Emerson Exchange 365

Selecting the Correct Overfill Protection Technology

One of the most popular session types at the Emerson Exchange is the Meet the Experts session where a panel of Emerson Experts provide short presentations followed by “no-holds barred” question from the session attendees.

This session, Selecting The Correct Overfill Protection Technology, features Emerson’s Teddy Tzegazeab, Christoffer Widahl, Paul Fadell, Carsten Thøgersen, Tadeu Bastista and Per Skogberg. Here is the abstract for this Meet the Experts session:

It is essential for companies to implement overfill protection systems to minimize risk and improve safety. As part of a complete overfill protection system, automation technology is increasingly being deployed to replace mechanical devices and human factors, but it is important to understand that there is no ‘one size fits all’ solution. Overfill prevention experts will discuss the different layers of protection required and outline the challenges posed by the most common applications and the technologies available - especially level sensors. They will also provide recommendations for the most appropriate solution for each application.

Tank Overfill Protection Expert Panel

Emerson’s Lydia Miller hosted the panel and opened with a poll question to gauge the focus on incorporating overfill prevention. It was rated as very important by nearly 3 quarters of the attendees. Per discussed the repercussions of an overfill condition. An overfill of a large tank means loss of product, cost of cleanup, and in extreme cases, regulatory fines, and accidents leading to injury and damage to the tanks and other assets. These situations also damage the company’s reputation and trust in the communities in which they operate.

Tadeu described the layers of protection for tanks. Basic process forms a layer with safety instrumented functions providing another layer. Each must be independent from one another to satisfy the requirements of the IEC 61511 global safety standard. The American Petroleum Institute (API) also defines the API 2350 standard for overfill protection for tanks in petroleum-based facilities.

Per noted that for some facilities, they run a manual operation where people are in the safety loop. The recognize the condition and take actions to avoid and overfill condition. This practice runs the risk of operator error and incorrect actions being taken. Carsten described and automatic overfill prevention system (AOPS) that is full automated. It is a safety instrumented function (SIF) which includes a sensor, final control element and logic solver. The components in this SIF must provide the risk reduction required for the overfill condition. The risk is a function of the content of the tank and layers of protection as identified in the HAZOP analysis.

Lydia asked a poll question about what type of level measurement devices are currently being used in overfill protection. The leading responses were radar, vibrating forks, and the Rosemount 2-in-1 radar measurement unit. The first main choice is whether to use a point level (on-off switch) or continuous measurement. Teddy explained that the switches must be tested by going to the top of the tank and testing periodically. The Rosemount 2140 vibrating fork switch provides continuous diagnostic to avoid climbing the tank to test. A continuous measurement provides ongoing information to the operations staff to identify impending issues.

The Rosemount 5900S 2-in-1 non-contacting radar is rated for use in safety applications up to SIL [safety integrity level] 3. Other radars can be applied in SIFs up to SIL 2 levels of risk reduction required. Per explained that 2-in-1 has two completely independent radar electronics to allow it to be applied in a single tank opening instead of the traditional means to use 2 or 3 separate radar transmitters. Often for these low-pressure tanks, adding more opening on top of the tank is not an option.

Paul explained the use of differential pressure (DP) level measurement technology. Capillaries from the tank pressure taps to the ports on the DP Level transmitter can have delays. The Rosemount 3051S Electronic Remote Sensor uses a pair of 3051S transmitters that can be directly connected to the tanks pressure ports to remove the capillaries and provide faster response to changes in level. DP level technology is quite mature and proven for many applications.

Lydia ask a poll question about what is being used as the logic solver for the overfill protection system. Frequent answers included DeltaV SIS, the basic process control system and PLCs. Tadeu explained that the first consideration is determining if the overfill prevention system requires a SIL rating. If yes, a safety instrumented system logic solver should be included that can provide the level of risk reduction required for the SIL rating.

A question was asked if separate logic solvers were required for each tank’s overfill protection loop. Tadeu explained that a single logic solver, totally separate from the basic process control system, can manage separate overfill protection loops. The DeltaV SIS logic solver is rated for applications up to SIL 3.

The focus then shifted to the final control element in the SIF. Carsten noted that for SIL rated applications, a separate automatic valve should be used. Many sites use manual valves operated by the plant staff. A Fisher 4320 wireless valve position monitor could be used to report the status back to the operators.

On the topic of proof testing, the scope includes the level measurement devices themselves and the full SIF. For some measurement devices, remote proof testing can be done from the logic solver to avoid climbing. This works by comparing the level values from the AOPS system with the regulatory control system levels. By placing a reflector at the high-level alarm point, the SIF can be tested by initiating a switch on the radar gauge from measuring the actual tank level to measure the reflector and verify that this alarm is going back to the operators.

For the final control element, when instrumented with digital valve controllers, partial stroke tests can provide proof test to extend the intervals between full stroke tests. If a high-level occurs and the SIF trips, then this can be counted as a full stroke test, since the action occurred as designed.

If you were in this session, what questions would you have for the panel of experts? Share your thoughts in a comment below!