Line Sizing and Pressure Drop Philosophy [PDF]

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LINE SIZING AND PRESSURE DROP PHILOSOPHY



The following guideline shall be used when selecting the minimum line sizes: • • • • • • • •



2" NB 2" NB 1½" NB ¾" NB 1” NB 2” NB 4" NB 4” NB



Minimum Minimum Minimum Minimum Minimum Minimum Minimum Minimum



nozzle size for vessels, tanks, heat exchangers process (hydrocarbon) line size utility line size bridle drain or pump casing vent / drain chemical injection. on pipe rack or pipe sleeper for (wrapped and coated) underground lines. for storm water runoff sewer drain lines



Sizing Methods Single phase line sizing can be done using different correlations for friction factor. The following roughness coefficients are typically considered for different pipe material: Pipe Material Carbon steel pipe Carbon steel corroded - pipe) Note-1 Stainless steel pipe Galvanized Steel Plastic Brass, Aluminum, Copper



Roughness (mm) 0.0457 0.457 0.0254 0.13 0.005 0.03



Note-1 : Higher roughness to be used for brownfield projects. Crane Data Book may be referred for selecting the pipe roughness. Crane Data Book may be referred for the equivalent length of pipe fittings and valves. DEP 31.38.01.11-Gen can also be referred to information is not available in crane handbook.



for Shell projects or in cases where such



For special pipe materials like GRE, cladded / coated pipes; Vendor provided data should be used.



Pipe Sizing Criteria for Single Phase Fluid The recommended velocities are based on vibration, noise and erosion considerations. The recommended pressure drop limitations are based on economic considerations and reasonable pump heads. It is difficult to establish the exact limits of min/max velocities and pressure drops analytically. The recommended values are based on common established engineering practices prevalent in the industry. Large size headers and bulk line pipes for pipeline transport are selected based on economic analysis of investment and operating cost, the guidelines in this document may not entirely apply to such situations. Line sizing criteria for both liquid phase and vapour phase flow regimes are described below.



Sizing Criteria for Liquid Phase Flow For a specific duty, line sizing criteria for liquid phase is tabulated below:



SERVICE



Pump suction, bubble point



LINE SIZE



 3”



RECOMMENDED MAXIMUM VELOCITY (m.s-1) 1.1



ALLOWABLE PRESSURE DROP MAX. (bar/100m) 0.045



(Note- 2,5) 4” – 8”



Pump suction, subcooled



1.4 – 1.8 ( Note-1)



10”



1.8



0.045 to 0.05 ( Note-1) 0.07



 3”



1.1



0.136



(Note-5) 4” - 8” 10”



1.4-1.8 ( Note-1) 1.8



0.136 to 0.226 ( Note-1) 0.226



 3”



1.8



4” – 8”



2.4-4.3



0.339-0.453



10”



4.9



0.453



Unit line liquid bubble point



 3”



1.1



0.1



or liquid with dissolved gas



4” – 8”



1.4 – 1.8 ( Note-1)



0.1



10”



1.8



0.1



Pump discharge (Note-6)



(Note-4) Gravity run lines



0.6 -2.5



0.339



0.04 to 0.09



Amine Solution: Rich Amine



1.5



Lean Amine



2.0



Sour water



2.0



Service water



 2”



1.5



0.35



(Note- 3)



3”-6”



2.0 – 3.0



0.35



8”- 12”



2.5 -4.2



0.35



 12”



3.0 - 5.0



0.35



Notes : 1)



Lower limit of velocity and pressure drop corresponds to lower sizes of pipe.



2)



Applicable to liquid feeding to thermosiphon reboiler and liquid containing dissolved gas.



3)



Higher velocity can be used in pipe if water quality is good.



4)



Unit subcooled liquid lines will be sized similar to pump discharge.



5) 6)



Line size must meet the pump NPSH requirement. Higher pressure drop and velocity can be allowed in Alloys/SS lines .



Sizing Criteria for Vapor Phase Flow Vapor flows in process piping can be split into two general categories: 1. Incompressible flow 2. Compressible flow The correct line sizing criteria must be selected, depending on the vapor flow type. Incompressible Vapor Flow Vapor flow is considered incompressible, when both the following criteria are met: Vapor velocity is less than 60 m/sec. Total pressure drop is less than 10% of the initial pressure. Normal in plant process and utility vapor flows fall in this category and the lines are sized for isothermal incompressible flow. Process and utility lines in vapor service (excluding flare, vent, PSV and depressurization lines) fall in isothermal incompressible flow. These lines can be sized in accordance with the criteria presented in the following table. The v2 criteria are governed by consideration of vibration and forces on piping supports. The v3 criteria are governed by consideration of noise levels. By exceeding these criteria, the energy of the fluid can damage the material (piping, fatigue of pipe supports, etc.) and personnel in the form of excessive noise. Line size to be selected based on recommended velocity and allowable pressure drop. Lines where pressure drop is not critical for compression power, should be sized based on maximum velocity.



For a specific duty, line sizing criteria for Gas phase is tabulated below: SERVICE



ALLOWABLE PRESSURE DROP MAX. (bar/100m)



RECOMMENDED MAXIMUM VELOCITY ms-1



RECOMMENDED MAXIMUM v² (kg.m-1.s-2)



15,000



Gas ; General (Note- 1,2) P  Atm



0.06



38 or 122/1/2



Atm < P  7 bar g



0.12



Whichever is



7 < P  70 bar g



0.45



lower



0.6% of upstream



= density in



pressure



kg/m3



0.011-0.022



40-60



atm < P  3.5 bar g



0.033



18-30



P > atm



0.15



12-15



P >70 bar g



Column gas outlet P  Atm



Compressor suction (Note-2) 0.07 Compressor discharge (Note-



0.12 to 0.22



2) Refrigerant Suction lines



4.5-10.7



Refrigerant discharge lines



10.7-18



Intermittent operation (antisurge, startup etc)



25,000



Notes : 1) The above indicated line sizing criteria are valid for continuous operation. In general, the pressure drop should be less than 10 % of the static terminal absolute pressure for long pipe segments and 5% for short segments. For intermittent operation, these limits may be exceeded, on a case by case analysis. 2) In addition to these criteria, flowing limitations on a noise point of view have to be considered (maximum V3 = 200,000 kg.s-3). V3 > 100,000 kg.s-3 may need acoustic induced vibration analysis. This requirement should be checked.



Compressible Vapor Flow When piping pressure drop is between 10% and 40% of the upstream pressure, the application of compressible or incompressible flow needs to be reviewed case by case. This situation occurs in relief and blow down lines and long pipelines. For proper sizing criteria of compressible flow, refer to the section on “Line Sizing Criteria for Flare and Relieving Devices”. Adiabatic condition is usually considered for short, well-insulated lines; while flow in long pipelines approaches isothermal condition. In general, isothermal pressure drop is greater than or equal to adiabatic pressure drop. If pressure drop in a pipe segment exceeds 10% of the upstream pressure, pipe shall be divided into segments to keep pressure drop in each segment below 10% of the upstream pressure (of each segment). This is usually done using a suitable computer program to account for changes on the vapor physical properties due to pressure change in the pipeline. Design Margin There is inevitably some inaccuracy associated with the line sizing as the design information is not always exactly the same as actual conditions. Therefore, it is a normal practice to consider design margin on the theoretical pressure drop calculations. In instances when there are no project specific requirements, the following design margins should be used over the “Normal Flow Rate” to obtain the “Design Flow Rate”: For all process lines: 10% For intermittent lines used during startup: 5% For all utility main headers and distribution lines: 20% (or future expansion requirements as applicable to specific cases shall be considered). For recirculating flow e.g. heating or hot oil : 25% For two-phase pressure drop calculations, a 50% design margin on the pressure drop is usually considered to allow for inherent inaccuracies in the calculation methodology. For very low pressure systems, margins are individually assessed especially for tank vent pipework. Pipe Sizing Criteria for Two Phase Fluid Many two-phase pressure drop correlations are quoted in the literature. However, for piping within plant boundaries where two-phase lines are short, pressure drop based on average density is generally adequate. For longer lines a more complex method such as modified Panhandle equations for wet gases or Beggs and Brill equation. Flow Regime The interaction of the liquid and vapor phases is complex and depending on vapor/liquid ratio, pipe size, and layout, the fluid flows in differing patterns or flow regimes. Where emulsions may form, the actual pressure drop may be higher than anticipated as the viscosity of the continuous phase would increase considerably When phase change is expected, unstable flow regimes may result in hydraulic and vibration problems.



Flow regime inside a pipe depends on flow rate and physical properties of gas and liquid phases as well as the pipe characteristics such as diameter, length and vertical/ horizontal orientation. These parameters decide the flow regime type i.e., mist flow, stratified flow, slug flow, etc. The transition from one flow regime to another is relatively gradual and is depicted in “flow regime map”. The boundaries shown in flow maps separating the different regimes should not be interpreted as sharp changes in flow pattern. The flow maps are generalized by using the gas and liquid Froude numbers based on the feed pipe velocity and diameter. The advantage of this general representation is that the flow maps are then unaffected by variations in flow conditions, physical properties and feed pipe geometry. This means that the flow maps can be used for a wide range of flow conditions, physical properties and feed pipe diameters. The gas and liquid Froude numbers are defined as follows: Gas Froude number: FrG = vG



G (L − G) gD



Liquid Froude number: FrL = vL



L ( L − G) gD



The flow regime map in a horizontal two-phase flow is shown in Figure-1.



Figure-1



The flow regime map in a vertical two-phase flow is shown in Figure-2.



This



Line Sizing Criteria For a two-phase fluid, the mixture density and velocity can be calculated as follows: ρm =



W WL/ρL + WV /ρV



Vm =



W / 3600 (ρm) (πxD2 / 4)



where:



ρL



Liquid Phase Density, kg/m3



ρV



Vapour Phase Density, kg/m3



ρm



Apparent Mixture Density, kg/m3



WL



Liquid Flow Rate, kg/h



WG



Vapour Flow Rate, kg/h



W



Total Flow Rate, kg/h



Vm



Mixture Velocity, m/s



D



Internal Diameter of the line, m



The fluid erosion velocity (the velocity above which erosion may occur) can be determined by the following empirical equation as per API 14E:



V = e



where



C



m Ve = Erosion Velocity, ft/s ρm = Apparent Mixture Density, lb/ft3 C = Empirical Constant



For solid free fluids, C= 100 for continuous service and C=125 for intermittent service are conservative. For solid-free fluids where corrosion is not anticipated or when corrosion is controlled by inhibition or by employing corrosion resistant alloys, values of C= 150 -200 may be used for continuous service. Values upto C=250 have been used successfully for intermittent service. For SI unit conversion, the empirical constant to be multiplied by 1.22. Example: If Ve is m/s, ρm in kg/m3 and empirical constant should 122 in the place of 100. API 14E recommends following: •



The fluid velocity must be lower than the erosion velocity.



This



•



If possible the minimum velocity in two phase lines should be about 10 ft/s (3.05 m/s) to minimize the slugging of separation equipment. This is particularly important in long lines with elevation changes.



•



If solids production is anticipated fluid velocity should be significantly reduced from above calculated erosion velocity. Different values of Empirical constant C may be used for specific applications if such recommendations are appropriate.



Accepted Flow Regimes •



For two-phase flow lines:



− where mixture is essentially gas, gas criteria will be used, − where mixture is essentially liquid, liquid criteria will be used. • For horizontal lines, stratified, mist, annular or bubble flow is acceptable. Slug and plug flow regimes shall be avoided.



•



For vertical lines, mist, annular or bubble flow is acceptable. Slug flow regime shall be avoided.



Simple Pressure Drop Models As per API 14E, the pressure drop for two phase flow in steel piping may be estimated using a simplified Darcy equation. 0.000336f W2



ΔP =



ρm di5 Where:



ΔP



= Pressure Drop, psi/ 100 ft



W



= Mixture Flow Rate, lb/hr



f = Moody friction factor, dimensionless ρm



= Apparent Mixture Density at flowing press and temp, lb/ft3



di



= pipe inside diameter , inches



The use of above equation should be limited to a 10% pressure drop due to inaccuracies associated with changes in density. If the Moody friction factor is assumed to be an average of 0.015 the above equation becomes: ΔP =



5x10-6 W2 ρm di5



Korf hydraulic software specifies several correlation methods to calculate the 2 phase pressure drop. They are Duckler method, Lockhart-Martinelli method, Chenoweth- Martin method etc. Appropriate method should be selected as per pipe geometry and corresponding method as recommended in Korf manual. Piping Arrangement for Two-Phase Flow Piping arrangement for two-phase flow shall consider:



This



•



Adequate support to avoid vibration.



•



Distribution



from



symmetrical



distribution to different branches.



two-phase



manifolds/



headers



shall



consider



equal/



Branched lines shall be



connected in horizontal direction and shall slope towards the equipment, if possible. Line Sizing Criteria for Flare and Relieving Devices This section describes sizing criteria for relief valve inlet/outlet lines and depressurization lines as well as flare and cold vent headers. Lines Upstream of Relieving Devices Pressure Safety Valves ( PSV) : Sizing of PSV inlet lines shall be based on the maximum relieving capacity of the PSV for the selected orifice. Line sizing shall be in accordance with the API 520/521 guidelines. •



P between the protected equipment and the PSV shall be less than 3% of PSV set pressure.



•



Inlet Pipe Diameter shall be greater than or equal to the PSV inlet size



Depressurization Devices Lines upstream or downstream of depressurization devices/blowdown valves shall be sized based on the calculated depressurization rates for the system and shall be in accordance with the criteria provided below: •



Minimum Line Size 2"



•



Max V² shall not exceed 200,000 kg/m/s2 Pressure drop criteria do not apply to these lines, the pressure loss shall be such that it does not impose any restriction on depressurization objectives.



•



Velocity shall be limited to 0.8 Mach.



•



Due to high velocities/V2/V3, Acoustic Induced Vibration analysis may be required.



Lines Downstream of Relieving Devices, Flare Headers Lines downstream of PSV, depressurization and other relieving devices and flares headers and sub-headers shall be sized in accordance with the following criteria: • •



Minimum line size 2". Back pressure consistent with relieving capacity of pressure-relieving devices and with design pressure of the protected equipment.



• Velocity and V² Sonic velocity is calculated as below:



This



Sonic velocity:



C=



105 * k * P







where: C



Sonic velocity, m/s



k



Cp/Cv



P



Pressure, bara







Density, kg/m3



Mach no



velocity/C



Lines downstream relieving devices and sub-headers: The basic criteria for sizing the discharge piping and relief header is the back pressure which may be developed at any point in the system does not reduce the relieving capacity of any of the relieving devices required to protect the equipment from overpressure. The back pressure must be maintained within specific safety valve stability limits to avoid chatter. 0.8 Mach maximum and V² < 200,000 kg.m-1.s-2 considering the maximum capacity of the relieving devices. The discharge shall be higher than or equal to outlet of relief devices. Flaring line downstream of pressure control valve shall be designed for a maximum velocity of 0.5 Mach (continuous flow) and maximum value of V² < 200,000 kg.m-1.s-2 Headers: 0.7 Mach maximum and V² < 200,000 kg.m-1.s-2 considering the maximum flow rate; however, a velocity of 0.8 Mach could be accepted for a long straight line without elbows and connections (e.g. stack, line on bridge ). The actual back pressure at the PSV outlet shall be checked to make consistent with back pressure limitations. Acoustic induced vibration study may be required for lines where V² values are high. PAM group will address the impact and propose the mitigation measures. If relief is a two phase/multi-phase, the following criteria may be followed: Potential slug/plug flow velocity shall be limited to 50 m/s Homogeneous flow ( i.e there is no slip between phases) maximum mVm² < 200,000 kg.m1.s-2



Utility Line Sizing Steam Lines The following sizing criteria may be used for preliminary pipe sizing of steam lines: Max ΔP, bar/100 m Pressure, barg Up to 2.0



Main Headers



Branch Lines



0.06



0.11



Recommended Max.Velocity m/sec Saturated : 60



This



2–7 7 – 20



0.09 0.11



0.17 0.23



20  P



0.23



0.34



Superheated: 75



For exhaust steam lines, pressure drop upto 0.33 bar/100 m can be considered. Condensate Lines Condensate piping is sized more generously than normal liquid service to provide allowance for flash steam that is generated. The condensate lines should also be checked for the startup load and piping shall be sized to provide adequate pressure drop for traps and control valves in the system, taking into account for flashing. Steam traps are usually sized for 200 to 300 % of normal flow. If significant amount of flash is occurring the two phase flow sizing criteria shall be used. Note: Mixing flashing condensate with sub-cooled condensate can cause a “water hammer” effect resulting in excessive pipe movement. Cooling Water/Sea Water Lines Cooling water headers and distribution piping is sized to keep the pressure drop per 100 m of piping less than 0.44 bar. Maximum velocity should be limited to 3.0 m/s for 12” and below. For 14” and above, maximum velocity should be limited to 4.5 m/s. Velocity restrictions may apply for large cement lined pipes and should be checked with suppliers. In general, higher pressure drop can be allowed for Corrosion Resistant Alloy (CRA) lines to reduce cost. Fire Water Lines Sizing of fire water lines shall be based on available system pressure and allowable flow velocities. In the ring main pipe work, the same sizing criteria as presented for “Cooling Water/Sea Water Lines” may be used. Downstream of the deluge, the flow velocities shall normally not exceed 10 m/s. Some areas may require velocities higher than 10 m/s in order to hydraulically balance the systems which is acceptable provided the reaction force within the system does not cause excessive stress in the pipe work or the supports. Oily Water Lines The lines for oily water to water treatment facilities shall be sized based on pressure drop available. Typically the velocity should not exceed 3 m/s.



This



Chemical Lines Chemical lines are usually designed for a maximum velocity of 1.5 m/s. Recommended maximum velocity in lines carrying chemicals are as below:



Instrument Air, Plant Air and Nitrogen Gas line sizing criteria shall be used for IA, PA and Nitrogen lines. Special Cases Reboiler Lines Reboiler Feed Line: Allowable pressure drop



0.03 to 0.07 bar/100 m



Allowable velocity



0.9 to 1.5 m/s



Reboiler Return Line: Allowable pressure drop



0.07 bar/100 m



Miscellaneous Fluids The following velocity criteria may be used in special service conditions: •



Liquid with sand: 0.8 m/s as min. velocity; 3 m/s max.



•



Solid-liquid mixture lines: 1 to 2.75 m/s max.



•



Pulverized catalyst standpipes (dense-phase flow): 1.7 m/s max.



•



Pulverized catalyst carrier lines (dilute-phase flow): 12 m/s max. Velocity of 7.5 m/s is preferred. However, for densities under 0.8 kg/m3 higher velocities may be used.



•



Cement pipe or coal tar Enamel liquid pipe carrying salt water : 4.5 m/s max.



•



Plastic pipe or rubber lined pipe carrying liquid in general : 3.0 m/s max.



Service



Recommended max velocity (m/s)



CS pipe carrying phenolic water



0.9



CS pipe carrying concentrated H2SO4



1.2



CS pipe carrying caustic solution



1.2



1.



SIZING VALUES



1.1



Pressure drop : AP It directly affects pump and compressor discharge pressure. So, its optimization is essential.



1.2



Fluid velocity : V



It must be limited, either for technological reasons concerning some pieces of equipment, or for speed of sound reasons (high velocity flows), or for static electricity reasons. 1.3



Kinetic energy: pV2/2 It must be limited, since it is an indicator of stresses to be withstood by piping. Noise and vibrations are also related to the fluid kinetic energy. In practice, reference is made to the pV2 •



2.



PROCESS LINES



2.1



Pump suction and discharge



Suction and discharge lines can be quickly and simultaneously sized using graph II-1. This graph shall only be used for preliminary sizing. Checking at contract stage shall be made on the basis of criteria developed hereafter. Beside velocity and pressure drop, NPSH is an additional sizing criterion for the pump suction line. In the case of centrifugal pumps, required NPSH increases when differential pressure or flowrate increases. In the case of reciprocating pumps (more specifically when there is a single piston), pulses make the NPSH problem more critical (since it also depends on the length and diameter of the suction line). In this latter case, it is necessary not to be too restrictive, and, at contract stage, to transmit isometrics of all suction lines of reciprocating pumps to the Mechanical Department. Often accepted criteria at pump suction are the following a)



Liquids at bubble point or with dissolved gases



Pressure drop Normal Maximum



0.6 bar/km 0.9 bar/km



Maximum velocity Line size up to 2" Line size from 3" to 1O" Line size from 12" to 18" Line size higher than 20"



0.6 mis 0.9 mis 1.2 mis 1.5 mis



-·-·...-·



GRAPH II.1- Pump suction and discharge - Quick sizing



.



.,,



...



*'



aoot



!



nu



aJ



,...,;.,



;:, 0 ti}



.



I'\.



...........



.



'



.. ..



"



ft



SJUI hJ!�OJ81\ .



,�



..



b) Liquids far from bubble point Pressure drop Normal Maximum



2.3 bar/km 3.5 bar/km



Maximum velocity Line size up to 2" Line size from 3" to 6" Line size from 8" to 18" Line size higher than 20"



0.9 mis 1.2 mis 1.5 mis 1.8 mis



It should be noted that if the liquid is very hot (near 300 °C or higher), it is recommended to remain conservative to provide an additional safety margin to take into account pressure drops due to expansion loops. Optimization of discharge lines is only the research of a compromise between investment and operating costs. Operating costs increase very fast when differential pressure rises. The following criteria are as a use considered at pump discharge Pressure drop



2.2



Normal Maximum



3.5 bar/km 7.0 bar/km



Maximum velocity



3.0 mis (4.5 mis in case of high flowrate)



Suction and discharge of compressors Regarding suction and discharge of compressors, kinetic energy is an important sizing criterion. Velocity must be limited to remain within acceptable vibration ranges (see Graph II-2). In case of low suction pressure, the acceptable pressure drop may be restrictive, and thus fix the suction line diameter. The use of pulsation dampers should allow flow pulses of a reciprocating compressor to be lowered to an acceptable level (sizing by the equipment Vendor). The engineer must not overlook sizing the bypass of a reciprocating compressor to allow starting at a very low compression rate.



1



2



3 � 100



'' �" ' '\



i"\..



r'\ I'



GRAPH Il-2



��



\�



� II,._



��



--�



� -!L! :::.



.. �



"' '.,,



..:::-



-



'",



'--



.Q



-:"'



-



�



.



RECOMMENDEO GAS VELOCITY FOR LINES ATTACHED TO COMPRESSORS



�



' \�



�



r'\



",�



, ...



��



�Ill



�



'' ' '"



r\.



"\



'"



" ',, '...



((l



� �



(!)



"



��



"" '"' " ' '" '



��



'\�



I'\'-



i"\



�



"IO



i\



� \



'., ...



""'



2



-3



Axial compressors: acceptable velocity in area delimited by lines 1 and 2. Reciprocating and Centrifugal compressors: acceptable velocity in area delimited by lines 2 and 3.



I



I



I



!Gas Density (kg/m') I



0.01



0.1



1



I



10



100



1000



2.3



Column draw-off



Draw-off flows either towards a pump suction or towards another vessel by gravity. In the first case, liquid is pumped at its bubble point (see paragraph 2.1). In the second case, for gravity flow sizing criterion, see paragraph 2.12. Currently accepted criteria for draw-off towards pump suction are the following: Pressure drop 0.6 bar/km 0.9 bar/km



Normal Maximum Velocity



A vertical downpipe shall be installed as close as possible to the tower, immediately after the nozzle. It shall be at least 3 m long, with the same diameter as the nozzle. Velocity in this downpipe shall be limited to 0.6 mis for piping up to 2" and to 0.9 m/s for those 3" and larger. The diameter may then be reduced, velocity criteria being then those accepted for liquids far from their bubble point, provided that pressure drop criteria defined above are complied with. 2.4



Column head



Graph II.3 gives the allowable pressure drop as a function of the column service pressure. It is also necessary to avoid vibrations for columns operating at high pressure with high density overhead vapors. So, it is necessary to check that the pV2 is not too high (:::::15 000 Pa). The velocity in vacuum distillation overhead line will be limited to 90 mis, and the line pressure drop limited to 5 mmHg maximum. GRAPH Il-3 - Allowable differential pressure 10



' '



........"....,,,...,



·····T···rTTT .. 111



..)



Allowablei\.P bar/km



/



vv



"



/



/ ..



I/



�--···--·



�



//v



7



/



r•bi p



0,1 0,1



�



---�--



10



� 111



100



2.5



Condenser outlet



If the effluent is totally liquid, gravity flow criteria are used (see paragraph 2.12). If the effluent is a gas/ liquid mixture, experience demonstrates that using the same criterion as for condenser inlet, with average density method, is satisfactory. 2.6



Reactor inlet and outlet



If the reactor is operating m liquid phase, it is then part of the pump discharge system (see paragraph 2.1). Very often, reactors operate in vapor phase (HDT, reforming for instance), or in mixed phase (HDS, hydrocracking, etc.). In this case reactor inlet/ outlet lines shall be sized on the basis of the pressure drop allocated to the reaction loop or, on the basis of a pV2 criterion. It may be useful to verify that flow modes ofliquid / gas phases are continuous. 2.7



Feed and return lines attached to forced flow reboilers



Feeding of a forced flow reboiler is simultaneously a pump discharge (see paragraph 2.1) and an exchanger inlet (paragraph 2.9). So, the design shall be based on the most stringent constraints. For return line, whether mixed phase or vapour phase, it is the pV2 that fixes the piping diameter (see paragraph 2.9) with, in the case of a mixed phase, the additional constraint to have a continuous flow regime (annular or dispersed in most cases). 2.8



Feed and return lines attached to natural circulation reboiler



The whole system must be sized so that the difference of hydrostatic pressure between inlet and outlet should balance the pressure drops. The pressure drop in line is only an element of a more global calculation that is described in details in the exchanger design guide to which one should refer for further details (Part 4 - Section 2 - Sub­ Section 2.2). The Heat Exchangers Department is responsible for the sizing of lines and determination of the relative position of the reboiler and the column. A final checking is made with lines isometrics.



For mixed phase return line, empirical formulae enable to estimate the optimum diameter and pressure drop:



Optimum diameter



0.737Qy 0.42 7



(



)0.167



� PL-X 2



AP



2 x 10- PmVm LID (to be compared to the average density method, Chapter III paragraph 1.3 .1)



Optimum diameter Qv Pv



In inches Vapor volume flowrate in m3/h Vapor density in kg/m3 Liquid density in kg/m3 Weight vaporized fraction Pressure drop in bar Average density of the mixed phase in kg/m3 Average velocity of the mixed phase in mis Piping length in m Pipe diameter in m



PL



X



AP Pm



Vm L D



Large capacity units shall be the subject of the utmost care. It is necessary to provide at project initial stage either a substantial column elevation (about 6 to 7 m) or to adopt a column bottom arrangement providing a substantial hydrostatic head (preferential circulation, once-through, feed from chimney tray). The purpose is to have a sufficient allowable pressure drop in the system. Indeed, the suitable type of flow (annular or dispersed) is generally attached to a substantial specific pressure drop: 0.5 to 0.8 bar/km according to the average density method. Actual allowable pressure drop in the return line of a natural circulation reboiler is approximately 0.20 to 0.40 bar/km. For a kettle type reboiler, velocity in feed line shall be limited to 0.6 mis and the pressure drop in vapour outlet line shall be between 0.2 and 0.4 bar/km. 2.9



Inlet and outlet nozzles of shell and tubes exchanger Because of erosion and vibrations, pV2 in inlet and outlet nozzles of shell and tubes exchangers, tubeside as well as shellside, shall be limited to 6000 (4000 in British units). This limit is recommended by TEMA (see paragraph 4.6.2. in TEMA, 1999 issue), at shell or bundle entrance. There is no requirement for tu beside in the TEMA. However, for the purposes of proper distribution of fluid in tubes, the same limit shall be considered for tubeside nozzles. In case of revamping, some latitude then exists regarding this limit (to be agreed with the Client). The diameter of inlet and outlet lines can be lower than nozzles diameter, but the reverse situation should not occur, as a rule.



2.10



Feed and outlet lines of a reboiling furnace



As the furnace is fed by a pump, sizing criteria attached to pump discharge will be applied for the feed line (see paragraph 2.1). The stream at outlet is in mixed phase. It is kinetic energy, i.e. pV2, which will fix the maximum velocity, and consequently the piping diameter. 2.11



Feed and outlet lines of a heating furnace



This case differs from the previous one in that the outlet stream can be also fully liquid or fully vapour. If the outlet stream is vapour, it is always the maximum pV2 that will govern. If the outlet stream is liquid, it is the pump discharge conditions that remain applicable. In the case of a mixed phase stream at outlet, there are very frequent examples met in refining industry : atmospheric or vacuum transfer lines. Experience gives pressure drop (AP) values for transfer lines. As example, for an atmospheric transfer line, the recommended values are : Small units Carton steel Alloy steel



3 bar/km 5 to 7 bar/km



Large units Alloy steel



2 to 3 bar/km



On the other hand, in large capacity units, the transfer line reaches a substantial length (an approximately 50 m long straight section). The pressure drop in this line must be limited in order to avoid cracking in the furnace in case of severe operating conditions with cracking sensitive crude oils. In this case, the total pressure drop in the transfer line is limited to 0.5 bar for atmospheric distillation and to 0.35 bar for vacuum distillation. It is not easy to correctly size the vacuum transfer line since the vaporization rate is very sensitive to pressure drop. Calculation must be iterative per sections. Allowable velocity may reach 80% of sound velocity. It is necessary to check that the diameter chosen is consistent with the type of flow, either annular or dispersed.



2.12



Gravity flow lines



The pressure drop is usually limited to 0.45 bar/km. Normal velocity : 0.6 mis. 2.13



Lines connecting two vessels at different pressure values



The maximum allowable pressure drop is directly given by the difference between the two pressure values ; maximum velocity will be deduced from the limitation of the pV2 due to vibration risks (< 15 000 Pa for a vapor or a mixed phase). For gases and steam, it may be allowed to exceed this limit when conditions allow it (steam let-down stations for instance) or require it (revamping for instance). However, 25 000 Pa is the value not to be exceeded. 2.14



Release to atmosphere



In case of process release to atmosphere (regeneration effluents from reactors, for instance), the same criteria as those indicated for flare lines shall be used (see paragraph 5). 3.



UTILITY LINES



Sizing of utility lines is within the duties of the Process Department. These diameters shall be estimated using fast estimation graphs. 3.1



Steam lines



The following recommendations shall be complied with



Steam Pressure



1 bar 10to30 bar 40 bar



Long line (iJ Short line Long line Short line



Allowable Pressure Drop (bar/km) Normal



Maximum



0.12 0.46 0.23 1.16 0.35 1.16



0.24 0.92 0.92 2.30 0.92 2.30



(1) These values may be adopted as a first basis



Maximum velocity (m/s) Line diameter



Saturated steam



Superheated steam



:,;;;2"



10



15



3" to 8"



30



40



40



60



� 10



II



The pV2 , shall be limited to 15 000 Pa as general rule. When there is an important available pressure drop (in a pressure let-down station for instance), higher pressure drops may be allowed and the pV2 limit can be extended to 25 000 Pa. Pressure drop in long lines feeding turbines shall be subjected to special care in order to deliver (and extract) steam under conditions specified for these machines.



3.2



Water and fuel oil lines The following recommendations shall be followed



SERVICE



ALLOWABLE PRESSURE DROP (bar/km)



ALLOWABLE VELOCITY (m/s)



Allowable pressure drop to be modulated according to each specific case. If the pumping station is close to units, a much higher value may be allowed. Generally, the velocity is the sizing criterion for large diameters.



COOLING WATER



•



•



1.5



Main headers between pumping station and units



Lines within units



"'1.5 2.5 to 3.5



(to be modulated) for long lines for short lines



1.5 to 3



1.5 2 2.5 3



2.5 to 3.5



SEA WATER SYSTEMS



REMARKS



for 2" for 3" for4" for 6" and higher



Small diameter lines used for cooling of rotating equipment must be conservatively sized at project initial stage since Vendor's data, provided much later, often cause unpleasant surprises. Not less than 2 m/s to prevent fixation of mussel, algae and other organisms.



BOILER FEED WATER



• •



Pressure lower than 50 bar Pressure higher than 50 bar



3 to 6 3 to 6



3.5 to 4.5



FUEL OIL



3.3



3.5 to 4.5 7 to 9



Air, nitrogen and fuel gas lines Use graph II-4.



3.4



Steam condensate lines Upstream steam trap, the line shall have the same diameter as the exchanger outlet nozzle. Downstream steam trap, lines shall be sized on the basis of a pressure drop of 0.2 to 0.3 bar/km.



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4.



LINES IN OFFSITES In offsites, the length of lines is the major element differing from the lines in units. The piping cost ratio shall be higher than in units. This is why an economic balance will favour smaller lines and costlier pumps.



4.1



Pump suction lines In the case of a storage tank feeding the pump with a significant head of liquid, sizing criteria for a liquid far from its bubble point will be selected. It should be noted that some clients impose velocity limits in their general specifications. In case of storage tank feeding the pump with low head of liquid, the main criterion to be considered is the pump NPSH. A particularly critical line is the suction line of the pump feeding the crude oil atmospheric distillation unit. Sizing the diameter of this line requires the following elements to be taken into consideration: ■ True vapor pressure (RVP +---➔ TVP correlation) calculated for maximum pumping temperature ■ Crude oil heating in line (for long lines in hot climate countries) ■ Pump required NPSH (high for high duty pumps) ■ Paraffin deposit in the line: this requires a safety margin to be considered



Note : In some cases, the suction line may be under vacuum. Some regulations prohibit vacuum conditions when the flowrate is measured by a meter (risk of air intake).



4.2



Pump discharge lines Selection of the diameter results from the following criteria: ■ Sizing recommendations set out in paragraph 2.1 ■ Economic balance ■ Surge pressure (water hammer) ■ Pump standardization



4.3



Connections between units and offsites



In most design studies, pressure available at battery limits is fixed a priori. Knowledge of the line routing enables to determine the actual pressure drop. Conventional pressures at battery limits are as follows: • Gasoline ■ Kerosine, jet fuel ■ Gas oil ■ Distillates ■ Residues ■ Bitumen / Furfural extracts ■ Waxes from dewaxing unit ■ Lube oil bases



4 4 4 5-7 5-7 10 7 7



bar g bar g bar g bar g bar g bar g bar g bar g



These values usually prove conservative with diameters calculated within units battery limits. Velocity limit is identical with that of pump discharge lines. REMARKS Limit velocity for kerosene I jet fuel In the case of lines outside units battery limits, it is required not to exceed a 3 mis velocity (with an average of 2 mis). At tank inlet (piping sections in tank dyked area) or at loading station inlet (over a 50 to 100 m long piping section), velocity shall be limited to 1 m/s. 5.



FLARE LINE Discharge lines of safety relief valves, flare headers and sub-headers shall be sized according to three simultaneous criteria : ■ ■



■



Pressure drop Fluid velocity Kinetic energy of the fluid



Pressure drop shall be limited by maximum allowable back pressure allowed at relief valve discharge. Velocity shall be limited to 0.3 to 0.5 times the critical velocity of the fluid, which can be calculated using the following formula, based on ideal gas hypothesis Ve y R Ta PM



In m / s Isentropic expansion coefficient (it is equal to specific heat ratio, Cp / Cv, in case of ideal gas) Ideal gas constant = 8313 J/0K/kmoles Fluid temperature upstream pressure relief valve in °K Fluid molecular weight in kg/kmoles



Pipe fittings generate turbulences the impact of which is the reduction of the effective flow area and the increase of velocity, with the risk of flowrate limitation if sound velocity is reached. Flare system sizing is the subject of a specific design guide and of a dedicated computer program,



6.



LINES TO BE CHECKED ON ISOMETRICS Proper operation of some equipment may only be ensured if pressure drops in some lines are properly checked. During detail engineering, it is necessary to perform a check, as detailed as possible, of these critical pressure drops, on isometrics. This mainly concerns the following lines • Suction lines of centrifugal pumps. • Suction lines of volumetric pumps : in this case, the NPSH problem is complex and much depends on the type of pump (reciprocating ... ). • Suction lines of low pressure compressors. • Recycling lines around a reactor (hydrotreatment, reforming) : the total pressure drop in lines and in equipment determines the differential pressure of the compressor. Experience has demonstrated that such calculation is required. The compressor is specified and ordered at contract initial stage. If pressure drop in lines proves higher than expected, it is easier to modify the diameter of lines and layout if necessary, than to modify the compressor. • Relief valves upstream lines when they are not located directly on the equipment item to be protected. • Lines attached to natural circulation reboilers. • Fractionation column overhead lines as well as draw-off lines. • Flare system lines.