Tanks are all about the raw materials, intermediates and finished products moved into, stored, mixed, moved out of or transferred between them. Because of their typically huge volumes, even small variances in how storage, custody transfer and other tank-based functions perform can mean significant swings in measurements and efficiency, and big gains or losses in revenues and profits.
That's why most operators and managers are understandably obsessed with the contents of their tanks, farms and fleets. Unfortunately, this focus means some might overlook the gases, vapors, atmospheres, empty spaces and resulting pressures in their tanks where materials and products aren't present, but which can result in several difficulties and potential safety issues. Fortunately, vapor-space management, tank blanketing and other solutions can solve these problems.
To help tackle these issues, Jeff Wolendowski, business unit manager at Novaspect Process Management in Schaumburg, Illinois, presented "Total Tank Management: Understanding and Managing the Vapor Space" on Jan. 20 at the ISA's Will-DuPage Section meeting in Plainfield, Illinois.
"Vapor space management is used in oil and gas, petrochemical, chemical, pharmaceutical, food and beverage, semiconductor manufacturing and any other applications where we need to preserve the quality of materials and products and protect the environment," says Wolendowski. "To protect high-temperature oil, flammable final products, hydrocarbon wastewater, volatile organic chemicals, adhesives and sealants, solvents, industrial coatings and other materials, we use tank blanketing that puts an inert layer, usually nitrogen, on top of the product or material to control vapor."
Wolendowski reports that the two main types of tanks are fixed-roof tanks with cones, domes and umbrella structures, and floating-roof tanks that rise or fall with varying liquid levels. Internal floating roofs are typically fixed cone-roof tanks with a floating roof inside, while external floating roofs just have a floating roof with no fixed roof above it. Typical tank systems include in-and-out pumps; temperature controls for a regulated steam loop; gauging for fluid levels, temperature, in-and-out flow rates and pressure; tank blanketing to add or remove inert gas; and overall distributed/digital controls of the gauges and pumps (Figure 1).
"Tank pressure needs to be controlled because tanks aren't perfectly sealed enclosures, so air and moisture can enter during temperature decreases and pump-out operations, and volatile emissions can escape during temperature increases and pump-in operations," explains Wolendowski. "To prevent air and moisture from getting in, a gas blanket at a slight positive pressure, usually less than 15 psig, can be applied inside the tank. To prevent emissions from escaping, pressure relief can be used, which vents relief gas or sends it to an environmental device before discharge to the atmosphere."
To control vapor space heating/expansion and cooling/contraction, Wolendowski adds that excess pressure can be relieved, and blanketing gas can compensate for pressure decreases. Pressure relief or compensation is also needed during pump-ins and pump-outs. "Controlling the pressure in the vapor space above the liquid is key, and it’s done with a combination of makeup pressure and pressure relief," explains Wolendowski. "A pressure-reducing regulator senses pressure decreases inside the tank, opens to let in more blanketing gas and closes when the pressure reaches setpoint. A relief regulator senses increasing pressure in the tank, releases vapor to atmosphere or an environmental device and closes when enough pressure is relieved. Both makeup and pressure relief work together to keep the tank holistically in balance."
In case of emergency
These two sides also cooperate when abnormal conditions require emergency venting by establishing proper setpoints for each. Setpoints for pressure relief (Depad) are higher than those for makeup pressure (Pad) to minimize use of blanketing gas, while setpoints for emergency venting for overpressure and emergency vacuum relief for underpressure are set outside of normal operating ranges. For proper operation, none of the setpoints should overlap.
While all tanks must have emergency pressure vacuum and relief venting, different liquid materials and products need different combinations of pressure control without blanketing, pressure control with blanketing, but not vapor recovery, or pressure control with blanketing and vapor recovery. Besides nitrogen, carbon dioxide, natural gas and fuel gas are used for blanketing, depending on which is compatible with a user's products, cost-effective and locally available.
"Tanks venting products should protect the tank from overpressure or over-vacuum, provide tight sealing at normal operating pressures, and meet increasing regulations and environmental concerns," adds Wolendowski. "These products include pallet-loaded, spring-loaded and dead-weight emergency pressure or pressure/vacuum vents, pressure/vacuum hatches, and inline or end-of-line vent valves."
Standards gain latitude
Beyond basic blanketing, Wolendowski reports that several primary standards govern tank management. For example, the American Petroleum Institute's API 2000 standard covers the normal and emergency venting requirements for aboveground liquid-petroleum product storage tanks and underground refrigerated storage tanks designed to operate at pressures from vacuum through 15 psig.
API 2000 was updated in 2009 in its sixth edition, which adopted the ISO 28300 standard issued in 2008 for venting of atmospheric and low-pressure storage tanks. Where earlier standards only dealt with contents pumping and vapor space heating/expansion and cooling/contraction, these newer guidelines can increase thermal inbreathing capacity of tanks by about 20-50% by also addressing the effect on tanks of their geographical latitude, average storage temperatures, vapor pressure and insulation, which can alter flow requirements. "For example, a tank without insulation, close to the equator, and with high vapor pressure may require higher flow," explains Wolendowski.
Saving blanketing gas
To help tanks operators and managers conserve blanketing gas and the cost of delivering it, Wolendowski advises applying only the minimum amount of gas pressure required to reduce the volume escaping through poorly sealed vents and other pathways. "Minimizing required gas pressure can be accomplished with properly sized, low-setpoint technology, such as regulators with low setpoints of about 0.5 inches water column," explains Wolendowski. "This can result in significant savings over time. Periodic monitoring and maintenance of the pressure-control system is also essential."
In addition, just as makeup and relief pressure setpoints shouldn't overlap, and emergency venting values should be set outside the normal operating range, this coordination can also decrease device cycling, reduce blanketing gas consumption and require less venting. For instance, if nitrogen costs $2 mcf, then a blanketing setpoint of 2.0 ins. water column and 5 mbar pressure through a 0.5-in. leak will cost about $5,000 per year, and a 1-in. leak will cost more than $15,000 per year.
For vapor space management and tank blanketing devices and gases to work effectively, Wolendowski adds that they need to be installed properly and placed correctly on their tanks. This means the tank-top is optimal; inlet and control lines slope into the vessel; there are at least 36 ins. between inlet and control lines and 15 ft. maximum length for the control line; the connect control line to the point pressure line is controlled; isolation valves are considered; inlet line strainer is recommended; and calibration and maintenance gauges are used (Figure 2).
"The best designed regulator for tank blanketing service won't perform well unless installation basics are observed and implemented, including control line diameter, length and placement, good design practices for outlet piping, and regulator placement and orientation," says Wolendowki. "However, there are significant challenges for proper installation due to the very low pressures used for positive pressure tank blanketing. These include downstream tanks’ pressures just above atmospheric; piping friction that can degrade the pressure-measurement signal back to the regulator; long piping runs on the regulator outlet that can delay delivery of makeup gas to maintain pressure; and regulators’ using the weight of internal parts to obtain desired setpoints."
Wolendowski adds that placement of the regulator in the piping system is an important consideration in how well it will regulate pressure. "Tank blanketing is considered to be a dead-end service," he says. "Demand is created by a volume (level) or thermal change in the tank, which equates to an on-off function, so the regulator must actuate open and closed with small changes in downstream pressure. Also, the distance between the sensing actuator and the pressure to be controlled is important."
Not surprisingly, placement requirements differ between internally and externally controlled regulators. Internal devices regulate controlled pressure just downstream of the body outlet, which is normally in the interconnecting piping to the vessel. In addition, long piping runs can cause inaccuracy in vessel pressure control, which normally affects the lowest pressures, such as those in the 0.5-5 in. water column range. Also, pressure in the vapor space will change first, but pressure must change in the outlet piping and register at the regulator actuator before it will supply gas to the vapor space, and low pressures exacerbate this issue. On the other hand, an externally controlled regulator will sense pressure where the pressure is to be controlled. So the sense line design is important to obtain the desired pressure control, and an outlet piping design should be considered for best results.
Meanwhile, an optimal tank top minimizes the outlet piping and sense line length, and places the regulator at the correct elevation. This is higher than the tank nozzle connection and tank sense line connection; one that promotes natural drainage of condensate back into the tank; doesn't create any traps or allow condensate to collect and drain back into the regulator. Freezing or trapped condensate in one or both lines will disrupt blanketing gas going into the tank vapor space.
Finally, the best practices for regulator installation include placing it close to the vapor space and keeping its outlet piping sense line lengths as short as practical; discouraging installations at ground level; not undersizing inlet or outlet piping; and understanding that deviation from best practices is possible by providing process requirements and elevations to the factory for discussion and solution. "Abnormally long runs of outlet and sense line piping will affect the regulator's ability to control the tank's vapor space pressure. In addition, operational problems can occur due to condensate collection, as well as more instability incidents and/or cycling," adds Welandowski. "Also, piping should not be less than the regulator's pipe size. It's also acceptable to swage-up on the regulator outlet piping to the tank."