Recently a near miss occurred while a derrick-man and mud engineer was trying to drain a tote of additive into a mud pit. Several attempts were made to drain the tank but due to the consistency of the fluid these attempts failed. The 2” vent on top of the tank was opened and a water hose was attached with a crow foot connection. The 2” drain valve was opened at the bottom of the tank. The tank drained for a short time, then the drain valve apparently became clogged with the fluid. There was no other vent on this tank open and this caused the water to pressurize the tank. The tank was not rated for pressure so when the rig’s drill water (approximately 80psi) was stopped from escaping it buckled the tank.