4 replies to this topic

[setak]

Hi,
Can anyone tell me how to calculate the thermal insulation thickness for a firewater storage tank, to prevent freezing.
Thank you in advance.

[Art Montemayor]

Setak:

You hold the key unknowns and data in your knowledge, so we can only indicate how to resolve the problem - without any specifics.

Firstly, I have to assume you have a continuous circulating fire water loop system: a centrifugal fire water pump is constantly circulating some water flow throughout the system - including the tank.

We don't know where you are located and what your climate conditions are, so I have to assume they can get severe - such as in Canada.

The logical way to ensure your firewater system is always prepared to operate effectively is to ensure that ALL the water in your system is in the liquid state. The commonsense way to ensure this is to keep your water inventory constantly moving during the winter. This means constant movement throughout your system. Do not leave any dead-headed or ultra low velocity sites within your entire inventory. You have to achieve successful adiabatic mixing of the entire contents, and this is a big contributor to keeping your water inventory as liquid, and not ice. Therefore, the physical design of the piping and tank are very important to ensure complete mixing. Once you achieve this, the next objective is to insulate the entire system and allow for a system heater - usually in the line or in the tank itself. The size of the heater and the amount (thickness) of insulation are related. This involves an optimization routine of calculations where you pit the amount of insulation to the cost of operating a heater. Depending on the size of your system, the weather conditions, the conditions of the equipment, and the cost of the heater heat input, you should be able to come up with an estimated optimum heater size. Do not forget to include the pump energy that is dissipated into the water due to pump throttling during circulation (assuming you are not using pump speed controls).

I would design for keeping the circulating water flow at 35 oF - minimum. The design temperature depends to a large extent on the severity of your winter. The more severe the climate, the higher the design temperature for the circulating fire water. The temperature also depends on how you will apply the fire water. If you are relying on sprinklers, you must allow for heat leaks through the sprinkler heads. If you are using fire water cannons ( or guns), then you must heat trace these constantly because of the difficulty of insulting these.

I hope these hints help out.

[setak]

Dear friend, Thank you for your helpful description.
Please let me explain more about my problem.
We have a 5000m3 firewater storage tank which is filled continiusly from a well throught a pipeline.
The owner asked me to insulate the tank to prevent freezing in the winter.
I know that the insulation can not prevent freezing but can prolong it, my question is the period which we shall consider for freezing in our calculation.
For example if we want to prolong the freezing 72 Hr we need more insulation thickness than 24 Hr.
Is there any reference or standard.
Thank you

[Zauberberg]

Check this software - it's quite good for insulation calcs: http://www.pipeinsul...4/download.html

[djack77494]

In very simple terms, you could calculate the heat losses when the tank contents are at 0 C/32 F. Then take the incoming water temperature and figure out how long it would take to cool to freezing based on the calculated heat loss. The heat loss, of course, depends on your insulation thickness, so that gives you the relationship between insulation and time to freeze.&nbsp; <BR><BR>An even better answer is to use the above PLUS Zauberberg's answer; do manual calcs and confirm using one of the excellent heat loss software packages available, many at no cost.

Edited by djack77494, 15 December 2009 - 05:53 PM.