Above Ground Storage Tank Purge Technology.

Explanation of the need for purging

• History
• Combustion Fire Triangle
• Rules and Regulations
• Purge Control Lay out and operations
• Vapors and gases

History:

Developed in the Mid 90’s Above Ground Storage Tank (AGST’s) inspections have developed by leaps and bounces.
The need to limit, or remove, individuals from having to enter the inside of the tanks pushed the development of technologies and automations to achieve this goal.
Important factors in these developments are: Security and safety, but also, and importantly, time and cost savings.

Putting personnel in the tank also requires for the tank to be emptied, whereby the product is pumped into another tank, the tank cleaned and inspected to make the entry safe, secure and practical.

OSHA lists confined space entry as one of the most hazardous industrial activities.

By putting a robot in the tank, the tests can be securely performed while the tank is in process if a number of security steps are put in place.
Depending of the product in the tank, a specific gaseous vapor will be emitted from the product.
This vapor will exist in the area above the liquid and is called the vapor space.

Flammable and combustible liquids do not, by themselves, burn.
It is the vapors given off by the liquid that form an ignitable mixture with air.

In the US, and Internationally, Above Ground storage markets require efficiency and safety; hence automations and the need for (our) robotics.
Not only do the robotics perform the tasks at hand safely, whilst the tanks are in process, it performs those tasks faster with better, more detailed results.

Combustion / Fire Triangle

The fire triangle or combustion triangle is a simple model for understanding the necessary ingredients for most fires
The triangle illustrates the three elements an explosion, or fire, needs to ignite:
One: an Ignition Source,
Two: Fuel
Three: An oxidizing agent (usually Oxygen).

An explosion or fire occurs when the elements are present and combined in the right mixture.

The Ignition source includes sparks, heat, static electricity or other means.
An explosion or fire can be prevented or extinguished by removing any one of the elements in the fire triangle. For example, covering a fire with a fire blanket blocks oxygen and can extinguish a fire.
In large fires where firefighters are called in, decreasing the amount of oxygen is not usually an option because there is no effective way to make that happen in an extended area.
Solution: By removing one of the three parts of these elements, we provide this safe environment for our robot(s) to operate in.

In our case we remove the Oxidizer.
We do this through purging, or removing the Oxidizer from the vapor space
We will go in more detail about this process in a bit

One additional step in this process is to prevent the robot from being powered before being lowered and submerged in the product.
(Some of the) Solutions here are
Floatation- or pressure sensors;  These are intrinsically safe and remotely controlled.
One other way to prevent the robot to be powered while not being submerged is by the use of our media sensors. If a liquid is detected, the robot is powered on.
   NOTE: These same sensors are actually capable of making a distinction between different liquids, assisting in determining what liquids are in the tank.
Other advantages: Some tanks contain multiple products, for instance water and oil. With our media and precision depth/pressure sensors we can now determine the level  (and with this the volume) of each layer.

The robotic vehicle carries a number of peripheral sensors to aid and abate in the effectiveness of its main prospect:
The ultrasonic testing of the steel bottom of the tank, providing information about corrosion, thickness and flaws.

Who is responsible for regulating aboveground storage tanks?

This depends on local rules and regulations.
For instance in the US: Aboveground storage tanks associated with commercial facilities or residential structures with 4 separate units or more are regulated by the State Fire Marshal and/or your local Fire Marshal.

Building codes, installation regulations and procedures for closing tanks involving flammable and/or combustible substances are adopted by the fire marshal.
For most of the applications that we have worked with, internationally and most specifically in the Kingdome of Saudi Arabia, the API 653 is accepted and followed, as dictated by Aramco’s standards.

There are two standards used to inspect Above-ground Storage Tanks (ASTs). The standards are API-653 and SP001 (Steel Tank Institute standard).
API-653 generally applies to larger field erected tanks constructed to the API-650 standard.
(SP001 only applies to tanks less than 50ft tall and less than 30ft in diameter. )

What is API 653
API 653 covers the Tanks inspections of tanks with specific size and application.
The API 653 is available for $ 255 and is in its fifth edition and comprises of 189 pages
This standard covers steel storage tanks built to API 650 and its predecessor API 12C.
It provides minimum requirements for maintaining the integrity of such tanks after they have been placed in service and addresses inspection, repair, alteration, relocation, and reconstruction.

The scope is limited to the tank foundation, bottom, shell, structure, roof, attached appurtenances, and nozzles to the face of the first flange, first threaded joint, or first welding-end connection.
Many of the design, welding, examination, and material requirements of API 650 can be applied in the maintenance inspection, rating, repair, and alteration of in-service tanks.
In the case of apparent conflicts between the requirements of this standard and API 650 or its predecessor API 12C, this standard shall govern for tanks that have been placed in service.

Purged Control System Lay out
                                        Set up and Operations

System Lay- out: (See image above)
Nitrogen is inserted in the vapor space of the tank, replacing Oxygen.
The outflow at the manway is measured using the O2 monitor.
If the levels of the Oxygen in the tanks outflow is measured to be at acceptable levels,
(About 2 % , down from about “normal” average levels of about 23 %) the O2 monitor goes to safe mode.

3 LEL sensors (LEL explained in next paragraph) are located around the tank and measure the gas levels.
These levels are compared in the Gas Monitor and, when within safe levels, those will also go to safe mode.

Note: The Gas Monitor compares (and displays) the measured values with calibrated values.
The Gas Monitor relays these modes onto the Main Controller.
Two Pressure sensors are located at the Manway and at the Umbilical, measuring the pressure levels.
Here too, when in safe levels, those will also go to safe mode.
The pressure ratings measured at the Umbilical Pressure Switch are (supposed to be) similar to the pressure ratings at the Manway, measured by the Manway Pressure Switch.

The Main Controller now allows for the Generator Control to be switched “On” when all sensors are in safe mode. Any “unsafe” sensors values will automatically disconnect the systems power.

Notes: In this system, two emergency over ride buttons are includes; one located on the Emergency Generator Emergency Control (GEC) Case and one on the Main Controller.

On the bottom of the Main Controller are 2 Uninterruptable Power Supplies (UPS).
As soon as the power to the system is cut off, the UPS will automatically switch on and takes over the power supply to the Gas and Oxygen Monitors.
This to make sure that the area remains under observation by the system.

Purged Control Set up

A. Manway is prepared for removal of the hatch Personnel removing the hatch do so upwind of the manway Respiratory protection is worn by all personnel when appropriate, and are equipped with personal 4-gas detectors
B. Once permanent hatch is removed, a flat temporary hatch, outfitted with a sealing gasket, is placed over the opening
          1. This temp hatch is outfitted with a flange to allow the Nitrogen tubing to be inserted
          2. It is also provided with tubing to connect the Manway Pressure Switch to.
          3. A second flange allows for the Umbilical to be guided through.
          4. A Third flange holds a tubing that is connected to the Oxygen sensor.

The Oxygen sensor will measure the outflow and monitor the O2 levels to be diluted to under 5 %.
Since the hatch will be opened to allow for the placing of the robot in the tank, the O2 levels will be lowered till about 2 % and continuously monitored by the Oxygen monitor.
The Robot will not be powered on during this process.

C.   The Nitrogen gas tubing is inserted into the tank and pushed to the opposite end of the vapor space.
       The Vapor Space is purged with Nitrogen, supplied from a Nitrogen generator or through the use of
       Nitrogen Cylinders.
D.   Robot and deployment tripod with sealing gasket are lowered just above the flat temporary hatch.
E.   Flat hatch is removed, and the deployment tripod is immediately lowered over the opening and secured to the manway.
F.   Tripod hatch is opened, and the robot is lowered into the tank via a manual winch
G.   Once the robot is completely below the manway, the hatch is closed 

Vapors and Gasses Flammable and combustible liquids do not, by themselves, burn.

It is the vapors given off by the liquid that form an ignitable mixture with air.

The flash point is the lowest temperature at which the liquid gives off enough vapor to produce an ignitable mixture with air.

The flashpoint of gasoline, for example, is about –40OC; the exact flash point varies with the grade of gasoline. This means that at temperatures as cold as –40OC, gasoline can still evaporate quickly enough that its vapors can create an ignitable atmosphere.

Material Safety Data Sheets (MSDSs) provide information such as a product’s flashpoint and any precautionary measures that should be taken when handling the material.
It is important to follow the advice provided in the MSDSs when using or working near a product.

Inert gasses •

An inert gas is a “Noble” gas (Group 18 elements)* that does not undergo chemical reactions under a set of given conditions.
The noble gases often do not react with many substances and were historically referred to as the inert gases. Inert gases are used generally to avoid unwanted chemical reactions degrading a sample.
These undesirable chemical reactions are often oxidation and hydrolysis reactions with the oxygen and moisture in air.
Purified argon and nitrogen gases are most commonly used as inert gases due to their high natural abundance (78.3% N2, 1% Ar in air) and low relative cost.
* The group 18 elements include helium, neon, argon, krypton, xenon and radon.

Nitrogen supply.
Nitrogen can be supplied by either using Industrial supply (Pressurized cylinders) Or generated at the site, through the use of Nitrogen generators.

LEL basics and levels • General: Before performing work involving a vessel, tank or piping system that contained or may contain hydrocarbons or other hazardous materials, the interior must be tested to determine if a flammable environment is present.
This testing must be done before work begins and is required at regular intervals or continuously while conducting work.
The most common unit of measuring the explosiveness is the percentage of the flammable mixture’s Lower Explosive Limit (LEL).
• The LEL is the minimum concentrations of a substance that, when mixed with air, may ignite. If there is too little fuel, the air/fuel mixture is considered too “lean” and will not burn.

The upper explosive limit (UEL) is the maximum amount of fuel that when mixed with air, can burn.
If there is too much fuel, the air/fuel mixture is considered too “rich” and will not burn .
The wider the explosive range for a particular substance, the more likely it is to burn or explode when mixed with air.

What's the definition?

• An LEL Monitor is an instrument used to detect hazardous levels of a combustible gas or solvent vapor in air, expressed in % LEL, or Lower Explosive Limit.
An LEL Monitor is also referred to as an LEL Gas Detection System, LEL Gas Detector or simply a fixed gas detection system.
Gas                    LEL Level       UEL Level
Acetone            2.6                          13
Acetylene         2.5                       100
Benzene            1.2
Gasoline            1.3
Methane           4.4
Hydrogen         4.0
Propane            1.7
Turpentine      0.8
Naphthalene  0.9
Octane               1.0
Crude Oil           1.1
Butane               1.9