Bed Limiter

A bed limiter is a grid laid on top of a packing bed to keep the packing from moving. In a packed column the gas travels upward through the bed while the liquid trickles down, and the faster the gas goes the harder it pushes up on the packing. Past a certain rate, or in a sudden surge, an upset, or a burst of vapour, that upward force can lift the top layers of packing, let them dance and rearrange, or even fluidize them like a boiling bed. Light packing, the metal and plastic kinds, is the most easily disturbed. When the bed shifts like this it stops being level and evenly packed, so the liquid and gas no longer spread across it properly and the separation suffers; the packing also grinds against itself and wears. A bed limiter prevents all of that. Sitting on the top surface of the bed, it holds the packing down and keeps it level and compact, while its open grid lets the gas and liquid pass through with almost no added resistance. It is a small, simple part that protects the performance of the whole bed.

They do the same job, holding the top of the bed down, but they are built and mounted differently, to suit the packing beneath. A bed limiter is light and is fixed to the tower wall, bolted to a ring or clipped in, so it does not rely on its own weight; it is the choice above metal and plastic random or structured packing, which is light and would be lifted easily. A hold-down plate is heavier and is not attached to anything: it simply rests on top of the bed and holds it down by its own weight. That free-resting design is used above ceramic packing, which is heavy and brittle, since a wall-fixed grid could trap the bed and let the ceramic crush itself as the bed settles, whereas a loose weighted plate can move down with the bed as it beds in. So the rule is: a fixed, light bed limiter over metal or plastic packing, and a free, heavy hold-down plate over ceramic. Tell us the packing and we will supply the right one.

They are a matched pair, working from opposite sides of the bed. A support plate sits underneath the packing and holds its weight up, carrying the load of the bed and the liquid it holds down onto the tower's support ring. A bed limiter sits on top of the packing and holds it down, stopping the gas from lifting it. One resists gravity from below, the other resists the gas from above. Both are open grids that let the gas and liquid through freely, and a well-designed bed has both: the support plate at the bottom keeping the bed off the floor of the section, and the bed limiter at the top keeping the bed from floating. They are usually made of the same material as each other for a given service. Send us the tower and the packing and we will size the support and the bed limiter together.

A bed limiter is made to match the packing and the chemistry, so it comes in stainless steel, 304 or 316L for corrosive duty, in carbon steel for mild service, and in plastic such as PP where the gas is corrosive but cool; a ceramic hold-down plate is used over ceramic packing. It is cut to the inside diameter of the tower, as a single grid in smaller columns or in sections that pass through the manway in large ones. Its place is at the very top of each packing bed, directly on the packing and just below the liquid distributor that feeds that bed; in a tall column with several beds, each bed has its own. Because the listing supplied gives no dimensions, material or model, the details here are the usual ones for this part, so please confirm the size, the material and the mounting for your tower before ordering. Send us the column diameter, the packing and the service and we will detail the bed limiter.