Chain falls, come-alongs and rigging inside boilers

Inside a boiler there is no crane. When the Riverbend crew replaced six boiler bank tubes in the spring, every tube, every scaffold board and every tool went in through a door or a roof opening and was moved by hand rigging: beam clamps on the structural steel, chain falls hung from them, lever hoists to pull pieces sideways, and a great deal of planning about where each load would hang at each moment. This fall, Ironbridge is back at Riverbend on the same contract to replace a section of waterwall in the Unit 2 furnace, and at Tamarack Pulp to replace floor tubes in the recovery boiler. Both jobs are hand rigging in a confined, crowded space, and they are where boilermakers do their most characteristic rigging work. This lesson covers chain falls, lever hoists and wire rope pullers, rigging points inside boilers, angled pulls, and transferring a load from one hoist to another.

Hand-operated hoists

Three kinds of hand-operated hoist do most of the work inside boilers. A chain fall, or manual chain hoist, lifts a load on a load chain when the operator pulls a hand chain; the gearing gives a large mechanical advantage, so one person can lift a tonne or more slowly. A lever hoist, often called a come-along, does the same with a ratchet lever and can pull in any direction, which makes it the tool for pulling a load sideways, tensioning a sling or fine-tuning a position. A wire rope puller, sometimes known by a maker's trade name, pulls a wire rope through jaws in a lever-operated housing and can pull long distances, because the rope is not wound on a drum. Electric and air-powered chain hoists are used where many lifts are needed or the hoist hangs out of reach.

Each hoist has a rated capacity marked on it and a load chain or rope matched to it. Hand hoists are designed so that one person pulling at a normal force will lift the rated load; if the hand chain or lever takes more than one person to pull, the hoist is overloaded or damaged, and extending the lever with a pipe, a cheater, is forbidden. Hoists are inspected before use and periodically under ASME B30.16 for chain hoists and B30.21 for lever hoists, and the Alberta Code's rigging rules apply to all of them.

Rigging points inside a boiler

The first question in boiler rigging is what to hang the hoist from. The boiler's tubes are not rigging points. They are thin, pressure-retaining and often weakened by erosion or corrosion, and a load hung from a tube can kink it, crack a weld or start a leak that appears only when the boiler is back in service. Approved rigging points are the boiler's structural steel, the buckstays that tie the furnace walls together where the owner has approved them, purpose-built lifting lugs or rails installed during construction, and temporary beams and lugs designed for the job. On Riverbend Unit 2, the original builder left lifting beams in the penthouse above the superheater headers, and Peg Lindqvist, the owner's inspector, keeps a drawing marked with every approved rigging point and its capacity.

Beam clamps attach to the flange of a structural beam and provide a lifting eye; they are rated for a load in a particular direction, usually straight down, and some are rated for limited angles. A beam clamp is fitted to a flange width within its range, tightened as the maker requires and checked that it sits square on the flange. Slings choked around beams are also used, with softeners on the sharp flange edges. Where no approved point exists, the lift engineer designs a temporary one, and it is installed and inspected before use.

Riverbend's rule, which Kiera hears from Dale and from Peg, is simple: every rigging point used inside the boiler is on the approved drawing or has an engineer's design, and if it is not, it is not used.

Moving a tube panel

The new waterwall section for Unit 2 is a tube panel, a set of tubes welded together side by side with membrane bars, about 2.4 m wide and 6 m long, weighing 1,850 kg. It is brought into the furnace in sections through a temporary opening cut in the roof, lowered with chain falls on the penthouse beams, and then moved sideways to the wall where it will be fitted. That sideways move is where the rigging gets interesting. The panel is hung from two chain falls on two different beams, and by paying out one and taking in the other, the crew can swing it across without anyone pushing it by hand.

When a load hangs from two hoists that are not directly above it, each hoist's chain runs at an angle, and the chain tensions are larger than the shares of the weight. The calculation is the same as for slings at an angle, but in the boiler the angles are often steep and unequal, and the tension in one chain can be close to the whole weight of the panel or even more. The crew must check both chain falls for the tension they will actually carry at each position along the move, not just the panel's weight divided by two.

At one stage of the move, the panel's lifting lug hangs 1.8 m to the right of beam clamp A and 3.2 m below it, and 1.2 m to the left of beam clamp B and 2.6 m below it. Dale has Kiera calculate the tensions.

# Two chain falls holding a tube panel inside the boiler (metric)
import math
W = 1_850                      # panel weight (kg)
# beam clamp positions and the panel lug, measured in the vertical plane (m)
A = (-1.8, 3.2); B = (1.2, 2.6); P = (0.0, 0.0)
def unit(a, b):
    dx, dy = b[0] - a[0], b[1] - a[1]; L = math.hypot(dx, dy); return dx / L, dy / L, L
ax, ay, La = unit(P, A); bx, by, Lb = unit(P, B)
# solve Ta*a + Tb*b = (0, W) for the two chain tensions
det = ax * by - ay * bx
Ta = (0 * by - W * bx) / det; Tb = (ax * W - ay * 0) / det
print(f"chain fall A: {La:4.2f} m of chain, angle {math.degrees(math.atan2(ay, ax)):5.1f} deg, tension {Ta:6,.0f} kg")
print(f"chain fall B: {Lb:4.2f} m of chain, angle {math.degrees(math.atan2(by, bx)):5.1f} deg, tension {Tb:6,.0f} kg")
print(f"sum of tensions {Ta + Tb:,.0f} kg for a {W:,} kg panel")
for cap in (1_000, 2_000, 3_000):
    ok = "OK" if max(Ta, Tb) <= cap else "too small"
    print(f"  {cap:,} kg chain falls: {ok}")

chain fall A: 3.67 m of chain, angle 119.4 deg, tension    957 kg
chain fall B: 2.86 m of chain, angle  65.2 deg, tension  1,119 kg
sum of tensions 2,076 kg for a 1,850 kg panel
  1,000 kg chain falls: too small
  2,000 kg chain falls: OK
  3,000 kg chain falls: OK

The two tensions add to 2,076 kg for an 1,850 kg panel, because the chains pull partly sideways against each other. Chain fall B, the steeper one at this position, carries 1,119 kg, more than 60 percent of the panel's weight. Rated 1,000 kg chain falls, which would have looked adequate for half the weight, would be overloaded. The crew uses 2,000 kg chain falls, and Kiera checks the beam clamps' ratings for the chain angles at the most extreme position of the move as well.

Transferring a load between hoists

Long moves inside a boiler are made in stages, with the load passed from one set of hoists to the next. A transfer is a controlled change of who holds the load. The next hoist is rigged and its chain taken up until it begins to carry load, then the first hoist is slowly paid out while the second takes in, until the second carries all the load and the first goes slack. During the transfer, both hoists share the load at angles, and the tension in each must stay within its rating at every point. The worst case is usually partway through, when the chains are at their widest angles.

Transfers fail when the crew hurries, when two people pull hand chains at once without coordinating, or when a hoist is unhooked before it is completely slack. The rule is one move at a time, called out by one person, with the load watched continuously. Nobody stands under the load, and nobody puts hands between the load and the tubes it is moving past.

Pulling at an angle and the effect of sheaves

A lever hoist or wire rope puller pulling a load across a floor or up a slope needs an anchor that can take the pull, and the anchor load can be much larger than the pull if the line runs around a snatch block. A line passing around a block and turning back on itself puts nearly twice the line pull on the block's anchor; a line turning through 90 degrees puts about 1.4 times the line pull. Riggers check the angle at every block and size the block and its anchor for the resultant, not for the line pull.

Rigging in a recovery boiler

At Tamarack Pulp, the recovery boiler's floor tubes are being replaced, a job done every few years because the floor sits under the smelt bed and corrodes. The space under the furnace is low and full of smelt spouts and their cooling water lines; above, the furnace is tall and dark. The crew lowers the floor tube panels through the furnace from a roof opening, with chain falls on lifting beams installed for the job and a powered hoist at the top for the long drop. Because the recovery boiler is a confined space with its own hazards, including the smelt-water explosion hazard covered in the first course, every rigging plan is part of the confined space entry plan, and all water lines in the area are isolated before work begins.

The rigging inside the recovery boiler also has to work around the scaffold. A tall scaffold fills much of the furnace, and the panels must be lowered past it through a planned opening in the scaffold's decks. The scaffold builder and the rigging crew agree on the opening before the scaffold goes up, and the panels' drop zone is barricaded at every scaffold level while a panel is moving.

Good practice with hand hoists

• Check the hoist's tag, capacity and inspection date, and inspect the hook latches, chain and brake before use.
• Hang the hoist from an approved rigging point, with the hook seated fully and the latch closed.
• Keep the load chain straight and untwisted; a twisted chain does not run properly over the sheave and can jump.
• Never wrap the load chain around the load as a sling; use a proper sling.
• One person pulls each hand chain, at normal force; if it takes more, stop and find out why.
• Never stand under a load hanging from a hand hoist; hand hoists can slip if their brakes are worn or contaminated.
• Do not leave a load hanging on a hand hoist unattended; land it or secure it with a second support.