The use of large capacity tanks and vessels is a common practice in a wide range of commercial and industrial enterprises.
Storage tanks hold liquids, compressed gases (gas tank) or mediums used for the short or long-term storage. Filling and emptying the tanks is accomplished through connected piping systems.
Tank protection equipment is fitted in various forms to protect against pressure, external ignition or contamination which could have detrimental effect on the products being stored or the vessel itself.
Atmospheric storage tanks are generally thin-walled and only designed to operate under low pressures. They are also primarily designed against the build-up of internal pressure and have extremely low resistance to vacuum.
Storage tanks – normal venting requirements
Changes in the liquid volume are caused by routine filling and emptying of the tank that inherently increase and decrease the vapour space
Changes in the temperature of the vapours and liquids in the tank are the result of variations in the ambient atmospheric temperatures. These fluctuations in temperature and volume also cause fluctuations in pressure.
Normal venting relief will allow the tank to breathe, preventing collapse or overpressure of the tank.
Pressure & vacuum relief valves are specifically designed to protect tanks from under or over pressurisation when correctly sized. They protect against the fluctuations in pressure associated with the general operation of a storage tank:
- Thermal Expansion
Elmac carry a full range of pressure only, and vacuum only relief valves, should these be required. Please get in touch with us today.
Storage tanks – the need for protection
Adequate tank protection is required to stop the catastrophic failure of a vessel.
A tank can be fitted with:
- a high level alarm, but this might be turned off due to faulty readings
- a pressure control system, which could be out of service
- a pressure relief valve with a flame arrester, which may be isolated after maintenance
Without any means to vent the build-up of internal pressure, when a product is transferred into the tank, it can easily become over-pressured until the roof ruptures, throwing it to the ground. And all despite only reaching a few psi over atmospheric pressure.
Collapsed storage tanks: vacuum hazards
Large storage tanks can easily collapse due to vacuum hazards. Things as innocuous as a nest of bees, or the rapid cooling of tank vapour space from a thunderstorm combined with a blocked or closed tank vent.
At Elmac, we have seen examples where, before the tank material was pumped out, a well-intended person had covered the tank vent to atmosphere with a sheet of plastic – probably to keep rain out, or to prevent debris from entering the tank. The result: complete collapse. Who would have thought that a thin sheet of plastic would be stronger than a large storage tank?
Well, it’s because large storage tanks are designed to withstand only a small amount of internal pressure, not vacuum (external pressure on the tank wall). It is possible to collapse a large tank with only a small amount of vacuum – the opposite of a soda can, for example: quite strong with respect to internal pressure, but it is very easy to crush an empty can.
So what can you do? Elmac recommends that you make sure that you keep a list of any covers left during maintenance or shutdowns and remove them before start-up. Never cover or block the atmospheric vent of an operating tank. And inspect tank vents routinely for plugging when in fouling service.
Storage tanks – emergency venting requirements
The temperature of the stored liquid and vapours may also increase as a result of the tank being exposed to an external fire.
Large volume of vapours will be generated as a result of this heat input, providing a means of discharging this large volume of vapours and prohibiting an increase of pressure within the tank is defined as emergency venting.
Emergency venting helps to protect the tank from catastrophic damage caused by an explosion which in turn protects the immediate vicinity of the tank where there may be people or other tanks.