It recognized that prevention provides the most basic level of protection, thus while using both terms "protection" and "prevention", the document emphasizes prevention. The standard"s scope covers minimum overfill and damage prevention practices for aboveground storage tanks in petroleum facilities, including refineries, marketing terminals, bulk plants, and pipeline terminals that receive flammable and combustible liquids. Use of this standard is intended for storage tanks associated with marketing, refining, pipeline and terminals containing Class I or Class II petroleum liquids. Use is recommended for Class III petroleum liquids. This standard does not apply to: underground storage tanks; aboveground tanks of US gallons liters or less; aboveground tanks which comply with PEI ; pressure vessels; tanks storing LPG and LNG; tanks at service stations; tanks filled exclusively from wheeled vehicles i. This standard recommends application of PEI Recommended Practice Recommended Practices for Overfill Prevention for Shop-Fabricated Aboveground Tanks for overfill protection where applicable for aboveground tanks falling outside the scope of this document.

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The loss of product is only a minor consequence of spilling petroleum liquids from a tank compared to potential outcomes such as lawsuits, fines, damage to reputation, fires, injury to personnel, a vapour cloud explosion VCE and possible facility closure.

Figure 1 shows the destruction of the Buncefield terminal VCE caused by overfilling a petrol tank. Filling a storage tank seems uncomplicated but repeating the process thousands of time, flawlessly, for hundreds of tanks over decades requires robust procedures, training, equipment, a good management of change MOC process as well as the right corporate culture.

The new 4th and 5th editions of API radically change that way the tank receipts are supposed to be handled from a best practices point of view and from lessons of the past. The 4th edition of the standard was published in May of , and the 5th edition is currently in the committee development and is expected to be issued this year or the next. The differences between the 4th and 5th editions of API are slight and are expected to clarify some problems of interpretation as well as to make the document more user-friendly.

Frankly, in terms of procedural or technical issues, not much will be changed between the 4th and 5th editions. This was because significant losses were occurring too often, caused by fires. Later, environmental protection was applied to overfill prevention practices in the 3rd edition of API published in January Note, however, that the principles can be applied to other tanks not covered by the standard and this is encouraged where appropriate.

The application of a maintenance programme must include training and written procedures, specific rules for communication between parties involved with the tank filling operations, measures that address normal and abnormal conditions, and initiation and maintenance of tank parameters i. Categories serve as a starting point for understanding system risk and for categorising the numerous tanks that a company has into manageable groups: A Category I system does not have transmittable level or alarms and is entirely dependent on the operator and manual gauging to prevent an overfill.

While this type of operation has been around a long time and can be highly reliable, it is limited to a low frequency and low rates of transfer that an operator is able to cope with. The Category II configuration uses level and alarm data, which can be transmitted to a control room.

Thus, the operator can deal with many more receipts at a high frequency when tank level and alarm data are transmitted to a control room. The Category II configuration dominated the large petroleum facilities up to and including the s.

The weakness of the Category II system was that it had no redundancy in terms of reliability. If the level alarm failed, then most likely there would be an overfill. Category III systems are the same as Category II, but they are much more reliable because of the redundancy in the alarm system. The failure of the high-high cannot be caused by any failure of the automatic level gauge or by the high alarm.

Most overfill prevention systems use the operator to control the receipt and terminate it before an overfill occurs. In the jargon of IEC for safety instrumented systems, this control is referred to as the Basic Process Control System BPCS and forms the most basic control over the process, whether it is an operator or some combination of operator and instrumentation.

A safety system can be applied to a hazardous process such as filling a tank, but it should be independent of the BPCS and no initiating event in the BPCS should affect the safety system. AOPS provides redundancy to the filling process, reducing the risk of an overfill event. AOPS can provide additional redundancy, but it can present many practical issues with implementation.

For example, an incorrectly designed AOPS applied to a pipeline could cause a pipeline rupture due to hydraulic shock. The use of safety instrumented systems and function in typical oil distribution terminals is often beyond the typical capabilities of the people and companies associated with designing and implementing these systems. Also, AOPS costs are high. The expense is not because of equipment or installation costs, but because a whole new process that applies to the entire lifecycle of the equipment including documentation and other management system elements such as testing and auditing functions.

Fortunately, AOPS is only one way to reduce risks. It may be the best solution in many cases, but certainly not all or even most.

There are many considerations when deciding to implement an AOPS and alternatives must be considered.



Answer: Absolutely. API RP does not reference the location or the type of site, but it does define which storage tanks the standards apply to, including the size of the tank, how you fill the tank and how much the tank can hold. Question: Are any compact probes or sensors available that can sense the presence of a very small amount of fluid i. Answer: If you have a very small amount of fluid in a pipe or on top of a roof, an ultrasonic gap switch can detect it — and may work more efficiently than a mechanical device.


API RP 2350


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