14.3 Tanks, Vessels & Containers: Installation, Inspection & Repair

Sub-tasks D-19.01, .03, .05 and .07 cover installing, diagnosing, maintaining and repairing process tanks and containers. The millwright's scope is mechanical: foundations, anchorage, nozzles and manways, agitators, level and vent hardware, and the rigging and confined-space work that surrounds any vessel job.

Codes and Jurisdiction

In Canada, any vessel over 15 psig must have a design registered with a Canadian Registration Number (CRN) and carries a nameplate stating the maximum allowable working pressure (MAWP), design temperature, manufacturer, serial number and CRN. A millwright must never weld on, alter, or drill into a registered pressure vessel. Repairs and alterations to pressure-retaining components are performed only by a shop holding the appropriate certificate of authorization, under a qualified procedure, with the provincial authority notified. Removing or defacing the nameplate is itself a violation.

Tank Construction and Appurtenances

                        ATMOSPHERIC STORAGE TANK (API 650)

               Conservation vent / PV valve      Gauge hatch    Emergency vent
                        |                            |               |
        +---------------|----------------------------|---------------|-------+
        |               v                            v               v       |  <- fixed roof
        |                                                                    |
        |  <- shell courses, thinner toward the top (hydrostatic loading)     |
        |                                                                    |
        |   [level transmitter]                              [ manway ]      |
        |                                                                    |
        +--[nozzle: fill]-------------------------------[nozzle: suction]----+
        =====  annular plate / bottom plate on a properly graded pad  ========
             (chime, sand or oiled-sand foundation, cathodic protection)
                        ------ DIKE / SECONDARY CONTAINMENT ------

Emergency Venting: A Life-Safety Item

An atmospheric tank has a very thin shell relative to its diameter and almost no vacuum capability. Two failure modes result:

• Vacuum collapse. Pumping out a tank whose vent is plugged, iced, or valved shut pulls a partial vacuum and implodes the shell. Tanks have been crushed by a few kPa of vacuum.
• Overpressure rupture. Filling faster than the vent can breathe, or fire exposure boiling the contents, over-pressures the tank.

A conservation vent must be inspected for screen plugging, ice, insect nests and stuck pallets. Never blank off, plug, or valve a tank vent, and never rely on a manway left cracked open as a vent.

Containment

Regulations across Canada require secondary containment — a dike, berm or double-wall — sized to hold at least 110% of the largest tank inside it, with a controlled drain valve kept normally closed and locked. This connects directly to sub-task A-1.03 (Protects the environment): an uncontained release into soil or a watercourse is a reportable environmental incident.

Foundations, Settlement and Installation

A tank is a heavy, thin-walled structure that is only as good as what it sits on.

• Foundation types: compacted granular pad with an oiled-sand cushion, ringwall (concrete ring under the shell with granular fill inside), or a full concrete slab for small tanks.
• Levelness: the bottom must be graded so product drains to the outlet and so shell settlement stays uniform. Differential settlement distorts the shell, jams floating roofs, and tears bottom welds at the shell-to-bottom joint.
• Settlement survey: elevations are shot at equally spaced points around the shell after hydrostatic test and periodically thereafter. Uniform settlement is tolerable; differential settlement is the problem.
• Anchorage: required where wind or seismic uplift, internal pressure, or empty-tank buoyancy in a flooded dike could lift the shell. Anchor chairs are welded to the shell and must be installed to the engineered detail.
• Hydrostatic testing: a new atmospheric tank is filled with water to prove the welds and to consolidate the foundation. Fill and drain rates are controlled and the vent path is verified open throughout — the most common way to destroy a brand new tank is to drain it with a closed vent.

Inspection Techniques

Inspection intervals for atmospheric tanks follow API 653 and are based on measured corrosion rate and remaining life; pressure vessels follow the provincial authority's schedule.

Confined-Space Entry for Vessels

Entering a tank or vessel is a confined-space entry, without exception. The full requirements of section 1.3 apply, and the vessel-specific points are:

1. Positive isolation. Close and lock the valves, then blind or spade every line, or use double block and bleed where the code permits. A closed valve alone is not isolation — valves leak.
2. Empty, flush, purge and ventilate. Steam or water washing may be required. Continuous mechanical ventilation must run throughout.
3. Test the atmosphere before entry and continuously during it: oxygen between 19.5% and 23%, flammable vapour below the permitted percentage of the lower explosive limit, and toxic gases such as hydrogen sulphide and carbon monoxide below occupational exposure limits.
4. Watch for inerted vessels. A nitrogen-blanketed tank is oxygen-deficient and immediately fatal on entry. Purge and verify oxygen before anyone enters.
5. Beware residual sludge. Sludge in a hydrocarbon or sour service tank continues to release vapour after the tank looks empty and may be pyrophoric — iron sulphide scale can ignite spontaneously when it dries in air, so it is kept wet during removal.
6. Permit, trained attendant, retrieval system and rescue plan in place before entry, with communication maintained throughout.
7. Hot work inside a vessel requires a separate hot-work permit, continuous gas monitoring, and a fire watch.

Common Repairs and Millwright Tasks

• Nozzle and manway replacement — cut out, fit and prepare for a certified welder; check for pipe strain on reassembly.
• Roof drain and seal replacement on floating-roof tanks.
• Agitator and mixer service — shaft straightness (runout typically within 0.25 mm), coupling alignment, seal or packing replacement, and steady-bearing condition. A bent agitator shaft creates a rotating unbalanced load that destroys the gearbox and the tank nozzle.
• Level instrument and vent hardware replacement.
• Internal coating and liner inspection support.

Anything that removes or replaces pressure-retaining metal on a registered vessel stops being a millwright task and becomes certified pressure-repair work.