This "TIPS" article applies to holding tank systems that are normally found on recreational and smaller commercial vessels.
When sewage is pumped into a holding tank air* is force out of the tank through the tank vent system. Likewise, when a holding tank is pumped by a Dockside pump out system, air enters the tank through the same vent system. If the vent system becomes clogged or restricted a vacuum (negative pressure) will be generated in the tank as the sewage is withdrawn. Dockside pumpout systems can generate a very high vacuum of more than twice the following example.
EXAMPLE: A tank is subjected to a vacuum of 10 inHG (inches/Mercury). This will result in a pressure of approximately 5 psi (pounds per square inch on the sides of the tank. A 100 square inch area (10X10) at 5 psi results in a force of approximately 500 pounds. This is why it is so important to have a properly designed, installed and maintained vent system. Anytime a vent system for any reason becomes (clogged or restricted) and cannot satisfy the "withdraw rate" (rate the sewage is being withdrawn) by the pumpout system a negative pressure (vacuum) is created in the holding tank. The value (Hg) of the vacuum generated will depend on the rate of "satisfaction" respective to the withdraw rate and vacuum potential
The quality of a holding tank system and its suitability depend upon the design, materials used and installation. NOTE: Most vent systems failures are caused be the accumulation of sewage debris due to overfilling of the holding tank.
NOW TO THE POINT: Stay/Side Systems recommends that a Sealand "Tank Saver" holding tank relief valve be installed to protect holding tanks and attached components from this potential problem. NOTE: Future "TIPS" pages will expand on the effects of this subject as it applies to different head systems and the configuration of these systems.
*Primarily sulfide gaseous odors resulting from Anaerobic decomposition.