Jan-2013

Loading up liquid

Issues that arise when transporting liquid sulphur away from the production site. It is a fairly common occurrence to see tank trucks or rail cars transporting liquids, but most of the time there is little thought to what is in them.

Joy Baker and Eric Harbaugh
Enersul

Article Summary

Often times it is molten sulphur being transported, which leads to questions regarding who is loading them, where is it going and why.

Large quantities of sulphur are produced around the world as a byproduct of oil and gas production, and the logistics of sulphur play a significant role in its entry to the market. Sulphur has to be transported from the production site to various destinations such as a storage location, rail transload station, solid sulphur forming facility or the end user. Because of the cost of forming, special infrastructure required for solid bulk handling and the cost of remelting, sulphur is commonly maintained in a molten state for truck, rail and (in some cases) water borne transport.

Distance, quantity and available transportation infrastructure dictate which mode of molten transportation is chosen. Low volume transport over short distances can be done via truck loading into specialty tank trailers. Rail is often preferred when larger quantities of molten sulphur need to be transported. Barge and vessel transport are also used for significant molten sulphur movement in various places around the world. This article will focus on rail and truck transport.

Considerations
A large variety of liquids are loaded in tank trucks and rail cars, ranging from asphalt and caustic to corn syrup and milk. However, there are special considerations for loading molten sulphur, beyond most other liquid loading. One considerable factor is that sulphur’s physical properties change significantly in a relatively small temperature range. Consequently, successful handling of sulphur in the molten state depends upon maintaining its temperature within a range of 260–310°F. As the temperature rises above 320°F, the viscosity increases abruptly, making the sulphur immobile. On the reverse side, sulphur freezes at ambient temperature and any solidification of sulphur in liquid handling equipment will result in damage to equipment and/or downtime to clear the resulting pipe blockage. Sulphur is a poor conductor of heat and once solidified, it is very difficult to melt.

Therefore, piping systems and loading arms are steam jacketed or heat traced to keep the sulphur molten during the loading process.

In steam jacketed pipelines, the molten sulphur is carried in an internal pipe surrounded by a larger pipe. The space between the pipes is filled with saturated steam (or occasionally thermal oil), which keeps the molten sulphur in the optimal temperature range. Heat tracing is either an electrical heating element or steam tubing run along the length of a pipe. Often, the construction cost of jacketed pipe is greater than heat tracing but it is typically the preferred choice since it is more effective and most loading sites have steam readily available.

Another concern for handling molten sulphur is the emission of hydrogen sulphide gas during loading of trucks or rail cars. Nearly all molten sulphur contains some level of H2S, and the agitation during loading will result in the release of this hazardous gas. During the loading process, proper venting of H2S to the atmosphere through a stack or to a scrubber or burner is essential for the safety of personnel. In addition, it is recommended that loading personnel wear a self contained breathing apparatus (SCBA) and do not rely strictly on mechanical means for their safety.

Liquid handling equipment
Because of the large volume of sulphur and other liquids loaded and transported safely to different parts of the world, a specialised market exists for liquid handling equipment that ensures safe and successful filling of the liquids.

Loading arms
The most efficient and cost effective method of loading sulphur in trucks or rail cars is by top loading arms. These top loading arms come in different styles and with various unique features. Factors in determining the appropriate loading system include the horizontal range to reach the compartments without respotting the vehicle and sufficient vertical movement for loading vehicles of varying heights.

The most basic top loading arm is the fixed reach arm, which is primarily used to load rail cars since they can typically be spotted accurately and loaded from a fixed distance.

Although its simplistic design is cost effective, it does not allow flexibility for mis-spotted cars or for loading multiple cars if they are not the standard length for which the facility was designed.

For loading applications where the spotting the vehicle is variable, a swivel joint loading arm should be considered. This arm is designed specifically for top loading applications where the secondary arm rotates in the horizontal plane, providing a spotting allowance that is not available in other loading arm designs. Although the swivel joint loading arm provides more versatility, it comes with an increased initial investment cost and additional maintenance consideration due to the wear and potential leakage on the swivel components. Another option is a telescopic loading arm, which incorporates a slide sleeve assembly that telescopes in and out to adjust for variations in truck heights. Its design lends itself to higher fabrication costs over the basis fixed reach arm, but has less maintenance concern than the swivel joint loading arm.

The application of steam jackets or heat trace to loading arms is critical in order to diminish the risk of sulphur solidifying within the arm. In any location, a minimum of steam or electric tracing is essential. Full steam or hot oil jacketing is recommended, especially for colder climates. Another consideration for loading arms is the materials of construction. For arms that are not jacketed or traced, aluminium is the optimal choice since it is corrosion resistant and relatively light. Jacketed arms are often fabricated from carbon steel, although 316L stainless steel is the ideal material for handling sulphur to minimise corrosion concerns.

Hatch interface
There are several options regarding the interface between the loading arm and the hatch of vehicles. Although a hatch interface is discretionary, especially if loading non-hazardous liquids, it is highly recommended. The advantages to a hatch interface are that it can be used to avoid excessive splashing, help control emissions from the tank as it is being filled and prevent foreign objects from entering the tank while loading. The hatch interface can be as simple as a plate welded to the loading arm or a more advanced design of a tapered seal.