3 transient flow

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Pipeline Engineering:

Transient Flow

Mike Yoon, Ph.D.



What’s Next?

• Transient Analysis

– Physical operation

– Why transient analysis?

– Transient model vs. Steady-state model

• Fundamental Principles

• Pipeline Transients

• Transient Control• Applications



• Both design and operation should address both safetyand economics or minimum cost operation.

• Pipeline system design is mainly concerned with line

sizing, equipment sizing and location, while systemoperation is concerned with pipeline system orfacility start-up and shut-down, product receipt anddelivery, flow rate change, emergency shut-down,equipment failure, etc.

Physical Operation



• A pipeline state is defined as a condition of a pipelineexpressed in terms of pressure, temperature, flow rateand fluid density at a given location and time.

• A steady state is a condition of a pipeline system that

does not change much over time, while a transient stateis an unsteady condition that changes with timebetween two steady states.

• A surge or water hammer is a transient that occursabruptly during changes from a normal steady state

flow in the pipe.• An upsurge occurs if the pipeline pressure increase

above the normal operating pressure of the pipeline.

• A down-surge is a pressure decreasing condition.

Definitions



• The steady-state assumption is not valid for short-termoperational study, because pipeline states in alloperations change with time.

• When an operation change takes place, the flow rate

and pressure change immediately, and subsequently thechange will have an impact on the pipeline system.

• With a steady-state assumption, the following problemscannot be addressed:

– Over or under pressuring along the pipeline,– Equipments such as pump/compressor tripping,

– Line pack/peak shaving,

– Potential column separation

Why Transient Analysis?



• Many pipeline failures, particularly for liquid pipelines,

occur because improper provisions are made to manage

transient related problems such as pump trip, line

rupture, etc.

• In order to manage them adequately, the following

operating conditions should be properly taken into

account in design and operational analysis:

– Changes to pump operations,

– Power failures,

– Valve operation,

– Line fills

Why Transient Analysis for Liquid?



• A transient model calculates time dependent flow,pressure, temperature and density behaviors by solvingthe time dependent flow equations discussed in the nextsection.

• Therefore, a transient model generates hydraulicallymore realistic results than a steady state model, andtheoretically the model is capable of performing notonly all time independent functions performed by the

steady state model but also time dependent functionssuch as effect of changes in injection or delivery,system response to changes in operation, and line pack movement.

Transient Model



• Study normal pipeline operations – Pipeline operation changesare simulated to find a cost effective way of operating thepipeline system. The transient model allows the operation staff to determine efficient control strategy for operating the pipelinesystem and analyzing operational stability.

• Analyze startup or shutdown procedures – Differentcombinations of startup or shutdown procedures are simulated todetermine how they accomplish operation objectives. Thetransient model can model a station, including the pump orcompressor unit and associated equipment.

• Determine delivery rate schedules – A transient model can beused to determine delivery rate schedules that maintain criticalsystem requirements for normal operations or even upsetconditions.

Transient Model: Capabilities



• Study system response after upsets – A pipeline system can beupset by equipment failure, pipe rupture, or supply stoppage.The transient model is used to evaluate corrective strategies bymodeling various upset responses.

• Study blow-down or pipe rupture – The transient model allowsthe operation engineers to study the effects of blow-down on acompressor station and piping or to develop a corrective actionwhen a leak or rupture occurs.

• Predictive modeling – Starting with current or initial pipelinestates, future pipeline states can be determined by changing one

or more boundary conditions.

• Note that a transient model is more complex to use andexecution time is longer than that of a steady state model. Itrequires extensive data, particularly equipment and control data.

Transient Model: Capabilities



Key Topics



– Conservation laws and equations

– Solution approaches


• Transient Control

• Applications



Governing Laws and Equations

• The pipeline state can be fully defined by fourindependent variables: pressure, temperature, flow anddensity. Four equations are needed to determine thevalues of these four variables: continuity equation,momentum equation, energy equation, and equation of state.

• The first three equations come from conservation laws:

– Continuity equation

– Momentum equation with friction factor– Energy equation with heat capacity and Joule-Thomson coef.

• Equation of state



Continuity Equation

• The conservation of mass equation is often referred to

as the continuity equation in fluid dynamics. A general

form of the one-dimensional continuity equation is

expressed as:

where ρ = density

t = elapsed time

v = velocity

0

xv

t



Line Pack Change

• The first term in the continuity equation represents 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 pipelineoperation.

• It should be distinguished from the line fill volume,

which is the quantity of liquid contained in a pipeline.

It is useful for batch pipeline operation.

• The second term represents the difference between

mass flow into and out of the pipe segment.



Momentum Equation

where: is densityD is pipe diameterP is pressuref is the f riction f actorh is elevationg is gravitational accelerationx is displacementt is elapsed timeV is velocity

02

||

D

V V f

x

hg

x

P

x

V V

t

V



Forces

• The first term is a force due to acceleration, and the

second term a force due to kinetic energy. These two

terms are inertial force.

• The third term is a force due to pressure differencebetween two points in a pipe segment

• The fourth term is a gravitational force.

• The last term is a frictional force, or Darcy-

Weisbach equation, opposing to the flow on the pipewall.



Reynolds Number

• Reynolds number relates density, viscosity, fluidvelocity and pipe diameter.

• Reynolds number is dimensionless.

• Re = ρ*v*D/µ = v*D/ ν, where D = inside diameter,v = fluid velocity, ρ = fluid density, µ = absoluteviscosity, and ν = µ/ ρ = kinematic viscosity

• Re < 2000 for Laminar flow regime

• 2000 < Re < 4000 for critical flow regime• Re>4000 for turbulent flow regime



Friction Factor

• Friction factor is a function of Reynolds number andpipe roughness

• Expressed in Colebrook-White equation

• Laminar flow is independent of pipe roughness• Partially turbulent flow is dependent on Reynolds

number and pipe roughness

• Fully turbulent flow is dependent only on pipe

roughness• The Moody diagram relates the friction factor in

terms of Reynolds number and relative roughness.



Moody Friction Diagram



Energy Equation

• Temperature change over time

• Rate of temperature change due to the net convection of fluid energy into the fluid element

• Change rate due to expansion/compression of the fluidincluding Joule-Thomson effect

• Heat flow to or from the surroundings

• Effect of work against or by gravity, which will heat thefluid going downhill and cool it going uphill.

• Heating due to friction, assuming that all the frictional heatis deposited in the fluid

02

||)(4)( 2

vvv

g

v DC

V fV

x

h

C

Vg

C D

T T k

x

VA

AC

T

x

T V

t

T



Energy Components

• Rate of temperature change due to the net convectionof fluid energy into the fluid element

• Rate of temperature change due to

compression/expansion of the fluid, including Joule-Thomson effect

• Friction heating, assuming that all the frictional heatis stored in the fluid

• Effect of heat flow to or from the ground• Effect of work against or by gravity, which will heat

the fluid going downhill and cool it going uphill



Fluid Properties

• Fluid properties include thermodynamic andrheological properties.

• Thermodynamic properties include density,

compressibility, heat capacity, enthalpy, and entropy.• These quantities can be derived from an equation of

state. However, appropriate correlations are used inpractice, particularly for liquid properties.

• Viscosity is a measure of fluid’s resistance to shearforce, expressed in absolute or kinematic viscosity.



Equation of State

• An equation of state describes the relationship between

pressure, temperature, and density or specific volume.

• Theoretically, all thermodynamic functions can bederived from the equation of state. In practice, it is very

difficult to obtain them, particularly for liquid

properties.

• There is no equation of state universally applicable toall products. Instead, there are many correlations

applicable to certain types of fluid.



Gas Equations

• Ideal gas law

• Real gas law with the compressibility factor (AGA-8)

• BWRS equation of state for light hydrocarbons

• Peng-Robinson equation of state for light

hydrocarbons

• SRK equation of state for light hydrocarbons



Liquid Equations

• A liquid equation of state is expressed in terms of bulk modulus and thermal expansion coefficient.They are slightly dependent on pressure and

temperature.• The density of liquids can be expressed by a Bulk

Equation of State.

• The API Equation of State for petroleum liquids isused for custody transfer. This equation takes intoaccount the dependence of bulk modulus and thermalexpansion coefficient on pressure and temperature.



What’s Next?



– Conservation laws and equations

– Solution approaches



• Applications



• Three conservation laws are sufficient to describe

the flow in any single phase pipeline systems. In

other words, no chemical reaction including phase

change takes place in the pipeline system.• Flow in the pipeline can be represented in one-

dimensional equations, and angular momentum is

negligibly small.

Assumptions



Explicit Solutions

• The values at the current time are explicitly

calculated from the values at the previous time step,

with the boundary conditions at current time.

• An explicit finite difference representation converges

as distance and time steps are small.

• These methods are limited to small time steps only,

depending on the smallest distance step in order to

maintain the stability of the solution.



Implicit Solutions

• An implicit solution evaluates the values at theadvanced point of time, instead of at the current time asin the explicit method.

• The implicit finite difference equations are expressed inlarge matrices, which are solved simultaneously forpressure, temperature, flow rate and density at everydiscretized point.

• The large matrices are normally solved by a sparse

matrix technique.

• Implicit solution techniques are flexible with time stepsand inherently stable always.



Hybrid Solutions

• These methods decouple the continuity and

momentum equations from the energy equation.

• Solve for pressure and flow rate implicitly and fortemperature explicitly.

• This method can work with large time steps.



Boundary and Initial Conditions

• Listed below are possible boundary conditions, amongwhich the first two boundary conditions are widelyused:

– Pressure - pressure boundary

– Pressure - flow boundary– Flow - pressure boundary

– Flow - flow boundary

• The initial conditions can be:

– A steady state– A pipeline state, either steady or transient, saved from a

previous simulation run

– An actual pipeline state downloaded from the host SCADA



• Transients are basically manifested in two types: pressuretransients and flow transients, which are different aspects of the same phenomena.

• Pressure transients occur when a change in energy occurs inthe pipeline which adds or remove energy from the pipeline,while flow transients occur when there is a change in flowrate by a change in energy.

• The main causes of transients in a pipeline are:– Change in valve settings (open or close)

– Starting or stopping of pumps

– Changes in pump speed or head

– Rupture, column separation, or trapped air

– Arrival of a batch interface at the pump

– Action of reciprocating pump

– Vibration of impeller in a centrifugal pump

Causes of Transients



Water Hammer

• Water hammer occurs, because the fluid mass before thestoppage is still moving forward with its fluid velocity,building up a very high pressure, when the pipe flow issuddenly stopped at the downstream end. It can causepipelines to break if the pressure is high enough.

• On the other hand, when an upstream flow in a pipe issuddenly stopped, the fluid downstream will attempt tocontinue flowing, creating a vacuum that may cause the pipeto collapse. This problem can be particularly acute if thepipe is on a downhill slope.

• Other causes of water hammer are pump failure, and check valve slam (due to sudden deceleration, a check valve mayslam shut rapidly, depending on the dynamic characteristicof the check valve and the mass of the water between acheck valve and tank).



Acoustic Speed

• The acoustic speed in a buried pipe can be calculated by

Where a = acoustic speedB = bulk modulus of fluid

ρ = fluid density

E = Young’s modulus of the pipe elasticity

D = inside pipe diametert = pipe wall thickness

µ = Poisson’s ratio of strain (0.3 for buried pipe)

)1)( / )( / (1 2

t D E B

Ba



Acoustic Speed



Potential Surge

• The initial pressure increase following flow stoppage is referredto as the potential surge.

• The magnitude of the potential surge is determined by theformula:

ΔP = ρav or ΔH = av/g

where ΔP = pressure increase

ΔH = head

a = acoustic velocity

ρ = density

v = fluid velocity before valve closure• Pressure wave propagates away from the source.

• The wave reflects back at a boundary point and the reflectedwave has negative head.



Propagation of Potential Surge



Critical Period

• The critical period is defined as the time that anacoustic wave travels from the source point to the

end point and then travels back to the source point.

• It is expressed as follows:

tc = 2L/C

where tc = critical period

L = distance between the source point of the

pressure wave to the end point where the wavebounces back.

C = acoustic speed



Pressure Wave Propagation



Flow and Pressure Dynamics – 2

• Under these conditions, the fluid in the pipe near the reservoirconnection is locally not in equilibrium since the reservoirpressure head is only H. Hence fluid begins to flow toward thereservoir, the region of lower head, as the stretched pipe forcesflow in that direction. In the absence of friction, this leftwardvelocity 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 inthe enlarged pipe cross section under the increased pressure head.

• The process continues to evolve with time. At time 2L/ a afterthe beginning, the pressure throughout the pipe has returned toits original value, but with the velocity reversed from its originaldirection. At this instant, the pressure wave undergoes areflection. The pressure head drops ΔH below the original steadyhead, and this pressure drop and closed valve cause the velocitybehind the wave front to return to zero. behind this negativewave the pipe cross section shrinks and the liquid expands.



Flow and Pressure Dynamics – 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

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 state that existed

before the valve was closed. This time interval is one full cycle

in a hydraulic transients that would, in the absence of friction,

continue without abating.



Potential Surge and Attenuation



Column Separation

• On the downstream side of the closed valve, thepressure behind the valve drops by the same amount asgiven in the above equation. If the pressure drops belowthe vapor pressure, the liquid vaporizes and columnseparation occurs.

• Column separation is a phenomenon that oftenaccompanies water hammer. It happens when a portionof the pipe is subject to low pressure.

• Column separation is the most serious consequence of down-surge. It is more likely to occur at high points or

knees (sharp changes in slope) in the pipeline.• Column separation can disrupt the operation of

pipelines and should be prevented from happeningthrough proper design and operation.



Column Separation



Collapse of Column

• The upstream column will be accelerated and thedownstream column decelerated if the backpressureincreases, and the upstream column overtakes thedownstream column. As a result, the column cancollapse if this process occurs quickly.

• If the difference in velocity at instant of collapse of thecavity is V, a head increase of a*V /2g may beexpected, where a is the acoustic velocity.

• When a separated column collapses, it can be

destructive. This head increase may be of sufficientmagnitude to rupture the pipe.



• Uneven fluid movements

• Unstable pressures

• Column separation

• Check valve slam or control valve oscillation

• Resonance in a station piping system, causing

unstable pressures

• Pump/compressor trip or pipeline shutdown due to

limited control capability: efficiency issue

• Pipe rupture or collapse: safety issue

Consequences of Transients



What’s Next?




• Surge Control

– Overview of surge control strategies

– Control devices

– Control of pumps

• Applications



• The main objective of surge control is to limit the

magnitude of the surge to within the allowable limits of

the pipeline system including pipe, pump/compressor,

and valves.

• There are two ways of managing pressure transients:

– Control of the surge

– Extra protection of the pipeline and equipment

• Surge control is important for the following operations:

– Start-up and shut-down operations

– Valve operations including pressure or flow control

– Injection and delivery condition changes

Objectives of Surge Control



• The main objective of pipeline and equipmentprotection is to preserve the integrity of the pipelinesystem and to prevent system failure when events occurwhich are beyond the control of operators.

• Operators must be able to protect the system duringany of the following conditions:

– Power failure

– Driver failure

– Valve failure

– Emergency shutdown valve operations

– Accidents

– Operator error

System Protection from Failure



• Transient control strategies and devices are discussed

in a general approach, because controlling transients is

mostly site specific.

• Control strategies include timing of control of pumps,

valve operations, adequate maintenance, and others,

while control devices include valves and tanks.

• If surges are expected to be severe, the magnitudes of

surges should be determined, the piping system be

reinforced, and a special control scheme be selected.

• Computer simulations are necessary to select reliable

and responsible control system.

Transient Control



Transient Control Options

• Design Phase: The continual pounding of surges cancause leaks and eventual pipe failure. At the locationswhere frequent surges are expected,– Sharp changes in slope (knee) are avoided,

– Thicker pipes are installed,– Control devices such as surge relief tanks are installed.

• Operation Phase:– Minimize engine failures to avoid pump trips,

– Open and close line or station valves slowly,

– Open and close pump station control valves gradually forfixed speed drives, or ramp speed up and down slowly forvariable speed drives.



What’s Next?





– Transient control strategies

– Control mechanisms and devices

– Control of pumps/compressors

• Applications



Control Devices

• The following mechanisms of controlling surge are inthe pipeline systems:– Valve movement

– Check valve

– Pump startup– Pump power failure

• There are four types of pressure surge control devices:– Pressure relief valves,

– Pressurized surge tanks,

– Rupture disks,

– Control valve with a PID controller.



Valve Movement

• The magnitude of the pressure waves depends on thetype of valve, the way in which the valve is moved, thehydraulic properties of the system, and the elasticproperties and restraint of the pipe system.

• The proper evaluation of the impact of valve movementon the pressures in a system depends strongly on theloss coefficient of the valve and position dependentcoefficients for the valve at various openings.

• In general, it is safe to close the valves slowly, longer

than 2L/ a. However, computer simulation studies arerequired to understand the system behavior in responseto various closure schedules and to implement aneffective and economical valve control system.



Check Valve

• Check valves can cause large transient pressure if theflow reversal through them can occur before the valveclosure is complete.

• Modern check valves do not slam. In some cases, aspring or weight causes the check valve to close at the

instant forward flow ceases, thereby preventing thereverse flow problem.

• Another type closes slowly, regulated by a dampingmechanism, to bring the reverse flow to rest gradually.

• The valve must either close quickly before a reverseflow can become large or close slowly over a timeinterval that is considerably greater than the critical timeof closure 2L/ a.

• The check valve problem is difficult to analyze.



Pump Startup

• As the pump starts up and comes “on line,” a positivesurge is created in the downstream line. The magnitudeof the incremental pressure depends on the suddenincrease in speed which occurs when the check valve isforced open and the liquid in the line begins to move.

• When there is no vapor in the pumping system, thepressure increase is generally not large.

• If there is vapor in the discharge region, substantialtransient pressures can be developed. Filling vapor

region can produce velocities that are above theexpected steady-state velocities. At the low flow thatgenerally exists early in the filling process, the pump isoperating on its curve at a point where the discharge isquite large.



Pump Power Failure

• The most severe transients upon power failure occurswhere the static lift is large; i.e. the pipeline profile risesrapidly immediately downstream of the pump station.

• If power is cut off, the pressure just downstream of the

pumps drops rapidly, and this pressure drop propagatesdownstream at the wave speed.

• This drop in pressure can cause extensive columnseparation and lead to subsequent cavity closure shocksof large magnitude.

• In addition, a flow reversal in the system occurs andlead to significant overpressures in the system, generallyin the vicinity of the pumps, if the transient is notproperly controlled.



Pressure Relief Valve (PRV) – 1

• PRVs are mechanical devices that are used to protect thepipeline system from excessive pressure. They areinstalled on points along the pipeline where maintenanceis easy.

• A PRV opens when the pressure exceeds a specifiedpre-set pressure. When the pressure increases above theset value, the valve opening force of the fluid is greaterthan the closing force of the spring, and the fluid flowsout the valve exhaust port. The valve closes when the

pressure in the line decreases below the set value.• A PRV can protect the pipeline from over-pressurizing

during pump startup. During startup, some of the flow isdischarged via the PRV to attached tankage.




• If the PRV is installed in the pump discharge 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.

• Pressure relief valves act to reduce upsurges, but do not

control the initial down-surge that occurs on pump shut-

down or power failure.

• Pressure relief valves are useful in short, steep pipe

profile where reversal of flow quickly follows power

failure or pump trip.




• Because upsurges travel atacoustic speed, a relief valve may not openquickly enough to prevent

a very short surge of highpressure.

• The PRV is designed for acertain flow rate. Anundersized PRV will not

be able to discharge at ahigh enough rate to reducethe line pressure.



Upsurge and Down-surge



Pressurized Surge Tank

• A pressurized surge tank or accumulator contains a gasthat absorbs the pressure surges and prevents thetransfer of a pressure waves to other parts of thepipeline system.

• Pressurized surge tanks prevent transients that arise inone section of the pipeline from being transmitted toanother section of the line.

• Pressurized surge tanks are reliable and require norepairs because there are no moving parts. However,

they are expensive to install. Regular maintenance isrequired to maintain the volume of gas in the tank.



Pressurized Surge Tank



Rupture Disks

• Rupture disks are non-mechanical pressure surge controldevices which consist of a bursting membrane designedto rupture at pre-set conditions of pressure.

• Rupture disks are an inexpensive substitute for other

pressure surge control devices. Like pressure relief valves, rupture disks usually require additional tankageto accept relief flow.

• Rupture disks must be replaced after being ruptured.Therefore, spare disks are required both in-line and in

storage.



Rupture Disk and Assembly



What’s Next?





– Overview of transient control strategies

– Control devices

– Control of pumps

• Applications



Transient Control: Start-up

• Pump start-up operations can cause a rapid increase in fluidvelocity that may result in an undesirable surge, but theyseldom cause a problem in actual operation.

• Surge controlling methods include:– If there are several pumps, start them one at a time at intervals

at least 2 times the critical period.– Open a control valve slowly (at least 2 times the critical

period) after the motor starts.

– Use a variable-speed drive for each pump ramped up to fullspeed slowly enough to avoid high surges.

• By interlocking the pump with control valves, transients canbe greatly reduced. Upon start-up, the pump operates againsta closed valve. As the valve opens, the flow into the pipelinegradually increases to the full pump capacity.



Transient Control: Power Failure

• Power failure at a pumping station causes pumptripping, and results in an initial rapid down-surge inthe discharge header and piping close to the pumpstation.

• The power failure cannot be fully avoided, but its effectcan be reduced by increasing the inertia of pump andmotor with a flywheel.

• Control valves cannot prevent down-surge on powerfailure.



Surge Pressure Generated



Compressor Failure



Station Control Valves – 1

• PID controllers are used to quickly stabilize adjustableparameters such as pressure or flow rate. PID stands forproportional, integral and differential. PID controllersoperate control valves for pressure, flow or otherparameters.

• They continually monitor the actual condition, compareit to the desired condition, and then adjusts the controlvalve. This monitoring and control provides faster andaccurate control of the pipeline at the control unit

location.• The effect of a change in a valve setting is localized

immediately, and can be adjusted before the largersystem is affected.



Station Control Valves – 2

• The controller makes a series of adjustments to thepressure or flow to produce a transition from one steadystate to another. This frees the operator to monitor theoverall system instead of paying attention to eachcontroller on the pipeline.

• There are several disadvantages to PID controllers:– Tuning controller parameters is tedious and difficult,

– The loss of both local and remote signals can occur duringoperation,

– PID controllers are expensive.



Station Control System



• Pump Station 2 was tripped, and 30 seconds later asecond pump at Pump Station 1 started. Describe

transient behaviors in the pipeline semi-quantitatively:

– List reasonable assumptions to answer the following questions.

– Calculate the acoustic speed and potential surge.

– Describe pressure behaviors between Pump Station 1 and

Station 2.

– Describe pressure wave behaviors downstream of Station 2.

– What action should the operator take in order to avoidvaporization in the pipeline?

– Is there any danger of over-pressuring the pipe? If so, why?

Homework 3: Pump Trip Questions



Pipeline Configuration

Delivery point0270Leg 15

Pressure reducing station350230Leg 14

220Leg 13

210Leg 12

Highest elevation point800200Leg 11

190Leg 10

180Leg 09

Pump station 2180170Leg 08

140Leg 07

130Leg 06

Low elevation point10120Leg 05

Pump station 16090Leg 04

60Leg 03

45Leg 02

Lifting point3030Leg 01

FacilityElevation (m)KMP (km)Pipe Leg



What’s Next?





• Applications

– Liquid pipeline

– Gas pipeline

– Demonstrations



• Determine pipe wall thickness by locating high pressurepoints in normal and abnormal operating conditions.

• Determine the location and size of a pressure relief

valve and tank.

• Determine the minimum and maximum allowable

transient pressures.

• Study the pressure effects of a valve closure on the

valve and pipeline.

• Design control system including pump and surge

control, pump station spacing, and a leak detection

system.

Applications to Liquid Pipelines – 1



• Study the effects of supply and demand changes on thepipeline and equipment.

• Study the effects of station operations including a

pump trip on the pipeline and equipment.

• Study the effects of pipeline leaks and rupture.

• Study the line purge and load during pipeline

commissioning.

• Use as a hydraulic training simulator for operation

staff.

Applications to Liquid Pipelines – 2



• Determine pipeline capability and optimum facilitylocations such as compressor stations.

• Determine the maximum allowable transient pressures.

• Design control system including compressor stationcontrol.

• Evaluate line pack change behaviors over timeincluding capacity determination.

• Determine operation strategies for supply and demandchanges.

• Study effects of station operations including acompressor trip on pipeline pressure and equipment.

• Use as the engine of a pipeline training system andother real-time modeling system.

Applications to Gas Pipelines



Transient Simulator

• A steady state is a special case of transient states. Atransient simulator may have two components: steadystate model and transient model.

• Therefore, such a transient simulator can perform allthe tasks that a steady state simulator can do, andmore. It can do the following:– Study pipeline operating efficiency – shows how operation

modes affect flow, pressure, and other pipeline parametersincluding operational stability.

– Analyze start-up or shutdown procedure – determines the

best way of starting or shutting down the system withvarious combinations of operating scenarios.

– Study system response after upsets – models various upsetresponses to determine effective ways of responding upsetconditions



Pipeline State



Real-Time Data

• Normally, the host SCADA collects real-time data andrefreshes 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 transferfrequency.

• Since the quality of real-time data is critical for accurate

and reliable results, real-time data received by the RTM

system should be validated before they are used for thereal-time model.



SCADA-RTM System Interface

• Since an RTM system uses real-time data, it must run inconjunction with the host SCADA, thus requiringhardware and software interface.

• The SCADA sends the collected real-time data to the

RTM database. When the RTM database is refreshed,the real-time model is executed with the real-time datato determine the pipeline state corresponding to thecollected real-time data. The updated state is stored inthe RTM database and certain data are sent to the host.

• The interface requirements include the data transfermechanism and data required by both SCADA and

RTM.



Hydraulic Profiles

• Hydraulic profiles help the operators to operate thepipeline safely by avoiding any limit violations such as

maximum and minimum pressures.

• Each scan, the real-time model generates the pressure,

temperature, flow, and density profiles over the entirepipeline.

• Since the amount of hydraulic data generated by the real

time model is very large, these profiles are plotted

together with the elevation profile, pump stations, batchlocations, and MAOP/LAOP lines.

H d li P fil



Hydraulic Profiles



Pressure Limit Violation

• The pressure limits include maximum allowable

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.

• Pressure limit violations can be of short-term or long-

term. The long-term violation should be avoided.

• Pipeline companies are required to record theviolation history and report to the appropriate

regulatory agency.

Sl k Li Fl D i



Slack Line Flow Detection• The phase of a fluid turns from liquid to vapor when the

pressure at a given temperature drops below thevaporization point of the fluid. A slack line is thecondition where a pipeline segment is not completelyfilled with liquid.

• It often occurs near high elevation drop points when thepipeline back pressure is low. Since the RTM calculatesthe pressure and temperature profiles, it can detect slack flow conditions and their locations.

• The problems caused by slack line conditions include:– Pressure drop is large due to constriction in slack regions

– Batch interface mixing increases

– Pipe metal fatigue rate increases



Hydraulic Gradient

Heavy Crude Batch

PS4 Densitometer

confirms location of batch

Slack line condition

T ki F ti



Tracking Functions

• The batch tracking data received from the real-timemodeling system may include the following:

– Line fill data, including the batch ID or name, product,location and volume, and estimated time of arrival (ETA),

– DRA concentrations if DRA is used,– Other contents and anomaly tracking,

– Pig locations and tracking,

– Tank inventory data, including the product and tank levelor volume,

– Meter data at lifting and delivery points to indicate thelifted or delivered volume of batches that are lifting and

delivering at the time the data was captured.



Batch Tracking Display



D R d i A (DRA)



Drag Reducing Agent (DRA)

• A drag reducing agent (DRA) is a long chain polymer.DRA dampens turbulence of the fluid near the pipelinewall, resulting in improved flow by reducing frictionalpressure drop along the pipeline.

• DRA is used in crude oil except heavy crude(*) andrefined products such as gasoline and diesel, and mainlyused to increase pipeline throughput.

• (*) ConocoPhillips has developed a new DRA that canbe effective for heavy oil transportation.

• DRA can be effective for reducing pipe friction for jetfuel, but it is not permitted to use yet because of safetyconcern.



Drag Reducing Agent (DRA)



C iti T ki



Composition Tracking

• The real-time model calculates density profiles andtracks compositions in the pipeline system.

• Composition data is made available at gas receiptpoints for volume correction and quality check.

• Composition tracking is required to correct flow ratesat meter station, calculate pipeline state and line pack accurately, and track gas quality accurately.

• At junctions, combined gas compositions arecalculated and tracked downstream of the junctions.

• Sour gas can be tracked using the compositiontracking function, and heating values can bedetermined if composition tracking data is accurate.

Pi li Effi i



Pipeline Efficiency

• Pipeline efficiency is defined as the ratio of its measuredflow rate to the flow rate predicted by the flow equationfor the conditions prevailing at the time of flowmeasurement.

• In practice, comparison of efficiencies calculated atdifferent flow rates is not easily possible, making itdifficult to implement a general performance factor forefficiency determination, because the flow and pressurechange and/or batch positions constantly.

• However, the behavior of efficiency loss can be detectedby comparing the long-term patterns of the frictionfactors during the same flow ranges.

Friction Factor Distribution



Friction Factor Distribution



Pump Performance Monitoring



Pump Performance Monitoring

• This application plots a centrifugal pump performancecurve, locates the operating point, and detects pumpperformance degradation. It has the followingfunctionality:– The dynamic plots of the current and historical operating

points are superimposed onto the pump curves. The curvesshow the minimum and maximum operating ranges.

– The operating point is determined in terms of the flow rate,head, and throttle pressure.

– The performance data can be be used to rerate the pump curveand to determine when maintenance is required, or to assessthe operator’s training requirement.

• To determine the pump operating points for parallelpump operation, the pump unit control strategy such asflow splitting must be known.

P U it St ti ti



Pump Unit Statistics

• The pump unit statistics is used to determine the pumpand driver maintenance schedule as well as review theperformance of each operator.

• The unit statistics may include the following:

– On-peak and off-peak run time with respective volumesmoved through the station

– Number of on-peak starts and total number of starts

– Total run time

– Date and time the unit was last running and started

– Suction, case, discharge, and throttle pressures– Measured input power, calculated output power and station

efficiency

– Limit violations and their counts

Compressor Performance Monitoring




• Monitor the compressor performance for efficientoperation and record compressor unit statistics forplanning maintenance and detecting potential unitproblems.

• Compressor operating points and their history areplotted in real-time on wheel map, showing head,speed, power and efficiency. This information is used toincrease compressor operation efficiency and preventsurge problems.

• The unit statistics maintains the compressor operatingdata such as number of starts, operating time, surgeviolation, etc., for maintenance planning.

C P f M it i




• This application monitors the performance of

compressor units for efficient unit operation and

maintains compressor unit statistics.

• The current and historical operating points are plottedon the compressor wheel map, showing flow,

pressure, speed, power and efficiency.

• It assists the operators in preventing compressor

damage by avoiding surge conditions and providesthe information on the maintenance schedule.

Compressor Monitoring Display



Compressor Monitoring Display

Compressor Unit Statistics



Compressor Unit Statistics

• Since compressors mostly use natural gas for their fuel,the main purpose of compressor unit statistics is todetermine the maintenance schedule.

• The required data are listed below:

– The number of unit starts issued to a compressor unit togetherwith the number of attempted and successful starts

– The accumulated compressor operating hours forcurrent/previous day, month and year

– The date and time the compressor was last served

– The number of surge control line violations and recyclingstatus

– Warning issued to the operator when the allocated number of annual starts for a unit is about to expire or have expired

Training System



Training System

• A full training system consists of a pipeline simulator, recordkeeping module and computer-based training (CBT) module.

• The simulator can be integrated with the host SCADA system totrain both hydraulics and SCADA operation. For the integratedtraining system, trainees use the SCADA screens and training

instructor a separate terminal. The simulator behaves like apipeline system providing measured values.

• The record keeping module records training session conducted,training module completed, and training session results.

• The CBT module provides trainees with various operating

scenarios including abnormal operations. It includes trainingmaterial on pipeline operations, hydraulics, equipment andfacility operations, SCADA, and other relevant topics.

Training System Environment



Training System Environment

Operations

TrainingSystem

Dispatcher

Terminals

PipelineDevices

Applications

Applications

SCADA

SCADA

TraineeTerminals

TrainingSimulator

Field

Protocols

InstructorTerminal

Protocol

Emulator

Training Objectives



Training Objectives

• Perform normal operations efficiently and safely

• Respond to abnormal operations including upsets and

emergencies

• Predict the consequences of facility failures• Recognize monitored operating conditions that are

likely to cause emergencies and respond to the

emergency conditions

• Understand the proper actions to be taken

Data Playback



Data Playback

• In addition to pipeline simulation, archived operationdata can be played on the host SCADA man-machineinterface, so that the trainees can learn both SCADAoperation and pipeline system responses to operationcommands.

• The training system normally increment playback timefaster or slower than real-time. The playback time isestablished by either the trainee or instructor.

• It is possible to rewind to the start of the playback

period and to fast forward/backwards to specificplayback times.

Trainee Interface



Trainee Interface

• In an integrated training system, the trainee interface isthe same as the SCADA’s man-machine interface, sothat it provides realistic training environment where thedispatchers control their pipelines using the samecontrol interface as the real pipeline.

• In addition to hydraulic training, the integrated systemhelps dispatchers to learn the operation of the SCADAsystem without interfering with actual pipelineoperations.

• In general, the training efficiency is higher with theintegrated system than with a simpler hydraulic-basedtraining system.

Instructor Interface



• Scenarios are managed through the instructor interface.– Selection of scenarios

– Copy/delete existing scenarios

– Replay/rewind a scenario

• The instructor interface has the capability to controlsimulation.– Equipment

– Leaks

– Execution speed and time step

– Batch injection

• Data can be displayed:– Graphic displays – profiles and trends

Tabular displays

Instructor Interface

3 transient flow

Documents