Comparison Of Two Options
Table 1. A system relying on an elevated vessel incurs far lower long-term costs.
Typically, purchase and installation only account for 20–25% of a pump’s lifecycle cost (which also includes both energy and maintenance expenses) over 20 years. In comparison, for a simple separator vessel, the 20-yr lifecycle cost generally splits closer to 75% capital and 25% maintenance.
Table 1 compares the costs of the two options. The installation cost is based on a fully engineered design and a ±10% cost estimate. The lifecycle costs are based on experience and rules-of-thumb for the equipment involved.
The elevated vessel boasts both lower capital and lifecycle costs. It’s the clear choice unless there’s an overwhelming reason for having a booster pump. The higher vessel also offers a safety benefit —elevating the vessel above 25 ft removes it from the standard pool-fire zone.
You can elevate a vertical vessel with a tall skirt or by installing it on a platform. A skirt is less expensive for most vessels but creates a confined space under the skirt. (A horizontal vessel usually doesn’t use a skirt, and so rarely results in a confined space.) Opting for a platform often simplifies maintenance and access.
Elevating the vessel makes it more obvious and easier to see over the plant fence-line. This may cause community relations issues. In this case, that wasn’t a concern.
A larger vessel, for example a storage tank, can change results. So, you must examine each case individually. However, unless you face constraints in modifying an existing plant, using booster pumps to solve NPSH problems often is an expensive choice.
ANDREW SLOLEY is a Chemical Processing Contributing Editor. You can email him at [email protected]. SCOTT SCHROEDER, Senior Consultant, Advisian, you can email him at [email protected].