GRP Tank Covers: How to meet regulatory requirements in a cost-effective way

Background

Where are GRP tank covers typically used?

GRP covers are widely used throughout the water-treatment industry due to their combination of mechanical performance, corrosion resistance, chemical resistance and low weight. The primary purpose of these covers is to help minimise the release of unpleasant odours to the neighbouring environment (hence these covers are also commonly referred to as odour-control covers). Depending upon the specific application and location, these covers may also be employed to keep foreign bodies out of the tank.

Are all GRP tank covers the same?

The short answer is no; there are various design and construction approaches, to suit the wide variety of tanks and other vessels which may be found in a typical water treatment plant. Broadly speaking, these can be grouped as follows:

• circular (or conical) tank covers;
• beam & infill tank covers; and
• bespoke tank covers.

In this article, we’ll base our discussion around circular (or conical) covers, but many of the topics apply equally to the other cover types.

What regulatory requirements apply to GRP Covers?

Whichever design is chosen, the covers will typically need to comply with the requirements set out in the Water Industry Mechanical and Electrical Specifications (WIMES) and the Eurocodes (BS EN 1991) which, between them, outline the loads which each cover must be able to withstand. These include:

• dead loads (i.e. self-weight of the cover in service and during installation or removal);
• access loads (including concentrated and distributed-personnel loads);
• wind and snow loads;
• impact loads;
• any other imposed loads from cover-mounted equipment;
• loads resulting from environmental/climatic conditions; and
• buckling load factors.

WIMES also specifies the maximum permitted deflections of the installed covers and outlines the safety factors which should be taken into account during the design phase.

What does the tank-cover industry look like?

With a relatively large range of suppliers to choose from (spanning GRP, metal and thermoplastic covers), tank cover buyers are often in a strong position when negotiating with tank cover suppliers and, rightly or wrongly, price can often end up being the key factor which influences purchasing decisions. Therefore, in order to remain competitive, fabricators of GRP tank covers must find the right balance between satisfying the WIMES and Eurocode requirements and minimising costs.

Challenges for the Fabricator

Demonstrating compliance

To demonstrate compliance with WIMES and the Eurocodes, fabricators are required to provide detailed structural calculations which show that the proposed cover design is able to withstand the anticipated load cases. Most fabricators outsource this task; often due to a lack of in-house structural engineering capability or, even if this capability does exist, because they would prefer to free up their staff to focus on other activities.

Some fabricators see a potential additional commercial advantage in this outsourcing, as it allows them to promote the fact that their cover designs are backed by structural analysis from an independent expert. Of course, this argument carries more much weight if the independent expert in question is working for a reputable firm with a demonstrated track record in the field.

Remaining competitive by providing a cost-effective solution

There are slight variations from manufacturer to manufacturer but, essentially, circular covers all have the same basic design (as shown below), whereby multiple wedge-shaped segments are produced (usually by hand-layup) and then bolted together along flanges to form a pitched roof over the tank. These flanges then act as ribs which help stiffen the final structure. In some cases, these segments will be bolted together off-site, with the whole cover then being transported to site and lifted in place by crane. In other cases, the cover will be constructed in situ.

Thickness, T	Laminate construction
T2	2 x layers of chopped strand mat (CSM)
T3	3 x layers of CSM
T4	4 x layers of CSM
T6	6 x layers of CSM
T2 + 6UD + T2	2 x layers of CSM 6 layers of unidirectional fabric 2 x layers of CSM
S-S and T-T represent beam sections of differing dimensions, both constructed using T2 laminate skins around polyurethane foam cores.

Laminate constructions

Circular tanks come in a variety of sizes, and diameters can range from one metre to over 30 metres. Fabricators, therefore, need a flexible manufacturing process which can accommodate this range. On the one hand, changing the diameter of the cover is relatively straightforward. In theory, all the fabricator would need to do is vary the length of each segment by adjusting the length of their wedge-shaped mould tool accordingly. Furthermore, to save on tooling costs (and save space in the workshop), the fabricator could just produce one tool suitable for their largest tank, then simply shutter it up to accommodate smaller wedges. However, this only works up to a point.

Let’s assume that Fabricator A wants to be able to offer covers suitable for the ‘full range’ of tank diameters. The largest of these might be 25 metres in diameter and, for whatever reason*, Fabricator A might decide that a cover of such size needs to be constructed from 24 individual segments, where each segment is manufactured using 12 mm thickness of GRP, with stiffening ribs positioned as shown in the above example. Then, at some point in the future, Fabricator A is approached by a potential customer who wants a tank cover which is just 5 metres in diameter. If Fabricator A were to use the same design and layup for this smaller cover, each segment would be unnecessarily small and, in all likelihood, the laminate would be unnecessarily thick – resulting in excessive materials usage and overcomplicated fabrication.

*for example, to achieve the required stiffness, or for ease of handling and transportation.

Clearly, then, a one-size-fits-all approach to the generic design is not appropriate. Instead, each fabricator will typically have a small range of generic designs. For example (as shown below), a 24-segment cover (for the largest covers) a 12-segment cover (for the smallest covers) and an 18-segment cover (for the intermediate sizes).

The role of the structural analyst

Understanding the customer requirement

This might seem like an obvious point, but what we are really talking about here is understanding the customer’s strategy in regard to demonstrating compliance with WIMES and the Eurocodes. To do this, we start by agreeing upon two things:

• the load cases; and
• the tank diameters to consider.

Identifying the relevant load cases

Access loads and impact loads are clearly defined within the standards and there are few instances where one could justify deviating from the given values. Likewise, the weight of the cover will be fixed (although it is important for the fabricator to specify whether the lifting load case should be considered and whether the cover is required to be self-supporting), as will any loads arising from cover-mounted equipment.

Wind and snow loads, however, are a slightly different matter. In both cases, the load case is dependent upon the installation location, taking into account the geographical area and the local environment. So, for example, the wind load in a built-up area in central London might be ~ 0.4 kNm^-2 whereas, in a coastal location in the North of Scotland, the wind load would be more like 1.4 kNm^-2. For the cost-conscious fabricator, this difference is sufficiently significant to consider a different laminate construction in each case…. so as not to use more material than is necessary for the more benign location. The downside of this approach is that each of these tank cover variants would require a separate set of structural calculations.

Then we need to consider item number 2 on our list…

Deciding which tank diameters need to be included in the analysis

The good news here is that it’s not always necessary to carry out structural analysis for each and every possible diameter of cover (assuming the laminate construction remains the same for each). Instead, it may be possible to consider just the largest possible diameter cover for each generic design, if we are comfortable that this represents the worst-case scenario. So, going back to our earlier example, Fabricator A’s largest generic design might be intended for tank covers between 15 and 20 metres. All else being equal, it would be reasonable to consider only the 20-metre variant when carrying out the analysis.

As with any other analysis, we start with a proposed design and laminate construction. We then begin to apply the various load cases and, where necessary, modify the design and or laminate construction as required.

Summary

What the fabricator must decide, therefore, is where the greatest cost savings are likely to lie. Is it better to design the covers for the worst case scenario in terms of wind and snow loads, and therefore only have to purchase a single set of calculations for a given cover diameter? Or would this, in the long term, actually end up costing the fabricator more money in unnecessary material costs, in which case the cheaper option would be to obtain separate calculations for each installation location? The reality, of course, is likely to be somewhere in the middle. In practice, therefore, many fabricators choose to obtain a single set of calculations to cover the majority of locations in which they anticipate having to install their covers. Any installations outside of these areas would then be considered ‘specials’, requiring separate calculations.

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