BOTTOM loading has significantly benefited the petroleum industry and has been good for the environment. However, all of the benefits can be lost if equipment at the loading rack is not properly specified and maintained.
That was the message delivered by two speakers during the Independent Liquid Terminals Association (ILTA) annual meeting, which was held June 14-17 in Houston, Texas. The speakers were Jerry Garteiser, technical specialist for Exxon Company USA, and Peter J Farrow, Heat & Process Equipment Company.
"Bottom loading has been a real boon for us," Garteiser said. "It is safer than top loading, and it has increased terminal efficiency. When problems occur, they can often be traced to the terminal piping.
"Thermal expansion must be factored into loading rack design. Product expands as it warms up, and it needs somewhere to go. The problem is most acute during warm weather.
"Terminal designers must provide pressure relief capabilities at various points in the loading system. This is especially important for piping that is above ground. Set-stop valves must be properly adjusted."
Check Valves Farrow recommended a series of check valves for pressure relief going all the way back to the storage tanks. "You don't need a large pressure-relief valve," he said. "A quarter-inch check valve is adequate."
Pressure relief is needed to protect the piping and hose that makes up the loading arms. Thermal expansion makes the hose stiff and puts a lot of stress on the pipe swivels and other mechanical components. The poppet valve i n the API (American Petroleum Institute) fitting at the end of the loading arm can be sheared by high line pressures, and evidence would suggest that this occurs relatively often. In many cases, the damage is wrongly blamed on the cargo tank that is being loaded.
The API standard calls for a loading pressure of 75 psi, but pressures as high as 500 psi are being observed. High pressures damage seals, and that leads to leaks. O-rings are being destroyed.
"High pressures just chew up the O-rings," Farrow said. "The energy released by the built-up pressure eats away and erodes seals. Ears and roll pins in the API fittings that connect to the cargo tank are worn down to the point that leaks result."
Garteiser said the pressure damage seems to be greatest at loading racks that handle heavy distillates. "We don't see as much evidence of leaks at gasoline racks," he said.
High line pressures aren't the only problems that damage hardware and lead to leaks at the loading rack. Control valves can become clogged with dirt and grit. Strainers need to be cleaned on a regular schedule. Farrow also recommended a strainer basket with smaller holes to catch a larger quantity of contaminants.