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Introduction

Crude oil blending equipment is designed and selected to ensure minimal pressure drop and

maximum reliability.

When the crude oil blender is started the required flow rate and component ratio is set by thecontrol system based on the ratio in the recipe. A density or viscosity analyser, installed at a

homogeneous point in the blender header, generates a control signal, which is used to continually

optimise the blended product by adjusting the component ratio. This ensures that the blended

product remains as specified at all times during the batch.

Major Components of Blending System :

The basic elements employed in each loop of a good in-line blending system are:

a) A strainer of suitable mesh to prevent debris from damaging the downstream mechanical

components.

b) A flow meter, accurate to 0.25% over a 10 to 1 range. The flow meter type will depend upon

each products characteristic such as viscosity and specific gravity.

c) A control valve (either pneumatically or electrically operated) with valve positioner to control the

rate of flow through the stream and maintain the correct proportions between streams. Control

valve sizing is critical to the correct performance of the system. Alternatively a pump fitted with

speed control is used, this is of particular benefit on small components with a high viscosity that

require a P.D. pump.

d) A non-return valve is fitted after the control valve to prevent back flow through the meters and

subsequent contamination caused by pressure imbalances during start up/shut down.

e) For systems requiring cleansing or purging after each batch a selection of vent and purge valves

are fitted at strategic points to enable the stream to be blown forward to the blend header and

backwards to the supply source.

f) An analyser of some form suitable to measure the quality of the final product is often fitted in the

blend header, on certain products mixing may well be required prior to analysing or final delivery.

Mixers are available in various forms:

Static ie. a spool containing specially designed baffles which mix and mingle the product, these

mixers use energy from the upstream pumping pressure thus will cause a pressure drop and are only

efficient at certain flow rates. Power, two major forms of power mixer are available, a blade type

mixer connected to a motor, and Jet mixers which recycle a small volume of the product through an

external pump and inject the product back into the line through specially designed jets to create

mixing.

g) To maintain good control during temperature variations it is advisable to fit temperature

transmitters to each stream, the signals from these are used by the control system to compensate

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the volume of liquid to standard conditions thus compensating for any effects of expansion or

contraction of the liquid.

Principle of Operation

The principle of operation for a ratio control in-line blender is that the flow of each component (e.g.Heavy Crude Oil and Lighter Crude Oil) is measured by a flow meter and controlled using a control

valve or a positive displacement pump. In operation the blender control system generates a demand

flow rate (called a set-point) for each component, which is compared to the measured flow. Any

deviation between the set point and the measured flow is stored in the blender controller memory

and an appropriate adjustment is made automatically to the control valve/pump to increase or

decrease the respective component flow to the required flow rate. If there is starvation of flow in a

stream, the control valve will open to compensate. However, if it reaches a point at which it can no

longer blend, the system will reduce the total blend rate to the maximum flow rate at which the

lagging component can maintain correct ratio and accuracy to keep the blend on specification

Ratio control in-line blenders operate on either a volumetric or mass ratio. This ratio is maintained

during blending by closed-loop control between the flow meters fitted to every stream, the blender

control system and the control valves or pump control used to adjust the ratio of each stream.

The in-line blender starts to produce the final product defined by a component ratio in the recipe,

but a viscosity analyser mounted at the exit to the blend header is used to continuously optimise the

quality of the final product. Product quality is continually maintained during the batch by the closed

loop control between the flow signals from the field equipment (flow meters, pressure, temperature

sensors and viscometer) and the control signals from the controller back to the field equipment. The

accuracy of the blender is determined by the accuracy of the viscosity measurement and controlsystem. All blenders are designed to minimise heating, noise and other process fluctuations, which

can affect viscometer measurements.

Blended products are volume corrected to standard conditions using API 2540 / IP 200. Loading and

blend documentation can be automatically produced in volume or mass units by the controller.

Viscosity trim blenders can be skid mounted (for installation on a barge, jetty or shore), mounted in

a container offering a portable, safe working environment or self-contained on a trailer allowing one

blender to be used at multiple locations.