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Storage/Loading/Unloading - Air Unloading
April 2, 2007


The following procedure is applicable to air unloading of tank cars or tank trucks. 

The procedure is general in nature and must be adapted to the specifics of each unloading facility and to the operators own procedures and rules.


Attendance by a qualified person during the entire unloading procedure should be required.  This person must be thoroughly familiar with the hazards of sulphuric acid and all operating, emergency, safety and first aid procedures recommended for its safe handling.

Safety and Preparation

Water in large quantities should be immediately available at the unloading site to wash down any acid that may be spilled.

A safety shower is recommended along with a signal system designed to get assistance if an emergency should arise.  The signal system and safety showers should be tested routinely. 

Signs restricting access to the area should be used.

The slow attack of sulphuric acid on mild steel results in the formation of hydrogen sulphide gas which is flammable and can form explosive mixtures with air.  Smoking or other sources of ignition should not be permitted near acid tank cars, and cars fittings should never be hammered or chiseled.

Unloading Procedure

Note:   Tank Truck Unloading - In some cases, the truck driver will normally connect the customer's transfer equipment to his the vehicle.  Unloading of the tank truck is the responsibility of the consignee.

Locating the Container

For tank cars, the train or engine crew should accurately spot the car at the unloading line.  The brakes should be set, and the wheels blocked by standard wheel clamps.  Blue warning flags and derails should be placed at least 1 1/2 car-lengths away.

For tank trucks the driver should position the truck accurately on the unloading line.  The vehicle should be switched off, the brakes set, and the wheels blocked.

Verification of Tank Contents

Verify the contents of the container by checking the shipping papers and container stenciling.  Make sure the receiving storage tank has sufficient outage to hold the contents of the container being unloaded.

Inspect Shipping Container

Inspect the shipping container for leaks or damage which could interfere with safe unloading.


Shipping containers as received will frequently be under pressure either because of thermal expansion of the acid or slow hydrogen generation.

Carefully open the air connection valve and vent any pressure in the container.  If there is no air connection, the safety vent should be removed.

If no venting occurs, check to make sure the air connection and valve are unobstructed.  Carefully loosen the fill hole cover bolt (do not disengage), and pry up slightly.  If any pressure is present, let it bleed off slowly until no pressure is in the tank, then open the fill hole cover.

            Caution:      All pressure must be vented before any container fittings are removed because of the danger of acid spray.  Do not remove the cap from the liquid eduction pipe until it is definitely known that all pressure has been vented from the container, and that the valve is closed (if so equipped).


Samples of the acid should be taken after the container has been depressurized, if required.  The locking hinge which holds the filling hole cover in position should then be loosened and the cover swung back.  The gas space must then be left to vent to atmosphere for 15 minutes before sampling is done. 

A hydrogen-enriched atmosphere can develop in the dome of containers which have been in transit for long periods in hot weather.  If a sampling device is lowered through the filling hole immediately on opening the cover, there is a risk that a stray spark could trigger an explosion.  The risk is eliminated by allowing the gas to escape through the opened cover for a few minutes. 

After sampling, the filling hole cover must be closed and the retaining clamp tightened to reseal the cover.

Acid Connections

Only after the container has been depressurized, then the acid connection can be made.  Close the liquid eduction valve (if so equipped), and carefully remove the cap from the valve, or from the liquid eduction pipe (if no valve is provided).  Inspect the condition of the threads, gaskets and liquid eduction pipe carefully.   Pipe threads in sulphuric acid service deteriorate rapidly, especially when exposed to acid and atmospheric moisture.  Connect the acid line to the eduction pipe.

Air Connections

The air supply must be non-contaminated, free from oil, moisture and foreign matter.  A suitable inert gas may be substituted for the unloading air if desired.

            Caution:      The air supply should never be connected before the acid unloading is connected.

The air supply must have a shut-off valve, a check valve, a pressure reducing valve (set at 172-193 kPag (25-28 psig)), a pressure gauge, a safety relief valve (set at 207 kPag (30 psig)), and a bleed off connection and valve.  Connect the air supply to the rail car.

Storage Tank

Ensure that the storage tank is ready to receive the acid.  The tank vent should be checked to ensure that it is open.

Final Check

Close the fill hole cover and inspect the system to be sure it is tight, and that all vent and bleed valves on the unloading line are closed, and all valves properly positioned to receive acid.


Slowly apply air pressure to the container by opening the isolation valve on the air supply line.  Care should be taken not to exceed 193 kPag (28 psig) air pressure.  If the air pressure is too high, the safety relief valve on the air supply will open.  If pressure cannot be maintained, check for leaks or a broken bursting disc on the shipping container.


Unloading of the acid in the container to the storage tank should now be underway. 

If any leaks should develop in the acid line, the air should be turned off and the air pressure released from the car before attempting to correct the leaks.  If it is necessary to break connections on the unloading line, extreme care should be taken in draining acid from the line, particularly from low spots.

If leaks develop or acid is spilled, promptly wash down any surface where acid has spilled with an excess of water being extremely careful to avoid getting water into the container, piping or into large accumulations of acid in holes or trenches.

Termination of Unloading

When the container is empty, a drop in the pressure and the sound of air rushing through the discharge pipe will be heard.  Shut off the air supply, then shut off the valve in the unloading line near the connection point.  Allowing some air to flow through the unloading line will help to clear the acid from the line.


Vent all pressure from the shipping container through the air supply line bleed valve.  Carefully open the fill hole cover and inspect the container to insure unloading is complete.  Disconnect the air supply line.

Disconnect Loading Line

When the unloading line has been completely drained, carefully disconnect the line from highest point first.  If the unloading line is not equipped with a drain and vent valve, acid should be expected at the disconnect points.

Sealing of Container

Close the fill hole cover tightly and replace the caps on the air connection valve and liquid eduction line.  Replace the valve covers.  Inspect the container and place proper DOT required placards on the container for the return shipment.