Pressure and vacuum are two big factors that can stress an industrial bulk container. Unless a tank is designed for specific operating pressure or vacuum, storage tanks including plastic and fiberglass are generally designed to be used at atmospheric pressure. Steel tanks can be introduced to a pressure of usually 2 PSI. In this article, we will concentrate on the influence of pressure and vacuum in plastic storage tanks. Introducing pressure or vacuum in plastic storage tanks can shorten the expected service life. In some cases, exceeding the vacuum and pressure design can result in a catastrophic failure of the tank.
What can cause pressure or vacuum in daily operation:
To avoid creation of pressure or vacuum in a tank, the plant engineer should develop an operation guideline and take into consideration the following:
- FILLING AND EMPTYING THE TANK TOO QUICKLY – It is strongly recommended that the tank will be filled and emptied in a slow rate to allow the venting system to displace the liquid volume with air. One needs to make sure that the venting system is allowing enough air movement to and from the tank.
- OVERFILLING THE TANK – This situation needs to be avoided since it can create pressure in the tank.
- PRESENCE OF HARSH CHEMICALS – Some chemicals release fumes by their nature. In many cases, these chemicals should not be openly vented to the atmosphere and are restricted by local ordinances. A scrubber system or other fume extracting devices are used to capture the fumes or odor. In the design of those scrubbers, we need to take into consideration and make sure that we are not restricting adequate air flow to and from the tank. A clogged scrubber and long piping to the scrubber are causes for restricting the air flow.
- TEMPERATURE – Elevated temperatures will increase the vulnerability of the tank. If a tank sits outdoors, pressure can build up and stress the container further.
The Human Factor
Poorly trained or inattentive operators may not know how much liquid is already in a chemical storage unit, or they may not know how to read the gauges that indicate this information. It could also be that they are at the end of their shift and make a mistake in their rush to finish up and get home for the day. The venting design of the tank should have a high margin of safety to avoid these situations that can cause pressure or vacuum.
Allowable Pressure/Vacuum Restrictions
Generally, rotationally molded tanks will have a more stringent restriction than thick wall extruded plastic tanks. Some rotationally molded companies will allow a maximum pressure of .36 PSI. Extrusion wound tanks can allow a maximum additional induced pressure of 1 PSI. This can be increased to 2 PSI with additional structural supports. We recommend, to always consult with the manufacturer regarding the maximum allowable pressure and vacuum in their tank.
Consider the Following Instrumentation
- Emergency pressure/vacuum relief valve
- Electronic monitoring of the liquid volume in the tank
- Visual monitoring of the liquid volume in the tank such as reverse float level indicators
Accumulating Damage Over Time
Unfortunately, stress cracks can form in plastic storage tanks yet still go unnoticed for long periods of time. Tanks need to be visually inspected periodically to look for any discoloration or cracks which indicates a development of stress. The manufacturer should be contacted and consulted in this situation.
Questions Regarding Pressure or Vacuum
Please feel free to contact us to discuss these import pressure and vacuum issues in more depth.