While in the process of cleaning the rig mud pump suction tank, two employees had entered the tank to perform the task. A Work Permit had been completed and a JSA pertaining to Lock-Out procedures had been reviewed prior to entry into the tank. However, it had been overlooked that the discharge line from #2 pump pressure relief valve discharged into this tank compartment. The #2 pump had not been isolated, nor had the pump bleed off line been opened. Fortunately, the Supervisor observed the danger and removed the employees from harm before and incident could occur.