Fran:
First, I am not an advocate of employing underground storage tanks for chemicals. I believe that you will find local regulations and environmental agencies frowning on this application – if not outright prohibiting it. There are other, more practical reasons for discarding the use of underground storage tanks – especially those made of steel. Some outstanding reasons are:
1. The inspection and control of potential corrosion is an ongoing headache; one can never be 100% sure of ensuring 100% mechanical integrity, simply because of the inability to visually inspect both the inside and the outside surfaces of the tank (I’m assuming the tank is
buried, not in a concrete pit; you didn’t specify which).
2. The possibility of a leak through the walls is a major environmental event; you have literally no means to contain it. All you can offer environmentalists is that you will make all efforts to monitor potential external leaks and react with immediate pump-out of the chemical to an available and ready, above-ground tank. Contamination of the earth would take place regardless of the speedy action and you require an investment in an
additional ready, available, above-ground tank. And, don't forget the obvious: how do you repair a wall leak? The answer is with a very costly shutdown and vessel entry.
3. The utilization of a reliable heating system is of prime importance for an underground tank – more than for an above-ground tank. For reliability, you pretty well have to discard the use of conventional, low-pressure steam and the customary steam trap. I am confident that you would have to employ a liquid heating fluid with recirculating pump(s) and the necessary instrumentation to ensure 100% operation and reliability in order to avoid a freeze-up within the underground equipment.
4. A required tank inspection is a dreaded job simply because of the need for a confined space vessel entry. For chemical service, this is a hard reality and a task that must be done from time-to-time in order to ensure the tank’s integrity. This is bad enough for an above-ground tank; for an underground tank, this is considered a virtual
NIGHTMARE. The personnel risks are compounded due to the need to ensure emergency evacuation through vertical manways that are oriented only in one direction: up and against gravity. Safety techniques to evacuate personnel through vertical openings are harder to carry out than for those in multiple, horizontal orientation.
5. The required pump(s) needed to transport the chemical almost have to vertical, submersible type. This is a special type of pump and requires that two of them be installed – in separate locations. With a conventional, above-ground tank, you can use one common suction header and conventional, centrifugal pumps.
6. The need to clean out the underground tank – to effect inspection, for example – can be a major undertaking and a headache – operationally and economically. How do you effectively drain and flush the damn thing? Obviously, you have to dedicate a lot of positive, engineered circulation of flush fluids and purging – more contaminated fluid to safely get rid of!
There are other reasons, but the above are my major concerns.
I don’t have any problems with a 16 oC freezing point fluid. I’ve succeeded in designing and modifying Phenol above-ground, API 620 storage tanks. Phenol has a freezing point of 41 oC, is an extremely hazardous material that can cause systemic poisoning, behaves like a Class IIIA combustible liquid and has a flash point of 79 °C. In spite of its multiple bad characteristics, Phenol is still handled above-ground – where it can be monitored and controlled for operability and safety reasons. I would recommend that you model your storage system in a similar manner. For a reference on Phenol, go to:
I realize your concerns may not relate to toxicity, but the operability and other safety concerns are mutual and common with the handling of Phenol.
I hope the above experience is of some guidance to your application. I do not apologize for this lengthy response because I strongly feel that the latent safety issues concerning the operation and maintenance of underground tanks are worth the effort.
