The design basis for many pressure-vessel foundations includes sufficient capability to handle the load of the vessel full of water. Designing to such a load enables hydrotesting the vessel full of water in one step after modifications. No rule forces a plant to build a foundation to meet the full-of-water load requirement. However, foundations for most storage vessels comply. It's normally expected that liquid will fill these tanks at some point. Even if the design liquid is less dense than water, typical practice is to assume the vessel may be filled with water.
Some foundations can't tolerate the load of a vessel completely full of water, though. Most commonly, these less substantial foundations are used with large distillation towers. Such towers rarely are completely full during operation. However, upset conditions that create high liquid level may allow the load in a vessel to exceed foundation limits.
Additionally, two other situations can create questions about acceptable foundation loads. Many older foundations included wood pilings. Monitoring may have shown damage to the pilings that limits allowable weight on them. In addition, foundation design criteria have changed over time. What may have been suitable in the past no longer may be judged sufficient. Standard practice is that an existing foundation that met requirements when built need not be changed even if it doesn't satisfy current ones — so long as the structure's load doesn't increase by more than a small amount. Despite this, a plant might choose to impose on critical equipment operating limits that reflect the more stringent requirements.
A large vacuum distillation tower (Figure 1) provides a recent example. It could exceed allowable foundation load if the liquid level got too high. So, a fully independent level measurement has been added as a component of a safety instrumented system (SIS). The SIS trips an interlock that shuts down the tower feed when the level rises too high.
Also, the logic of the existing level controls has been augmented. It now includes alarms on the level and procedures for operator action; addition of a duplicate bottoms level measurement using an alternative technology (gamma ray) with voting for level control; auto-start of standby bottoms pumps; and automatic alternative bottoms dispositions. These modifications aren't part of the SIS. However, they do count as safeguards that make over-filling less likely.
High level stems from either too much into or not enough out of the tower. The unit and control system changes all focus on ensuring the not-enough-out event doesn't occur. The new SIS controls address shutting down the inlet flow if the outlet flow is too low despite all the modifications.
The crucial measurement for the SIS shutdown is differential pressure over the entire vertical height of the vessel. This includes both liquid height in the bottom plus pressure drop across the tower. For a packed tower, this is a reasonable approach. Increases in packing pressure drop boost liquid hold-up in the packed beds. More liquid in the packed beds raises the weight on the foundation as well.
Pressure, whether differential or absolute, is given in force per unit area. Knowing the force, you can estimate the weight on the foundation from liquid. At first glance, no correction for liquid density is required. This is true as long as the tower has only one diameter. However, the situation becomes more complex in multiple diameter towers. For the same force, a lower density liquid will have a higher level. Because the tower shown has a larger diameter middle section, a higher liquid level increases the relative amount of the larger diameter that's filled. Therefore, in this case, a less dense liquid will have more total volume for the same pressure. More total volume ends up with more weight on the foundation.
Adding an appropriate margin to the differential pressure set point obviates using density corrections in the SIS. Extra margin accounts for the effect of pressure drop and liquid volume in the different diameter packed beds.
Correct selection of instrumentation for the SIS achieved the desired safety integrity level. Damage or modification to an existing structure or foundation certainly may demand modifications like this. However, the better solution for a new vessel is to require the foundation to bear the water-filled load. One-time foundation costs are cheap compared to the installation and ongoing costs of a SIS.
ANDREW SLOLEY is a Contributing Editor to Chemical Processing. You can e-mail him at email@example.com