Ball valve seat wear on a cement line

Where the valve sits

Cement is made by moving powder. Raw meal is ground and homogenised, preheated and calcined, ground again, stored and packed. Between those stages the material is pushed through pipework by air. Pulverised coal reaches the kiln the same way.

Every one of those streams is abrasive. Cement dust, mineral powder and pulverised coal are hard silicate particles travelling at conveying velocity. A valve in that line is not holding a fluid back. It stands in a sandblast whenever it is open. When it is closed, it seals against a surface that sandblast has been working on. The same duty turns up in thermal power and in metallurgy, wherever powder is conveyed.

This customer is a major cement manufacturer in South-East Asia, running several thousand tonnes a day.

How ball valve seat wear takes a line down

Ordinary metal-seated ball valves do not survive this. The particles score the ball and the seat, and what starts as scratching becomes pitting. The sealing interface that was lapped flat at the factory is no longer flat, and the valve stops shutting.

A ball valve that will not shut does not announce itself with a bang. It leaks continuously past a closed seat. Material goes where it is not supposed to go, and is lost. Worse for the plant, the process either side of that valve loses a control point. The operator is asking for closed and getting something else.

The plant was seeing severe sealing surface wear after only a few months in service. Each replacement took several hours, and on a line producing thousands of tonnes a day the cost of the fault is entirely in those hours. The valve itself is a rounding error beside the tonnage that did not get made.

What we supplied

Not a harder valve of the same design. A different sealing structure.

The ball and the seat are fully encapsulated, lined and enclosed in tungsten carbide or zirconia ceramic. Both are harder than the silicate particles doing the damage. Hardness above the abrasive is the whole point. A surface softer than what is hitting it wears by definition. A surface harder than it keeps its geometry and its finish instead of losing them by degrees.

Encapsulation matters as much as the material. A hard facing with an exposed edge only moves the failure to the edge. The particles find the softest thing in the flow path and work on that. With the sealing structure fully enclosed there is no exposed weak point for erosion to start at. A hard metal seat works against a wear-resistant lining, precision-ground to fit. The valve carries two layers of protection rather than one coating to lose.

The bore is full and straight through. A step or a reduction is somewhere powder slows down, and somewhere powder slows down is somewhere it packs and eventually blocks. A full-bore path keeps flow resistance low, and keeps the valve from becoming the place the line chokes. The materials are verified for composition and hardness under material testing, and the ground sealing interface is checked against the drawing under dimensional inspection.

What it changed

Seat life went up several times over, and the replacement shutdowns that had been arriving every few months stopped arriving on that schedule.

Maintenance spend came down with it, but that is not how the plant counts it. What it got back was scheduled operation — the ability to plan a stop rather than take one, on a line where hours are tonnes.

Why this one came to us

We develop the structure and the materials ourselves, which means we can be asked why rather than what.

Erosion is not one mechanism. Particle size, hardness, velocity and angle of impingement each change what fails and where. A valve specified without knowing them sounds right on paper and wears out in the field anyway. So the work runs in that order. Diagnose the duty as it actually is, design the sealing structure around it, and only then choose the wear-resistant material to suit.

If your cement line is being stopped every few months to change a valve, the valve is probably not the thing to change.