Rules of Thumb: Tanks and Vessels

Stephen Hall discusses the golden rules for design

VESSELS provide many functions in a chemical plant. They have many forms and sizes, and are fabricated from nearly any structural material. Vessels contain valuable inventory that can be hazardous to life and property if released, so it’s important for process engineers to pay close attention to the use, sizing, and design of all of the vessels in the plant. This article presents some useful rules of thumb for vessel design.

You should strive to minimise the size of vessels, while maintaining the desired plant functionality. The primary reasons to minimise vessel size (and number) fall into two categories: safety and cost.

Safety of vessels mostly pertains to the integrity of the tank, its ability to fully contain its fluids in the face of process upsets, degradation of the tank or components, physical damage, and external events such as fire. Another important consideration is how to safely access the inside of the vessel for inspection and maintenance after it is installed. Asphyxiation is a concern: well-positioned access flanges should be provided along with a means to ventilate the tank interior and install personnel harnesses. Ergonomics are another aspect for discussion, to include operational, sampling, instrument calibration, and maintenance procedures.

Balance plant operating requirements with economic considerations, but never sacrifice safety to save money. Operating requirements are derived from the material and energy balances, anticipated throughput variations (eg weekend shutdowns, discontinuous flow through unit operations), physical size of the facility (eg ceiling heights, door widths), size of trucks or rail cars delivering raw materials, desired inventory level (eg 1 week, 1 month), hold times for inspection, and release of product, etc.

Vessel size has a ripple effect on the size of piping, pumps, heat exchangers, agitators, and other related vessels. Ancillary costs such as structural supports, skirts, ladders, platforms, insulation, and painting are directly affected. The maximum allowable diameter of a shop-fabricated cylindrical vessel is usually determined by how the vessel will be transported to the plant site. Carefully assess any vessel that is proposed to be in the 12-14 ft diameter range to be sure it can be transported and installed. Larger tanks will likely need to be fabricated at the plant site.

Inventory held in the tanks, whether raw material, intermediate, or final product, is an operating cost that can significantly affect the cost of goods sold. Other operating costs are affected by vessel size, including operator hours, cooling water, steam, and power.

Large atmospheric storage tanks such as the kind you see at oil refineries are designed by specialists who follow strict codes. These range in size up to 200 m³, with diameters often reaching 20 m.

Smaller (<60,000 L) atmospheric tanks and pressure-rated vessels are often specified by generalist process and mechanical engineers. The following “rules of thumb” may be useful:

• They must be designed, fabricated, and tested in accordance with applicable codes.
• The user is responsible for specifying loadings that are used to calculate the vessel wall thicknesses and reinforcements. Factors include: internal/external pressure; ambient and operational temperatures; static pressure and mass of contents in operating and test conditions; wind and earthquake conditions; reaction forces and moments resulting from supports, attachments, piping, agitators, thermal expansion, etc; corrosion; fatigue; and decomposition of unstable fluids.
• The aspect ratio (vertical straight-side height divided by diameter) is usually between 1:1 to 1.5:1. Taller vessels, with aspect ratios ranging to about 4:1, are used when necessary to maximise heat transfer through a jacket, to maximise contact time of a sparged gas, or for other process requirements.
• ASME F&D heads (torispherical) are usually specified for pressures to 20 bar. Ellipsoidal (2:1) heads are used for pressures from 20–100 bar. Very high pressure applications, above 100 bar, utilise hemispherical heads. Conical bottoms are used for some crystallisers when it is desired that precipitates flow freely to the bottom outlet nozzle.
• The working volume of an agitated vessel should be about 80% of the volume measured at the top tangent line. If the aspect ratio exceeds 2:1 then multiple impellers may be needed.
• Determine heating and cooling duty using factors including control of process exotherms, heat-up and cool-down loads (time-based), boiling, thermal losses to the environment, and heat input from agitators and pumps.

This is the first in a series that provides practical insights into on-the-job problems. To read more, visit the series hub at https://www.thechemicalengineer.com/tags/rules-of-thumb

Disclaimer: This article is provided for guidance alone. Expert engineering advice should be sought before application.