ALTHOUGH bottom loading tank trailers provide a faster loading in a safer vapor-rich environment, the procedure presents some problems because technology improvements enable pumps to move more product at a faster rate. If preventive equipment does not shut down the loading system in time, an overflow can occur.

The issue was discussed at the Independent Liquid Terminals Association International Operating Conference June 9-10 in Houston, Texas.

Peter Farrow of Allesco and Phil Wetmore of Chevron Texaco discussed overflow prevention methods addressed by the American Petroleum Institute's (API) Recommended Practice 1004, Eighth Edition, published in January 2003. They presented highlights of the major changes, but noted that the document itself should be consulted for precise requirements and recommendations. More information about the publication can be found on the API Web site at api.org/cat.

The RP1004 covers interfaces, such as product, vapor, and overfill prevention, between the fixed facility, which is regulated by the Environmental Protection Agency, and tank trailers, which are regulated by the Department of Transportation, according to the information presented at the meeting.

In the new recommended practice, liquid loss when disconnecting the coupler from the adapter (three consecutive times) has been reduced from an average of 10 cubic centimeters to five cubic centimeters.

API envelope

The API envelope for the positioning of API adapters on tank trailers was raised by six inches. This change will permit adapters to be mounted directly into the side of the tank compartment.

In previous API editions, the minimum requirements for control of liquid loading were a preset loading-island meter with a set-stop valve. The new RP adds the requirement of an independent secondary control system to prevent overflows. This may be accomplished by using a second control valve on each product riser, which stops all product flow on one loading lane. The use of a secondary tank valve and pump shutdown also can be applied. This approach shuts off product to all lanes at the loading rack.

In addition, the loading rack primary shutoff control and secondary shutoff control must be separate independent devices.

Secondary shutoff control is typically accomplished by a thermister or optical high-level sensor in the tank compartment. If a sensor is activated, a signal is sent to close the pre-set control valve to stop product flow. In previous API editions, no allowance was made for failure of the preset control valve to close.

Experience has indicated that pre-set valve failure is not uncommon, and can be caused from valve wear, controlling pilot valve failure, or trash buildup.

Minimum outage volume

Another issue addressed in RP1004 is minimum outage volume. It establishes a quantity of 60 gallons minimum outage considering maximum flow rates at loading racks and typical valve closure times.

Thermister and optical high-level sensors should be positioned so that there is room for an additional 60 gallons in the compartment. Unlike outage requirements for thermal expansion, the 60-gallon minimum outage requires sensors at different depths. To accomplish this, each sensor must be positioned according to the tank's actual capacity chart, while making allowance for tank shell or domelid mounting.

A 750-gallon compartment will require a lower sensor than a 3,000-gallon compartment. (Inspections and adjustments of the sensors can be easily and quickly accomplished during the annual vapor tightness testing.)

Current thermister connectors feature green connectors with two locking pins that mate into two j-slots. This arrangement can cause intermittent electrical contact with worn connectors. The new RP enables future use of thermister connectors with four j-slots/locking pins. The additional pins will provide improved reliability for thermister systems.

Optical connectors must be blue color-coded, and use three j-slots and locking pins.

Loading islands equipped with a secondary overfill control system that will accept both thermister-type and optical-type systems, and will automatically configure itself for proper operation of either type system, are to be equipped with a black color-coded plug on the island, featuring eight electrical contacts and four attachment locking pins.

Data communications

Data communications using the thermister or optical connectors may be conducted through auxiliary pin 9, or a new optional eleventh pin. If optional pin 11 is used, the data shall be in a serial format compatible with known industry standards, or a publicly documented format.

The vapor-recovery system pressure in the trailer shall not exceed a pressure of 27 inches water column when there is a maximum pressure of 18 inches water column at the truck vapor connection to the loading rack. This may require the use of more than one vapor hose when loading multiple compartments simultaneously, especially when the product flow rates are at the higher end of the acceptable range.

The specification for the trailer vapor recovery adapter was changed from MIL-C-27487 to CID-A-A-59326.

Bottom-loading facilities in areas that do not require recovery of truck-vapors displaced during loading shall provide a discharge standpipe or other method to remove vapors from the loading area. Precautions shall be taken to avoid any vapor discharges that could cause human health, fire, or environmental hazards.

Minimum requirements for liquid flow control are: a preset loading-island meter, a valve that provides a positive means of loading a predetermined quantity, and an independent automatic secondary shut-off control system. The maximum volume for shutdown of the secondary shut-off control system is 60 gallons.

Vehicles and loading islands with thermister overfill control systems must use green electrical plugs and sockets, featuring four j-slots for locking pins.

Loading islands with systems that automatically configure for thermister or optical systems shall have black color-coded plugs featuring four j-slots for locking pins.

Bottom loading terminal systems shall be designed for a minimum 75 psi and shall be leak-free at 1.5 times the rated pressure. Adequate pressure relief shall be provided to ensure that the maximum pressure due to thermal expansion does not exceed 75 psi.

The entire vapor recovery system must be designed and operated to remain in compliance with 40 CRF 60 Subpart XX, which requires vapor back pressure, as measured as close as possible to the vapor connection on the trailer, not to exceed 18 inches water column under any loading condition.

A typical configuration for a loading rack arm has certain recommended features: a shear spool or breakaway coupling may be used to minimize damage and product spillage caused by drive aways. This is typically located between the elbow and the API coupler.

Also, if a valve is used to facilitate maintenance of the coupler without draining the entire arm, it shall be rated for a minimum of 75 psi. If a sight glass is used in the loading rack system, it should be pressure rated for a minimum of 75 psi.

To assure that the pressures never exceed 75 psi, the use of a pressure gauge equipped with a maximum indicator is recommended in the loading arm system. Periodic visual inspection of the loading arm system is recommended.