Trip tank is a small tank which has a capacity of 20 – 50 bbl and its shape is tall and shallow because it can effectively detect volume changes. The trip tank system has the ability to continuously fill the well and take return back to the tank. With this capability, it will keep the hole full all the time and the volume changes either increasing or decreasing can tell the condition of the well.
The diagram (Figure 1) below demonstrates how the trip tank is lined up.
Figure 1 – Trip Tank Line Up To Continuously Fill The Hole
Each trip tank has a pump which will suck the fluid from the tank and pump into the well via the fill up line connected to a bell nipple under the rig floor. The fluid return will flow back via a return line and back to the trip tank. The float in the trip tank is connected to the wire and the position of the float will represent the trip tank volume indicator. What’s more, nowadays several rigs have installed the electronic instrumentation for the accurate volume measurement. This will help personnel on the rig track what is going on the well very quickly and accurately. As you can see, the complete system allows personnel to monitor the well.
The trip tank must be maintained in order to avoid solid build up, pump and valve failure, leakage, etc. Moreover, it is very critical to frequently check the float and the electronic instrument to see if they are in good condition.
Stripping operation requires a separate trip tank which has very small capacity of 3 to 4 bbl therefore it is not recommended to use the normal trip tank for this operation. The small volume tank, called “strip tank”, has more accuracy and suite for the operation.
How The Trip Tank Monitor The Well For Well Control
Trip Out of Hole
While pulling out of hole, each stand of drillstring pulled out must have the same amount of drilling fluid to replace the drillstring volume. For instant, each stand should take around 0.8 bbl. If you pull 10 stands out of hole, you should see at least 8 bbl of mud volume decrease in the trip tank. If you see the volume displacement less than what it should be, it indicates that the well is swabbed in.
Figure 2 - Trip Tank While Tripping Out
Trip In Hole
While tripping in hole, mud will be pushed out of the well to the trip tank because steel displacement will replace the drilling fluid in the well. The volume displacement should be the same as the steel displacement. If the volume displacement is more than the steel displacement, the well may has some unwanted kick in the well.
Figure 3 - Trip Tank While Tripping In
Flow Check
While flow checking, the volume in the trip tank should be at the same level. There should not be any changes. Increasing volume in the trip tank means the well is flowing. Conversely, if the volume decreases, the well has static loss.
Figure 4 – Trip Tank While Flow Checking
Reference books: 
Well Control Books