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1 Copyright 2008 by ASME

Proceedings of IPC08

7th

International Pipeline Conference

September 29-October 3, 2008, Calgary, Alberta, Canada

IPC2008-64161

CUT POINT OPTIMIZATION OF DIESEL OIL - GASOLINE INTERFACES

Sergio D. Gavn

Repsol YPF

La Plata, Argentina

ABSTRACT

This paper presents the technical solution developed by

Repsol YPF in one of its pipeline systems to reduce

contaminated product generation as a result of the interfaces

that are generated between diesel oil and gasolines during

transport.

INTRODUCTION

The refined white product pipeline where this new tool

will be applied has its origin pump station in the city of Villa

Mercedes, one intermediate delivery station in the city of Junn,

and its terminal station in La Matanza, a total of 663 km (12

inch diameter) with nineteen mainline block valves and the

elevation profile shown below (Figure 1).

0

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038

75

112

149

186

224

261

299

336

373

410

446

482

519

556

594

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kilometers

meters

-Figure 1-

This system transports 90,000 m3 of refined white

products (diesel oil and gasolines) per month.

Due to the pipelines low transportation volume and to

Repsol YPFs configuration of its pipeline network, it is not

necessary for this system to be continuously pumping. It is in

operation 75 % of the time, and during this period its flow ratis below the systems maximum capacity. On average, its flow

rate is 130 m3/h, being able to deliver fully in Junn, delive

one part in Junn and let the other flow to La Matanza, o

pump from Junn to La Matanza.

There exist three options for pumping the product from th

origin pump station located in Villa Mercedes, namely: 1) b

gravity 2) taking advantage of excess pressures and derivin

from another system belonging to Repsol YPF, for which Vill

Mercedes works as an intermediate delivery and repumpin

plant 3) from a tank with turbopumps.

Normal operating pressure is 8 kg/cm2 (785 kPa) in Vill

Mercedes, 20 kg/cm2 (1960 kPa) in Junn and 12 kg/cm2

(1175 kPa) in La Matanza (manometric values).

For this pipeline the use of small batches of about 50 m3 o

JP-1 jet fuel (corks, buffers) between diesel oil an

gasolines in order to minimize contamination of both product

was not successful due to the many stops and starts of thi

system, which cause important compressions an

decompressions (e.g. Figure 2), and the negative slope that it

elevation profile shows, resulting in interfaces of greater siz

without reducing the diffusion between both product

(interfaces).

Villa Mercedes

La MatanzaJunnn

Proceedings o f IPC20087th International Pipeline Conference

September 29-October 3, 2008, Calgary, Alberta, Canada

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0

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023

46

69

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kilometers

meters

0

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Pressuere(kg/cm2)

Pressuere curv e (Q=130 m3/h)

Pressuere curv e (Q=0 m3/h)

-Figure 2-

The most significant cause of interfase increase is when

the system stops and the interface remains at the greater slope,

and it is even worse when diesel oil is above gasoline.

Also, the small size of the batches does not make it

possible to absorb the interface on the product tanks inspecification.

Separating scrapers (pigs) did not show good results

either, mainly due to the fact that they moved backward or

forward as a consequence of the refined white product

pipelines particular elevation and the constant starts and stops

of the pipeline.

For these reasons the pipeline operates by sending the

initial and final parts of the interface to a product tank in

specification and its central portion to a contaminated one.

We refer to diesel-gasoline interfaces as having two cuts,(Figure 3) since their central part is segregated to a

contaminated product tank to be subsequently recovered

through proportioning in excess-quality products, or else, by

sending it to refinery in order to be reprocessed, that is to say,

there are two changes of tank.

Volume

Density

-Figure 3-

The outer lines indicate the start and finish of th

interface, while the two central lines show the portion o

interface that is sent to the contaminated product tank.

On the other hand, interfaces between different types o

gasolines are referred to as one cut (Figure 4) since one par

of the interface goes to the previous product tank and the otheto the subsequent product tank in the batching sequence, bu

neither of them is segregated to a contaminated product tank

that is to say, there is only one change of tank.

Volume

Density

-Figure 4-

Before applied the new methodology presented in thi

paper some measures intended to reduce the amount of volum

of diesel-gasoline interfaces may be taken. The most urgent on

should be to increase the size of the batches in order to reduc

the number of interfaces, thus for the same transported volume

the number of batches and the quantity of interfaces will b

lower.

However, this measure helps to reduce the volume o

contaminated product but not to eliminate it, and consequently

it is necessary to take further measures to reduce the interfac

portion that is segregated to a contaminated product tank.

NEW METHODOLOGY

In the past, tank changes were performed upon reaching

pre-defined density value as measured by the plant`s interna

densitometer. This value was constant and did not vary. Tank

changes always occurred at the same value.

The developed improvement consisted of optimizing th

first and the second interface cut points with a view to

maximizing the volume of mixture that can be absorbed by th

tanks in specification, thus reducing the volume o

contaminated product to be recovered in the Refinery

The used methodology calculates the composition of both

products in the interface within the distribution manifold i

1st cut 2nd cut

Cut

Villa Mercedes

Junnn

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real time, making it possible to quantify the volume of diesel

oil and gasoline present in the mixture that enters each tank.

The cut point is determined by the maximum volume of

diesel that the gasoline tank can admit and vice versa, without

leaving any of them in out-of-specification condition (RON,

MON, sulphur content, cetane, flash point, etc). The well-known mixing indexes, such as those of the Petrofine Manual

or other oil companies are used to do this.

In order to define that volume, the following aspects are

taken into account: the quality/volume of the product in the

receiving tank, whether there is interface in the pipeline

entering the tank, its composition, and the quality/volume of

the product to be received. The Test Laboratory is responsible

for this task, which must be performed before the interface

arrives at the plant. Once these values have been defined, they

are communicated to the Operations Area so that cuts are

performed following the Laboratory s technical

recommendations.

Facilities diagram

See Annex A

On line software

The software program is set up on the plant operative PC

where software iFIX runs and collects data from the PLC

which receives field instrumentation data (volume flow meter,

mass flow meter, densitometers, etc). The interface composition

calculation is visualized in real time on this PC so that the

operator makes the change of tank when the cut point value isreached.

See Annex B Operative screen

See Annex C Tuning screen

Interface analysis

Assuming an interface between two consecutive products A

and B, the calculation of interfaces determines the percentage

and volume of each of the products entering the tank at every

instant of time.

The percentages could be calculated using different process

variables such as density, sonic velocity, absorbance, viscosity,

etc, but density follows lineal mixing models so it becomes

easier to do the calculations.

In order to detect the beginning of the interface, the

programme continuously senses the density value on line at

intervals of 2 seconds. Upon verifying that density is 1 kg/m3

higher (or lower) than the reference value in five consecutive

samples, the programme is activated (the reference value is th

average density during the last minute). See Annex C.

The reason for using said logic as well as a reference valu

is to prevent the programme from starting accidentally due t

very small peaks in the density of the product that do no

correspond to the beginning of the interface.

Once the programme has been started, one interval wit

the following characteristics is obtained every two dat

samples:

Volume

Density

-Figure 5-

averageDensity: (Dens n+1+ Dens n)/2

averageFlow rate: (Q n+1+ Qn)/2

Time interval: Hr n+1 Hr n

Interval volume: averageFlow rate x Time interval

% of A: (averageDens - Dens B) / (Dens A- Dens B)

% of B: 100% - % of A

Vol of A: Interval volume x % of A

Vol of B: Interval volume x % of B

Dens A and Dens Bvalues refer to the values of pure A and

B products respectively.

The value Dens Autilized in the calculations is the averag

density of A in the last ten minutes (300 samples), up to th

moment when the interface begins. This value remains fixe

once the interface begins and is no longer recalculated. In thi

manner, it is possible to perform calculations for %A and %B

with a representative value of the density of the produc

entering the plant.

Bs density value (Dens B) should be entered manually by

the Operator in iFIX depending on the density value that th

product is expected to have. This value can be obtained from

the information submitted by intermediate pumping stations

from the laboratory test certificate for that batch issued upon it

entrance to the pipeline, or automatically, by reading the dat

from the Batch-Tracking of SCADA, or the density valu

observed on the plants out station densitometer. This last cas

n-1, n, n+1, n2+2

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is valid only when the interface volume is lower than the

pipeline volume existing between the location of the out station

densitometer and the internal one. Otherwise, at the beginning

of the reception of the interface within the plant there would

not be a stabilized value (density of pure B product) on the out

station densitometer.

As the mixture is being received, the volumes of A and B

accumulate and appear on the terminals operation screen.

In a `two-cut interface, the first change of tank is carried

out (manually by the operator, or automatically, depending on

the established set point value) when the value of maximum

amount of B in A, as defined by the laboratory, has been

reached. At this point the software starts to count again from

zero the accumulated volumes of A and B sent to the new tank,

displaying on screen the first cut values accumulated up to that

moment. In this manner, one can know accurately the amount

of product A and that of product B sent to each container /

vessel during operation.

There exist two alternatives to make the second change of

tank:

a) Use typical density values according to experience.

b) An analysis tool (resembling the online analysis tool)

which utilizes historical data from the plants out station

densitometer can be used. This tool makes it possible to

simulate the tank change points and associate them with

density values. The first point is not essential because the on-

line calculation makes it possible to make the decision. On the

contrary, the density value obtained for the second cut point isindeed important because this is the value at which the tank

change shall occur when the interface is being received.

An Excel file which imports historical data from SCADA

and automatically performs interface calculations is used. Data

importation takes just one second since the GE Proficy

Historian data management system application Historian

Excel Add-In is used. The amount of time available to perform

these calculations will depend on the pipeline flow rate and

volume existing between the out station and internal

densitometers. For example, if the volume is 100 m3 and the

flow rate is 130 m3/h, the amount of time available to make the

simulation is 45 minutes.

Consequently, it is very important that both the plants out

station densitometer and the internal densitometer are well

calibrated and that there are no differences between their

readings when the same product is passing through them, in

order to be able to take the tank change density simulated by

means of data obtained from the out station densitometer as a

valid reference.

When the result of simulating the cut points shows tha

both points cross each other, i.e., the second cut point is befor

the first cut point, then that interface will be a one-cut

interface. In these cases, economic variables will be considered

and the cut will be performed at a point that maximizes th

volume of the product with the highest economic value.

In order to detect the final point of the interface, th

software verifies that the difference between the referenc

value density and the instantaneous value is less than 0.5

kg/m3 in 60 samples (during 2 minutes).

Considerations

The logic for detecting the beginning and the end o

interfaces through the use of reference values, quantity o

samples, number of times that one condition must be verified

etc. are empirical adjustments that must be made for each

particular pipeline (in the same manner as the tuning of a PID

control loop).

On the other hand, the values of maximum concentratio

of B in A and vice versa provided by the laboratory allow for a

safety margin to prevent any tank from being out o

specification (e.g. if flash ponit specification is 45C

Laboratory calculates the value using 46C).

Further uses

Once the interface has finished, a report is printed. Sai

report shows the volume of a product transferred to another

according to the point where the batch was ended, and th

following one was begun, on the flow computer (SybertrolOmni, etc). That is to say, the volume of B that was counted a

A and vice versa. These volumes, which are called transfers

represent the degraded volume of each product.

It is extremely important to record transfers in th

accounting system in order to keep a strict control of th

Custody Transfer at product level.

The programme also provides data about the volume an

composition of the interface that remains in the tank

admission line. This information is useful in order to know

both the quality and the quantity of the product that will b

swept into the tank in the next reception and, consequently, i

should be taken into account by the Laboratory at the momen

of determining the maximum volumes of A and B allowed i

each tank.

In addition, the function of calculating volumetri

shrinkage due to the mixture of both products in the interfac

was added to the software, in accordance with API Manual o

Petroleum Measurement Standards. Chapter 12 -Calculation o

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-Table 2-

Method comparison

By comparing both tables it can be observed that the

interface volume that was segregated to the contaminatedproduct tank was reduced from 93,494 liters in case a) to

44,477 liters in case b), a total of 52 % less of product to be

recovered.

Economic benefit

The use of this work methodology entails numerous

economic benefits that translate into several types of savings:

operating, fixed capital and product degradation. This

methodology helps to:

Reduce contaminated product recovery operations

through subsequent injection in excess-quality tanks

Reduce the transport of contaminated product to the

Refinery or pumping to another terminal tank where it

is more possible to recover it due to the fact that their

tanks are over quality specification.

Reduce operations related to reprocessing at the

Refinery

Reduce the products fixed stock with its associate

financial cost

Reduce contaminated product generation

CONCLUSIONS

Adopting this work methodology makes it possible t

minimize contaminated product generation, obtaining a

economic benefit mainly due to the savings in operating cost

related to its recovery.

ACKNOWLEDGMENTS

Special thanks to the Instrumentation and Automati

Control team from the Terminal and Pipeline Department o

Repsol YPF (Adrin Zoppi, Marcelo Leissa, Marcelo Bertrand

Gustavo Diogo and Germn Markowski) that worked in th

programming of the change detection logic and itimplementation in each plants iFix systems.

We also want to thank Maximiliano Gonella, Metrology

Coordinator, for his participation.

REFERENCES

API Manual of Petroleum Measurement Standards

Chapter 12 -Calculation of Petroleum Quantities- Section 3

Volumetric Shrinkage Resulting From Blending Ligh

Hydrocarbons With Crude Oils-

API Manual of Petroleum Measurement Standards Chapte14.6. Continuous Density Measurement

API Manual of Petroleum Measurement Standards Chapte

5.3-Metering. Section 3-Measurement of Liquid Hydrocarbon

by Turbine Meters

API Manual of Petroleum Measurement Standards Chapte

6.1-Metering Assemblies. Section 1-Lease Automatic Custody

Transfer (LACT) Systems

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ANNEX A

FACILITIES DIAGRAM

PLC

iFIXOperative PC

iHistorianAdministrative PC

SCADAServer

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ANNEX B

OPERATIVE SCREEN

Annex B shows an example of diesel oil gasoline interface.

(1) Maximum volumen of B that can admit A

(2) Maximum volumen of A that can admit B

(3) A and B volumen that was sent to the contaminated product tank

(4) Reference density of A product (pure)

(5) Reference density of B product (pure)

(6) Transfer 1. Net value of the transfer that were made within A and B (B to A and A to B). It is registred to a better control of

Custody Transfer

(7) Transfer 2. It is the contaminated product composition. It is registred to a better control of Custody Transfer

(8) Volumetric shrinkage resulting from blending A+B

(9) A and B on line percentage

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

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ANNEX C

TUNING SCREEN

Documents

IPC2008-64161