Part 1 and Part 2 sized the pump and heater in the feed section. This part sizes the tank they draw from: T-101, an atmospheric storage tank that holds the feed and decouples the reactor from deliveries. A tank seems simple, but its size, its level settings and its wall thickness each come from a different question.
Step 1: how much holdup?
Holdup is set by what the tank has to absorb. For a feed tank, the usual questions are: how long must the plant keep running if a delivery is late, how big is a delivery, and how much time do operators need to respond to an alarm. This example takes 8 hours of feed at full rate:
Working volume = 6,250 kg/h × 8 h / 850 kg/m³ = 58.8 m³
Not all of a tank is usable. The bottom is below the pump suction, and the top is reserved for alarms, trips and vapour space. Allowing for 80% of the total volume to be usable, the tank needs about 73.5 m³. If deliveries arrive by tanker, also check that one full delivery fits above the normal level; that can govern instead.
Step 2: shape
For a vertical tank, the diameter is often fixed by the space available and the height follows. With a 4.0 m diameter, the height needed is 5.85 m, rounded up to a 6.0 m shell. A height of one to two diameters is common for tanks of this size.
Step 3: level settings
A tank's levels are as much a part of its design as its volume. Each one has a job, and the gaps between them give operators time to act.
| Level | Height | Purpose |
|---|---|---|
| High-high (HHLL) | 5.3 m | Trip: closes the inlet valve to prevent overfill |
| High (HLL) | 5.0 m | Alarm: stop filling |
| Normal (NLL) | 3.2 m | Typical operating level |
| Low (LLL) | 1.2 m | Alarm: arrange a delivery or reduce rate |
| Low-low (LLLL) | 0.8 m | Trip: stops P-101 before it loses suction |
The low-low trip protects the pump from running dry, and its height must keep enough liquid above the pump for the NPSH calculated in Part 1. The high-high trip protects against overfill, which is one of the most common causes of tank incidents. Both are candidates for the independent protection layers examined in a HAZOP.
Step 4: wall thickness
Atmospheric storage tanks in North America are commonly designed to API 650, which sets the shell thickness of each course from the liquid head it has to hold. The one-foot method gives the required thickness of the bottom course as:
t = 4.9 × D × (H − 0.3) × G / Sd + CA
with t in mm, D the diameter and H the liquid height in metres, G the specific gravity, Sd the allowable stress in MPa and CA the corrosion allowance. For T-101, with G = 0.85, Sd = 137 MPa and a 1.5 mm corrosion allowance:
t = 4.9 × 4.0 × (6.0 − 0.3) × 0.85 / 137 + 1.5 = 2.2 mm
That is less than the minimum shell thickness the standard sets for a tank of this diameter, 5 mm, so the minimum governs. For small tanks this is usual: the walls are thicker than the liquid pressure needs, for handling, welding and stiffness. Pressure vessels, such as reactor R-101, are designed to a different code, ASME Section VIII, which uses a different formula based on the internal design pressure.
What else the tank needs
- Venting. An atmospheric tank breathes as it fills, empties and warms. Vents are sized for normal breathing and for emergency venting in a fire, so the tank is not pressurized or pulled into vacuum.
- Containment. A bund or dike sized to hold the tank's contents, with allowance for rainfall, as local regulations require.
- Inerting. A flammable feed may need a nitrogen blanket so that the vapour space never contains an ignitable mixture.
- Nozzles. Inlet, outlet, drain, vent, level instruments, temperature and a manway, each located and sized on the tank drawing.
Key points
- Holdup comes from what the tank must absorb: late deliveries, delivery size and operator response time.
- Usable volume is less than total volume; allow for the space below the pump suction and above the high levels.
- Level settings are design decisions; the gaps between them buy operators time.
- For small atmospheric tanks, the code minimum often sets the wall thickness, not the liquid head.
This completes the series. For what protects this equipment if things go wrong, see pressure relief basics.
The plant, fluid properties and equipment in this series are an illustrative example. The calculations are preliminary sizing estimates. Final sizing needs vendor data, the applicable codes and a qualified engineer.
