Steam-heated storage tanks are critical to manufacturing processes, and prioritizing reliability in tank-system design and operations can mitigate unwanted issues
Storage tanks are essential to the chemical process industries (CPI), and they require significant capital investment to ensure optimal installation and continued reliable operation. Many of these storage tanks are heated by steam to maintain liquid viscosity or product integrity. The concept that storage tanks just maintain liquid stocks and are relatively simple equipment may confound some when dramatic issues occur (Figure 1). This article describes some specific issues the author has encountered at various chemical processing plants, and provides guidance for troubleshooting problems with steam-heated storage tanks.
FIGURE 1. Although storage tanks are common and critical equipment in the CPI, they can experience many significant issues
Severe hammer in a tank coil
An example of a severe issue personally reviewed by the author was in the late 1980s regarding a large tank in a petroleum refinery in the northeastern U.S. It was only about three months before the issue occurred that the tank drainage problems were reviewed and discussed with the process engineer, who ultimately decided that the system was working “well enough.” The specific issue at hand was that the condensate header-system backpressure had increased from 40 psig to over 60 psig, resulting in the electric condensate pump set (used for pumping the condensate into the return) ceasing to work. In a “cost-saving” effort, the engineer decided to bypass the pumping system and discharge the condensate directly into the highly pressurized condensate header.
Additionally, each steam-heating coil discharged directly through a straight horizontal line into the steam trap, with no drop-down into the trap. Discharging directly into a trap in such a manner can create a steam-lock condition. A steam lock will occur when the inlet piping to the steam trap is configured in such a way that steam vapor is somehow filling the steam-trap body and stifling — or even fully preventing — condensate from entering the trap. The result is that the trap locks shut until the steam in the trap body and inlet piping condenses (Figure 2) [1], allowing for condensate to finally enter the trap and be drained from the system.
FIGURE 2. Vapor or steam locking of a steam trap can occur when there is a long horizontal run-up to the trap
While reviewing the bypassed system, it was noted that significant waterhammer occurred within the internal coils and also at the steam traps. Still, the site’s process engineer remained unfazed by any concern about both items (Figure 3). Nevertheless, during each subsequent visit, the hammer was brought up with the engineer and subsequently dismissed because he felt that the system was “working.”
FIGURE 3. Hammer and coil damage experienced in internal steam coils can be the result of improper design
That is, until about three months later, when the engineer called and said he should have listened to the concern. Apparently, the hammer became so severe that it knocked one of the steam coils off its support and caused steam to leak directly into the stored liquid — making some of it boil and subsequently damage the tank top. According to the engineer, the tank had to be drained, the top repaired, insulation replaced, the tank pressure-tested and the test water treated at the sewage plant before final discharge. The exact cost was never provided, but the initial estimate of approximately $3,000,000 (circa 1988) is certainly memorable. Ultimately, the issues with the trapping and condensate system were corrected as originally recommended. A key takeaway from this experience is that if left unaddressed, hammer in a tank coil can lead to serious consequences and should be mitigated with high priority.
Steam coils in extreme cold
Another storage-tank problem reviewed was an asphalt storage tank in an extremely cold climate where thermal maintenance had been an ongoing issue. In this case, ascertaining the cause of the problem and its mitigation were relatively easy (Figure 4). There were three main issues related to the steam coils.
FIGURE 4. This asphalt tank was experiencing multiple hammer and control issues, even though the steam pressure was high
First, the coils were drained by subcooling, bimetal-style traps. They also discharged horizontally straight into the traps via a 2-in. line. Finally, a method was needed that could drain condensate in the event of loss of positive differential-steam pressure.
Bimetal traps can significantly subcool condensate by as much as 50 to 100ºF, and can also cause difficulty with maintaining consistent high temperature [2]. Additionally, a long, straight horizontal run into the trap can cause a steam-lock condition, as previously explained (see Figure 2). Finally, positive steam-pressure differential is lost when a system shutdown occurs. This means that the coils can flood, lose temperature and corrode. If the temperature drops low enough, such as when the tank is empty, the coils can freeze and split if not fully drained.
Figure 5 shows the troubleshooting recommendations in a three-dimensional (3D) detail drawing. The bimetal traps were replaced with float and thermostatic traps that can discharge condensate without backup. Additionally, there is a vertical drop-down into the traps to allow for condensate to easily enter the trap body and mitigate against a steam-lock condition. In the drawing, it can also be seen that two of the traps (high-pressure models) discharge into the return line and two of the traps (lower-pressure versions) discharge to atmosphere.
FIGURE 5. A 3D drawing can help installers better understand how to properly design pipe systems
The high steam pressure during normal operation prevents the lower-pressure traps from operating due to a pressure-block condition (thus preventing drainage). When the steam pressure drops into the rated operating range of the lower-pressure traps, they can open and discharge to atmosphere or grade.
This same dual high-pressure/low-pressure drainage design configuration can also be used with certain insertion-tank coils or shell-and-tube heat exchangers to mitigate corrosion (Figure 6). Because a rapid drop in steam pressure can cause the coils to go into vacuum conditions, a vacuum breaker is commonly recommended as shown.
FIGURE 6. Designing for coil drainage on shutdown can be an important consideration
Intermittent operation
Another issue encountered involved a steam heat exchanger in a petroleum refinery in the Caribbean where the tube set was corroding, requiring replacement every year or two. The process only needed to operate four hours per day, so during the “rest” period, the condensate was absorbing CO2 and O2, thus forming corrosive carbonic acid that rotted the coils. The use of a lower-pressure steam trap discharging to grade on shutdown was recommended to mitigate the corrosion.
Steam heaters and stall
FIGURE 7. Bayonet heaters provide more filling head to the steam trap, but can present some other problems
Insertion-bayonet tube-side steam heaters (Figure 7) are essentially similar to steam shell-and-tube heat exchangers, which means that stall can be an issue that occurs when heat demand lessens [3– 6]. As illustrated in Figure 8, when the steam pressure (red line) is higher than the backpressure (green line), the pressure differential is positive and only a steam trap is normally required for condensate drainage (purple triangle). When the steam pressure is negative (dark diagonal blue line intersecting the green backpressure line is beneath the green backpressure), the differential is negative and condensate can only be drained using a pump or pump-trap system (light blue triangle).
FIGURE 8. Condensate can drain from coils when the pressure differential is positive. Conversely, condensate can back up when the pressure differential is negative
Stall is a condition where the steam pressure exiting the coil has become equal to or less than the backpressure of the condensate return system. When stall occurs, condensate backs up in the coil with various issues resulting, such as poor temperature control, corrosion or hammer.
A common solution to overcome the effects of stall is to incorporate a pumping system that discharges condensate into the higher backpressure system.
Another mitigation option is to drain condensate to atmosphere (where allowed, and if the condensate amount is small). In such instances, the combined high- and low-pressure trap arrangement can be used. However, it is mostly undesirable to discharge condensate to grade, so a pumping solution is needed.
Gravity drainage
Some insertion heaters may be at sufficient vertical height to enable gravity drainage of condensate into a trap and pump set or pump/trap system — this can be a benefit when possible. Gravity drainage is highly preferred, because anytime condensate remains backed up into a coil, its temperature can drop rapidly and significantly, making it acidic and corrosive in the process.
In the case of a steam trap and pump arrangement, if the heater condensate outlet is located high enough, condensate can drain into the trap by gravity, which can then drain into the condensate-pump receiver by gravity [7]. Such systems must be carefully designed to enable gravity flow, otherwise a manometer effect can occur with the condensate and cause the coils to remain at least partially flooded. An arrangement with a steam trap and condensate pump is preferred when there are multiple coils in the tank, with each coil being drained through an individual steam trap and then collected and returned through a condensate pump set.
In some instances, the insertion-heater condensate outlet is not vertically high enough to enable full gravity flow into a trap and pump set. In such cases, a combination pump/trap system may be used, but a crucial point to note is that the insertion heater itself must have a tapping on the outlet side (just below the pass partition) to enable proper system balancing from the heater to the pump/trap’s reservoir [4, 5]. Without such a proper balance point, it is probable that the system will not drain freely and will not effectively fill the pump/trap reservoir, causing condensate to back up in the coil, which can lead to other problems.
In certain applications, thermosiphon effects from convection off of internal coils or bayonet heaters cannot provide the required circulation or temperature homogeneity throughout the tank liquid or on the wall. In such cases, a skin-heater system can be attached to the external tank wall, as shown by the blanket coils in Figure 9. However, unlike internally submerged coils, which have full surface-area contact with the stored liquid, only the surface area of the external coils that makes direct contact with the tank wall can be considered to conduct heat to the tank and stored fluid. This makes it essential to optimize the steam heat going into the external coils or skin heaters.
FIGURE 9. Skin heaters benefit when steam supply is optimized and condensate drainage avoids backup
Figure 9 represents a graphic detail design provided to mitigate issues experienced at a refinery in a far northern location. Not only was the condensate drainage system inadequate, but the steam supply also experienced certain deficiencies.
Optimization for an external skin/blanket heater system starts with high-quality steam entering the coils. In this sense, many of the requirements for effective steam-heated air or process coils also apply. It may require that the steam traps installed in at least three drainage locations before the coils are functioning properly to mitigate large amounts of undrained condensate from reaching the coils. It may require that a steam separator is used prior to the entrance of steam to the coils to optimize the heating capability on the coils’ internal surface for most effective heat transfer. Commonly, a steam air vent can be incorporated at the entry point to remove as much air as possible before it enters the coil. Certainly, air-venting capability is prescribed at the coil exit if the heat quality is to be optimized. The mitigation recommendation for the northern refinery is shown on the steam inlet side in Figure 9 with vertical supply headers used in conjunction with proper steam trapping at the riser base to preclude large amounts of condensate reaching the coils during operation.
The tank was not particularly wide, the steam pressure not too high and the condensate loads relatively small, so this enabled an interesting opportunity for condensate drainage. Rather than requiring a large condensate vessel to handle flash steam, the condensate could be collected in a condensate manifold that was able to flash as required and supply condensate into the pump (Figure 9 red inset, expanded detail view on right).
Sulfur storage
Some storage tanks, such as those containing sulfur, can have severe thermal-maintenance requirements — not only for the sulfur itself, but also for the tank walls. For example, sulfur tanks can experience multiple issues aside from just sulfur freeze-up. They also have to keep the sulfur hot (around 275ºF) to keep it flowing and maintain approximately 260ºF for the walls and vapor space to mitigate against corrosion, sulfur buildup, pyrophoric FeS creation and fires or explosion [ 8]. To sustain critical internal liquid and wall temperatures, it is common to require high heat transfer, such as shown by the strap-on channel jacket-type heating elements on the outside of the tank, supplementing the heat supplied by the internal coils (Figure 10).
FIGURE 10. Sulfur storage tanks require critical maintenance considerations to preclude serious issues
Edited by Mary Page Bailey
Acknowledgement
Special thanks to Ametek CSI, TLV Corp., and Justin McFarland for the kind provision of graphics used within this article, and Norm White for his kind review and comments.
References
6. TLV Corp., Calculator: Stall Point, www.tlv.com/global/US/calculator/stall-point.html?advanced=on, July 2022.
Author
James R. Risko (Email: [email protected]; Phone: 704-641-8959) is the retired president of TLV Corp, Charlotte, NC, formerly responsible for U.S. and Canadian operations. He has 47 years of experience with steam systems, authored more than 60 technical articles, provided webinars to over 3,500 attendees globally and presented for numerous industry organizations and conferences, including the Kister Distillation Symposium, Distillation Experts Conclave, Fractionation Research Inc., AFPM, AIChE, the Ethylene Conference, RefComm, IPEIA, IETC, eChemExpo, AEE World and ASHRAE. He co-invented the world’s first combination pump/trap and created the “Extended Stall Chart” for draining stalled coils, heat exchangers, and reboilers, the “Drop-down Loop Seal” concept to help mitigate hammer in vertical risers of flashing condensate lines and the 2-bolt combined steam trap strainer-connector. A past chairman of the FCI, he has been selected to receive its 2024 Lifetime Achievement Award. Risko is currently an Advisory Board member of both the Texas Industrial Efficiency Energy Program (TIEEP) and the TEES Industrial Energy Technology Conference (IETC).