COPYING AND DISTRIBUTING ARE PROHIBITED WITHOUT PERMISSION OF THE PUBLISHER
Material selection for low-temperature low -temperature applications Texas 07.02.2004 | Kumar, S., Consultant, Houston, Texas Analysis shows conventional metallurgy is usually adequate for process upsets
Keywords: A void selecting expensive materials of construction for short-term, low-temperature low-temperature exposures of vessels and piping. Short-term low temperature temperature generally occurs during blowdown of a section of the process. Straight lowtemperature data from a depressurization study might appear to exaggerate the threat of brittle fracture. However, detailed study reveals that low-temperature exposure is generally not as adverse as suspected, and that conventional metallurgy is usually adequate for the application. Scenarios leading to low-temperature low-temperature exposures are not unusual in oil and gas plants—it might be a long-term normal process or a short-term abnormal contingency (see Appendix 1). The immediate concern of any lowtemperature exposure exposure is selecting compatible metallurgy. Cryogenic exposure adversely affects the metal properties such that the material strength (brittle fracture resistance–toughness) –toughness) becomes questionable, particularly under high pressure (see Appendix 2). A perfect material selection requires process knowledge and material science. At times, over-conservative material selection results, due to lack of coordination between the process engineer and material specialist. As per general convention in an engineering design company, a material specialist is responsible for selecting appropriate material and examining its worthiness for low-temperature conditions, conditions, based on the minimum min imum design temperature ( MDT ) as specified by the process engineer. The source of over-conservatism over-conservatism starts when the process engineer nominates the vessel MDT vessel MDT to to be equal to minimum temperature temperature of the process fluid. However, often in reality, MDT reality, MDT is is not equal to process temperature. As will be discussed later, particularly during short-term blowdown, reasonable credits are available to avoid the potential low-temperature scenario that can raise the vessel MDT vessel MDT much much above the actual process fluid temperature. This article explores useful information from ASME and other relevant codes, and integrates it with a lowtemperature study. Joint knowledge of process and material engineering helps a process engineer maintain an integral and economical design. Generally, as a quick rule of thumb, any process event that would expose the metal to colder than –20°F will necessitate a low-temperature low-temperature study to justify use of the proposed material of construction. Severity of lowtemperature exposure depends on the following factors:
State of the process fluid within the vessel: liquid/gas. Heat transfer (cold fluid to metal) for liquid is faster than that for gas. Fluid pressure—the higher the coincident pressure, the higher the potential of brittle fracture in m etal Fluid temperature—the temperature—the colder the temperature, the more severe the risk of brittle fracture Material metallurgy and thickness. The impact of thickness and metallurgy on low-temperature failure is discussed later (See Appendix 2).
Cold liquid service. Given the higher heat transfer coefficient in liquid, it is reasonable to equate the material temperature to the liquid temperature. However, However, concessions in the minimum design metal temperature ( MDMT ,
due to low pressure) can be taken, as will be discussed later.
Cold gas services. In gas service, however, unlike liquid, the film heat transfer coefficient is relatively very poor. So, it is highly unreasonable to equate the MDMT to the temperature of the cold gas, unless the vessel is insulated and the cold service is long term. Indeed, the gaseous low-temperature scenario needs detailed study to determine a realistic MDMT and to select an appropriate construction material. The following example helps one understand the implication. A low-temperature study indicates that cold blowdown gas can expose the discharge pipe to –200°F at the end of 15 minutes blowdown, the coincident pressure being only 20 psig (less than 30% of the pipe design pressure). On the face of this example, without any study, one can nominate the MDMT of the pipe to be –200 °F, and can propose use of stainless steel. The question: How realistic is this MDMT and the resultant material selection? A detailed study, however, reveals even normal carbon steel ASTM106 (Gr. B) with MDMT of –20°F would be suitable for the service. The detailed low-temperature study essentially comprises the following two different approaches. They may be used in conjunction or in isolation.
Heat transfer model: The heat transfer (HT) model helps us determine the realistic wall temperature that a vessel/pipe section can attain within the specified short term. As will be shown later in detail, the HT model incorporates a dynamic heat balance over a unit section of pipe. Coincidental concession: This approach allows some concession in MDMT (reduced MDMT ), should the coincidental operating pressure be lower than the design pressure. However, this approach needs special attention, as will be discussed later.
Heat transfer model. The model is based on the basic premise that the wall temperature progressively dips due to net heat loss from the wall metal. The net heat loss is a balance of the heat loss to cold gas and the heat in-leak from ambient. The heat loss rate from metal to gas fundamentally depends on the temperature and film heat transfer coefficient of the cold fluid. The heat in-leak rate from ambient air depends on the temperature difference between ambient and metal, with the ambient (outside) heat transfer coefficient and air temperature being nearly constant. Fig. 1 shows the simultaneous relationship and behavior of the heat transfer system.
Fig. 1
The rate of both heat transfers varies with time.
As is evident from curves A and B, the rate of both the heat transfers varies with time, and so does the heat loss/gain from the metal. Therefore, a dynamic heat balance is needed to estimate the variation in metal temperature along time. A dynamic HT model is presented for cold gas flowing though a discharge pipe downstream of a BDV assembly.
Here we will model the heat transfer around a unit section of tail pipe. The model assumes that the vessel/pipe is not lagged with any insulation or fire blanket. The tail pipes are more prone to low-temperature exposure than the upstream inlet pipe/vessel during the blowdown. Blowdown gas conditions and properties can be extracted from any depressuring software. Heat in-leaks occur simultaneously from ambient to pipe, and pipe to cold gas. Initially, there is an imbalance of heat transfer in favor of heat loss to the pipe. As a result, the pipe starts getting cold. Much later, however, both the heat transfers balance, and the pipe temperature stops declining. After the pipe gets cold enough, the heat imbalance slightly shifts in favor of ambient heat gain to pipe. Eventually, depending on the time, the pipe temperature increases marginally. The reason for heat imbalance in favor of heat loss to pipe is due to a significantly higher gas-side heat transfer coefficient than that of the ambient side (Fig. 2).
Fig. 2
The gas-side heat transfer coefficient is significantly higher than the ambient side.
The model is based on a unit length of pipe (immediately downstream of BDV) and is represented by the following equation: Heat loss from pipe = inside heat transfer – outside heat transfer We will break the total time span of blowdown into equal increments of one minute. Incremental heat gain/loss during the nth minute from a unit section of pipe is given by:
Incremental temperature change in the pipe section as a result of Eq. 1 is given by:
Pipe temperature at an n th instance of time can be represented by the following:
Eq. 3, simultaneously with Eqs. 4 and 8, can be integrated numerically by writing a suitable VB code in a macro module of Excel. The VB codes, as developed by the author, provide the pipe temperature profile over a specified time (Fig. 3).* The two R.H.S. expressions of Eq. 1 are separately elaborated and discussed in detail:
Inside heat transfer. The heat transfer rate from a unit pipe section to cold gas varies with time and depends on the following time-dependent (dynamic) variables:
Gas-side heat transfer coefficient continuously decreases with flowrate. Gas temperature decreases with time. Pipe temperature varies with time.
The instantaneous heat transfer rate at nth time is given by:
The instantaneous heat transfer coefficient at nth time can be estimated from the following:
The thermodynamic properties of the cold gas can be taken from simulation and assumed constant amid the varying gas flowrate and temperature. The blowdown flowrate decreases from an initial peak to a minimum rate within the specified blowdown time. The mass rate, particularly during blowdown, is an exponential function of time and can be fit to the following equation:
where constant C is given by following:
As the blowdown progresses, the gas continues to get colder. The declining temperature generally follows a straight line with respect to time. The instantaneous gas temperature at nth time can be estimated by the following:
The instantaneous temperature of the unit section of pipe, T mn, is estimated via solving Eqs. 3, 7 and 8 simultaneously at the nth stage of time, using VB codes.
Outside heat transfer. The inside heat transfer from pipe metal to the gas will make the pipe progressively colder. The cold pipe in turn will be heated by the ambient heat transfer. Instantaneous heat transfer rate at nth time from ambient to the unit section of pipe is given by:
The terms such as ambient temperature and ambient heat transfer coefficient in E q. 8 are assumed constant. The ambient heat transfer coefficient for the cold surface (being heated by ambient air) is smaller than that for the hot surface (being cooled by ambient air). The reason being, the former does not include the phenomenon of radiation and natural convection, which generally cast a significant effect on heat transfer. Conventional correlations for estimating outside ambient heat transfer coefficient based on wind velocity, as given in literature (particularly Fig 2.91), are generally for hot surfaces. Using the conventional correlations, therefore, might result in potential error in end results. Unfortunately, availability of correlations in the engineering literature for estimating ambient heat transfer coefficient for a cold surface is very poor. However, a simple experiment as set up by the author, indicates that the value varies between 0.5 and 2 Btu/(h ft 2 °F). It is reasonable to use 1.0 Btu/(h ft 2 °F). Again, instantaneous temperature of the unit section of pipe, T mn, is estimated via solving Eqs. 3, 7 and 8 simultaneously at the nth stage of time using VB codes. The following example shows the application of the HT model. A tail pipe of a BDV discharges cold gas over a 15-minute period. Data required for calculation are given below. The pipe is standard 4-in. SA 106 ( MDMT = –20°F).
Refer to the nomenclature for descriptions of symbols and units. This problem was solved by applying a heat balance every second for 15 minutes via VB macro codes in an Excel spreadsheet. The results are presented in Fig. 3. The metal curve indicates that the minimum temperature the pipe can attain is –70°F at the 12th minute. Then after 12 minutes, the gas heat transfer coefficient drops to such a value that outside heat transfer exceeds the inside heat transfer, and effectively the pipe starts gaining heat from ambient.
Fig. 3 The VB codes provide the temperature profile over
a specified time.
It is evident from this example that the metal temperature can never reach the minimum gas temperature (–200° F). The coincident pressure at –70°F of metal temperature is nearly close to atmospheric where coincident ratio would be less than 0.35. As a result, as will be discussed later, we can further apply concessions in the MDMT of SA106. Finally, we can nominate the reduced MDMT of SA106 being –155°F, where this material i s obviously suitable at minimum operating temperature (–70°F, as predicted by HT modeling) during blowdown. At this stage, we can definitely rule out use of any exotic metallurgy or stainless steel.
Concession in MDMT . The MDMT is the minimum limiting temperature the vessel/pipe material can sustain (without having to be impact tested) at full design pressure ( DP ). The MDMT value can be obtained from the applicable curve as given in ASME (Piping: B31.3-2002 Fig. 323.2.2A, Vessels: USC-66.1). The MDMT is a vessel/piping material property that is dictated by the ASME curves or impact testing. The lowest process temperature never fixes the MDMT ; rather it does help, in conjunction with MDMT , in determining the integrity of the material. Quite often, we encounter a situation where the lowest process medium temperature is even lower than the MDMT limits set by the curves. But such a situation generally occurs at much lower pressure than the vessel DP . This low operating pressure is defined as the coincident pressure. To justify suitability of the same vessel material whose DMT is warmer than the process temperature, the ASME (B31.3-2002 and UCS-66.1) and API 579 allows for concession in MDMT at coincidental low operating pressure. It is believed that the coincident low pressure will induce low stresses (the general primary membrane tensile stress) in the vessel such that MDMT can be reduced without any brittle fracture. The "coincident ratio" is defined as the ratio of the required thickness corresponding to the low coincident pressure (without CA ) to the nominal thickness used for construction. The coincident ratio and the corresponding MDMT concessions are applicable when the low stresses are greater than 8 ksi ( $ 35% of DP ). In a simplified way, the coincident ratio can be coarsely estimated as the ratio of operating to design pressures. Traditionally, the concession has been applied in determining suitability of existing equipment for anticipated low-temperature service. The draft API 579 presents a methodology for retrospective determination of critical exposure temperature (CET ) and reduced MDMT . The CET is defined as the lowest process temperature at which the equipment metal will be exposed to at a coincident pressure greater than 35% of equipment design pressure. The CET may be a single temperature or an envelope of temperatures and coincident low pressures. A minimum safe operating temperature ( MSOT ) curve between reduced MDMT and coincidental low pressures can be established. Thus it follows that two curves can be constructed: CET and MSOT . Suitability of the material and safe operation is indicated when the CET curve stays above the MSOT curve. Even though these guidelines are intended for retrospective assessment of existing equipment in low-temperature service, they can be used for the design specification of new equipment, too, with an important operational note. The operating manual must be incorporated with repressuring procedures as follows. During the repressuring (above the coincident pressure) for resumption of normal service, the equipment must be warm enough such that the combination of coincident pressure and vessel temperature always stays above the MSOT curve. In other words, if the equipment is subject to process temperature below MDMT at low pressure as discussed previously, it must be allowed to re-warm before it is repressured. The methodology for applying concession in MDMT is presented in Fig. 4 and explained through the following example: A 42-in. ID fuel gas scrubber ( DP = 890 psig) is fabricated from SA516 plate (grade 70, normalized). The short-
term vessel blowdown prior to maintenance is anticipated to expose the vessel to the following conditions, as extracted from the simulation program. Suitability of the vessel needs to be reviewed during the blowdown scenario.
Fig. 4
Methodology for applying concession in MDMT.
Methodology: CET curve: Construct a CET curve (coincident pressures vs. temperatures, as given in Table 1). The figures of low temperatures and corresponding coincident should be estimated at above 35% of the DP .
Therefore, the coincidental low pressure cut-off point for the CET curve = 0.35 3 890 = 312 psig. SOT curve: Construct an MSOT curve, following the steps given below. This curve indicates the relationship between various coincident operating pressures (Table 1) and corresponding reduced MDMT .
Vessel wall thickness: DP = 890 psig R = 21 in. E = 100% S = 20,000 psi CA = 0.118 in. Vessel wall thickness is given by the formula:
Substituting the values in thickness formula, we get: X = 1.079 in. The nearest standard plate (SA516) available is 1.25 in. Therefore, used thickness of material, X n = 1.25 in. Following that, now estimate the required thickness based on the corresponding operating pressure without CA, as inputted in the second column of Table 2.
The MDMT at 1.25 in., using the ASME curve D = –20°F. Construct the calculation Table 2.
Inspecting Table 2 indicates that the CET data are warmer than the reduced MDMT data. Hence, the selected material SA 516 is suitable for this contingency. However, the operating manual should be modified with the following repressuring instruction. Repressurize the vessel in the warm c ondition such that any temperature and pressure combination must stay above the MSOT curve. Table 2 results are plotted in the CET/MSOT curve (Fig. 5).
Fig. 5
Results of Table 2.
APPENDIX 1. LOW-TEMPERATURE CONTINGENCY A process upset during normal operation is not unusual. The effects of process upsets are variations in process conditions (pressure temperature or flow). Generally, most process upsets can be attenuated by using pressure safety valves. But unfortunately, upsets that result in exposing metal to low temperature can only be handled by integrity of the material; no mechanical device can protect the system from brittle fracture. The following upsets can contribute to low-temperature contingency:
Emergency blowdown: An emergency situation such as confirmed fire and gas detection, the hydrocarbon inventory of the system has to be vented out within a specified time to avoid risk of vessel rupture and the resultant escalation of fire. Also, a system can be blown down for planned maintenance. In either case, contents of the system lose internal energy and get progressively colder with declining pressure. Operational blowdown: During planned maintenance, the system needs to be emptied of its hydrocarbon inventory. This type of blowdown is generally controlled such that any cryogenic exposure can be eliminated. However, use of the same orifice (designed for emergency blowdown) for operational blowdown will give rise to low-temperature concern. Leaks: Leaks can occur as a result of nozzle gasket or control valve failure, or pipeline rupture. The effects are the same as from blowdown. Ambient: The lowest ambient temperature might jeopardize cryogenic i ntegrity of the material.
APPENDIX 2. ppendix 2. A BRIEF OVERVIEW OF MATERIALS Steels used for construction are ductile at warm temperatures, and the stress-strain curve depicts the following tensile regions:
Elastic region in which deformation is reversible and time independent Plastic region where metal deformation occurs after the stress is removed Permanent set region that represents the "yield strength" of the metal.
Usually a small tensile specimen of steel stretches plastically by a large amount before breaking. Yet large pressure components (fabricated vessel/pipes) from the same material sometimes fracture without much plasticity at nominal stresses below the "yield stress" for the following reasons. Such fractures in the pressure components are caused by minor cracks that result from fabrication, welding or low-temperature brittleness. Nonetheless, the material of pressure components, unlike that of the specimen, is additionally exposed to hoop stresses. The steels exhibit a characteristic temperature range called "ductile brittle transition temperature" ( DBT ), below which they are susceptible to brittle fracture. If a vessel/pipe is operated below DBT , the material loses its ductility and, consequently, might result in a potential crack or geometrical notch. The crack or flaw thus generated, however minor, acts as a stress concentrator and induces a brittle type fracture. The crack tends to grow slowly and progressively under regular service load as the stress continues to concentrate around it. Eventually, the crack penetrates through the wall cross-section and results in brittle fracture failure. The lower the temperature for a given steel, the greater the potential for brittle fracture. The DBT varies with the quality of material. Material toughness is an index to the brittle fracture: the lower the temperature, the poorer the toughness. Material toughness can be inferred from impact test results. The purpose of a Charpy impact test is to determine worthiness of a material at the intended low-temperature operation. The potential for brittle fracture is greater for thick vessel pipes for the following reasons. The inside and outside temperature difference results in a thermal gradient across the wall thickness. The thermal gradient induces differential expansion of material across the thickness that might eventually result in a crack. As a result of such a
cascading effect, the higher the wall thickness, the greater the risk of brittle crack in a material. In a nutshell, the potential for brittle fracture is subject to the type of steel and the wall thickness. The MDMT is defined as the cold limit of the vessel material at full operating pressure. The MDMT is an index for suitability of the proposed material without any risk of brittle fracture. However, reduced MDMT is possible at lower operating pressures. Usually the mechanism and determination of brittle fracture with respect to the material type and thickness and the operating temperature are very complex. In an attempt to eliminate the complexity and simplify the determination of brittle failure, ASME has provided simple and quick-reference curves (Fig. UCS-66 and 323.2.2A: DMT vs. Nominal Thickness vs. Type of Material). Use of the curves is very simple as follows (refer to Fig. 5 also):
Determine the lowest service temperature. Determine material thickness based on the nominated system design pressure. Determine the type of material to be used for the system. Locate the above three combinations on the MDMT curves. If the combination is above the material curve, the proposed material is suitable for the intended cold service. If the combination is below the curve, then the material needs to be upgraded or impact tested for the desired MDMT .
Following are the useful extracts from the ASME curves: For thickness # 10 mm, coincident ratio = 1, and the coldest process temperature . –20°F; the MDMT can be nominated to –20°F and the following carbon steel material may be used: SA 516 (grades 65 and 70, if not normalized) for pressure vessels and SA 106, API 5L for pipes. For thickness # 10 mm, coincident ratio = 1, and the coldest process temperature . –55°F; the MDMT can be nominated to –55°F and the following carbon steel material may be used: SA 516 (all grades, if normalized) for pressure vessels and SA 333 for pipes. For thickness # 10 mm, coincident ratio between , 1 and $ 0.35, and the coldest process temperature . –155°F; consider taking a concession in MDMT to justify use of the above materials. For thickness # 10 mm, coincident ratio, 0.35, and the coldest process temperature . –155°F; the MDMT can be nominated to –155°F and the above materials are suitable. For thickness . 10 mm, refer to the curves to determine the realistic MDMT , take the concession in MDMT if applicable. The basic methodology is same as described for thickness # 1 0 mm. All the above guidelines are applicable if the requirement of impact testing is not considered. If the material is impact tested at the coldest process temperature, the MDMT can be directly equated to the impact test temperature. HP
NOMENCLATURE AND SYMBOLS
BIBLIOGRAPHY ASME, Section VIII, Div. I. Kern, D. Q., Process Heat Transfer, McGraw-Hill Book Company.
Sunil Kumar is senior process engineer working with Worley Limited (Oil/Gas), Perth, Australia. Prior to that, he has served Bechtel at various places in the world. Mr. Kumar has 15 years' process and system design experience in the onshore/offshore oil and gas industries. His key experience is in low-temperature studies and flare system modeling. He has his master's degree in chemical engineering from Harcourt Butler Technological Institute and is a member of IEAust (The Institution of Engineers Australia). Mr. Kumar can be contacted at:
[email protected].
