Response Time To Tank Level Alarms

Editor's note: This is our fourth post on API 2350, which covers best practices for preventing tank overfills in petroleum facilities. The first post introduces the Buncefield Incident as a major impetus for the creation of API 2350. You can find the second post on mandatory management and operations practices for preventing tank overfills. The third entry, on tank operation and alarm automation categories, can be found here. Our final post on API 2350 covers Levels Of Concern within a petroleum storage tank and the necessary alarms at each one.

Now that we have a better understanding of the tank categories, I would like to focus on response times. Click here if you would like to review the tank categories before I move on, as you will need to know these for this discussion.

The response time is the time you have to stop the tank from filling before the tank overfills after the high high level alarm initiates.

To understand this better, let’s discuss the high high alarm. The high high alarm is defined by the standard as a level of concern (LOC). Level of concerns (LOCs) are theoretical liquid level positions that are important to the operation of a tank. API 2350 requires you to define the following LOCs and equip them with the appropriate alarms.

• Critical high (CH): This is the level point where an overflow occurs. Obviously, an alarm is not required for this level since it would be too late to take action if activated.
• High high (HH): This is the only level point where an alarm is required by the standard and plays a key role in the response time. The actual point where this needs to be placed depends on the tank category.
• Maximum working level (MW): This level point should be your highest level point under normal conditions. It is your target for a full tank.

So, given the above information, the response time can be described as the time it takes the level to rise from the high high LOC and the critical high LOC.

To pin-point where your high high alarm should be located you will need to have a good understanding of the rate of the rising liquid. You will want to have this documented for each tank.

API 2350 allows you to determine your own response times but will require extensive documentation. Initially you may decide to conform to the minimum default response times as outlined in the standard. For these, the response time for the tank is dependent upon the tank’s category.

For category 1 tanks, the minimum response time is 45 minutes. This means that after the HH alarm is initiated by an employee, it will take the rising level 45 minutes before an overflow occurs.

Category 2 tanks are allowed response times of 30 minutes and category 3 tanks allow 15 minutes. Notice that the higher the tank category the higher your HH level alarm may be located. This is because the higher category tanks are equipped with more reliable level gauging equipment.

Again, these response times are the minimum default required by the standard, and are quite long. So to maximize your tank capacity while conforming to API 2350, you will need to provide the appropriate documentation. To dramatically decrease your response time, you may also consider upgrading your tank level measurement technology.

If you have any questions about response times or anything else with tank overfill avoidance, feel free to contact us.