40 (25Cr-20Ni). However, in recent years more advanced
alloys, such as HP mod (25Cr-35Ni Nb) as well as numerous other proprietary foundry compositions are
replacing HK 40 for this application.
Table 1 lists some of these proprietary alloys and their compositions including HP mod micro (25Cr-35Ni
Nb,Ti) and IN 519 (24Cr-24Ni Nb). It is estimated that an entire reformer assembly may cost 25% of the total
cost of a steam methane heater. From a life - cycle basis there is a strong incentive to select optimum
materials. This is illustrated from three different perspectives as shown below.
Table 1
A: Extended life for comparable cost
Creep properties (creep rupture data) obtained from several of the suppliers was used to compare
these alloys with HK 40 on a performance basis. Figure 3 shows the average 100,000 hour creep rupture life
vs. temperature (data provided by supplier foundries).
Figure 3 shows how service life for an HK 40 tube in a steam-methane reformer furnace varies with temperature and pressure. Point "A" on the figure is for a tube that is designed for 950C (1750F), 350psi, and a 10-year service life. An increase in temperature of about 55C (100F) above design can shorten the life from 10 years to approximately 1.4 years (point "E") with HK 40. Points "B" ,"C", "D" and "E" show the improvements that can be realised in terms of temperature increase at the same pressure, or pressure increase at the same temperature for IN 519, HP mod, respectively.
A cost comparison can be made between HK 40 and the 25Cr/35Ni modified alloys for a five inch diameter tube length (three cast tube sections) about 12 meters (40 feet long). Table 2 shows that for a minimum sound wall (MSW) thickness of 10mm (0.4 inches) HP mod and HP mod micro are essentially the same cost as HK 40 at 15mm (0.6 inch) MSW.
Using the creep data in Figure 3 it can be shown that tube life for the 10mm (0.4 inch) thick HP mod alloys will be much greater than for the HK 40 with an MSW of 15 mm (0.6 inches). For example a 13 cm (five inch) diameter tube with operating conditions of 950C (1750F) and 2.4Mpa (350 psi) tube life will be nearly three times greater for the 10mm (0.4 inch) MSW HP mod micro than for the 15mm (0.6 inch) MSW HK 40 alloy. Clearly the cost comparison highly favours the HP modified alloys.
B: Cost comparison for same tube size
A cost comparison can also be made with cast tubes of equal size at different operating temperatures. Table 3 compares major candidate alloys using net present value (NPV) for a 30 year period using a discount rate of 12%. For this application creep failure is the only significant life limiting factor.
Minimum creep rupture data from alloy manufacturers were used to estimate tube life. Two operating temperatures were considered - 1750F and 1800F both at 400psi.
Net present value cost estimates show that for 1750F HP mod and In 519 are much better investments than HK 40 even if periodic comprehensive inspections at a cost of $50k each are included (Case a). At 1800F HP mod micro alloy is found to be the better investment (Case b).
C: Extended tube life related to thermal changes
Other factors effecting creep life help make the use of the HP mod alloys more attractive Figure 4. Schillmoller (3) demonstrated the effect of thermal cycling on tube life for HK 40 (lower curves) and HP mod alloys (upper curves) for a given application.
It was shown that for the same MSW the relative tube life of HP 40 mod was about three times that of HK 40, (comparing point "A" and point "C").
Also shown are the effects of switching from HK 40 to the HP mod based on the same, steady state design conditions. Point "A" for HK 40 at 19mm (0.75") can be decreased to 13mm (0.50"), point "B", with HP mod. Use of thinner tubes provides a better rate of heat transfer, making the process more efficient. Alloy tube life can be doubled, since the thinner tubes can better cope with stresses during startup and shutdown.
Also, the higher-nickel alloy (35%Ni in the HP mod alloy vs. 20%Ni in the HK 40) will have a lower coefficient of expansion, which will aid in coping with thermal stresses.
Inspection of steam methane reformer cast heater
tubes
There are many steam reformer furnaces in service that are approaching their expected design lives
that will need to be assessed to determine whether there is any further remaining life. In other cases,
because of more severe operating conditions (than design) there is simply the need to assess tube condition
to determine whether there is any remaining life. Hence the need for testing.
Various non-destructive testing methods have been used in order to detect creep fissures. Of significant value is the fact that the critical flaw size prior to failure can be 1/3 the wall thickness with about 10,000 hours remaining life, Figure 2.
Radiography (RT) has been used effectively on isolated areas that have been identified where damage may have occurred. Figure 5A shows an unfailed tube that contains significant creep fissuring. Figure 5B shows a tube with less damage. In both cases the tubes did not contain catalyst. RT has the potential for use as a final discriminator after damage has been reported by way of a screening method that has examined all tubes in question.
Some inspection methods have been used extensively in the past few years for creep fissure detection in cast reformer tubes. Ultrasonic testing has been used as a screening method to automatically scan tube lengths. Probably the most effective way to inspect cast reformer tubing is to use a combined inspection method. Recently, a comprehensive inspection method "H" scan has been developed that combines three methods of detection: ultrasonic (scattering and wall thickness), eddy current, profilometry (4).
Case 2: Coker Heater Tube Alloy Selection
Many refineries have delayed coking units in order to upgrade reduced crude as well as other heavy
bottoms to more valuable distillate by-products and a coke by-product. The charge is fed directly to the
bottom of the fractionate where light hydrocarbons are flashed off and heavy residue is passed on to a
heater.
The heated residue is then introduced into coke drums where the residence time is sufficient to form coke product. A vital part of this process is thermal cracking which is provided by coker heaters. These heaters operate in a carburizing environment at a design pressure of approximately 1.3-2 MPa (250 - 300 psi). Over a period of time, usually several months, coke build-up in the coker heater tubes leads to a pressure drop of 1-1.4MPa (150-200 psi) at the outlet. Eventually a decoking operation is required when a maximum tube metal temperatures at the outlet has been reached.
Failure modes and materials of
consturction
Until recently 9Cr - 1 Mo (9 Cr) alloy has been used for coker heater tubes. Tube life has varied depending on the level of severity. A typical failure can be caused by stress rupture brought on by a combination of pressure related stresses and internal stresses caused by carburization and coke on the inside surface of the tube. Sometimes local hot spots develop that cause premature stress rupture failures. In other cases embrittlement caused by carburization will lead to a premature tube failure brought on by thermal stresses caused during shutdowns. Many tube failures have occurred when depth of carburization of about 1/2 wall thickness has been reached.
Coker heater temperatures need to be measured and controlled to assure that tube metal temperature limits are not exceeded at end of run when coke build-up has occurred. With 9Cr-1Mo alloys a limiting temperature of 700C (1300F) is usually required. Wth increasing demands for higher levels of efficiency and throughput comes the need to handle higher tube metal temperatures. For this reason 18Cr - 8Ni austenitic stainless steel alloys, specifically Type 347 H, are now starting to be used for this application.
Optimum alloy selection
The 9 Cr alloys are susceptible to metal loss caused by oxidation on the external surface in this service. Oxide build-up (on the outside of the tube) can also lead to less efficient heat transfer with an increase in fuel costs. Metal loss caused by oxidation (fireside) is negligible up to 820C (1500F) with 18Cr - 8Ni austenitic stainless steel alloys. Type 347H as well as all other 18Cr-8Ni stainless alloys have much higher resistance to carburization than 9Cr.
Although carburization stresses are very hard to estimate it is clear that internal stresses caused by changes in volume, resulting from carbide formation, can result in additional creep stress. Furthermore, recent field experience suggests that coke build-up occurs at a slower rate with Type 347 H stainless. Slower rates mean fewer decoking operations in the life cycle of a coker heater tube which offers an economic advantage regarding equipment availability.
Type 347 H stainless offers a much higher resistance to stress rupture. In order to compare tube life with 9Cr an example is used: 9 cm (3.5 inch) diameter tube that is 6 mm (0.25 inches) thick with an outlet pressure of 1.4 MPa (200psi). In most real life cases it is unlikely that the entire tube life will be spent at the higher temperatures. It is more likely that these conditions will be experienced when coke build-up has occurred at outlet tubes resulting in a pressure drop and a temperature build-up or as a result of a local hot spot. If we choose a scenario where heater tube operation is 5,000 hours at 700C (1300F) and the remainder of time at 675C (1250F) tube life would be about 40,000 hours with 9Cr steel. With Type 347 H stainless a tube life exceeding 250,000 hours would be expected for the above conditions.
Certain disadvantages should also be considered with either 9 Cr or the above stainless steel alloys. The 9 Cr alloy can be air hardened (with a commensurate loss in toughness) by rapid cooling from the above operating temperatures. There is a similar concern with Type 347 H which can become sigmatized after long term exposure at in the 700C (1300F) temperature range. Therefore there could be a need to control shut down conditions to ensure that a rapid cool down does not occur for either alloy. One additional precautionary measure is recommended with Type 347 H stainless alloys to protect against down time stress corrosion cracking - use of neutralizing solutions during down times.
Table 4 provides a qualitative comparison between Type 347H stainless steel and 9 Cr. Similar comparisons can be developed for other refinery heaters such as visbreaker heaters. Here again, austenitic stainless steels have a significant advantage over the more traditional 9Cr alloys at temperatures of 700C (1300F) and above.
The advantage of Type 347 H stainless steels over 9Cr steel for a coker heater application must be considered in light of the increased cost. Current cost estimates indicate that Type 347 H is approximately 1.2 to 1.5 times more expensive than PWHT 9Cr. When actual costs are taken into account, that are associated with the above damage mechanisms, a life cycle cost comparison can be made between 9 Cr and Type 347H stainless.
The comparisons shown in Table 5 are based on a coker heater application.where decoking/pigging was
carried out on a frequent basis with 9Cr when temperatures reached a little over (700C)1300F at end of run
(EOR) . Because of better high- temperature properties a higher end of run temperature can be used with Type
347H stainless. Even with a limiting temperature the 9 Cr tube replacement would be required primarily due to
stress rupture limitations and oxidation.
Table 5 : Life Cycle Cost (1) (2)Comparison for Coker Heater Tubes Using Type 347
stainless steel and 9 Cr ' 1Mo alloys with Net Present Value (NPV)
|
Case A, Coker Heater Operation at TMTave= 1235F, Pave=200psi |
Timing of expense |
Type 347 |
NPV |
9 Cr 1Mo steel |
NPV |
|
|
Initial Purchase Cost |
Current |
$800 |
$800 |
$600 |
$600 |
|
|
Retube heater based on stress rupture life/9Cr-1Mo |
Every 10 years |
$600 |
$435 |
|||
|
Cost for off-stream decoking/pigging |
9Cr/2yrs |
$80 |
$178 |
$80 |
$393 |
|
|
347/4yrs |
||||||
|
Carburization of heater tubes with tube failure/ consequence |
Random |
none |
none |
$280 |
||
|
Probability of Carburization of heater tubes with tube failure. |
Random |
none |
none |
1/100 |
||
|
Cost of Risk |
Random |
none |
none |
$40 |
||
|
Stress corrosion cracking of stainless tubing/ consequence |
Random |
$280 |
none |
none |
||
|
Probability of stress corrosion cracking w/ soda ash wash |
Random |
1/1000 |
none |
none |
||
|
Cost of Risk |
Random |
$10 |
||||
|
Total Cost |
$988 |
$1468 |
||||
(1) US $ thousands
(2) over 30 year period
Also carburization is a concern. This is taken into account as a cost of risk. Because of the risk of stress corrosion cracking (polythionic stress corrosion cracking) of Type 347H stainless a cost of risk must be included. The Type 347 stainless is not stress rupture limited even over a 30 year period. Using NPV for a 30 year period with a discount rate of 12% the overall cost comparison shows that Type 347H is a better choice for this specific application.
Tube retirement is usually based on creep damage as measured by strain. For wrought tubing such as 9 Cr a typical retirement criterion would be 5% local strain (local bulge) and 3% strain for average tube deformation. Tube wall thickness measurements for metal loss caused by oxidation also supports tube retirement decisions. To date there has been very little success in measuring carburization in ferritic alloys.
Conclusions
1. When using a life cost cycling approach for alloy selection the key drivers are damage mechanisms and failure modes. For steam reformer heater tubes there is only one key driver - creep rupture. For delayed coker heaters there are several key drivers including: creep rupture, oxidation and carburization as well as coke formation.
2. Where possible quantitative or semiquantitative life estimates provide support to selection of an optimum alloy.
3. Reliability considerations are also important in this type evaluation and selection of key inspection methods can have an important effect on reliability.
4. Two examples for elevated temperature service were used to demonstrate the advantages of higher nickel alloys for heater tubes using remaining life and reliability considerations.
Acknowledgements
The authors wish to thank Nickel Development Institute for support in preparation of this paper. Bob
Sims of Becht Engineering and John Jones of DONCASTERS Paralloy are also acknowledged for their helpful
comments.
References
[1] J. R. Sims and C. Becht, "Pressure Vessel Maintenance", Chemical Engineering, pp 68-78, July
2000.
[2] C. E. Jaske and F. A. Simonen, "Creep Rupture Properties for Use in the life Assessment of Fired Heater Tubes" , 1st international Conference on Heat Resistant Materials, Lake Geneva WI, (1991).
[3] C. M. Schillmoller, "HP modified Furnace Tubes for Steam Reformers and Steam Crackers", Nickel Development Institute Technical Series, No 10,058.
[4] B. E. Shannon, "Assessing Creep Damage in Steam/Hydrogen Reformer Catalyst Tubes", ASME Pressure
Vessels and Piping Conference, Seattle, Washington, July 2000.
The material presented in this publication has been prepared for the general information of the reader and
should not be used or relied on for specific applications without first securing competent advice. The Nickel
Development Institute, its members, staff and consultants do not represent or warrant its suitability for any
general or specific use and assume no liability or responsibility of any kind in connection with the
information herein.
 |
 |
Figure 2: A typical metal cask for traditional beers.
|
Figure 3: The dispense of beer from a traditional cask.
|
Table 1: Weights and nominal capacities of typical metal beer casks.
| CASK |
NOMINAL CAPACITY |
MATERIAL |
WEIGHT |
WEIGHT |
|
Litres (Imperial Gallons) |
kg (lb) |
kg (lb) |
||
| Firkin |
41 (9) |
Stainless Steel | 11 (25) | 54 (119) |
| Kilderkin |
82 (18) |
Stainless Steel | 23 (51) | 107 (236) |
| Barrel |
164 (36) |
Aluminium | 29 (63) | 197 (435) |
| Hogshead |
246 (54) |
Aluminium | 39 (85) | 291 (641) |
Volume
