Same idea inside the machine, different way out of it
Both machines start the same way. A ready-mix truck discharges into a hopper, and a piston draws the concrete out of that hopper and pushes it forward under pressure. Most use a pair of pistons working against each other, so one is filling while the other is pushing, and a valve flips the flow between them. A paddle keeps the hopper stirred so the load does not sit still and come apart. Up to that point there is nothing to choose between them.
The difference is entirely in what happens after the pressure. On a boom pump the concrete travels up a folding articulating arm through rigid pipe, called slick line, with a short length of flexible hose at the tip. On a line pump it travels through hose laid on the ground, in sections that clamp together, from the machine to wherever the pour is. One route goes up and out over. The other goes along.
Over the top, or around the side
That difference in route is the whole practical difference. An arm exists to solve a clearance problem. Something tall or wide is parked between the concrete and its landing spot, and the fix is to travel above it. Upper decks, tall walls and large commercial pours are what the trade built it for.
Hose on the ground solves a different problem, and it is one of access at ground level. The forms are out of the truck's reach but well inside walking distance, and the line follows the two people who carry it. Through a gate, along a side yard, in a door or a window opening, down a bank into a footing trench. Which one your own pour wants turns on the direction the concrete has to move from wherever a truck can park, and there is a page on this site that walks that decision properly instead of in passing.
- Level with the truck, past something sitting beside the pour: hose on the ground
- Below the truck, down a bank or inside a basement wall: hose on the ground
- Above the truck, onto a deck or over a building already standing: an arm
Reach is measured two different ways, which is where the comparison goes wrong
People compare these machines by reach and then get confused, because the word does not mean the same thing twice. A boom pump is described by its arm: how far it folds out and how high it can put the pipe end. That figure is a property of the machine and it is fixed on the day it arrives.
A line pump has no equivalent number, because its reach gets built out of hose sections on the morning of the pour. Sections go on to get around a house and come back off when the forms sit closer than the plan assumed. What limits it is not a length printed on a spec sheet, it is resistance. Every foot of hose, every bend and every foot of rise makes the machine work harder. That is a planning problem rather than a wall, which is why the answer to how far a line will go is always a conversation about the route.
The setup footprints are nothing alike either. An arm has to be braced down. Outriggers reach out from the truck and carry serious weight on a small number of pads, which means solid, fairly flat ground under every one of them. A trailer pump just gets parked, and the rest is hose two people can carry.
What we run, and when we tell you to hire the other machine
What we tow to a job is a trailer-mounted line pump running 2.5 and 3 inch line, with our own operator at the machine and our own hose hand at the placing end. There is no arm on it. There is no boom pump in our yard either, and we are not about to dress our equipment up as one.
So there are pours that are not ours, and we would rather say that on the first phone call than after ready-mix is on the road. When the concrete has to land on an upper deck, sit on top of a tall wall, or be reached out across a building that is already up, the right advice is to hire a crew that owns an arm. When it has to go around, through or down instead, that is our work, on our own jobs and as the pumping sub on somebody else's.