The packing support plate holds the whole bed of packing in an ammonia stripping column, so it has to be strong enough for the weight and open enough to let air and liquid pass. In a replacement quotation for a landfill leachate plant, the upper plate was 3.7 m in diameter, made from 3 mm 304 stainless steel with a tie brace and a separate support lattice welded to the column on site.
The challenge: a thin plate carrying a heavy bed
An ammonia stripping column passes leachate down through a bed of packing while air moves up through it. The packing creates a large surface area for the ammonia to transfer from liquid to air. The packing sits on a plate near the bottom of each bed. The plate is perforated or open, so it does not block flow, but the bed above it is heavy, particularly when wet.
When a plate corrodes, distorts or fails, packing can fall, flow becomes uneven and the column loses performance. Replacement means entering the column, so the part needs to be right first time.
The approach: a plate and a lattice, fabricated off site
The plate
The plate quoted was 3.7 m in diameter, in 3 mm 304 stainless steel, with a stainless steel tie brace. A plate this thin cannot carry the load across such a span unaided.
The lattice
A separate support lattice is supplied to let the plate hold a greater weight. The lattice has to be welded into the column on site, which is why installation was excluded from the supply: it is a fit-out job inside a vessel, requiring a welder and a safe entry procedure.
The same design for the lower plate
A second quotation, for another line, offered the same size of lower packing support plate with its lattice, plus a replacement 316 stainless steel feed pipe for the column. Both were manufactured in Thailand, with four to six weeks to make and about two weeks of sea freight.
The packing itself
A further quotation covered replacement packing: 38 mm (1.5 inch) polypropylene saddle rings, supplied by the cubic metre, with delivery of five to six weeks to port and no installation. Saddle rings are a standard random packing shape. Polypropylene is light and resistant to many chemicals, which keeps the load on the support plate down compared with a heavier material.
Why it works
Splitting the job into a plate, a lattice and a feed pipe lets each part be fabricated and checked off site. The lattice, welded to the shell, gives the thin plate the support it needs to carry a heavy bed. The feed pipe was specified in 316 stainless steel.
In the UK, entry into a vessel is a confined space activity, with duties under the Confined Spaces Regulations. See the Health and Safety Executive’s confined spaces guidance.
What to consider for your own project
- Know the loaded weight, including wet packing and liquid hold-up.
- Plan the entry. Welding and fitting a lattice means confined space work.
- Check how the parts will get in. A 3.7 m plate has to pass through the existing access openings, so ask how it will be delivered.
- Replace packing and support together if both are due, to save a second shutdown.
- Allow lead time. Fabrication and sea freight add up to about two months.
For the design of the stripping process itself, see our post on ammonia stripping waste heat.
FAQ
What does a packing support plate do?
It holds the packing bed in place and lets liquid and air pass through.
Why is there a separate lattice?
The lattice carries the load so that a thin plate can span a wide column.
Is installation included?
Not in this quotation. The lattice is welded to the column on site.
What packing was used?
38 mm polypropylene saddle rings in the related packing quotation.
Replacing column internals?
Send us your column diameter and packing details and we will advise on the support design.