Thickener tank design options for operational and cost efficiency

By Gena Hart*

In such challenging times, reviewing and questioning the way we ‘normally’ do things can result in simple, clever and more cost-effective options. Really looking at the various options in thickener tank design, for example, can bring substantial rewards.

Thickener design and construction – what are your options?
1. Piece small fabrication – comprising many small individual pieces.
This option is cheap to fabricate, easy to transport but expensive to construct due to increased handling, fitting and welding at site. Steel tanks are commonly delivered in small pre-cut sections. Section sizes are often determined by standard plate dimensions and practical workshop fabrication methods. Transport limitations can also determine plate dimensions for delivery and site erection.
For piece-small delivery of tanks as described above, the ex works price is low. Site fabrication and surface treatment however, involve a significant investment of time, resources and, consequently, cost. Plates must be welded and usually site surface treatment will involve blasting and either painting or rubber lining.

2. Piece large fabrication – comprising large sections, major items pre-assembled.
Here the risk of misalignment of mating parts is reduced, and there is reduced installation time. This option, however, is difficult (but not impossible!) to transport. For example, a recent Outotec 22 metre diameter thickener was delivered in two halves to site. Obviously the access route to site had to be very carefully mapped out, ensuring all roads, bridges etc could cope with the diameter of this piece large fabrication. Once the pieces arrived, welding together at site took just 14 days.

3. Fully pre-assembled – one, fully constructed thickener
Cheap to fabricate and construct (depending on diameter), very difficult to transport. However, generally speaking, workshop fabrication results in a better quality final product.

4. Bolt together – large modular pieces, higher capital cost to fabricate due to significant number of bolts and flanges.
The higher initial capital costs, however, can translate into substantial overall project savings as hundreds of metres of site-welds and painting are replaced with a simplified fitting and bolting process.
For a typical welded thickener installation time of 12 weeks, it’s common for an equivalent bolted design to take 4 to 6 weeks, with less manpower. This option is proving to be especially popular in the mineral sands industry.

Benefits of the bolt together design include:
• Better value proposition overall as reduced total costs. Ex works cost is higher but as shop fabrication is significantly cheaper than site, with associated time benefits, overall savings can be impressive;
• Large reduction in site erection time and manpower. We would expect a 43m thickener to be fully erected within approximately 6-8 weeks;
• Shorter delivery time on an installed basis due to reduction in site time;
• The majority of the fabrication is performed in the workshop, providing better control of both fabrication and surface treatment quality;
• Increased safety by reduction in site installation processes – no welding and no/minimal site surface treatment (blasting, painting, blast containment);
• Low risk install - less time on site, little/no surface treatment, minimised exposure to inclement weather, wind (blast containment) and also minimised exposure to industrial disputes.

Design standards
As there is no specific published methodology for the design of a bolted thickener tank, Outotec has made significant investment conducting research into the behaviour of flanged tank joints. Through extensive testing and analysis, the company has developed specialised methods for the design of bolted tank connections.
Quality and structural integrity is ensured through meeting design requirements of particular codes and standards. Outotec’s flanged tanks meet strength requirements of API650 (welded steel tanks) and AS3990 (allowable stress design for steelwork).

Flange joint sealing
Joint sealing is another important consideration in the design of bolted tanks. AS1210 (pressure vessels) is referenced as part of Outotec’s design philosophy, ensuring bolted joints are designed for no leakage.
Joint surfaces on the inside of Outotec bolted tanks are usually finished with sealant, selected specifically to meet process and application requirements. To maintain structural integrity, no sealant is applied to mating faces of flanges.

Manufacturing
Obviously, a reputable, experienced manufacturer is vital to ensuring a quality product and is particularly important in the case of bolted tank design. If your thickener supplier has a long, proven track record in bolted tank design, this will help ensure their manufacturer will provide a quality product. Reputable manufacturers will use purpose-built jigs for floor plate assemblies to achieve complete accuracy. They should also insist on including trial assembly of components for dimensional checks and to ensure problem-free assembly on site.

Installation
This step is particularly critical and it is strongly advised that you use either your technology supplier for installation or at least for installation supervision. An installation by an inexperienced workforce will potentially undo all savings from the bolted design option, not to mention OH&S considerations.

Bolted tank construction is an attractive option in many industries. Benefits include faster installation, lower cost and better quality control. Although the ex-works capital cost (fabrication) can be higher, the overall (fabrication & install) value for money can be better with a bolted design. Given that the construct and install phase of a thickener’s birth constitutes 75 per cent of the time (and time = money), it makes sense to not just look at a thickener’s capital cost, but also take installation time, risk and final quality of workmanship into consideration. In these challenging times, looking at options outside the box can bring real rewards.

* Gena Hart is currently the technology leader in thickening for Outotec in Perth, Australia.
For more information contact tel: +61 (0)2 9984 2500 or email: laura.white@outotec.com