Pipeline Engineering: Transient Flow Mike Yoon, Ph.D.
What’s Next? • Transi nsient An Analysi lysiss – Physic Physical al oper operat atio ion n – Why transie transient nt analys analysis? is? – Transient Transient model model vs. vs. Steady Steady-stat -statee model model
• Fun Fundam damenta entall Pr Prin incciple ipless • Pipe Pipelline ine Tra rans nsie ien nts • Transi nsient Contr ontrol ol • Applications
Physical Operation • Both Both desig design n and and operat operation ion should should addr address ess both both safet safety y and economics or minimum cost operation. • Pipeli Pipeline ne system system design design is mainl mainly y conce concerne rned d with with line line sizing, equipment sizing and location, while system operation is concerned with pipeline system or facility start-up and shut-down, product receipt and delivery, flow rate change, emergency shut-down, equipment failure, etc.
Definitions • A pipeli pipeline ne state state is defin defined ed as as a cond conditi ition on of a pipeli pipeline ne expressed in terms of pressure, temperature, flow rate and fluid density at a given location and time. • A steady steady state state is a condi conditio tion n of a pipeli pipeline ne sys system tem that does not change much over time, while a transient trans ient state is an unsteady condition that changes with time between two steady states. • A surge surge or wate waterr hamm hammer er is a transi transient ent that that occurs occurs abruptly during changes from a normal steady st eady state flow in the pipe. • An ups upsurge urge occurs occurs if the the pipel pipeline ine pressu pressure re incr increas easee above the normal operating pressure of the pipeline. • A down down-su -surge rge is a pressur pressuree decr decreas easing ing condit condition. ion.
Validity of Steady State Analysis • Under Under a stea steady dy state, state, pressu pressure re and flow remain remain constant from one instant to another (i.e. independent of time). • A pipe pipeline line system system design design can be base based d on a stead steadyystate assumption. In general, the assumption is valid when the system is not subject sub ject to sudden changes in flow rates or other operating conditions over a short period of time. • Steady Steady-sta -state te cond conditi itions ons are invalid invalid for for shor short-t t-term erm operation analysis and even for designing control systems, testing the capability of the t he system, testing the level of safety, locating l ocating facilities, etc, because these behaviors are time-dependent.
Why Transient Analysis? • The stea steady dy-sta -state te assum assumptio ption n is not not vali valid d for short-t short-term erm operational study, because pipeline states in all operations change with time. • When When an operat operation ion change change takes takes plac place, e, the flow rate rate and pressure change immediately, and subsequently the change will have an impact on the pipeline system. • With With a steady steady-sta -state te assum assumptio ption, n, the the followi following ng probl problem emss cannot be addressed: – – – –
Over or under under pressuring pressuring along along the the pipeline pipeline,, Equipment Equipmentss such as as pump/co pump/compre mpressor ssor trippin tripping, g, Line Line pack/p pack/peak eak shavin shaving, g, Potent Potential ial colum column n sepa separat ration ion
Why Transient Analysis for Liquid? • Many pipelin pipelinee failure failures, s, partic particular ularly ly for for liquid liquid pipel pipeline ines, s, occur because improper provisions are made to manage transient related problems such as pump trip, line rupture, etc. • In order order to mana manage ge them them adequate adequately, ly, the foll followi owing ng operating conditions should be properly taken into account in design and operational analysis: – Chang Changes es to to pump pump opera operation tions, s, – Powe Powerr fai failu lure res, s, – Valv Valvee oper operat atio ion, n, – Line fill ills
Why Transient Analysis for Gas? • A transi transient ent analys analysis is is benefi beneficia ciall for design designing ing large large gas pipeline systems, because: – Steady state design design for for gas pipelin pipelinee systems systems tends tends to yield yield larger pipe diameter, higher compressor power requirement, and incorrect compressor station location. – To achieve achieve the the maximum maximum capacity capacity,, the steady state design design of gas pipeline systems may be based on peak load, while the transient state design may be based on average load. The maximum capacity can be achieved by managing line pack in gas pipelines. – The transien transientt analysis analysis provides provides realistic realistic inform informatio ation n for control system design, peak shaving study, survival time analysis, etc.
Transient Model • A trans transien ientt mode modell calcu calculate latess time time depen dependen dentt flow, flow, pressure, temperature and density behaviors by solving the time dependent flow equations discussed in the next section. • Theref Therefore, ore, a transi transient ent mode modell genera generates tes hyd hydrau raulica lically lly more realistic results than a steady state model, and theoretically the model is capable of performing not only all time independent functions performed by the steady state model but also time ti me dependent functions such as effect of changes in injection or delivery, system response to changes in operation, and line pack p ack movement.
Transient Model: Capabilities • Study Study normal normal pipeli pipeline ne opera operatio tions ns – Pipeli Pipeline ne opera operatio tion n chan change gess are simulated to find a cost effective way of operating the pipeline system. The transient model allows the operation staff to determine efficient control strategy for operating the pipeline system and analyzing operational stability. • Analy Analyze ze sta start rtup up or or shut shutdo down wn pro proce cedu dure ress – Diff Differ eren entt combinations of startup or shutdown procedures are simulated s imulated to determine how they accomplish operation objectives. The transient model can model a station, st ation, including the pump or compressor unit and associated equipment. • Deter Determin minee deli deliver very y rate rate sched schedule uless – A tra transi nsient ent model model can be used to determine delivery rate schedules that maintain critical system requirements for normal operations or even upset conditions.
Transient Model: Capabilities • Study Study system system respon response se after after upse upsets ts – A pipe pipelin linee syste system m can can be upset by equipment failure, pipe rupture, or supply s upply stoppage. The transient model is used to evaluate corrective strategies by modeling various upset responses. • Study Study blow-d blow-dow own n or or pipe pipe ruptu rupture re – The transi transient ent model model allows allows the operation engineers to study the effects of blow-down on a compressor station and piping or to develop a corrective action when a leak or rupture occurs. • Predic Predictiv tivee mode modelin ling g – Starti Starting ng with with curr current ent or initia initiall pipe pipelin linee states, future pipeline states can be determined by changing one or more boundary conditions. • Note Note tha thatt a tra trans nsie ient nt mod model el is mor moree com compl plex ex to use use and and execution time is longer than that of a steady state model. It requires extensive data, particularly equipment and control data.
Key Topics • Transi nsient An Analysi lysiss • Fun Fundam damenta entall Pr Prin incciple ipless – Conser Conserva vatio tion n laws laws and equa equatio tions ns – Solu Soluti tion on app appro roac ache hess
• Pipe Pipelline ine Tra rans nsie ien nts • Transi nsient Contr ontrol ol • Applications
Governing Laws and Equations • The pipeli pipeline ne state state can can be full fully y defi defined ned by four independent variables: pressure, temperature, flow and density. Four equations are needed to determine the values of these four variables: continuity equation, momentum equation, energy equation, and equation of state. • The first first three three equat equation ionss come come from from conse conserva rvatio tion n laws: laws: – Cont Contin inui uity ty equa equatio tion n – Momen Momentum tum equat equation ion with with fric frictio tion n factor factor – Energy Energy equation equation with heat heat capacity capacity and Joule-Tho Joule-Thomson mson coef. coef.
• Equation of of st state
Conservation Laws • Conser Conservat vation ion of mass: Mass Mass cannot cannot be create created d or destroyed. The net net change rate of the fluid flow in a segment of pipe is equal to the net packing rate of the fluid in the segment of pipe. • Conser Conservat vation ion of momentum omentum:: The rate rate of change change of momentum equals the sum of forces. Newton’s second law of motion is applied to the fluid element in pipelines. • Conser Conservat vation ion of energy energy:: The net rate rate of energy energy transport across a pipeline section is the same as the rate of energy accumulation within the pipeline section. The energy includes the internal internal energy, energy, compression or expansion energy and kinetic energy.
Continuity Equation • The conser conservat vation ion of mass mass equat equation ion is is often often referr referred ed to as the continuity equation in fluid dynamics. A general form of the one-dimensional one-dimensional continuity equation is expressed as:
t
v
x
0
where ρ = density t = elapsed time v = velocity
Line Pack Change • The first first term term in the cont continu inuity ity equa equatio tion n repre represen sents ts the the change of mass in a pipe segment. It is often called line pack change. The line pack can be increased or decreased due to pressure and temperature changes. The line pack change is useful for gas pipeline operation. • It shoul should d be distin distingui guishe shed d from from the line line fill fill volum volume, e, which is the quantity of liquid contained in a pipeline. It is useful for batch pipeline operation. • The second second term represe represents nts the differe difference nce between between mass flow into and out of o f the pipe segment.
Momentum Equation
V t
V
V x
P x
g
h x
where: is density D is pipe diam diam et er P is pressure f is the f riction ri ction f actor h is elevation g is gravitational acceleration x is displacement t is elapsed time V is velocity
f V | V |
2 D
0
Forces • The first first term term is is a forc forcee due due to to accel accelera eratio tion, n, and and the the second term a force due to t o kinetic energy. These two terms are inertial force. • The third third term term is a forc forcee due due to to pressu pressure re diffe differen rence ce between two points in a pipe segment • The The fourt fourth h term term is a grav gravita itati tion onal al for force ce.. • The The last last term term is a fric fricti tion onal al for force ce,, or Dar Darcy cy-Weisbach Weisbach equation, equation, opposing opposing to the the flow on the pipe pipe wall.
Reynolds Number • Reynold Reynoldss numb number er rela relates tes density density,, viscos viscosity ity,, fluid fluid velocity and pipe diameter. diameter. • Reyn Reynold oldss num number ber is dimens dimensio ionl nles ess. s. • Re = ρ*v*D *v*D/µ /µ = v*D/ v*D/ ν, where D = inside diameter, v = fluid velocity, ρ = fluid fluid densi density, ty, µ = absol absolute ute viscosity, and ν = µ/ ρ = kinematic viscosity • Re < 2000 2000 for Lam Laminar inar flow flow regim regimee • 2000 2000 < Re < 400 4000 0 for for cri critic tical al flow flow reg regim imee • Re>4 Re>400 000 0 for for turbu turbule lent nt flow flow regim regimee
Friction Factor • • • • • •
Fricti Fric tio on fac facttor is a fu functi nctio on of of Rey Reyno nold ldss num numbe berr an and pipe roughness Expre pressed sed in Colebr lebro ook-W ok-Wh hite ite equa quatio tion Lamin aminar ar flow low is ind independ pendeent of pipe ipe rou roughnes hnesss Part Pa rtia iall lly y turb turbul ulen entt flo flow w is is dep depeenden ndentt on on Re Reynold noldss number and pipe roughness Full Fully y turb turbu ulen lent flow flow is depen epende dent nt only only on pip pipe roughness The Moody oody dia diagram gram re rela lattes the fr fric icti tio on fac facto torr in terms of Reynolds number and relative roughness.
Moody Friction Diagram
Energy Equation 4k (T T g ) Vg h fV 2 | V | V 0 D C v C v x 2DC v t x C v A x
T
T
T
(VA)
• Temp Temper erat atur uree cha chang ngee ove overr time time • Rate Rate of temp tempera eratur turee chang changee due due to the the net net conv convec ectio tion n of fluid energy into the fluid element • Chang Changee rate rate due due to expan expansio sion/c n/comp ompre ressio ssion n of the the flui fluid d including Joule-Thomson effect • Heat Heat flow flow to or from from the the surr surrou ound ndin ings gs • Effect Effect of work work aga agains instt or by gravity gravity,, which which will will heat heat the the fluid going downhill and cool it going uphill. • Heatin Heating g due due to frict friction ion,, assumi assuming ng that that all all the the frict frictio ional nal hea heatt is deposited in the fluid
Energy Components • Rate Rate of tem tempera perature ture change change due due to to the net conv convec ectio tion n of fluid energy into the fluid element • Rate Rate of tempe tempera ratur turee cha chang ngee due due to compression/expansion of the fluid, including JouleThomson effect • Frictio Friction n heatin heating, g, assu assumi ming ng that that all all the the fricti frictiona onall heat heat is stored in the fluid • Effe Effect ct of hea heatt flow flow to or from from the grou ground nd • Effect Effect of work work agains againstt or by gravity gravity,, which which will will heat heat the fluid going downhill and cool it going uphill
Ground Heat Transfer • Use Fourie Fourier’s r’s law of heat heat conduc conductio tion, n, describ describing ing the flow of heat from pipeline to ground • Ground Ground heat heat transf transfer er tak taking ing into into accou account nt the the heat heat transfer through pipe, insulation, and soil • Grou Ground nd tem temperat peratur uree along along the the pipe pipeli line ne is not not normally measured, but an important parameter for designing a pipeline system. s ystem.
Fluid Properties • Fluid Fluid proper propertie tiess include include thermo thermodyn dynam amic ic and rheological properties. • Therm Thermody odynam namic ic propert properties ies includ includee densit density, y, compressibility, heat capacity, enthalpy, and entropy. • These These quan quantit tities ies can can be be deriv derived ed from from an equat equation ion of of state. However, appropriate correlations are used in practice, particularly for liquid properties. • Viscos Viscosity ity is a meas measure ure of fluid fluid’s ’s res resista istance nce to shea shearr force, expressed in absolute or kinematic ki nematic viscosity.
Equation of State • An equa equatio tion n of state state descri describes bes the the rela relatio tionsh nship ip betw betwee een n pressure, temperature, and density or specific volume. • Theore Theoretic tically ally,, all all therm thermodyn odynam amic ic funct function ionss can can be derived from the equation of state. In practice, it is very difficult to obtain them, particularly for liquid properties. • There There is no equat equation ion of of state state unive universa rsally lly applic applicabl ablee to all products. products. Instead, there are are many many correlations correlations applicable to certain types of fluid.
Gas Equations • Ideal gas law • Real Real gas gas law law with with the the com compress pressib ibili ility ty facto factorr (AGA (AGA-8) -8) • BWRS BWRS equati equation on of state state for for ligh lightt hyd hydroc rocarbo arbons ns • Peng Peng-R -Rob obins inson on equ equat ation ion of stat statee for for ligh lightt hydrocarbons • SRK SRK equa equatio tion n of stat statee for ligh lightt hydro hydrocar carbo bons ns
Liquid Equations • A liqui liquid d equat equation ion of state state is expre expresse ssed d in terms terms of bulk modulus and thermal expansion coefficient. They They are are slig slightl htly y depend dependen entt on pressu pressure re and and temperature. • The density density of liqu liquids ids can be expre expresse ssed d by a Bulk Bulk Equation of State. • The The API API Equa Equatio tion n of of Sta State te for for petr petrol oleu eum m liqui liquids ds is used for custody transfer. This equation takes into account the dependence of bulk modulus and thermal expansion coefficient on pressure and temperature.
What’s Next? • Transi nsient An Analysi lysiss • Fun Fundam damenta entall Pr Prin incciple ipless – Conser Conserva vatio tion n laws laws and equa equatio tions ns – Solu Soluti tion on app appro roac ache hess
• Pipe Pipelline ine Tra rans nsie ien nts • Transi nsient Contr ontrol ol • Applications
Solution Methods • The contin continuit uity, y, momen momentum tum,, and and energy energy equati equation onss are non-linear, so they can be solved only in a numerical approach using a computer. • The set of thes thesee equat equation ionss are discre discretiz tized, ed, dividi dividing ng pipelines into finite intervals. • The The discr discret etiz ized ed equa equati tion onss are are sol solve ved d num numerica erically lly for pressure, temperature, flow rate, and density over a time step. • Commo Common n soluti solution on appro approach aches es are: are: Expl Explicit icit soluti solutions, ons, Implicit solutions, Hybrid solutions, and Method of characteristics
Assumptions • Three Three cons conserv ervati ation on law lawss are sufficie sufficient nt to to descr describe ibe the flow in any single si ngle phase pipeline systems. In other words, no chemical reaction including phase change takes place in the pipeline system. • Flow in the pipeli pipeline ne can be repr represe esente nted d in oneonedimensional equations, and angular momentum is negligibly small.
Explicit Solutions • The The valu values es at at the the curr curren entt time time are exp explic licitl itly y calculated from the values at the previous p revious time step, with the boundary conditions at current cu rrent time. • An expli explicit cit fini finite te differ differenc encee repres represent entati ation on conve converge rgess as distance and time steps are small. s mall. • These These meth methods ods are are lim limited ited to sma small ll time time steps steps only only,, depending on the smallest distance step in order or der to maintain the stability of the solution.
Implicit Solutions • An implic implicit it soluti solution on evalua evaluates tes the values values at the advanced point of time, instead of at the current time as in the explicit method. • The imp implic licit it finite finite diff differen erence ce equat equation ionss are expr express essed ed in in large matrices, which are solved simultaneously for pressure, temperature, flow rate and density at every discre discretize tized d point. point. • The large large matric matrices es are are norm normally ally solved solved by a sparse sparse matrix technique. • Implic Implicit it soluti solution on tech techniq niques ues are are flexib flexible le with with time time steps steps and inherently stable always.
Hybrid Solutions • Thes Thesee meth method odss decou decoupl plee the con contin tinuit uity y and and momentum equations from the energy equation. • Solve Solve for for pres pressur suree and and flow rate rate implic implicitly itly and for temperature explicitly. • This This meth method od can can work work wit with h larg largee tim time step steps. s.
Method of Characteristics • The charac character terist istics ics meth method od conv convert ertss the the conti continui nuity ty and momentum equations into four total differential equations. • The four four char charact acteri eristi sticc equati equations ons are are solve solved d explic explicitly itly for pressure and flow rate. • The soluti solution on procedu procedures res are are sim simple ple for for a sing single le flui fluid d pipeline, but many characteristic lines are required for a batch pipeline. • Time Time step step should should be constra constraine ined d by the shortes shortestt distance step and acoustic speed of the fluid in order to maintain the stability of solutions.
Boundary and Initial Conditions • Listed Listed below below are possib possible le boun boundary dary condit condition ions, s, among among which the first two boundary conditions are widely used: – – – –
Pressu Pres sure re - pres pressu sure re bou bound ndar ary y Pres Pr essu sure re - flow flow boun bounda dary ry Flow Flow - pres pressu sure re boun bounda dary ry Flow low - flow low boun bounda dary ry
• The The ini initi tial al con condi diti tion onss can can be: be: – A ste steaady sta state – A pipeline pipeline state, state, either either steady steady or transient, transient, saved saved from from a previous simulation run – An actual actual pipeline pipeline state state downloa downloaded ded from from the the host SCADA SCADA
What’s Next? • Transi nsient An Analysi lysiss • Fun Fundam damenta entall Pr Prin incciple ipless • Pipe Pipelline ine Tra rans nsie ien nts – Caus Causes es of tran transi sien ents ts – Acou Acoust stic ic spee speed d – Pote Potent ntia iall sur surg ge – Pressu Pressure re wave wave prop propag agati ation on – Conseq Conseque uence ncess of of tra transi nsient entss
• Transi nsient Contr ontrol ol • Applications
Causes of Transients • Transi Transient entss are basical basically ly manife manifeste sted d in two two types: types: pres pressur suree transients and flow transients, which are different aspects of the same phenomena. • Pressu Pressure re trans transient ientss occur occur when when a chang changee in energy energy occurs occurs in in the pipeline which adds or remove re move energy from the pipeline, while flow transients occur when there is a change in flow rate by a change in energy. • The The main main cau cause sess of tran transi sien ents ts in a pipe pipelin linee are: are: – – – – – – –
Change Change in valve valve setting settingss (open (open or or close) close) Starti Starting ng or stop stoppin ping g of of pumps pumps Chang Changes es in in pump pump spe speed ed or or head head Rupture, Rupture, column column separation separation,, or trapp trapped ed air air Arrival Arrival of a batch interface interface at the the pump pump Action Action of reci recipro procat cating ing pump pump Vibration Vibration of impelle impellerr in a centrif centrifugal ugal pump
Transient Properties • A trans transien ientt (surg (surgee or waterh waterham amme mer) r) is is a pres pressur suree wave. • Pressur Pressuree waves waves propa propagate gate at the the acous acoustic tic veloci velocity ty of the fluid in a pipe. • Initial Initial magn magnitu itude de of of a pres pressur suree wave wave is is propor proportio tional nal to acoustic velocity and fluid velocity. • The magnitud agnitudee atte attenua nuates tes as the pressu pressure re wave wave moves away from the source of the transient. • Small Small amo amount unt of vapo vaporr in the liquid liquid can alter alter the acoustic velocity.
Water Hammer • Water Water hammer hammer occu occurs, rs, beca because use the fluid fluid mass mass befor beforee the the stoppa stoppage ge is still still moving forwar forward d with its its fluid veloci velocity, ty, building building up a very high pressure, pressure, when when the pipe flow flow is sudd sudden enly ly stop stoppe ped d at the the dow downs nstr tream eam end. end. It ca can n ca caus usee pipelines to break if the pressure is high enough. • On the the oth other er hand, hand, when when an upst upstre ream am flow flow in in a pipe pipe is suddenly suddenly stopped, stopped, the fluid fluid downstream downstream will attempt attempt to continue flowing, creating a vacuum that may cause the pipe to collapse. This problem can be particularly acute if the pipe pipe is on a downhil downhilll slope. slope. • Othe Otherr caus causes es of wat water er ham hamme merr are are pum pump p fail failure ure,, and and chec check k valve slam (due to sudden sudden decelerati deceleration, on, a check valve valve may slam shut rapidly, depending on the dynamic characteristic of the check valve and the mass of the water between a check valve and tank).
Acoustic Speed • The ac acous oustic tic spee speed d in a buried buried pipe pipe can can be calcul calculate ated d by a
B
1 ( B / E )( D / t )(1 2 )
Where a = acousti acousticc speed speed B = bulk modulus of fluid ρ = fluid density
E = Young’s modulus of the pipe elasticity D = inside pipe diameter t = pipe wall thickness µ = Poisson’s Poisson’s ratio ratio of strain strain (0.3 for for buried buried pipe) pipe)
Acoustic Speed
Potential Surge • The initial initial pressu pressure re increase increase following following flow stoppage stoppage is refer referre red d to as the potential surge. • The magnit magnitude ude of the the pote potenti ntial al surg surgee is dete determi rmined ned by by the the formula: ΔP = ρav or ΔH = av/g
where ΔP = pressure increase ΔH = head a = acoustic velocity ρ = density v = fluid velocity before valve closure • Pressu Pressure re wave wave pro propag pagate atess away away from from the the source source.. • The wave wave reflec reflects ts back back at a boun bounda dary ry point point and and the the refle reflecte cted d wave has negative head.
Propagation of Potential Surge
Critical Period • The critic critical al period period is define defined d as the time time that that an an acoustic wave travels from the source point to the end point and then travels back to the source point. • It is expre xpress ssed ed as foll follow ows: s: tc = 2L/C where tc = critical period L = distance between the source point of the pressure wave to the end point where the wave bounces back. C = acoustic speed
Pressure Wave Propagation
Flow Flow and and Pres Pressu sure re Dyna Dynami mics cs – 1 • Assume Assume tha thatt the valv valvee is close closed d instan instantan taneou eously sly.. At the valv valve, e, the water velocity is suddenly forced to zero. As a result, the head at the valve abruptly increases by an amount ΔH = av/g. • The increa increased sed hea head d immedi immediate ately ly crea creates tes two two other other chan change gess at the valve; the pressure increase slightly s lightly enlarges the pipe and also increases the density of the fluid. The amount of the stretching of the pipe depends on the diameter and thickness of the pipe and on the compressibility of the pipe material and the liquid, but it normally changes changes by less than one-half percent. • The rise rise in press pressur uree head head causes causes a shar sharpp-fro fronte nted d pressu pressure re wave wave to propagate upstream at speed a. The wave front reaches the reservoir L/ a seconds after valve closure. At that instant, the velocity is zero throughout the pipe, the pressure head is everywhere H + ΔH, the pipe is enlarged and the fluid is compressed.
Flow Flow and and Pres Pressu sure re Dyna Dynami mics cs – 2 • Under Under these these cond conditio itions, ns, the the fluid fluid in the the pipe pipe near near the rese reservo rvoir ir connection is locally not in equilibrium since the reservoir pressure head is only H. Hence fluid begins to flow toward the reservoir, the region of lower head, as the stretched pipe forces forces flow in that direction. In the absence of friction, this leftward leftward velocity is equal in magnitude to the original steady velocity as it is driven by the same head increment ΔH; and the source of the liquid for this flow is the compressed liquid that is stored stored in the enlarged pipe cross section s ection under the increased pressure head. • The proces processs conti continue nuess to evol evolve ve with with time. time. At At time time 2L/ 2L/ a after the beginning, the pressure throughout the pipe has returned to its original value, but with the velocity reversed from its original original direction. At this instant, the pressure wave undergoes a reflection. The pressure head drops ΔH below the original steady head, and this pressure drop and closed valve cause the velocity behind the wave front to return to zero. behind this negative wave the pipe cross section shrinks and the liquid expands.
Flow Flow and and Pres Pressu sure re Dyna Dynami mics cs – 3 • By time 3L/ a, this negative wave has reached the reservoir, and the velocity is everywhere zero. However, the pressure head at the reservoir is again not in equilibrium with the reservoir head, so fluid is drawn from the reservoir into the t he pipe at velocity v. Behind the new, advancing wave, the head is in equilibrium with the reservoir head. • At time 4L/ a, the wave has reached the valve; at this instant all variables have returned to the original steady stead y state that existed before the valve was closed. This time interval is one full cycle in a hydrau h ydraulic lic transients that would, in the absence of friction, continue without abating.
Attenuation and Line Pack Change • The magnitud agnitudee of the potenti potential al surg surgee reduc reduces es as as it travels. This reduction is called attenuation. • An incr increas easee occur occurss in volum volumee stored, stored, which which is calle called d line packing. • The pres pressur suree increa increases ses,, the the pipe pipe wall wall expa expands, nds, and and the fluid is compressed.
Potential Surge and Attenuation
Column Separation • On the downst downstrea ream m side of the the clos closed ed valve, valve, the pressure behind the valve drops by b y the same amount as given in the the above above equation. equation. If the pressure pressure drops below below the vapor pressure, the liquid li quid vaporizes and column separation occurs. • Colum Column n separ separati ation on is a phen phenom omeno enon n tha thatt often often accompanies water hammer. hammer. It happens when a portion of the pipe is subject to low pressure. • Colum Column n separa separatio tion n is the the most most seri serious ous conseq consequen uence ce of of down-surge. It is more likely to occur at high points or knees (sharp changes in slope) in the pipeline. • Colum Column n separ separati ation on can disrup disruptt the the operat operation ion of pipelines and should be prevented from happening through proper design and operation.
Column Separation
Collapse of Column • The upstre upstream am column column will will be accele accelerat rated ed and the downstream column decelerated if the backpressure increases, and the upstream column overtakes the downstream column. As a result, the column can collapse collapse if this proces processs occurs occurs quickly. quickly. • If the the differe difference nce in velo velocity city at insta instant nt of colla collapse pse of of the the cavity is V, a head increase of a*V /2g may be expected, where a is the acoustic velocity. • When When a sepa separa rate ted d colu colum mn colla collapse pses, s, it can can be be destructive. This head increase may be of sufficient s ufficient magnitude to rupture the pipe.
Consequences of Transients • Unev neven flui luid mov movem emen ents ts • Unsta nstab ble pre pressure ress • Colum lumn se separatio tion • Check Check valve valve slam or contro controll valv valvee osci oscillat llation ion • Resona Resonance nce in a statio station n pipin piping g system system,, causi causing ng unstable pressures • Pump/c Pump/com ompre press ssor or trip trip or pipe pipelin linee shu shutd tdow own n due due to limited control capability: efficiency issue • Pipe Pipe rupt ruptur uree or coll collap apse se:: safe safety ty issu issuee
What’s Next? • Transi nsient An Analysi lysiss • Fun Fundam damenta entall Pr Prin incciple ipless • Pipe Pipelline ine Tra rans nsie ien nts • Surge Control – Overview Overview of surge control control strategi strategies es – Cont Contro roll dev devic ices es – Cont Contro roll of of pum pumps ps
• Applications
Objectives of Surge Control • The main obje objecti ctive ve of of surge surge contr control ol is is to limit limit the magnitude of the surge to within the allowable limits of the pipeline system including pipe, pump/compressor, and valves. • There There are are two two ways ways of manag managing ing pressur pressuree trans transien ients: ts: – Cont Contro roll of of the the surg surgee – Extra protectio protection n of the the pipelin pipelinee and equipment equipment
• Surge Surge contr control ol is impor importan tantt for the the follow following ing oper operati ations ons:: – StartStart-up up and and shut-d shut-down own oper operati ations ons – Valve Valve operation operationss including including pressure pressure or flow flow control control – Injection Injection and delivery delivery condition condition changes changes
System Protection from Failure • The main obje objecti ctive ve of pipelin pipelinee and and equipm equipment ent protection is to preserve the integrity of the pipeline system and to prevent system failure when events occur which are beyond the control of operators. • Operat Operators ors must must be able to prote protect ct the the syst system em during during any of the following conditions: – – – – – –
Pow Power failu ailure re Driv Driver er fa fail ilur uree Valv Valvee fail failu ure Emergency Emergency shutdown shutdown valve operation operationss Accidents Oper Operat ator or er erro rorr
Transient Control • Transi Transient ent contr control ol strat strateg egies ies and and device devicess are are discu discussed ssed in a general approach, because controlling transients is mostly site specific. • Contro Controll strateg strategies ies include include timing timing of of control control of pump pumps, s, valve operations, adequate maintenance, and others, while control devices include valves and tanks. • If surge surgess are expec expected ted to be severe severe,, the the magni magnitud tudes es of of surges should be determined, the piping system be reinforced, and a special control scheme s cheme be selected. • Compute Computerr simulat simulation ionss are neces necessary sary to selec selectt reliab reliable le and responsible control system.
Transient Control Options • Design Design Phase: Phase: The The cont continu inual al poun poundin ding g of surges surges can cause leaks and eventual pipe failure. At the locations where frequent surges are expected, – Sharp Sharp change changess in slope slope (knee) (knee) are are avoide avoided, d, – Thicke Thickerr pipes pipes ar aree insta installe lled, d, – Control Control devices devices such such as surge surge relief relief tanks tanks are installed. installed.
• Opera rattion ion Phase: – Minimize Minimize engin enginee failure failuress to avoid avoid pump pump trips, – Open and close line or station station valves valves slowly, slowly, – Open and and close close pump pump station control control valves valves gradual gradually ly for for fixed speed drives, or ramp speed up and down slowly sl owly for variable speed drives.
What’s Next? • Transi nsient Ana Analy lysi siss • Fun Fundam damenta entall Pr Prin inccipl iples • Pip Pipeli eline Tra rans nsie ien nts • Transi nsient Contr ontrol ol – Trans Transien ientt contr control ol strat strateg egies ies – Contro Controll mechan mechanism ismss and device devicess – Contro Controll of pumps pumps/co /compr mpress essor orss
• Applications
Cont Contro roll Devi Device cess • The follow following ing mecha mechanism nismss of cont control rollin ling g surge surge are in the pipeline systems: – – – –
Valv Valvee mov movem emen entt Check va valve Pum Pump star startu tup p Pump Pump powe powerr fai failu lure re
• There There are are four four types types of pres pressure sure surge surge cont control rol device devices: s: – – – –
Pressu Pres sure re reli relief ef val valve ves, s, Pressu Pressuriz rized ed surge surge tanks, tanks, Rupt Ruptur uree disk disks, s, Contro Controll valve valve with with a PID PID contro controlle ller. r.
Valve Movement • The magnit magnitude ude of the the pres pressure sure waves waves depen depends ds on the the type of valve, the way in which the valve is moved, the hydraulic properties of the system, and the elastic properties and restraint of the pipe system. • The proper proper eval evaluat uation ion of of the the impac impactt of valve valve movem movement ent on the pressures in a system depends strongly on the loss coefficient of the valve and position dependent coefficients for the valve at various openings. • In gener general, al, it is safe to close close the valves valves slowly slowly,, longer longer than 2L/ a. However, computer simulation studies are required to understand the system behavior in response to various closure schedules and to implement an effective and economical valve control system.
Check Valve • Check Check valve valvess can cause cause large large transie transient nt pressu pressure re if the flow reversal through them can occur before b efore the valve closure is complete. • Modern Modern check check valves valves do not not slam slam.. In some some cases, cases, a spring or weight causes the check valve to close at the instant forward flow ceases, thereby preventing the reverse flow problem. • Another Another type type clos closes es slow slowly, ly, regulat regulated ed by by a damping damping mechanism, to bring the reverse flow to t o rest gradually. • The valve valve must must either either close close quick quickly ly befo before re a revers reversee flow can become large or close slowly over ov er a time interval that is considerably greater than t han the critical time of closure 2L/ a. • The check check valve valve pro proble blem m is diff difficul icultt to analyz analyze. e.
Pump Startup • As the the pum pump star starts ts up and and com comes “on “on lin line, e,”” a posi positiv tivee surge is created in the downstream line. The magnitude of the incremental pressure depends on the sudden s udden increase in speed which occurs when the check valve is forced open and the liquid in the line begins to move. • When When ther theree is no vap vapor or in in the pumping pumping system system,, the pressure increase is generally not large. • If there there is is vapor vapor in the the disch discharg argee regio region, n, subs substan tantia tiall transient pressures can be developed. Filling vapor region can produce velocities that are above the expected steady-state velocities. At the low flow that generally exists early in the filling process, the pump is operating on its curve at a point where the discharge is quite large.
Pump Power Failure • The most most seve severe re tran transie sients nts upon upon power power failure failure occurs occurs where the static lift is large; i.e. the pipeline profile rises rapidly immediately downstream of the pump station. • If power power is cut cut off, off, the pres pressur suree just just downs downstre tream am of the the pumps drops rapidly, and this pressure pr essure drop propagates downstream at the wave speed. • This This drop drop in in press pressure ure can cause cause exte extensi nsive ve colum column n separation and lead to subsequent cavity closure shocks of large magnitude. • In addi additio tion, n, a flow flow rever reversal sal in the the syst system em occurs occurs and and lead to significant overpressures in the system, generally in the vicinity of the pumps, if the transient is not properly controlled.
Pressure Pressure Relief Relief Valve Valve (PR (PRV) V) – 1 • PRVs PRVs ar aree mec mecha hanic nical al dev devic ices es tha thatt are are used used to pro prote tect ct the the pipeline system from excessive pressure. They are installed on points along the pipeline where maintenance is easy. • A PRV PRV open openss when when the the pres pressure sure exceed exceedss a spec specifie ified d pre-set pressure. When the pressure increases in creases above the set value, the valve opening force of the fluid is greater than the closing force of the spring, and the fluid flows out the valve exhaust port. The valve closes when the pressure in the line decreases below the set value. • A PRV PRV can can protec protectt the pipeli pipeline ne from from over-pre over-pressur ssurizi izing ng during pump startup. During startup, some of the flow is discharged via the PRV to attached tankage.
Pressu Pressure re Relief Relief Valve Valve (PRV) (PRV) – 2 • If the the PRV is inst installe alled d in the pump pump disch discharg argee line line between the pump and the discharge control valve, it functions as a bypass valve during pump startup. To prevent a pump from operating near shut-off head, the PRV is set to open at a desired pressure. • Pressur Pressuree relief relief valves valves act to redu reduce ce upsur upsurge ges, s, but but do not control the initial down-surge that occurs on pump shutdown or power failure. • Pressur Pressuree relief relief valves valves are useful useful in shor short, t, stee steep p pipe pipe profile where reversal of flow quickly follows power failure or pump trip.
Pressure Pressure Relief Relief Valve Valve (PR (PRV) V) – 3 • Beca Becaus usee upsu upsurg rges es trav travel el at acoustic speed, a relief valve may not open quickly enough to prevent a very short surge of high pressure. • The PRV PRV is is des desig igne ned d for for a certain flow rate. An undersized PRV will not be able to discharge at a high enough rate to reduce the line pressure.
Upsurge and Down-surge
Pressurized Surge Tank • A press pressuriz urized ed surg surgee tank tank or accu accumu mulat lator or conta contains ins a gas that absorbs the pressure surges and prevents the transfer of a pressure waves to other parts of the pipeline system. • Pressur Pressurize ized d surge surge tanks tanks preve prevent nt trans transient ientss that that arise arise in in one section of the pipeline from being transmitted to another section of the line. • Pressur Pressurize ized d surge surge tanks tanks are are relia reliable ble and and requ require ire no repairs because there are no moving parts. However, they are expensive to install. Regular maintenance is required to maintain the volume of gas in the tank.
Pressurized Surge Tank
Rupture Disks • Rupture Rupture disks disks are are non-m non-mecha echanic nical al press pressure ure surge surge control control devices which consist of a bursting membrane designed to rupture at pre-set conditions of pressure. • Rupture Rupture disks disks are are an inexpe inexpensi nsive ve subs substit titute ute for othe otherr pressure surge control devices. Like pressure relief valves, rupture disks usually require additional tankage to accept relief flow. • Rupture Rupture disks disks must must be be repla replaced ced after after bein being g ruptur ruptured. ed. Therefore, spare disks are required both in-line and in storage.
Rupture Disk and Assembly
What’s Next? • Transi nsient An Analysi lysiss • Fun Fundam damenta entall Pr Prin incciple ipless • Pipe Pipelline ine Tra rans nsie ien nts • Transi nsient Contr ontrol ol – Overview Overview of transie transient nt contro controll strateg strategies ies – Cont Contro roll dev devic ices es – Cont Contro roll of of pum pumps ps
• Applications
Transient Control: Start-up • Pump Pump start start-up -up operati operations ons can can caus causee a rapi rapid d increa increase se in in fluid fluid velocity that may result in an undesirable surge, but they seldom cause a problem in actual operation. • Surg Surgee cont contro rolli lling ng metho methods ds incl includ ude: e: – If there there are several several pumps, pumps, start start them one at at a time at interval intervalss at least 2 times the critical period. – Open a control control valve valve slowly slowly (at least least 2 times times the critic critical al period) after the motor starts. – Use a variab variable-sp le-speed eed drive drive for for each pump pump ramped ramped up to to full speed slowly enough to avoid high surges.
• By inte interlo rlockin cking g the pump with with contro controll valves valves,, transi transient entss can can be greatly reduced. Upon start-up, the pump operates against a closed valve. As the valve opens, the flow into the pipeline gradually increases to the full pump capacity.
Transient Control: Shut-down • Norm Normal pump pump shut-d shut-down own may may also also cau cause se surg surges. es. They They can be controlled to remain within acceptable limits by the following methods: – Turn pumps pumps off off one at a time at at intervals intervals at least least 2 times times the the critical period. – Close a contro controll valve slowly (at least least 2 times the the critica criticall period) before the motor is stopped. – If pumps pumps are equippe equipped d with variable variable-spe -speed ed drives, drives, ramp ramp down slowly.
• By inter interloc lockin king g the pum pump p with control control valv valves, es, tran transie sients nts can be greatly reduced. Upon shut-down, the control valve slowly closes to decelerate the flow, after which power to the pump is shut off, but not until the valve is fully closed.
Transient Control: Power Failure • Power Power fail failure ure at a pumping pumping station station causes causes pump pump tripping, and results in an initial rapid down-surge in the discharge header and piping close to the pump station. • The power power failur failuree canno cannott be fully fully avoide avoided, d, but but its its effec effectt can be reduced by increasing the inertia i nertia of pump and motor with a flywheel. • Contro Controll valve valvess canno cannott preven preventt down down-sur -surge ge on on powe powerr failure.
Surge Pressure Generated
Compressor Failure
Station Con Conttrol Valves ves – 1 • PID control controller lerss are used used to quic quickly kly stabil stabilize ize adjust adjustabl ablee parameters such as pressure or flow rate. PID stands for proportional, integral and differential. PID controllers operate control valves for pressure, flow or other parameters. • They They conti continua nually lly monit monitor or the actua actuall condit condition, ion, compare compare it to the desired condition, and then adjusts the control valve. This monitoring and control provides faster and accurate control of the pipeline at the control unit location. • The effect effect of of a chan change ge in in a valv valvee settin setting g is locali localized zed immediately, and can be adjusted before the larger system is affected.
Station Con Conttrol Valves ves – 2 • The control controller ler makes makes a series series of adju adjustm stments ents to the the pressure or flow to produce a transition from one steady state to another. This frees the operator to monitor the overall system instead of paying attention to t o each controller on the pipeline. • There There are are seve several ral disadv disadvant antage agess to PID control controller lers: s: – Tuning Tuning contro controller ller parame parameters ters is tedious tedious and and difficu difficult, lt, – The loss loss of both both local local and remote remote signa signals ls can occur occur during during operation, – PI PID D contro controlle llers rs are are expen expensiv sive. e.
Station Control System
Homework 3 (2 Pts): Pump Trip • A pipe pipeli line ne compa company ny ope opera rate tess a 20” 20” liqu liquid id pipe pipeli line, ne, transporting 19oAPI crude. The pipe length is 270km and pipe wall thickness 0.25”. The detailed pipeline profile is given in the next slide: • Required data – Hour Hourly ly flo flow w rat rate: e: 700 700m m3 /hr – Poiss Poisson on’s ’s ra rati tio: o: 0.3 0.3 – Young Young’s ’s modulu modulus: s: 2.2x10 2.2x108kPa – Pipe Pipe gra rade de:: X56 X56 – Heavy Heavy crude crude bulk bulk modul modulus: us: 1.5x 1.5x10 106kPa
Homework 3: Pump Trip Questions • Pump Pump Statio Station n 2 was was tripp tripped, ed, and and 30 second secondss later later a second pump at Pump Station 1 started. st arted. Describe transient behaviors in the pipeline semi-quantitatively: – List reason reasonable able assumptio assumptions ns to answer answer the followin following g questions. questions. – Calculate Calculate the the acousti acousticc speed speed and potential potential surg surge. e. – Describe Describe pressure pressure behav behaviors iors betwee between n Pump Station Station 1 and and Station 2. – Describe Describe pressure pressure wave wave behavio behaviors rs downstre downstream am of Station Station 2. – What action action should should the the operat operator or take take in order order to avoid avoid vaporization in the pipeline? – Is there there any danger danger of overover-pressu pressuring ring the the pipe? If If so, why? why?
Pipeline Configuration Pipe Leg
KMP (km)
Elevation (m)
Facility
Leg 01
30
30
Lifting Lifting point point
Leg 02
45
Leg 03
60
Leg 04
90
60
Pump Pump station station 1
Leg 05
120
10
Low elevation point
Leg 06
130
Leg 07
140
Leg 08
170
180
Pump Pump station station 2
Leg 09
180
Leg 10
190
Leg 11
200
800
Highest elevation point
Leg 12
210
Leg 13
220
Leg 14
230
350
Pressure reducing station
Leg 15
270
0
Delivery point
What’s Next? • Transi nsient An Analysi lysiss • Fun Fundam damenta entall Pr Prin incciple ipless • Pipe Pipelline ine Tra rans nsie ien nts • Transi nsient Contr ontrol ol • Applications – Liqui iquid d pip pipel elin inee – Gas Gas pipe ipeline line – Demo Demons nstr trat atio ions ns
Applica Application tionss to Liqu Liquid id Pipeli Pipeline ness – 1 • Determ Determine ine pipe pipe wall wall thic thickne kness ss by locatin locating g high high pressu pressure re points in normal and abnormal operating conditions. • Deter Determ mine the the loca locatio tion n and and size size of of a press pressur uree relief relief valve and tank. • Determ Determine ine the minim minimum um and maxim maximum um allowab allowable le transient pressures. • Study Study the pressur pressuree effec effects ts of a valve valve closur closuree on the valve valve and and pipeli pipeline. ne. • Design Design contro controll system system includ including ing pump pump and and surge surge control, pump station spacing, and a leak detection system.
Applica Application tionss to Liqu Liquid id Pipeli Pipeline ness – 2 • Study Study the effects effects of supp supply ly and and dem demand and chan changes ges on the pipeline and equipment. • Study Study the effects effects of stat station ion operat operation ionss includ including ing a pump trip on the pipeline and equipment. • Study Study the effects effects of pipe pipeline line leaks leaks and ruptur rupture. e. • Study Study the the line line purg purgee and and load load dur durin ing g pipel pipelin inee commissioning. • Use Use as as a hyd hydra raul ulic ic tra train inin ing g sim simula ulator tor for for oper operat atio ion n staff.
Applications to Gas Pipelines • Determ Determine ine pipel pipeline ine capa capabil bility ity and optim optimum um facil facility ity locations such as compressor stations. • Determ Determine ine the the maxim maximum um allow allowab able le transi transient ent pres pressur sures. es. • Design Design contro controll system system includ including ing compres compressor sor statio station n control. • Evalua Evaluate te line line pack pack chang changee beha behavior viorss over over tim timee including capacity determination. • Determ Determine ine oper operati ation on stra strateg tegies ies for for supply supply and demand demand changes. • Study Study effects effects of stat station ion operat operation ionss incl includi uding ng a compressor trip on pipeline pressure and equipment. • Use as the the engi engine ne of of a pipeli pipeline ne trai trainin ning g system system and other real-time modeling system.
Transient Simulator • A stead steady y stat statee is a spec special ial cas casee of transie transient nt stat states. es. A transient simulator may have two components: steady state model and transient model. • Theref Therefore, ore, such such a tran transie sient nt sim simulator ulator can can perfor perform m all all the tasks that a steady state simulator can do, and more. It can do the following: – Study pipeline pipeline operating operating efficien efficiency cy – shows shows how how opera operation tion modes affect flow, pressure, and other pipeline parameters including operational stability. s tability. – Analy Analyze ze startstart-up up or shutd shutdown own proc procedu edure re – determ determines ines the the best way of starting or shutting down the system s ystem with various combinations of operating scenarios. – Study Study syste system m respon response se after after upset upsetss – models models vari various ous upse upsett responses to determine effective ways of responding upset conditions
Advantages of Transient Simulator • Advantages – The simulato simulatorr is capable capable of gener generating ating realisti realisticc informati information, on, representing the true pipeline conditions, either normal operating or upset situations, if accurate data is provided. – It can can be used used as a training training tool tool for for the pipelin pipelinee operato operators. rs.
• Disadvantages – It is more more compl complex ex to use use and required required data may may not be be available to ensure accurate modelling.
Real-Time Model • A realreal-tim timee model model is is a transi transient ent mode modell that that runs runs in realrealtime. It assists the operators to analyze the pipeline system performance. • The model model contin continuou uously sly synchr synchroniz onizes es to to the the actua actuall pipeline state through real-time real -time measurements received from the host SCADA. • It gener generate atess the the curre current nt pipe pipeline line states states in in the pipeli pipeline ne system in real-time. The pipeline states include the measured and modelled flow, density, pressure and temperature profiles. • It can can track track batche batches, s, dete detect ct lim limit it viola violatio tions, ns, and determine pump operating points.
Pipeline State
Real-Time Data • Norma Normally lly,, the host host SCA SCADA DA colle collects cts real-t real-tim imee data data and refreshes its real-time database at regular intervals. The SCADA system transfers the current data with time tag from the real-time database to the RTM system database. The scan time dictates the data transfer frequency. • Since Since the the quality quality of real-t real-tim imee data data is critic critical al for for accura accurate te and reliable results, real-time data received by the t he RTM system should be validated before they are used for the real-time model.
SCADA-RTM System Interface • Since Since an an RTM RTM system system uses real-t real-tim imee data, data, it it must must run run in in conjunction with the host SCADA, thus requiring hardware and software interface. • The SCADA SCADA sends sends the the coll collect ected ed real real-tim -timee data data to to the the RTM database. When the RTM database is refreshed, the real-time model is executed with the real-time data to determine the pipeline state corresponding to the collected real-time data. The updated state is stored in the RTM database and certain data are sent to the host. • The interfa interface ce req require uirem ments incl include ude the data data transfe transferr mechanism and data required by both SCADA and RTM.
RTM Applications • Press Pressur uree lim limit it vio viola lati tion on dete detect ctio ion n • Slac Slack k line line flo flow w de detect tectio ion n • Tracking ing fun funcctions – Batc Batch, h, DRA DRA and and con conte tent nt trac tracki king ng – Pipeline Pipeline efficien efficiency cy and pig tracking tracking
• Moni Monito tori rin ng func functi tio ons – Pump Pump perfo performa rmance nce monito monitorin ring g – Compre Compresso ssorr perform performan ance ce monitor monitoring ing
• Training System
Hydraulic Profiles • Hydrau Hydraulic lic prof profiles iles help help the operat operators ors to oper operate ate the pipeline safely by avoiding any limit violations such as maximum and minimum pressures. • Each Each scan, scan, the real real-ti -time me model model gen genera erates tes the the pres pressur sure, e, temperature, flow, and density profiles over the entire pipeline. • Since Since the the amoun amountt of hydr hydrauli aulicc data data gener generate ated d by the real real time model is very large, these profiles p rofiles are plotted together with the elevation profile, pump stations, batch locations, and MAOP/LAOP lines.
Hydraulic Profiles
Pressure Limit Violation • The pressu pressure re limits limits includ includee maxim maximum um allowa allowable ble operating pressure (MAOP) and minimum operating pressure. MAOP is determined by the pipe strength, design factor and elevation, while minimum operating pressure by the vapour pressure of the product. • Pressur Pressuree limit limit violat violation ionss can can be of short short-te -term rm or longlongterm. The long-term violation should be avoided. • Pipeli Pipeline ne compan companies ies are require required d to to reco record rd the violation history and report to the appropriate regulatory agency.
Slack Line Flow Detection • The The pha phase se of a flui fluid d turn turnss from from liqui liquid d to vapor vapor when when the pressure at a given temperature drops below the t he vaporization point of the fluid. A slack line is the condition where a pipeline segment is not completely filled with liquid. • It often often occu occurs rs near near high high elev elevati ation on drop drop point pointss when when the the pipeline back pressure is low. Since Si nce the RTM calculates the pressure and temperature profiles, it can detect slack flow conditions and their locations. • The proble problems ms caus caused ed by by slack slack line line condit condition ionss inclu include: de: – Pressure Pressure drop drop is large large due to constr constrictio iction n in slack slack regions regions – Batch Batch inter interfac facee mixing mixing incr increa eases ses – Pipe Pipe metal metal fatig fatigue ue rate rate incre increase asess
Hydraulic Gradient
Slack line condition
Heavy Crude Batch
PS4 Densitometer Densitometer confirms location of batch
Tracking Functions • The batc batch h tracki tracking ng data data recei received ved from from the real real-tim -timee modeling system may include the following: – Line fill data, data, includin including g the batch batch ID ID or name, name, product, product, location and volume, and estimated time of arrival (ETA), – DRA conc concent entra ratio tions ns if DRA is is used, used, – Other Other conte contents nts and and anomaly anomaly tracking tracking,, – Pig locati location onss and and trac trackin king, g, – Tank invent inventory ory data, data, including including the the product product and and tank level level or volume, – Meter data at at lifting lifting and delivery delivery points points to indicate indicate the the lifted or delivered volume of batches that are lifting and delivering at the time the data was captured.
Batch Tracking Display
Drag Reducing Agent (DRA) • DRA DRA can can help help incre increase ase through throughput put in liqui liquid d pipeli pipelines nes and reduce operating costs. • The effecti effectiven veness ess of DRA DRA is is measu measured red in term termss of the the reduction in frictional losses in the pipeline. The effectiveness varies with the DRA concentration, viscosity of the solvent fluid, pipeline temperature, fluid velocity, and pipeline diameter. • Degrad Degradati ation on of the effe effecti ctiven venes esss depend dependss on the the lengt length h of the pipeline and the amount of shear due to pipeline facilities such as pump stations and valves.
Drag Reducing Agent (DRA) • A drag drag re red ducin ucing g ag agent ent (D (DRA) RA) is a lon long g cha chain in poly olymer er.. DRA DRA dampen dampenss turbu turbule lenc ncee of the flui fluid d near near the the pipe pipeli line ne wall, resulting resulting in improved improved flow by reducing reducing frictional frictional pressure drop along the pipeline. • DRA DRA is used used in crude crude oil oil exce except pt heav heavy y crud crude(* e(*)) and and refined products such as gasoline and diesel, and mainly used to increase pipeline throughput. • (*) Cono ConocoP coPhil hillip lipss has has devel develope oped d a new new DRA DRA that can be effective for heavy oil transportation. • DRA DRA can can be effect effective ive for redu reducin cing g pipe pipe frictio friction n for jet fuel, but it is not permitted to use yet because of safety concern.
Drag Reducing Agent (DRA)
Benefits of DRA • The benef enefit itss of of usi using a DRA ar aree as as fo follow llows: s: – Incre Increase ase in pipe pipelin linee cap capaci acity ty – Reduction Reduction of of the waiting waiting time time for tanker tanker loading loading/off /offload loading ing – Maintainin Maintaining g the through throughput put during during pump mainten maintenance ance for for derated lines – Use smalle smallerr number number of Bypassing Bypassing pump stations stations – Ener Energy gy Sa Savi ving ngss
• DRA DRA is expens expensive ive,, but but is requ required ired in small small amount amount in the order of 20 part per million (ppm) to achieve the desired result. • Due to high higher er flow flow capa capacit city, y, the pote potenti ntial al for for surge surge can can be increased on pipelines which were not designed for high velocities.
Composition Tracking • The real real-tim -timee model model calcul calculate atess density density profile profiless and and tracks compositions in the pipeline system. • Composi Composition tion data data is is made made avai availab lable le at at gas gas recei receipt pt points for volume correction and quality check. ch eck. • Composi Composition tion track tracking ing is requir required ed to to correc correctt flow rates rates at meter station, calculate pipeline state and line pack accurately, and track gas quality accurately. • At juncti junctions, ons, combine combined d gas gas composi compositio tions ns are calculated and tracked downstream of the junctions. • Sour Sour gas gas can be be track tracked ed using using the com compos posit itio ion n tracking function, and heating values can be determined if composition tracking data is accurate.
Pipeline Efficiency • Pipeli Pipeline ne effic efficien iency cy is is define defined d as the the ratio ratio of of its its measu measured red flow rate to the flow rate predicted by the flow equation for the conditions prevailing at the time of flow measurement. • In prac practice tice,, com comparison parison of effic efficienc iencies ies cal calcula culated ted at at different flow rates is not easily possible, making it difficult to implement a general performance factor for efficiency determination, because the flow and pressure change and/or batch positions constantly. • However However,, the behavi behavior or of efficie efficiency ncy loss can be detec detected ted by comparing the long-term patterns of the friction factors during the same flow ranges.
Friction Factor Distribution
Friction Factor after Pigging
Pump Performance Monitoring • This This applic applicati ation on plot plotss a centri centrifug fugal al pum pump p perform performance ance curve, locates the operating point, and detects pump performance degradation. It has the following functionality: – The dynamic dynamic plots of of the curre current nt and histor historical ical operatin operating g points are superimposed superimposed onto onto the pump pump curves. The curves show the minimum and maximum operating ranges. – The opera operating ting point point is determ determined ined in terms terms of of the flow flow rate, rate, head, head, and throt throttle tle pressure. pressure. – The performa performance nce data data can be be be used to rerate rerate the pump curve curve and to determine when maintenance is required, or to assess the operator’s training requirement.
• To dete determ rmine ine the pump pump operat operating ing points points for para parallel llel pump operation, the pump unit control strategy st rategy such as flow splitting must be known.
Pum Pump Unit Unit Stat Statis isti tics cs • The pump pump unit unit statis statistic ticss is used used to to determ determine ine the the pump pump and driver driver maintenance maintenance schedule schedule as well as review review the performance of each operator. • The unit unit stat statist istics ics may incl include ude the follow following ing:: – On-pe On-peak ak and offoff-pea peak k run time time with with respec respectiv tivee volume volumess moved through the station – Number Number of on-peak on-peak starts starts and and total total number number of of starts starts – Total tal ru run tim timee – Date and time time the the unit was last last running running and started started – Suction, Suction, case, case, dischar discharge, ge, and and throttle throttle pressu pressures res – Measured Measured input input power power,, calculated calculated output output power power and and station station efficiency – Limit Limit viol violat ation ionss and their their counts counts
Compressor Performance Monitoring • Monito Monitorr the the com compres pressor sor perform performance ance for efficie efficient nt operation and record compressor unit statistics for planning maintenance and detecting potential unit problems. • Compres Compressor sor opera operatin ting g point pointss and and their their history history are plotted in real-time on wheel map, showing head, speed, power and efficiency. This information is used to increase compressor operation efficiency and prevent surge problems. • The unit unit stati statistic sticss maint maintain ainss the com compres pressor sor oper operati ating ng data such as number of starts, operating time, surge violation, etc., for maintenance planning.
Compres Compressor sor Perfor Performa mance nce Monitor Monitoring ing • This This appl applic icat atio ion n mon monito itors rs the the perfo perform rman ance ce of compressor compressor units units for efficient efficient unit unit operation operation and maintains compressor unit statistics. • The curre current nt and and histo historic rical al opera operatin ting g point pointss are are plott plotted ed on the compressor wheel map, showing flow, pressure, speed, power and efficiency. • It assi assist stss the the ope opera rato tors rs in prev preveentin nting g com compr pres esso sorr damage damage by avoiding avoiding surge conditi conditions ons and provides provides the information on the maintenance schedule.
Compressor Monitoring Display
Compressor Unit Statistics • Since Since com compres pressors sors mostly mostly use natur natural al gas gas for their their fuel, fuel, the main purpose of compressor unit statistics is to determine the maintenance schedule. • The The req requi uire red d dat dataa are are liste listed d bel below ow:: – The numbe numberr of unit unit starts starts issued to to a compres compressor sor unit toge together ther with the number of attempted and successful starts – The accum accumulate ulated d compresso compressorr operati operating ng hours hours for for current/previous current/previous day, month and year – The date date and and time the compr compressor essor was last last served served – The numbe numberr of surge surge control control line line violatio violations ns and recy recycling cling status – Warning Warning issued issued to the the operato operatorr when when the allocate allocated d number number of annual starts for a unit is about to expire or have expired
Training System • A full full traini training ng syste system m consist consistss of a pipeli pipeline ne simu simula lator tor,, record record keeping module and computer-based training (CBT) module. • The simula simulator tor can can be integ integrat rated ed with with the the host host SCADA SCADA syste system m to train both hydraulics hydraulics and SCADA operation. operation. For the integrated integrated training system, trainees use the SCADA screens and training instructor a separate terminal. The simulator si mulator behaves like a pipeline system providing measured values. • The reco record rd keeping keeping mod module ule recor records ds train training ing sessio session n conduc conducted ted,, training module completed, and training session results. • The CBT modu module le provi provides des traine trainees es with with vari various ous opera operatin ting g scenarios including abnormal operations. It includes training material on pipeline operations, hydraulics, equipment and facility operations, SCADA, and other relevant topics.
Training System Environment Pipeline Devices
SCADA Field Protocols
Applications
Operations
Dispatcher Terminals
Training System Training Simulator Instructor Terminal
SCADA Protocol Emulator
Applications
Trainee Terminals
Training Objectives • Perform Perform normal normal oper operati ations ons effici efficient ently ly and safely safely • Respo Respond nd to abnorm abnormal al opera operatio tions ns inclu includin ding g upset upsetss and emergencies • Predic Predictt the the con conseq sequen uences ces of faci facility lity failur failures es • Recogn Recognize ize monito monitore red d opera operatin ting g condi conditio tions ns that that are likely to cause emergencies and respond to the emergency conditions • Under Underst stan and d the the prop proper er act actio ions ns to to be tak taken en
Data Playback • In add addit itio ion n to pipe pipeli line ne sim simulat ulatio ion, n, arc archi hive ved d oper operat atio ion n data can be played on the host SCADA man-machine interface, so that the trainees can learn both SCADA operation and pipeline system responses to operation commands. • The trai trainin ning g system system normally normally increm increment ent play playback back time time faster or slower than real-time. The playback time is established by either the trainee or instructor. • It is possib possible le to to rewin rewind d to the the star startt of the playba playback ck period and to fast forward/backwards to specific playback times.
Trainee Interface • In an inte integra grated ted train training ing syst system em,, the train trainee ee interf interfac acee is the same same as the SCADA’s SCADA’s man-mach man-machine ine interface, interface, so that it provides realistic training environment where the dispatchers control their pipelines using the same control interface as the real pipeline. • In addi additio tion n to hydra hydraulic ulic traini training, ng, the the integ integrat rated ed system system helps dispatchers to learn the operation of the SCADA system without interfering with actual pipeline operations. • In gene general ral,, the the train training ing efficie efficiency ncy is highe higherr with with the the integrated system than with a simpler hydraulic-based training system.
Instructor Interface • Scenar Scenarios ios are are manag managed ed thro through ugh the instru instructor ctor interfa interface. ce. – Sele Select ctio ion n of scen scenar ario ioss – Copy Copy/delet /deletee existi existing ng scena scenario rioss – Replay Replay/re /rewin wind d a scenar scenario io
• The instruc instructor tor inte interfa rface ce has has the the capa capabil bility ity to to control control simulation. – – – –
Equipment Leaks Execut Execution ion speed speed and and tim timee step step Batc Batch h inj injec ecti tion on
• Data ata ccaan be be dis disp play layed: ed: – Graph Graphic ic displa displays ys – profil profiles es and trends trends – Tabu Tabula larr disp displa lay ys
