1417 Pipeline Surge Analysis Studies Atmos_PSIG

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Copyright 2014, Pipeline Simulation Interest Group

This paper was prepared for presentation at the PSIG Annual Meeting held in Baltimore,Maryland, 6 May – 9 May 2014.

This paper was selected for presentation by the PSIG Board of Directors following review ofinformation contained in an abstract submitted by the author(s). The material, as presented,

does not necessarily reflect any position of the Pipeline Simulation Interest Group, its officers,or members. Papers presented at PSIG meetings are subject to publication review by EditorialCommittees of the Pipeline Simulation Interest Group. Electronic reproduction, distribution, orstorage of any part of this paper for commercial purposes without the written consent of PSIGis prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300words; illustrations may not be copied. The abstract must contain conspicuousacknowledgment of where and by whom the paper was presented. Write Librarian, PipelineSimulation Interest Group, P.O. Box 22625, Houston, TX 77227, U.S.A., fax 01-713-586-5955.

Abstract

Pipeline Surge Analysis Studies require the

hydraulic simulation of pressures and flows in

fluids caused by the transient operations of pumps

and valves. Pressure surges can cause significantdamage to pipelines producing pipeline leaks,

cracked pump casings, contamination and

environmental damage. Without adequate surge

protection this will result in significant downtime in

process plants and distribution systems, and the

reduced life expectancy of the pipeline.

This paper discusses pipeline surge analysis and

looks to address the challenge of efficiently

reviewing entire pipeline networks for pressure

surges in order to comply with the Department of

Transport regulatory requirements. To reduce the

effort required in conducting pipeline surge analysis

studies for pipeline design, operational changes, and

product changes, a surge analysis program has been

developed to automate the procedure from the

scheduling of simulation scenarios to the creation of

the surge analysis report.

Why Surge Analysis?The consequences of pipeline failure may be

catastrophic, and strict regulatory requirements are

in place to ensure the safe operations of pipelines.The UK Pipeline safety regulations state that:

“The pipeline operator shall ensure that no fluid is

conveyed in a pipeline unless the Safe Operating

Limits (SOL) of the pipeline have been established

and that a pipeline is not operated beyond its SOL.”

(Ref 2)

This pipeline safety regulation can be found (with

words to the same effect) globally in numerous pipeline safety regulations. A few examples are:

• U.S. Department of Transportation's Pipeline

and Hazardous Materials Safety

Administration

• British Standard Code of Practice for Pipelines

• European Harmonised Standard Petroleum and

Natural Gas Industries – Pipeline

Transportation Systems

The recognised standards often allow short

excursions of pressure above Maximum Allowable

Operating Pressure (MAOP). The pipeline may

therefore, for limited periods, see pressures above

MAOP and still be in code. The SOL, above which

the pipeline is not allowed to run under any

circumstances, is therefore higher than MAOP. The

SOL for the maximum pressure is typically 10% to

15% larger than the MAOP limit, depending upon

PSIG 1417

Pipeline Surge Analysis StudiesGarry Hanmer - ATMOS International LimitedSusan Bachman and Gregory Lind – Enterprise Products



2 Garry Hanmer, Susan Bachman, Gregory Lind PSIG 1417

local pipeline safety regulations.

Pipeline failures are rare occurrences, but have the

potential to cause extensive property damage, loss

of life and shut down of pipeline facilities for

extended periods of time.

The causes of pipeline failure can be internal or

external. Internal causes include events such as

pressure surge and material defects. External causes

include events such as earthquakes and third party

intervention.

Pressure surge in a pipeline system can produce

pressures in excess of the allowable maximum andminimum pipeline pressures. High pressures can

damage pumps, valves and other pipeline objects,

along with the potential to rupture the pipeline. Low pressures can lead to pipeline collapse, and

cavitation. Vapour cavity closures can produce

high shock pressures.

Transient pressure surges occur in pipelines because

of sudden changes in the fluid flow velocity. Thesechanges in flow velocity can occur due to

operations such as valve movements, pump power

failures, and pump start-up. Pressure surges canoccur in all fluid pipeline systems resulting in

pipeline fatigue and pipeline failure. Pressure surge

may be avoidable through sufficient pipeline

assessments and protection.

The consequences of a pipeline failure can be

tragic. On Saturday 1 June 1974, a 20 inch pipeline

in Flixborough, UK ruptured. Within one minute ofthe rupture, approximately 40 tonnes of

cyclohexane leaked from the pipeline, forming a

vapour cloud 200 metres (650 feet) in diameter.

This vapour cloud ignited resulting in injuries to

thirty-six workers, and fatalities to twenty-eight

workers. Offsite a further fifty-three injuries were

reported and varying levels of damage to property

were registered up to three miles away. No one

escaped from the control room, where all eighteen

personnel were killed (Ref 1).

The rupture occurred due to an over pressurization

of the pipeline. An assessment of the technical

failings concluded that a plant modification had

occurred without a full assessment of the potential

consequences, where only a limited amount of

calculations were conducted on the integrity of the

pipeline. No pressure testing was carried out on the

modification to the installed pipework.

Detailed pipeline assessments may be time-consuming, requiring large amounts of calculations

to be computed for each pipeline section. This isdue to the large variety of different operating

conditions which must be analyzed within the surge

analysis study, where each valve closure, pump tripand fluid change must be considered.

Pipeline simulation packages can be used to run

pressure surge analysis on pipeline sections to

provide accurate transient surge predictions and

analysis. A surge analysis application has beendeveloped to directly interact with the pipeline

simulation tool in order to schedule dynamic

simulation scenarios, run transient simulations,

conduct initial analysis of the simulation results and

create a surge analysis report. This whole procedure

is automated into a single step through the

utilization of the surge analysis program, allowing

the surge analysis study to be conducted in a much

timelier manner, making the surge analysis study a

much more efficient process.

What Causes Surge?Pressure surge refers to the pressure produced by a

change in velocity of the moving fluid that results

from events such as the shutting down of a pumping

station or pumping unit, unstable controls,

oscillations in tank levels, vapour pocket collapse,

the closure of a valve, or any other sudden blockage



PSIG 1417 Pipeline Surge Analysis Studies 3

of the moving fluid. These pressure surges may

occur in all fluid pipeline systems and can result in

pipeline fatigue and pipeline failure. The effects of

valve movement, pump trip, and pump start-up will

be considered below.

Valve Movements

One of the most common causes of pipeline

pressure surge is the movement of a valve. Anyvalve movement causes pressure waves to

propagate through the pipeline system. The

magnitude of this pressure wave depends on severalfactors. These factors include the type of valve, how

quickly the valve is moved, the hydraulic propertiesof the system, and the elastic properties andrestraint of the pipeline.

A sudden valve closure, such as an emergency

shutdown valve closure with a short valve closuretime creates the same behaviour as a slamming

check valve. Check valves can cause large pressure

surges if the flow back through them can occur before the valve closure is complete. This may

occur following a pump failure where the pump

discharge pressure drops rapidly resulting in thecheck valve closure.

Most modern check valves do not slam. A spring or

weight are commonly used to close the valve at theinstant when the flow ceases, while other check

valves close slowly, regulated by a damping

mechanism, to bring the flow to rest gradually.Even for these check valves there may be some

elastic energy in the system which will cause a

pressure surge.

Therefore it is important to ensure that the valve

either closes quickly before a reverse flow can

become large or closes slowly over a time intervalthat is considerably greater than the critical time of

closure as given by Equation 4 below. Otherwise a

high pressure could occur at the time of the valveclosure.

Pump Trip

Pipelines with a large static lift and where the

pipeline elevation profile immediately downstream

of the pumps rises rapidly can observe severe pressure surge following an event such as a pump

power failure. When a pump stops, the pressuredrop propagates down the pipeline. This pressure

drop may result in cavitation and cavity closure

shocks. A flow reversal may occur, resulting in over pressurisation of the system if the transient is not

controlled correctly. This over pressurisation tends

to occur in the vicinity of the pumps.

The magnitude of a pressure surge depends upon

the fluid compressibility, its density, and themagnitude of the change in the flow velocity. Themagnitude of the pressure surge caused by a sudden

change in fluid velocity can be approximately

expressed by Joukowsky's Law (Equation 1), whichis based on the total conservation of kinetic energy

of motion into pressure head.

Pump Start-up

When a pump starts up a positive pressure surge is

created in the downstream pipeline section. The

magnitude of this pressure increment depends uponhow quickly the velocity is increased when the

check valve is forced open and the fluid in the line

begins to move.

Pump start-up pressure surge typically occurs when

the pipeline is not completely primed for the start-up, such as following the tripping of a pump in the

event of a power failure. The fluid may come to restin the pipeline following the possible formation of

vapour cavities. When pumping is resumed these

vapour cavities collapse resulting in transient pressures developing. (Ref 3)

What are the Effects of Surge?Pressure surges within pipeline systems can lead to

severe damage and the potential failure of pipeline

equipment. Pressure surge damage can occur due to pressures in excess of the maximum allowable

pipeline pressures, resulting in damage to pumps,

valves and the pipe. Damage may also occur due to

pressures below the minimum allowable operating




pressures, where low pressures can result in

cavitation and pipeline collapse. Failures can occurunder a number of different situations, including:

•

Failure of static components through fatigue,

erosion or corrosion

• Failure of dynamic components leading to

high fatigue loads on other components

• Failure of the piping system due to extreme

pressures or temperatures

Transients in pipeline systems can cause the local

absolute pressure of the fluid to approach its vapour

pressure. At the vapour pressure of the fluid, gases

begin to come out of the solution. If a pipelinetransient results in a drop in pressure which is

severe enough to cause the pressure to reach thevapour pressure, then the fluid boils (cavitates)

forming pockets of un-dissolved gases and vapour

(column separation).

When the local absolute pressure increases, the

cavitation bubbles collapse rapidly and violently.During the collapse of the cavitation bubbles, the

vapour bubbles will transform themselves back into

a liquid state. There is a large volume changeduring this transformation, where the collapsing

bubbles release a large amount of energy resulting

in pipeline damage. Damage due to cavitation caninclude, material fatigue, component damage and

cavitation erosion.

When the vapour pressure of a homogenous fluid

such as water is reached, the entire fluid begins to

change phase, resulting in the formation of large

vapour cavities. For non-homogenous fluids such as

a hydrocarbon, only the light ends (such ascondensates) with low specific volume are affected.

‘Figure 9- Fluid Vapour Pressures’ shows thevapour pressures of Ethane, Iso-Butane, Propane,

Iso-Pentane, Pentane and Hexane.

How to Mitigate Surge?There are many devices and procedures which may be used in order to mitigate unacceptable levels of

pressure. Surge mitigation is a safety critical

requirement and should be treated with the highest

level of importance. A surge mitigation device or

procedure which does not perform when requiredmay result in catastrophic consequences. The

following devices and procedures will be analyzedin the following sections; valve movements, relief

valves, surge tanks, increased pipeline diameter andincreased pipeline wall thickness.

Valve Movement

The impact of valve movements varies significantly

between different types of valves. Gate valves for

example must be nearly closed before it generatesenough head loss to decrease the velocity by a

significant amount.

The sudden closure of a valve causes an increase in pressure head to occur at the upstream location and

a decrease in pressure head ( H Δ ) in the

downstream location, which propagate at a speed(c). Using the linear momentum equation the

change in pressure head can be calculated:

⎟ ⎠

⎞⎜⎝

⎛ +

Δ−=Δ

c

V

g

V c H 1

*

(Equation 1)

In most rigid pipe situations V/c is less than 0.01

and Equation 1 is therefore often reduced to:

V g

c H Δ−=Δ

(Equation 2)Where:

GravitygWavespeed c

ChangeVelocityV

Change Head essure H

==

=Δ

=Δ

Pr

From this equation it can be seen that a change in

velocity results in a change in pressure head. The

calculation depends upon the wave speed as

calculated in (Equation 3).

There are several equations which can be used to

calculate the speed of the pressure wave. These can




vary depending upon fluid types and pipe

properties. For an approximation of thin-walled pipes the wave propagation speeds can be

calculated using:

2

1

1 −

⎟⎟ ⎠

⎞⎜⎜⎝

⎛ ⎥⎦

⎤⎢⎣

⎡+= φ ρ Ee

D

K c

(Equation 3)

Where:

Factor stra

ThicknessWalle

Elasticityof Modulus E

Diameter Pipe D

Modulus Bulk K

Density

Wavespeed c

intRe

=

=

==

=

=

=

φ

ρ

The wave will subsequently reflect from upstreamand downstream and may then result in excessively

high or low pressures on either side of the pumps.

The wave reflection time can then be calculated

using the following equation:

c

LT

2=

(Equation 4)

Where:

Wavespeed c =

LengthPipe=L

TimeReflectionWave=T

Thus according to (Equation 4), the configuration

given in ‘Figure 1- Single Pipe’ would show

conditions at the supply occurring L/c seconds afterthe conditions at the demand.

The effects of the pressure head change can be

demonstrated by varying the valve closure time.‘Figure 2- Upstream Pressure when the Valve is

closed within Different Time Periods’ shows the

simulated effects of different valve closure times on

the pressure immediately upstream of a valve. A

linear valve curve was used for the purposes ofgenerating this data. The figure shows valve

closure times of one second (red), five seconds(blue), ten seconds (pink), thirty seconds (green),and sixty seconds (orange).

The increasing valve closure time in each case

shows a decreasing peak pressure. The simulatedfluid velocities for each of these valve closure times

within ‘Figure 3- Upstream Velocity When the

Valve’ shows the velocity is reduced at lower rateswith the increasing valve closure time, resulting in

the lower pressure. Figure 3 shows valve closure

times of one second (red), five seconds (blue), tenseconds (pink), thirty seconds (green), and sixty

seconds (orange).

Relief Valve

Relief valves allow for fast acting pressure relief

within the pipeline. Relief valves open when a pre-

defined pressure is exceeded and range frominexpensive and simple spring loaded discs to more

expensive and complicated systems designed to

operate within milliseconds. The purpose of relief

valves is to provide an escape for the flowing fluidso that a sudden change in velocity and consequent

change in pressure do not occur.

‘Figure 4- The Effect of a Relief Valve’ compares

the simulated upstream pressure of a one second

valve closure on two similar networks. The onlydifference between these two networks is the

presence of a relief valve in one networkimmediately upstream to the closing valve. The redtrend shows the pressures without the relief valve,

and the green trend shows the pressures with therelief valve. It can be seen that the chosen relief

valve has a significant effect on lowering the peak pressure at the upstream location of the closing

valve.

Surge Tanks

Surge tanks can be used to mitigate both high and

low pressures. They may act as temporary storage

devices for excess liquid which has been divertedfrom the main pipeline flow. This diversion allows




for a more gradual change in velocity in the pipeline

and a reduction in the magnitude of transient pressure waves.

Surge tanks may also be used to supply liquid to the pipeline to prevent excessive deceleration and low

pressures. Surge tanks can be used to act as

damping devices on a pipeline where velocities go

back and forth frequently.

Increased Pipeline Diameter

Increased pipeline diameters may be included in the

pipeline design to reduce potential surge pressures.This is done by reducing the fluid velocity resulting

in a reduced change in momentum to bring the fluidto rest.

Increased pipeline diameters may also have a

negative impact due to the reduction of the

frictional damping of the pressure fluctuations. Thefollowing formula can be utilized to calculate the

pipeline design pressure:

T J LF D

t S P ****

**2=

(Equation 5)

Where:

3)(TableFactorDerating eTemperatur T

2)(TableFactorJointlLogitudina=J

1)(TableFactorLocation=L

1)(TableFactorDesign=F

Diameter PipeExternal=D

ThicknessWall=t

StrengthYield MinimumSpecified =S

PressureDesign=P

=

The calculated design pressure observed when

varying external pipe diameter can be seen in

‘Figure 5- External Diameter Vs. Design Pressure’.This figure utilizes the equation above and shows

that as the external diameter is increased, the design

pressure decreases. An increase to the pipe wallthickness may be used to balance the reduced

design pressure produced by the increased diameter.

Note that ‘Figure 5- External Diameter Vs. DesignPressure’ assumes that all other factors within the

equation remain constant, for example as the pipeline diameter is increased, the pipe wallthickness remains constant.

Pipeline Wall Thickness

A stronger pipeline may be necessary where othersurge mitigation techniques are not possible.

Stronger pipelines may be achieved through

increased wall thickness for example.

Increased pipeline wall thickness is a more costly

method in terms of initial capital, although it doesnot require the same level of further maintenance

and testing as other mitigation methods require.

(Equation 5) may be used to show the effects of anincreased pipeline design pressure in relation to

pipeline wall thickness.

‘Figure 6- Pipe Wall Thickness Vs. DesignPressure’ shows the calculated effects on the design

pressure to a variation in pipeline wall thickness.

Note that this figure assumes that all other factors

remain constant and the only variable is the pipewall thickness.

How to Perform Surge Analysis?When analyzing the pipeline for pressure surge

scenarios, it is important to ensure that all of the

potential hazards and threats to the pipeline which

result in pressure surge are addressed. Each

pipeline will be unique in this respect; however the

following is a sample of scenarios which should be

considered:

1. What if power fails to the motors driving the

pumps?

2. What if the power fails to the motors driving the

pumps?

3. What if the pump delivery valve closes in a

given number of seconds?

4. What if one pump trips and another keeps

running?




5. What if a pump is restarted within a given

number of seconds after being tripped?

6. What if a control or emergency shut-down valve

is closed rapidly?

7. What if an operator opens/closes a valve too

quickly?

8. What if a given pipeline component

malfunctions?

9. What if the demand on the system is increased?

Should there be any changes to the pipeline design

or operating conditions; the pressure surge analysis

should be conducted again on all parts of the

pipeline system. This pressure surge analysis shouldinclude all pipeline and operating condition

modifications and should monitor the effects ofthese modifications on all parts of the network and

not just the modified section of the pipeline.

This analysis should ensure that the design data is

correctly input and attempts should be made to

ensure that the design data is reliable. Unreliable

design data can have a significant impact on thereliability of the analysis. This design data includes

items such as, flow rates, component operatingcharacteristics, material specification and fluid properties.

An Example Surge Analysis

The procedure for a manual surge analysis study

utilizing some of the equations discussed above will

be followed for a simple pipeline. A valve closureat the outlet of a short crude oil pipeline will be

studied. There is a safe operating limit on the

pipeline equal to a head of 1000 meters (3280.83

feet) for the crude oil which will be transported inthe pipeline. Table 4 details the pipeline properties

which will be used for the surge analysis study

calculations.

In order to calculate the Joukowsky’s head, the

wave propagation speed and the fluid decelerationmust first be calculated. The wave propagation

speed can be calculated as follows:

s ft

smc

c

Ee

D

K c

/64.3368

/76.1026

1*7.12*2.0E11

4.914

1.3E9

1840

1

2

1

2

1

=

=

⎟⎟ ⎠

⎞⎜⎜⎝

⎛ ⎥⎦

⎤⎢⎣

⎡+=

⎟⎟ ⎠

⎞⎜⎜⎝

⎛ ⎥⎦

⎤⎢⎣

⎡+=

−

−

φ ρ

In order to calculate the fluid deceleration it is

assumed that the fluid velocity will decreaselinearly over the valve closure time. This

assumption may result in inaccuracies within the

calculated values. The magnitude of the

inaccuracies will depend upon the actual valve type being used on the pipeline.

2

*

r

qV

π −=Δ

(Equation 6)

Where:

RadiusPiper

RateFlowq

VelocityFluid V

=

=

=Δ

This gives:

s ft

smV

V

/44.26

/06.8

4445.0*

5

2

−=

−=Δ

−=Δπ

Joukowsky’s law may now be used to approximatethe head increase for the given valve closure:




ft

m H

H

V g

c H

45.2767

52.843

06.8*81.9

76.1026

=

=Δ

=Δ

Δ−=Δ

Before this value can be compared to the pipeline

design pressure, the initial pipeline head at the

location of the valve closure must be added to the

Joukowsky’s head. This will give the maximum

head value as:

ft

m Head Max

Head Max

Head Joukowsky

Head Initial Head Max

17.4726

54.1440

52.84302.597

==

+=

+

=

It can be seen that the calculated head is above the

design head for the required fluid and there is now a

requirement for a surge mitigation procedure ordevice. This can be done be controlling the valve

movements, such as the manipulation of the valve

closure time.

Joukowsky’s law assumes that the valve closesinstantaneously. A modification of this law can bemade for short pipes to include the valve closure

time, where the wave reflection time given in

(Equation 4) is sufficiently less that the valveclosure time. The 500 meter pipeline given in Table

4 was used to demonstrate this equation, andcompared against simulated data. An approximation

for valve closure times of 1 second, 2 seconds, 3

seconds, 4 seconds and 5 seconds can be given bythe equation:

dt

dV

g

c H −=Δ

(Equation 7)

Rearranging this equation to give the required valve

closure time; an approximation of 2.78 seconds

ensures that the pipeline pressure does not rise tounacceptable levels following the valve closure with

a safety factor of 10%. This equation assumes a

linear fluid deceleration for the entire valve closuretime, which is an unrealistic assumption. For a gate

valve for example, only the last 2-5% of the valve

closure motion is critical for determining the

maximum pressure and different valve types will produce different results.

The length of the pipeline section immediately

upstream of the closing valve (to the next upstream

constraint or boundary condition) will also have aconsiderable effect on the maximum head, which is

also not taken into account.

As shown above, although it is possible to perform

simple surge analysis based on various equations,

the use of an accurate hydraulic simulation softwareis essential for pipeline design, expansion study and

operations planning. A few benefits of hydraulic

simulation analysis are:

• Accurate assessment of pressure surges caused

by different operating scenarios, based on the

exact pipeline material, dimensions, fluid

properties, equipment type and location

• Effective transient simulations and analysis of

emergency situations such as power failure, and

equipment malfunction

• Timely analysis and automatic report of a large

quantity of surge scenarios

How to Automate Surge Analysis?To carry out surge analysis, the entire pipeline

network must be included for the effects of pressure

surge, including any branches. Hydraulic




simulations are an effective way of conducting this

analysis, where the simulation may be used to

analyze the entire pipeline network as a whole or

various subsections of the network.

A hydraulic simulation software that can accurately

model the effects of the pressure surge should be

used. The following elements should be considered

when selecting a hydraulic simulation software for

surge analysis:

• Variable knot spacing allows for accurate

analysis of the pressure surge, minimizing

interpolation and errors

• Variable time steps allow for accurate

analysis of the pressure wave propagation

• Reverse velocity modelling allows dynamic

check valve modelling

• Pump modelling allows accurate spin –up

and spin-down times

• Control logic for accurately simulating

pipeline control systems during surge events

•

Valve characteristic inputs allows foraccurate valve closure modelling

Surge Analysis Tool

While a hydraulic simulation software is effective at

identifying pressure surges, the effort required in

conducting surge analysis studies can be extensive

and time consuming. This is due to the large

variety of operating conditions that must be

analyzed. This issue is only amplified for large

pipeline networks and poses a significant challenge.

To reduce the effort required in conducting pipeline

surge analysis studies for pipeline design,

operational changes, and product changes, a surge

analysis program has been developed to automate

the procedure from the scheduling of simulation

scenarios to the creation of the surge analysis

report.

The surge analysis program has been exclusively

designed for the purpose of surge analysis,

simplifying and automating the process of providing

the necessary submissions to the regulatory

authorities.

The surge analysis program will analyze all selected

parts of the pipeline sequentially without any

further input from the user. This can be an

extremely efficient method for analyzing very large

pipelines and networks, allowing the users to

conduct other tasks while the calculations are being

conducted by the software. The user will receive areport in their selected format when the analysis is

complete. This report outlines each surge scenario

with the resulting pressure and produces tables and

trends of the data.

The automatically generated surge analysis report is provided in a format that the regulatory authorities

and surge analysis engineers can easily scrutinise.

‘Figure 7- Surge Analysis Sample Pipeline

Network’ shows a sample pipeline configuration fora pipeline with four intermediate pumping stations.

‘Figure 8 – Surge Analysis Report’ shows the

automatically generated report from the surgeanalysis tool for the sample pipeline configuration,

where examples of the tables and trends are

displayed.

The automatically generated report shows a

selection of information within a tabulated formatfor each pressure surge scenario. This information

includes:

• Software version for traceability

• Project name and description

• Date report is generated

• Name of closed valve \ tripped pump

• Indication of MAOP \ SOP violation

• Time \ Location \ Pressure of MAOP \ SOP

violation




• Pump trip times

The report also generates a selection of trends for

each pressure surge scenario including:

• Pressure trend at MAOP \ SOP violation

• Pressure trend upstream of valve closure \

pump trip

• Maximum piezometric pressure profile for

each scenario

• Minimum piezometric pressure profile for

each scenario

• Scenario flow trends

This information is automatically generated withoutany further interaction with the pressure surge

analysis engineer. Pressure trends at the location ofthe MAOP \ SOP violations are automatically

generated at the point of the violation. There is no

requirement for a reporting point to be pre-definedwithin the simulation at the location of the

violation. This prevents a duplication of the surge

analysis study, where an initial analysis isconducted to determine the location of the

violations and a second study is conducted tointerrogate the pressure trends at the location of any

violations.

In the event of a failure within any of the surge

analysis scenarios this information can be used todetermine which surge mitigation procedure or

device to incorporate. The minimum and maximum

piezometric pressure profiles as determined by thehydraulic analysis results may be used with

(Equation 5) for example to calculate the required

wall thickness for a given design pressure. Increaseswithin the pipeline wall thickness are normally not a

practical solution for an existing operational

pipeline, due to the costs associated with the

modification. Existing operational pipelines areusually ‘looped’ as a way to increase capacity for

example. Increased pipeline diameter is therefore

typically only considered during the pipeline design phase.

The pressure surge analysis study may then

automatically rerun the schedule of scenarios withthe applied pressure surge mitigation procedures

and devices incorporated. An updated report will

then be generated for the modified scenarios.

A pipeline configuration report may also be

generated from the hydraulic simulation software to

be included within the pressure surge analysisreport. This configuration report will provide

detailed information regarding the pipeline

configuration, such as pipe lengths, diameters, wallthickness, pump and valve performance curve data

and the model boundary conditions for each model

item.

A surge analysis study could take several days,

weeks or even months to conduct depending upon

the size and complexity of the pipeline. Runningeach individual pressure surge scenario,

interrogating the simulation results and generating a

surge analysis report are all routine tasks which can be automated. The automation of these tasks often

allows the surge analysis study to be conducted

within a few hours. This can reduce costs andenhance opportunities significantly. The enhanced

opportunities could be from the additional revenuegenerated by completing the surge analysis studytimely for an increase in pipeline capacity earlier, or

the ability to provide temporary capacity increases

to fulfil a one-off demand which may not have been

possible if the surge analysis study were to takeseveral days or weeks to conduct. The automated

interrogation of results also allows for the removal

of human errors from the procedure, thus increasingthe accuracy of the analysis. The selection of anincorrect location of the peak pressure, for example,

could have catastrophic consequences.

Pipeline Logic Control

During a pressure surge a flow reversal may occur,

which could result in the over pressurization of the

system if the transient is not controlled correctly.Pipeline pressure surge can be controlled providing

the procedures are fully understood and planned in

advance. Planned procedures may assist with

minimizing the impact of a pressure surge. Thisrequires that a previous analysis of the system




including the pipeline controls has been conducted.

In the event of a pipeline operator closing a pipelinevalve too quickly for example, the pipeline control

logic may be replicated and where necessarymodified to safely respond to the valve closure,minimizing the pressure surge and reducing the

likelihood of cavitation.

Advanced pipeline control logic may be included inthe hydraulic simulation to achieve the most

accurate representation of the pipeline system

response. This control logic includes feedbackcontrol elements such as PID control blocks.

‘Figure 10: Control Diagram Editor’ shows a PID

control block embedded within the hydraulicsimulation software, where feedback control of the

pipeline system is being used to automate the

response of pipeline components to the effects of a pressure surge. Utilizing control elements within a

hydraulic simulation allows the simulation to

replicate the physical control system on the pipeline

network.

Pipeline pressure surge analysis may therefore be

conducted on the pipeline control system in addition

to the pipeline. This allows the pressure surgeanalysis study to monitor the effects of the control

system and indicate much safer methods foroperating the pipeline network.

Case StudiesTwo surge analysis case studies will be considered

in this section. Case 1 considers a capacityexpansion of an existing low sulphur diesel

pipeline, and case 2 considers a new pipeline design

study.

Case 1

Consider a pipeline modification study for an

existing low sulphur diesel pipeline with 100 km(62.14 mile) in length and 16 inches in diameter.

The pipeline requires an increase in the existing

flow rate. This increases the volumetric flow ratefrom 0.15 m3/s (5.3 ft3/s) to 0.2 m3/s (7.06 ft3/s).

The pipeline has a booster pump and a mainline

pump located at the pipeline inlet and anintermediate pumping facility located 50 km (31.07

mile) downstream. ‘Figure 11: Case 1 Pipeline

Modification Surge Analysis’ shows the pipelineconfiguration.

There is a maximum allowable operating pressureof 6550 kPa (950 psi) and a minimum pressure at

the pipeline outlet of 450 kPa (65 psi).

Steady state analysis of the pipeline indicates that

the pipeline discharge pressure at the main pump

station is required to be 5100 kPa (740 psi) and a

discharge pressure at the intermediate pump stationis required to be 4500 kPa (652.67 psi). A dynamic

surge analysis study can now be conducted on the

pipeline. Three pressure surge events will beconsidered:

1. Pump trip at the pipeline inlet

2. Pump trip at the intermediate pumping station

3. Sudden valve closure at the pipeline outlet

The steps demonstrated within the ‘An Example

Surge Analysis’ section above could take several

days to perform for this modification. A hydraulic

simulation utilizing the surge analysis program can

perform this operation in a number of minutes. Thisallows the surge analysis study to be much more

cost effective and allows the results to be obtainedmuch sooner. The pipeline operator would

therefore benefit from the cost savings of

conducting the studies and would be able to utilizethe increased pipeline capacity much sooner,

generating additional revenue.

‘Figure 12: Case 1 Pipeline Maximum PressuresViolation’ shows an automatically generated trend

from the surge analysis tool for the peak pressurelocation on the pipeline. An image of this trend isappended to the surge analysis report which is

produced by the surge analysis programme. The

blue trend shows the pressure over time at thelocation of the peak pressure during the simulation

run. The red and orange trends show the MAOP

and SOP limits respectively. It can be seen that the

peak pressure exceeds the MAOP limit when thevalve is closed suddenly at the outlet.




In order to mitigate the pressure surge an increase in

the valve closure time could be made. Each of thesimulation scenarios can be rerun automatically

with the applied change. ‘Figure 13: Case 1Pipeline Maximum and Minimum Pressures’ is a pipeline profile trend from the report produced by

the surge analysis programme following the applied

change. This trend shows the maximum (red trend)

and minimum (green trend) pressures for each pointalong the selected pipeline section, for the entire

duration of the hydraulic simulation. It can be seen

that the increased valve closure time has the desiredeffect on lowering the peak pressure below the

MAOP setpoint.

This peak pressure could be lowered further through

the use of the advanced control logic within the

hydraulic simulation. This could be done bytripping the upstream pumping facilities following

the valve closure. This logic control could then be

implemented on the physical pipeline.

Case 2

Consider a pipeline design study for a new crude oil

pipeline with 60 km (37.28 mile) in length and 16

inches in diameter. The pipeline requires a pumpingstation to be located at the main inlet, and has a tie-

in point at 20 km downstream. There are 8intermediate block valve stations along the pipeline.

‘Figure 14: Case 2 Pipeline Design Surge Analysis’

shows the pipeline configuration.

There is a maximum flow rate at the pumping

station of 0.15m3/s (5.3ft/s) and 0.04m3/s (1.41ft/s)at the tie-in point, and a discharge pressurerequirement at the pipeline inlet of 10100 kPa (1450

psi). There is a maximum allowable operating pressure of 10200 kPa (1480 psi).

Steady state analysis of the pipeline indicates that

the pipeline pressures and flows are sufficient to

meet the production targets. A dynamic surgeanalysis study can now be conducted on the

pipeline. Eleven pressure surge events will be

considered:

1. Pump trip at the pipeline inlet

2. Sudden valve closure at the 8 intermediate block

valve facilities3. Sudden valve closure at the pipeline inlet and

outlet

Usually this analysis could take several days to

weeks to conduct. The surge analysis tool can be

used to conduct the required pressure surge analysis

in a much more efficient manner, reducing theanalysis time to several hours.

The one step solution provided by the surge analysis programme prepares a detailed analysis of the

pipeline pressures and flows for the required

pipeline section. The automatically generated reportshows that the required valve closure time produces

peak pressures of 12100 kPa (1750 psi). This

pressure is above the MAOP threshold of 10200kPa (1480 psi), and also above the SOP threshold.

In order to mitigate the pressure surge an increase in

the valve closure time could be made, however thismay be limited by the emergency shutdown

requirements on the pipeline. A second option is

for a surge relief tank to be located immediately

downstream of the pumping station with a relief pressure of 10100 kPa (1465 psi), and an additional

tank located at the first and second block valvestations. ‘Figure 15: Case 2 Pipeline Design Surge

Analysis with Surge Relief’ shows the modified

configuration with the pressure relief valves andsurge relief tanks installed.

‘Figure 16: Case 2 - The Effect of a Relief Valve onPressure’ shows the effects of the surge relief valvesin reducing the pressure surge to below the MAOP

limit. The green trend shows the pressure at the pipeline high pressure location without the reliefvalve, and the blue trend shows the pressure at the

same location with the relief valve. The orange

trend shows the MAOP setpoint. It can be seen that

the chosen relief valve has the desired effect onlowering the peak pressure below the MAOP

setpoint.

As with ‘Case 1’, the peak pressure could also be

lowered further in ‘Case 2’ through the use of the




advanced control logic within the hydraulic

simulation. This could be done by tripping theupstream pumping facilities following the valve

closure. This logic control could then beimplemented on the physical pipeline.

ConclusionsSurge analysis studies are an essential part of

pipeline design and operations planning. Hydraulic

analysis of the pipeline network allows for potential

pressure surge risks to be identified and for surge

mitigation measures to be designed and

implemented.

In order to avoid pipeline damage resulting from

pressure surge, it is necessary to determine if there

is likely to be a pressure surge. This may be

determined through the use of pipeline simulation

tools, where valve closures and pump station

shutdowns may be simulated and the resulting fluid

behaviour analyzed.

To prevent an increase in fatigue damage to the pipeline, devices and procedures such as valve

opening and closing times, pressure relief valves,surge tanks, increased pipeline diameter and

increased pipeline wall thickness can be used as

surge mitigation measures. Efforts however should be made to ensure that these measures agree with

emergency procedures.

To reduce the effort required in conducting pipeline

surge analysis studies, routine procedures may be

automated utilizing a surge analysis tool to scheduleand run simulation scenarios, interrogate the

simulation results and generate a surge analysis

report.

This paper has demonstrated that the whole surge

analysis procedure can be automated into a single

step, reducing the time from several day or weeks to

a few hours. In addition, it improves the accuracy of

the analysis by removing human errors from the

process. The reduced time in completing such surge

analysis can help increase revenues by running

pipelines at higher capacities sooner and reduce the

cost of such analysis.

Author BiographyGarry Hanmer is a Principal Project Engineer atATMOS International in Manchester, United

Kingdom. Garry Hanmer has over 8 years’

experience in the pipeline industry with anemphasis on pipeline hydraulic simulation. He also

has experience in development and delivery of

pipeline operations and integrity managementsoftware systems. Garry Hanmer has a Master of

Engineering in Aeronautical Engineering (MEng

Hons) from the University of Salford, UK.

References

1.

Flixborough (Nypro UK) Explosion 1st June1974. http://www.hse.gov.uk

(http://www.hse.gov.uk/comah/sragtech/caseflix boroug74.htm)

2. Pipeline Pressure Limits - Pipelines Safety

Regulations 1996 http://www.hse.gov.uk(http://www.hse.gov.uk/pipelines/resources/pipe

linepressure.htm)

3. Entrapped Air in Pipelines, Martin C.S., 1976

4. Hydraulics of Pipeline Systems. Bruce E.Larock, Roland W Jeppson, Gary Z Watters.

5.

Pipeline Design and Construction – A PracticalApproach. M. Mohitpour, H. Golshan, A.Murray.

6. Fluid Transients in Pipeline Systems. A.R.D

Thorley



FIGURES

Figure 1- Single Pipe Example for Water Hammer Analysis

Figure 2- Upstream Pressure when the Valve is closed within Different Time Periods

Figure 3- Upstream Velocity When the Valve is Closed within Different Time Periods




Figure 4- The Effect of a Relief Valve on Pressure

Figure 5- External Diameter Vs. Design Pressure




Figure 6- Pipe Wall Thickness Vs. Design Pressure

Figure 7- Surge Analysis Sample Pipeline Network




Figure 8- Surge Analysis Report

Figure 9- Fluid Vapour Pressures




Figure 10: Control Diagram Editor

Figure 11: Case 1 Pipeline Modification Surge Analysis




Figure 12: Case 1 Pipeline Maximum Pressures Violation

Figure 13: Case 1 Pipeline Maximum and Minimum Pressures




Figure 14: Case 2 Pipeline Design Surge Analysis

Figure 15: Case 2 Pipeline Design Surge Analysis with Surge Relief

Figure 16: Case 2 - The Effect of a Relief Valve on Pressure




T ABLES

Design Factor * Location (Gas and Liquid))

Application Class 1 Class 2 Class 3 Class 4Gas (nonsour)

General & cased crossings 0.8 0.72 0.56 0.44

Roads 0.6 0.5 0.5 0.4

Railways 0.5 0.5 0.5 0.4

Stations 0.5 0.5 0.5 0.4

Gas (sour service)

General & cased crossings 0.72 0.6 0.5 0.4

Roads 0.6 0.5 0.5 0.4

Railways 0.5 0.5 0.5 0.4

Stations 0.5 0.5 0.5 0.4

High Vapour Pressure

Liquid General & Cased crossings 0.64 0.64 0.64Roads 0.64 0.64 0.64

Railways 0.5 0.5 0.5

Stations 0.64 0.64 0.64

Low vapour pressure liquid

All but uncased RR crossings 0.8 0.8 0.8

Uncased railroad crossings 0.8 0.5 0.5

Table 1 – Design Factors for CSA Z662-07

Pipe Type CSA Z662-07

Seamless 1.0

Electric Welded 1.0

Submerged arc welded 1.0

Furnace butt welded 0.6

Table 2 – Longitudinal Joint Factors for CSA Z662-07

Temperature (C) CSA Z662-07

> 120 1.0

150 0.97

180 0.93

200 0.91

130 0.87

Table 3 – Temperature Derating Factors for CSA Z662-07

SI Imperial

Pipe Diameter 0.9144 m 36 in

Pipe Wall Thickness 0.0127 M 0.5 in

Pipe Length 500 M 1640.41 ft

Pipe Modulus of Elasticity 1300000000 Pa 188549.06 psi

Pipe Inlet pressure 5101325 Pa 739.88 psi

Pipe Steady State Flow 5 m3/s 176.57 ft3/s

Restraint Factor 1 - 1 -

Fluid Density 840 Kg/m3 52.43 lb/ft3

Table 4 – Pipeline Properties

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