Liquid Static Measurement and Tank Gauging

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Liquid Static Measurement and Tank Gauging

Every time the product changes ownership measurement of the

product and custody of the product are transferred to another party.

Quantities typically measured for custody transfer and

monitoring/controlling facilities:

·

Volume flow rate accumulated at transfer points

·

Pressure at custody transfer and pressure

control points (pump stations/peak points)

·

Temperature at custody transfer and temperature

control points ( injection/delivery)

·

Density at the custody transfer and batch

lifting/delivery points

·

Quality at the custody transfer and batch

lifting/delivery points

Measurement Systems are installed for:

·

Custody transfer

·

Inventory control

·

Batch size determination

·

Leak detection and line balancing purposes

Measurement is done statically or dynamically.

Static: involves measuring tank volume and its changes over

time

Dynamic: involves measuring hydrocarbons under flowing

conditions by a meter

Process Control and Internal Product Transfer Measurement:

can use vortex shedding and orifice meters.

Static Measurement:

requires exacting attention to the detail and precision in

the calculation process and the underlying supporting processes.

Supporting processes include:

·

Tank calibration

·

Calibration of temperature and pressure

transmitters

·

Densitometer calibration

·

Tank gauging equipment verification or

calibration

Tank Calibration:

Manual Tank Strapping Method

(MTSM):

·

Tanks must be calibrated after they have been

filled at least one with a liquid of density equal to or greater than that of

the liquid it will hold in the future.

·

Hydrostatic test is applied to all

new tanks

·

In order to determine tank capacities

dimensional measurements and determination of weights are necessary using the

following methods

o

Circumferential measurement: uses steel tape aka

tank strapping

o

Height

o

Deadwood

o

Floating roof – weight of the roof material

o

Tank bottom survey

·

Wall thickness of the circumferential plates and

of the paint used on the tank walls is necessary to be able to achieve the

correct internal volume

Optical Reference Line Method

(ORLM)

·

Uses an optical device but requires manual

strapping of a reference circumferential course.

·

The strapping of the first ring is set 20% below

the top horizontal weld seam and acts as a datum

Optical Triangulation Method (OTM)

·

A laser is used to externally calibrate vertical

cylindrical tanks by measuring angles.

·

No access to the tank roofs is necessary

·

A measured reference is necessary by strapping

at a location near the bottom ring

·

Two stations are present per ring and is

established at 20% of the distance from the upper and lower horizontal weld

seams for each tank ring.

Electro-Optical Distance Ranging Method

(EODRM)

·

Uses internal measurement procedures outline in

API MPMS chapter 2

·

An electro-optical distance-ranging instrument

capable of precision angular measurement and a laser beam emitter are involved.

·

Distances and slope angles are measured at a

predetermined number of target points around the internal circumference of the

tank

·

Placed on the tank floor and stabilized to

prevent movement. It should be located at or near the center of the tank. All

target points are sighted along the horizontal plane at each course location.

Tank

Capacity Table:

They’re

determined from the dimensional measurements obtained through the calibration

procedures.

Recalibration

of tank required establishing revised tank capacity tables

Liquid

Calibration of Tank:

If

the tank shapes are irregular or unknown or inaccessible liquid calibration may

be required this takes place using a positive displacement meter.

Tank

Gauging:

Manual Tank Gauging:

manual

gauge tape must be calibrated and certified to a national standards.

Some

operations don't allow open access to tank contents because of hazardous vapors

or for emission control purposes. In these cases it is desirable to implement

manual closed tank gauging systems.

Servo Tank Gauge:

Automatic

tank gauge (ATG) involves a physical connection of the servo instrumentation to

a level sensor or float or a physical connection to a floating roof.

Servo

tank gauge can provide a product level indication as well as product density,

can be used to determine Net Standard Volume of the tank contents (NSV)

When

using Servo tank gauge on a floating roof tank, a gauging platform is required

in order to mount the gage over the product. (Tank roof for IFR or actual

platform)

Stilling

Well: provides stability and calm to the product surface allowing for more

accurate measurements.

Accurate

temperature information is accomplished by the installation of multiple

resistance temperature detector RTD elements each of a different length in a

prove that traverses the entire height of the tank.

Microprocessors

determine the RTDs and average their temperature.

Radar Tank Gauge:

Very

reliable and suitable for custody transfer purposes in tanks with open

visibility to the liquid.

Non

contact device

Needs

to be installed with instruments of similar accuracy in the measurement of

product temperature and density. Stilling well is recommended for radar level

measurements.

Hybrid Tank Measurement System (HTMS):

Combines

accurate level gauge instrumentation with temperature sensors and pressure

transmitters.

Uses

level-based and mass-based systems it can obtain tank level, temperature, mass

and density.

Components

of Hybrid Tank Measurement System:

·

An automatic tank gauge

·

Tank thermometer system

·

Tank bottom pressure sensor and transmitter

·

A processor that provides an accurate conversion

of pressure to density

These

four components calculate NSV.

Calculation

of Tankage Volumes:

NSV

is the primary unit of measurement for custody transfer and product inventory

control. It is the equivalent volume of a liquid at its base temperature and

pressure conditions. Doesn't include non-merchantable items

To

obtain NSV you must:

Obtain TOV; total observed Volume,

determined by measuring the liquid height of the tank. You reference the liquid

height in the tank to the calculated volume from the tank capacity table.

GSV:

Gross standard volume, the total volume of petroleum liquids and S&W

TCV:

Total Calcualted volume

TOV:

total observed volume