Piston tanks are horizontal tubes that have a stabilized
piston with seals that separate the air pressure behind the piston
from the product between the piston and the tank outlet. This
stabilized piston keeps the product separate from the air pressure
used to push the piston. Instead of depending on the product to
flow or slump into the pump suction, this type of trailer extrudes
it from the trailer by the force of the stabilized piston to the
pump suction. After unloading, no significant product exists inside
the tank.
Since piston bulk tanks were first developed, improvements continue to be made in their design. These improvements have enhanced piston position monitoring and piston seal reliability.
Single Compartment vs. Triple Compartment Piston Bulk Trailer Tanks
Product Monitoring
It is essential both during loading and unloading to monitor the position of the piston correctly. Standard Varec tank gauges have always monitored piston position in the past. A strong cable is attached to the back of the piston and extends out of the trailer to the Varec tank gauge. These gauges were designed for use in vertical tanks. Use in horizontal tanks is more difficult. Strong tension must be maintained on the cable to avoid inaccurate readings. The cable can slump giving a false reading. They also need routine maintenance to remain effective. Far too often they cease to work properly.
Several attempts were made through the years to
find a non-mechanical way to monitor. All failed because of the
sidewall interference encountered when using a sender/receiver
in a long narrow tube. Laser units were investigated to eliminate
this problem. When coupled with a custom designed microprocessor,
these devices provide a more accurate and dependable piston position
monitor. Our improved version of the electronic laser gauge system
now allows our customers to monitor the unloading process remotely
as well as at the tanker.
Piston Seal
Unexpected and catastrophic seal failures presented another problem in the operation of the piston tanks. Modifications continued throughout the 70s. By the 80s all seals used were pneumatic. Air pressure within the seals provided the necessary force to keep the seal effective when the tank diameter or roundness changed. As a result the seals were susceptible to unexpected and immediate failure. When there was a rupture in either the seal or its valve, the air pressure was immediately lost making the piston totally ineffective.
New seals have been developed and patented. These seals used compressed foam rather than air to expand the outer seal tightly against the sidewall. These foam seals are even more effective in piston tank service and have eliminated the fear of unpredicted seal failure.
