At Rapallo, we spend a lot of time dealing with tanks, designing them, inspecting them, maintaining them, and fixing them. Spending so much time with and around tanks has given us a good understanding of exactly why and how they should be managed, which is why today we are answering one of the most common questions we get.
“What’s going to happen if I don’t do that bit of maintenance on my tank.”
This question is asked of us at Rapallo quite often, and for good reason, maintaining tanks can be expensive and the tanks themselves are often massive structures typically made from thick plates of high-quality steel, fibreglass, or plastic material and on the initial impression it can be hard to comprehend how something like that can fail. But they do, and when they do it usually costs a lot of money.
So let us take a deep dive and have a good look at this question: What actually IS Going to happen if you forget to do all the tank maintenance? Firstly, we need to talk about what tanks do, and what sort of conditions they operate in.
Storage Tanks – What do they do?
Storage tanks, like the name implies are used to store things usually some type of liquid, they can be constructed out of steel, fibreglass or plastic, we will be considering steel (and stainless steel) tanks. These tanks can hold capacities of over 1 million litres. The sorts of liquids stored in these tanks could be anything from plain old water, process solution (sometimes at pressure and/or high/low temperatures), a flammable substance such as fuel, or a dangerous chemical such a cyanide solution or concentrated acid solution. Ultimately the purpose of the storage tank is to hold a significant quantity of potentially dangerous liquid either stand alone or in a circuit or process.
Storage Tanks – More than a big bucket
The main forces acting on the tank are those caused by hydrostatic pressure, this is the pressure caused by the weight of the liquid in the tank. As anybody that has carried a large bucket of water knows liquids such as water are actually pretty heavy. Storage tanks must have the strength to hold up to thousands of kilograms of pressure created by this weight, for quick reference every litre of water weighs 1 kilogram. Thus, a tank holding 100,000L of water is supporting 100,000 kilograms of weight.
Even knowing the weight that is inside a tank it’s easy to try and think of a storage tank as a large bucket, and thus easy to forget that to keep them in good working order they need proper maintenance, buckets don’t need maintenance after all. However, due to the difference in scale, the forces on a storage tank are proportionally much larger than a bucket. This is because of the square-cube law, which describes the relationship between the volume and surface area of a shape as its size increases. To put things in basic terms, as the size of a tank increases the weight of the water inside the tank gets heavier much faster than the supporting surface between the tank and its contents.
To use an example, imagine we have a bucket that holds approximately 12.5L or 12.5kg of water, that bucket might be 40cm wide and 10cm tall, and the surface area of the walls touching the liquid would be equal to 0.1257 square meters. This means for each kg in the tank we have 0.01 square meters of walls holding it in.
If we now consider a tank that holds approximately 12,500L or 12,500kg of water that tank would have its dimensions increased proportionally, thus it would be 4m wide, and 1m tall, in this case, the surface area of the walls touching the liquid is equal to 12.57 square meters, which means for each kg in the tank we only have 0.001 square meters of walls holding it in.
This contact surface per kilo of the larger tank is only 10% of the smaller bucket, meaning our materials must be at least 10x stronger to compensate. This same law is true of the tank supports and footings that must avoid cracking under the weight.
Storage Tanks – Why do they need to be maintained?
Back to our initial question “What’s going to happen if I don’t do that bit of maintenance on my tank.” The truth is there probably won’t be any problems – at least not in the short term. Storage tanks are a bit like strong people at the gym, at the start of the workout they’re strong and lifting heavy weights is no problem, but over time if care isn’t taken, they can grow fatigued and eventually fail. Storage tanks are the same, whilst in good condition they won’t have any issues meeting their operating specification but over time corrosion, or fatigue can creep in which may eventually cause a failure.
For steel storage tanks corrosion can be a particularly silent killer, it’s difficult to imagine how forgetting to paint a tank can be the cause of its demise, however in many cases this is all tanks to eventually cause a major tank failure. Once paint has been removed corrosion of the steel can occur and leading to cracking or thinning of the steel and eventual failure.
Some storage tanks are open to the atmosphere. Others may have purposely designed roofs for containment or to provide protection from environmental elements. Maintenance of tank roofs is equally important.
Thus far we have been talking about the tank’s structure but much of this applies to the foundations as well, for instance, concrete can degrade over time and can eventually crack and fail if not attended to. Ground movement/instability if not arrested could jeopardise tank integrity. In a best-case scenario, operation needs to be shut down for a period to rectify the fault, however in a worse case scenario catastrophic failure can occur; in this scenario, there is the possibility for fatalities of nearby workers, irreparable environmental damage, and the loss of the millions of dollars of value in both production and reputational terms.
Storage Tanks – How to keep them in their best condition.
The manufacturer of the storage tank should have an inspection and maintenance routine set out within their documentation that outlines the best way to care for their particular storage tank, in general, it will include various inspection activities at different periods in the tank’s life to look for and find signs of damage, some of these may simply be a visual inspection whilst others could involve complicated non-destructive testing (NDT) methods such as Ultrasonic thickness testing (UTT) or Magnetic flux leakage floor scanning (MFL). Further discussion of inspection and maintenance activities will be included in a future blog or contact Rapallo if you need immediate assistance.
If the manufacturer has failed to provide instructions on the best way to care for their storage tank the next best thing is to get guidance from competent professionals. Rapallo does offer services to optimize tank maintenance strategies and can carry out any inspection and testing you require, having dealt with so many tanks you can be rest assured that we know what we’re talking about when it comes to tanks.