A double mechanical seal 6 surrounds the rotatable shaft 3 where it enters the pump 1
. The double seal 6 is typical in that it includes inboard and outboard seal faces and a gland housing which, together with the shaft, define an internal cavity through which barrier fluid is allowed to circulate. The barrier fluid is stored in a header vessel or tank 7 and is directed to the double seal 6 by means of a downward inlet pipe 8. The inlet pipe 8 has external copper fins 8a attached to it to increase the rate of heat loss from the barrier fluid. The barrier fluid is recirculated back to the header tank 7 by an outlet pipe 9, provided with fins 9a, which joins the top of the header tank 7. The header tank is mounted on a frame 11 which is rigidly connected to the bed plate 12.