April 2021

Process Control, Instrumentation and Automation

Take control of tank pressure

Maintaining pressure control in bulk storage tanks is a vital challenge faced by operators of both process plants and storage terminals.

Macedonia, D., Emerson

Maintaining pressure control in bulk storage tanks is a vital challenge faced by operators of both process plants and storage terminals. Although they may not get the same level of attention as other parts of the plant or facility, storage tanks present many inherent risks to personnel, the environment and equipment if not properly maintained.

The tank farm of a refining or chemical complex is a relatively low traffic area and is often maintained by junior operators. The devices maintaining tank pressure are usually located on the top of the tank and only accessible by tall stairs or ladders, scaffolding or catwalks. As a result, these devices are often out-of-sight and out-of-mind, presenting operators with increased risk. Many tanks do not have the capability to signal an abnormality to the crew on shift, so issues may linger unresolved for long periods of time (FIG. 1).

FIG. 1. Hydrocarbon storage tanks are often installed far from a control room, making it difficult to monitor their operation.

According to American Fuel and Petrochemical Manufacturers (AFPM) statistics, 12% of Tier 1 and Tier 2 safety incidents involve storage tanks, with refineries and terminals accounting for 74% of these incidents.1 A large percentage of incidents can be attributed to personnel error or equipment failure, most of which result in fires or explosions.

In addition to equipment damage, downtime and possible personnel injury, these incidents can put the general public at risk. According to the U.S. Chemical Safety and Hazard Investigation Board’s analysis2 of the U.S. Environmental Protection Agency (EPA) Toxics Release Inventory (TRI) Database3, 77% of bulk terminal locations are located within one mile of communities of 300,000 residents or more.

For these and other reasons, it is vital to monitor and control tank pressure in hydrocarbon plants, facilities and storage terminals.

Issues and risks

Operators must ensure each tank operates in its normal pressure band, with the appropriate equipment installed to prevent overpressure or vacuum conditions. An overpressure condition could lead to a tank rupture, and an underpressure condition could lead to an implosion (FIG. 2). Failure to properly control a tank’s pressure could result in significant financial risk due to lost product and/or  catastrophic event, such as a fire or spill.

FIG. 2. Several layers of protection are recommended to protect tanks from overpressure and underpressure conditions.

Tank farm operators commonly have metrics in the following areas, each of which are addressed by proper tank pressure control (FIG. 3):

FIG. 3. Poor control of tank pressure creates multiple risks. Sources: NYMEX spot price, July 2020; U.S. EPA and the U.S. Chemical Safety and Hazard Investigation Board.
  • Product loss—Even with depressed oil prices, the approximate value of crude or naphtha can be worth several million dollars for an 80,000-bbl tank. Contamination or spillage of this product would negatively impact a terminal’s financial results.
  • Emissions—From an environmental perspective, tank farm operators need to minimize emissions and corresponding risks to the general public. The U.S. EPA levied more than $470 MM in penalties related to pollution in 2019, and that amount will only increase over time.3
  • Personnel safety—Tanks present occupational challenges to operators. Inspections and maintenance of devices at the top of the tank present fall hazards for personnel. In addition, personnel may be exposed to vapors vented from the tank.

Multi-level protection

Tanks are typically blanketed (or padded) at a slightly positive pressure with nitrogen. Blanketing maintains the purity of the product from contaminants, isolates the tank from atmospheric air and moisture, and maintains oxygen levels low enough to prevent ignition.

Tank blanketing and vapor recovery (de-pad) regulators, pressure/vacuum relief valves (PVRVs) and emergency relief vents should be installed on tanks and used together at staggered setpoints (FIG. 4). This will ensure protection from conditions above or below the normal pressure range, either of which could compromise the tank’s contents or its physical integrity.

FIG. 4. Full array of devices used together to control pressure of fixed-roof storage tanks.

The environment inside a tank can be quite complex, which is not necessarily apparent from the outside. Fixed-roof tanks at a process facility or storage terminal can be filled with a wide range of liquids—from chemicals to finished products to crude oil—and can contain volatile vapors under potentially caustic or hazardous conditions. Therefore, it is best to think of a tank as an ecosystem and not as a collection of individual components.

During normal operation, the vapor space inside a tank may expand or contract due to pumping fluids in or out of the tank, or as ambient temperature changes. These tanks are typically blanketed (or padded) at a slightly positive pressure with an inert gas. Conversely, the tank may also be equipped with de-pad or vapor recovery devices to relieve tank pressure when it reaches the upper end of the operating band.

It is completely normal for tanks to ‘breathe’ during normal operation, whereby blanketing gas is drawn into the tank and vented out of the vapor space. During breathing and other conditions, the tank blanketing system maintains the tank’s pressure within a desired control band to ensure tank integrity and the quality of its contents. Operation of emergency vents should never be considered a normal event since these devices are the tank’s last line of defense.

Maintenance of tank pressure is accomplished by regulators, with selection of the correct type a key factor.

Pilot-operated advantages and design recommendations

Pilot-operated regulators are ideal devices for tank blanketing applications. Accuracy is paramount due to the low pressures needed in these applications, and fast speed of response is required to account for pump cycling and changes in temperature. These devices have much tighter control and cycle less frequently than control valves used for the same purpose, making them the right choice in these types of applications.

One of the most important design features of pilot-operated regulators is pressure amplification by the pilot, called ‘gain.’ This feature amplifies a small change in outlet pressure to a much larger change in the loading pressure of the regulator, which controls its operation. This improved accuracy at low setpoints results in a bonus of using less nitrogen or blanketing gas, making the tank less expensive to operate (FIG. 5).

FIG. 5. Tank blanketing regulators sense tank pressure and provide inert blanketing gas at a low setpoint to minimize the amount of gas consumed.

Purge meters are also recommended in tank blanketing systems where the process media is corrosive, volatile or can solidify in the lines. Purging maintains a low flow of blanketing gas through the main and sensing lines to isolate the regulator from the downstream process fluid, enhancing the service life of the materials used in the unit and preventing foreign material from the tank backing up into the lines.

Reactive vs. predictive maintenance

To ensure each tank is operating per design, preventative maintenance and inspections should be performed for tank blanketing regulators and other tank top components at regular intervals. Maintenance spend is a large percentage of operating costs in any process facility or terminal, but it can be minimized with the right monitoring and control devices and systems.

The goals of any maintenance program are to maintain equipment without excessive cost, while maximizing uptime. Corrective or reactive maintenance costs are often substantially higher than preventative maintenance costs, so the latter method is strongly preferred.

According to an ARC Advisory Group study, the global process industry loses about $20 B due to unscheduled downtime—around 5% of its annual production.4 Every plant operator can recall several instances where an emergent maintenance item ended up causing significant downtime, but many of these incidents can be avoided with proper care and attention, facilitated by careful device selection.

Proper product design leads to improved maintenance practices related to tank devices. Some pressure regulators on the market can be installed ‘at-grade,’ meaning they can be set up, checked and calibrated from the ground. This makes maintenance much easier, quicker and safer because technicians can work with their feet on solid ground. To determine if this is a possibility, plant personnel must analyze the specific system and process conditions for the tank blanketing regulators under consideration, often with assistance from vendors.

To provide remote visibility to control room personnel, many pressure control devices available are wireless-ready, and they can be used to remotely monitor various parameters in the tank or its support systems. Although each of these devices could be monitored via traditional wired means, this is often cost prohibitive due to the difficulty of installing wired infrastructure from tanks back to the control room.

Wireless devices can be added to tank vents and regulators to indicate position. Information collected from such an installation helps determine baseline operation of regulators and vents, indicates if both devices are open at the same time (which should not be the case), shows if a vent fails to reseat and alerts if an emergency vent were to open.

Many of these status readings are useful as leading indicators for whether maintenance or inspections should be scheduled for later work or must be performed immediately. With respect to vent monitoring, wireless devices also help plants compile an auditable record to present to government regulators regarding emissions.

Regulators in action

Utilizing tank blanketing regulators as part of a complete tank pressure control system yields financial benefits due to improved operations while minimizing risk. Several vendors offer tank walkdowns to provide a review of tank pressure control devices. Walkdowns can be as narrow or broad as desired, but typically include a review of each tank’s physical condition, operability and sizing.

During a tank survey of 40 tanks at a U.S. Gulf Coast chemical plant, plant personnel needed a focused analysis of the costs used for nitrogen blanketing, which were higher than expected. Upon review, it was found that the setpoints of tank blanketing regulators and PVRVs were too close together on several of their tanks.

As a result, the two devices were occasionally open at the same time, thus venting excess nitrogen into the atmosphere during normal operations. By implementing recommendations to adjust the setpoints of their devices, as well as by installing pilot-operated tank blanketing regulators to maintain lower blanketing pressures in the tank, the plant saved more than $500,000/yr in nitrogen costs and reduced emissions.

Takeaway

Fixed-roof storage tanks present various challenges to operators, many of which can be mitigated with a well-designed tank blanketing system. Tanks may often be overlooked at industrial facilities, but technology exists in the market to provide complete pressure control, along with visibility of tank pressure control system operation.

Tank blanketing and vapor-recovery regulators are only a part of a complete system to control the complex tank ecosystem. These devices—along with vent valves, hatches, overfill protection device and others—provide operators with the reliability they need to properly operate and maintain these assets, while minimizing risks to personnel, equipment, the environment and the community. HP

LITERATURE CITED

  1. AFPM, Process Safety Performance Metrics, https://www.afpm.org/safety-programs/safety-statistics-programs/process-safety-performance-metrics
  2. U.S. Chemical Safety and Hazard Investigation Board, “Final investigation report: Caribbean Petroleum tank terminal explosion and multiple tank fires,” October 2009, online: https://www.csb.gov/caribbean-petroleum-refining-tank-explosion-and-fire/
  3. U.S. EPA, EPA announces 2019 annual environmental enforcement results, U.S. EPA website: https://www.epa.gov/newsreleases/epa-announces-2019-annual-environmental-enforcement-results
  4. ARC Advisory Group, “Process Industry Downtime and Key Performance Metrics Study,” 2017, online: https://www.arcweb.com/blog/process-industry-downtime-and-key-performance-metrics

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