The design temperature of flare systems A key objective when setting flare system design conditions is to maintain the integrity of the system during fire relief PAUL DAVID Paul David Process Ltd
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great deal has been writ- process engineer, faced with This can lead to the somewhat ten about the minimum tables of relief case data, has to uncomfortable result that the design temperature place a number in the box on PRV re case operating tempertemperof are and blow-down syssys - the line list that says ‘Design ature is higher than its design tems because cold tempera- Temperature’. It is a number temperature. If during the re the vessel tures can cause brittle failure or for which he or she will be held over-stressing of are piping. accountable. PRV opens to relieve vapour, The design temperature of the temperature of the uid Less has been written about setting the maximum design tem- process equipment is usually entering the are system may perature of are system piping. the higher of: the maximum be much higher than the vesAmerican Petroleum Institute normal operating temperature sel design temperature. Such (API) Standard 521,1 which is plus a margin (typically of the a possibility gives rise to the used throughout the indus- order of 25°C); or the highest question: should the design try by process engineers, gives temperature expected during temperature of the are system only general guidance on how start-up, shutdown or upset. be higher than the design temto approach the task of setting Upset conditions include the perature of the vessel it serves? the (mechanical) design tem- operation of the pressure relief A re on a hydrocarbon propro perature. The standard advises valve and for equipment concon - cessing unit usually means that the extremes of temper- taining a saturated liquid the that a loss of containment ature of the uids entering temperature is higher at relief has already occurred. When the header should be consid- pressure than at normal pres- a vessel containing hydrocarhydrocarered, heat transfer analysis sure. The selection of relief bon is subject to heat input may be performed, it is comcom - valve body and ange ratrat- from a major re, further failfailmon to exclude the re case, ing are normally based on the ure (for instance at the vessel and careful analysis is required. equipment design conditions. anges) should not be unexunex There is considerable scope The re contingency is not pected. Although this results for interpretation by individindivid- normally considered when setset - in an escalation of the re, the uals and companies of how ting the design temperature incident is still contained in the this is to be implemented. The of the equipment or its associassoci - same plant area. author’s experience is that com- ated pressure relief valve (PRV). If hot vapour relief to the pany standards differ in their The re relief temperature for are system causes sufcient approach and in the level of a heavy hydrocarbon mixture, stress at any point in the are detail they offer, sometimes with a wide boiling range, can header (by thermal expanexpangiving rise to more questions be very much highe higherr than the sion) then signicant damdam than answers. The renery equipment design temperature. age can occur. This may cause
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loss of containment at a location remote from the original re. The result would be a major escalation of the incident. It is therefore not illogical that parts of the are system should have a higher design temperature than that of the equipment from which the re relief stream originates. However, the header piping in on-plot are systems is highly constrained (by many piping branches) and likely to be large in diameter. Setting an unreasonably high design temperature will result in major mechanical design problems. There is no industry wide practice for setting design temperature based on re case data. This scenario requires careful consideration to avoid either an inadequate specication or an infeasible specication. API Standard 521 states that it is common practice to exclude the re relief scenario when specifying the design temperature of the are headers. It may be a common practice but, in the author’s experience, it is certainly not ubiquitous. To consider re case for the majority of renery PRVs but to ignore it when specifying the disposal system to which they discharge might be considered questionable. After all, a re on an oil renery is a reasonably foreseeable contingency. The following sections assume that the re case will not be ignored and consider some of the issues that will therefore arise.
carbons will have very high tents since thermal cracking is calculated re relief tempera- endothermic. tures. The heavier the hydrocarIf the calculated re relief bon and the higher the PRV set temperature is higher than pressure, the higher will be the 350°C it should be treated with relief temperature. If we take the extreme caution. average boiling point as indicative of the temperature a vessel Vessels containing vapour might be expected to reach, then When vessels containing only a distillate stream with an aver- gas or vapour are subject to age boiling point of 300°C might re heat input, very high ini be over 400°C at a relieving tial relieving temperatures can pressure of 5 barg. The design be calculated depending on of the are sub-header piping the ratio of normal operating would be very challenging at to relieving pressure. In some this sort of temperature. cases the calculated relief temActually, the relieving tem- perature will be infeasibly high perature is unlikely to reach and failure of the vessel would 400°C and it is therefore have occurred before relief. In unlikely that the are piping any case the mass relief rate is would ever reach this temper- likely to be low and the relief ature. Hydrocarbons tend to relatively short lived. Failure start cracking if the tempera- of the vessel is likely unless ture exceeds a value of around the vessel is effectively cooled 350°C; if the liquid in a vessel by re-water. In either case the is boiling at 350°C it is likely relief will cease. For these reathat some cracking is occurring sons, relief from a vessel conat the vessel walls. The lower taining vapour only is unlikely molecular weight materials to heat up a signicant porproduced by cracking will tend tion of the are piping and to reduce the effective vapour is unlikely to determine the pressure of the liquid and make design temperature of the are it unlikely that the temperature system. will continue to rise in the way predicted from the feed stream What is the temperature boiling range. downstream of the pressure Depending on hydrocarbon relief valve? type and molecular weight, it Overall, the ow through a may well be that the hydrocar- relief valve is considered to be bon in the vessel will become approximately isenthalpic – the supercritical at the relieving nozzle ow is isentropic, but pressure and will no longer this does not continue through boil. Prediction of what hap- out the valve. Therefore it is pens inside the vessel is now usual to expect a temperature even more difcult since crack - drop across the relief valve due ing will still occur (or increase to the pressure falling at consince the wall temperature is stant enthalpy. This is typilikely to increase). The for- cally of the order of 1.5°C per What is the fire case relieving mation of light hydrocarbons bar drop across the PRV for a temperature? will tend to increase the crit- hydrocarbon stream. Wetted wall vessels Many renery vessels contain- ical pressure of the mixture The ow velocity at the relief ing wide boiling range hydro- and also cool the vessel con- valve outlet is almost always
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higher than at the relief valve inlet. Rigorous ow simulators will indicate a further temperature drop below the stagnation temperature which would be calculated by a process simulator at the valve outlet. (Stagnation temperature is the temperature that would occur if the ow was brought to rest isentropically.) This additional temperature drop will generally not amount to more than a few degrees and should not be accounted for in simple heat transfer calculations to estimate the wall temperature. Due to the velocity prole, the velocity of the uid next to the wall is zero and the uid temperature at the wall would be nearer the stagnation temperature than the bulk owing temperature for adiabatic ow.2 The ow is not actually adiabatic, since heat transfer through the pipe wall occurs, but the principle that the velocity related temperature drop is not fully experienced at the wall remains. As vapour ows through pipe of constant diameter, its pressure will fall and its velocity will increase – this results in a reduction in temperature. This effect may briey be reversed when a ow from a small pipe enters a larger header, causing the velocity to reduce. Any fall in temperature due to velocity increase in the are system will normally be less than 10°C for heavy hydrocarbons at typical back pressures. Again, care should be taken in accounting for this temperature drop – the uid temperature close to the wall would be higher than the bulk temperature for adiabatic ow. Note: API Standard 521 recommends the use of isothermal
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ow calculations for estimat- lower than at the PRV outlet ing PRV back pressures from – heat transfer, limited by the the are piping. While this is outside co-efcient, is slow. The reasonable in that it results in a wall temperature in the rst conservative back pressure esti- segment is also not much lower mate (for gases at above ambi- than the PRV vapour outlet ent temperature) isothermal temperature. ow calculations are not genAs we move downstream erally suitable for estimating through the are system, this the are temperature pro- process keeps repeating itself. le. Isothermal ow is equiv- The gas and wall temperaalent to adiabatic ow with tures rise later (due to the the exponent in the equation larger upstream mass of metal PVγ = constant set to a value of to be heated) and the steady1. For heavy hydrocarbons, the state temperature is lower (due ideal gas value of γ is not much to the greater upstream heat above 1, which is why the cal- transfer area). The rate of both culated temperature drop (in these effects is dependent on the absence of heat transfer) is the relief case and the system usually small. geometry. However, on a typWhile all these effects are ical process unit the time to directionally ‘helpful’ for the steady state is still likely to be are system, their combined measured in minutes and the effect is relatively small. steady-state temperature is When hot gases ow through likely to be well within 50°C of a PRV into the are system, the the PRV outlet temperature. Off-plot are headers are heat transfer from the gas to the cold pipe will initially result often of considerable length, in relatively rapid cooling of are designed with considerable the hot gas. The heat transfer exibility, and have far fewer coefcient between the ow- connections. They are downing gas stream and metal is stream of the sub-header, often likely to be limited by the foul - downstream of a large knocking factor (renery are lines out drum, and the header itself are typically heavily scaled). has considerable mass and relNevertheless the inside coef- atively large heat transfer area. cient will be well over an order All the considerations for the of magnitude greater than the sub-header apply but, for the outside coefcient and this off-plot header as a whole, the results in the pipe wall heating rate of temperature rise will quickly. Since the outside coef - be signicantly slower and the cient is very low, the steady- steady-state temperature will state pipe wall temperature is also be lower. quite close to the relief stream temperature. Near the PRV, What is the hazard? the steady-state temperature Process engineers are accuswill be reached within minutes. tomed to think of the anges as Once the steady-state tempera- the weak points in piping systure is reached in the pipe seg- tems and most of us are quite ment just downstream of the capable of assessing their presPRV, the gas temperature into sure-temperature rating. For the next segment is not much are sub-headers with multiple
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branches, the thermal expanHowever, 350°C would be a essary to be careful with this sion of the header can cause very high design temperature approach – the uid ow and stresses for which the ange for the process unit are sub- heat transfer in the are system pressure-temperature rating is header or downstream piping can be modelled with rigour not directly relevant. It is the and renery headers gener- but the process engineer should temperature of the pipe wall ally have design temperatures question whether the model (causing thermal growth) that lower than this. There are sev- of the process vessel, during is important, not necessarily eral reasons for this, including the re, is equally rigorous. the temperature of the anges. those that have been discussed The calculated re case piping Resultant over-stresses, which above: metal temperature is used by might occur due to re relief, • There will be some heat loss/ the stress engineer to ensure the can be in the sub-header or one temperature drop as the vapour integrity of the are system and of the branches and can only be ows through the PRV and a suitable mechanical design predicted by a full stress anal- are piping temperature is back calculated. ysis. This stress analysis will • If the relief valve opens inter- All short-term overstress allowreect the fact that only part of mittently then are piping ances in the relevant piping the sub-header may be at high may not reach its equilibrium design code should be taken temperature, depending on the temperature into account for the re case. location of the re. Where the • The relief stream from For units where the re failure occurs is not necessar- the vessel in question may relief temperature is assessed ily intuitive. It may well be that mix with other material at 350°C, a sub-header design the failure is at a ange, but the as it enters the sub-header temperature in the range ange that fails may not be one • It takes time for the vapour 250°C to 300°C may not be that experiences the highest to heat the metal of the are unexpected. temperature. The stress may be sub-header. The off-plot are system caused by the growth of piping The heat-up time may also design temperature is found elsewhere. allow the re brigade to start by similar considerations and Failure in the are sys- cooling sprays on the area sur- methods to those applying to tem during a localised re is a rounding the re. Although the sub-headers. Generally, the highly undesirable event. The the are header is unlikely to design temperature is lower are is an important safety be their priority, cooling of the than for the on-plot piping system. As the re scenario process equipment in the re since there is more opportunity unfolds, operators may vent area should reduce the relief for heat loss in the upstream gas from numerous locations rate. piping. into the are. Failure will result There are two methods by It is not normally considin highly ammable or toxic which the design temperature ered that the PRV tail-pipe or gas venting to atmosphere at an of the are sub-header is usu- are headers will experience unexpected location. ally set: re engulfment. On-plot are 1. Large companies sometimes sub-headers will normally be have a ‘not to be exceeded’ at an elevation above the 7.6m Setting the flare system design value based on experience. This (25ft) normally considered for temperature The are system does not usu- value is derived from many re relief calculations1 and PRV ally have a design temperature years of operating multiple pro- tail-pipes should be situated lower than the owing temper- cess units and is known to be above their respective headature for non-re contingencies. practical for design from the ers. Direct radiation is thereSetting the design tempera- piping stress viewpoint. fore not normally taken into ture of the PRV tail-pipe to a 2. The second method is based account when determining pipvalue of 300°C-350°C, based on on rigorous uid ow with heat ing design temperatures. Where the re case, might not be con- transfer calculations to give an there are particular concerns, sidered unreasonable for equip- expected metal temperature for it may be advisable to provide ment containing heavy or wide the are sub-header during the re-proof insulation although boiling hydrocarbon streams. worst case scenario. It is nec- this is unusual on are piping.
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The provision of any insulation, of course, affects the heat loss calculations discussed above. Conclusion It is important to know the objective when setting are system design conditions. For large hydrocarbon processing plant, one key objective is to maintain the integrity of the are system during re relief. The consequences of the re may cause upsets or shutdowns on other process units and the are system is a key util ity under these conditions. A are system failure may result in a large release of amma ble material at an unexpected location. The author is aware of an incident where a huge reball was caused by a process unit venting into a damaged are system. Although in this case the damage was not caused by thermal expansion it emphasised the importance of maintaining are system integ-
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rity, particularly during upset ble for the are piping stress conditions. analysis. The use of relieving temperatures, and hence are design References Petroleum Institute temperatures, of higher than 1 American around 350°C are unlikely to be Standard 521, Pressure relieving and warranted for typical renery Depressuring systems. Sixth Edition, Jan 2014. hydrocarbon streams. For other 2 Shackelford A, Temperature Effects materials, the behaviour of the for High-velocity Gas Flow, Chemical material at elevated tempera- Engineering, Jan 2015. tures needs to be reviewed. Design temperature of Paul David is Director and Process downstream sub-headers and Engineer with Paul David Process, headers is based on the max- providing process engineering and overpressure protection services. He imum expected metal tem- holds a bachelor’s degree in chemical perature. This is often set by engineering from the University of Bath experience since the exact and has over 30 years’ experience in the behaviour of the relieving industrial gases, chemical and oil refining material when heated by re industries. For more than 20 years he and the actual contingency is worked at a major UK oil refinery. not known until it happens. In the absence of extensive, releLINKS vant experience, the large simulation effort required should More articles from the following be tempered by good judgecategories: ment. The data generated durCombustion Systems and ing the re case study should Engineering be documented and reviewed Emissions Reduction with the engineer responsi-
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