Terminal Operators Face Loading Rack Changes
Sep 1, 2000 12:00 PM, MODERN BULK TRANSPORTER STAFF
Several equipment proposals are being considered that, if eventually implemented, would affect tank truck carriers, and terminaling and storage facilities. At the same time, federal rules already in effect have resulted in new components designed to meet the requirements.
The American Petroleum Institute (API) has formed a task force to revise Recommended Practice (RP) 1004, under which bottom-loading dry disconnect equipment is standardized. Depending on what the final revision entails, terminaling and storage facilities may have to make adjustments in loading racks.
Al Mosser of Chevron and Peter Farrow of Heat & Process Equipment Company discussed the possibilities June 12 at the International Liquid Terminals Association annual meeting in Houston, Texas.
Some gasoline tank trailer manufacturers have increased the size of the vapor vent valve, piping, and other components in the vapor path from the compartment to the vapor connection, said Farrow. This creates a higher vapor pressure at the truck vapor line connector and requires appropriate steps to minimize the system back pressure at the rack vapor hose and loading arm.
Many terminals and/or local authorities require a check valve at the truck end of the vapor hose to prevent operator exposure and environmental discharges. Many of the older vapor fittings have small internal vapor passages and can introduce pressure drops of five to 10 inches water column. Recent vapor coupler designs have reduced this drop and will be required in many terminals. Similarly, it is difficult to keep within the 18 inches water column pressure limit using a three-diameter-inch vapor hose unless loading rates or number of compartments being loaded simultaneously are minimal.
Even with a check valve at the truck end of a vapor hose, a second vapor check valve is recommended at the rack to protect the area from vapor hazards in the event a truck is driven off with the vapor line connected, and the hose or arm breaks.
Another change affecting terminals applies to the prospect of eliminating product from lines underneath petroleum trailers. Although there has been no official government action on this issue, the industry is taking a hard look at the issue.
If connectors on the trailer are raised about six inches because of eventual wet line requirements, as has been suggested, loading arms may have to be adjusted accordingly, said Farrow.
Both Farrow and Mosser pointed out that loading arms require careful placement and routine maintenance to perform safely. Some minor changes in valving and piping can greatly reduce product leaks and spills. Equipment retrofitting would be quite easy to implement when designing a new facility, and the incorporation would greatly reduce downtime and potential product spills, they said.
Mosser, long a proponent for adequate overfill prevention devices, noted that overfills can be significantly lessened when tank truck probes are installed at proper depths in the compartments.
The current version of RP 1004 states that the secondary control consists of a level sensor that signals high level and activates a control valve to shut off flow. In most terminals, the secondary shutoff control system signals the same control valve that is used to control the load.
In the event that the compartment overfill is caused by failure of this control valve to fully close, giving it a second closure signal will not prevent a spill. Only if the closure signal is given to a second or backup valve will the product flow be stopped.
The proposed revision to RP 1004 specifically requires that detection of an overfill shuts down the loading (control) valve and an independent shutoff valve. The independent shutoff valve may be a second valve installed in the same riser piping as the loading (control valve), or some other existing valve at the storage tank (Fig. 9). This arrangement insures that the secondary overfill device has the redundancy necessary to reduce overfills.
Installing a second (independent) shutoff valve at each loading arm limits any shut down due to an overfill signal to the loading bay with the problem truck. Closing an existing valve (Fig. 10) and the associated pump in the tank farm results in shutting down loading to all bays at once with the accompanying inconvenience to other trucks. In practice, this secondary shutoff valve should close in four to six seconds in order to prevent an overfill while not causing a large shock wave in the terminal piping, said Mosser.
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