There are two types of stuffing boxes used in the centrifugal pumps and other rotary pumps aboard ship. One type uses a mechanical seal, and the other type uses gland packing. Nowadays almost all new ships have mechanical seals on all the rotating pumps due to very strict pollution laws like Marpol 73/78 and others. Any fluid leakage has to be further collected, treated and filtered, with oil stored for discharge to shore reception facilities. Water is to be discharged according to the regulations by approved filtering equipment under a 15 parts per million guideline. It amounts to a lot of work, and any marine engineer would vouch for that.
However in certain applications, gland packing is still used where a mechanical seal may not work properly. In certain corrosive environments, for example, where a mechanical seal may corrode and fail, a gland packing may serve better. In applications like globe, gate, ball valves, and valve cocks gland packing is used because mechanical seals require a rotary motion for successful sealing action. It is for this reason that reciprocating pumps use gland packing. Sometimes a ship owner may want gland packing to be installed on his ships to cut costs. Also where skilled man power is not available, it is easier to open the gland to change the gland packing, than to open the whole pump for changing the mechanical seal. Also mechanical seals are very expensive as compared to gland packing. Old ships use gland packing extensively. However a certain amount of skill is required to cut the joints and in their installation and removal, as otherwise it is difficult to get consistent results and long life.
The Advantages of Gland packing over Mechanical Seals
Although considered old and low-end technology, the advantages of the gland packings over the mechanical seals are as follows:
It is an extremely reliable sealing method.
It is very simple to install and maintain.
It tolerates poor mechanical conditions like off-center shafts and worn down anti-friction bearings better than mechanical seals.
Works better in abrasive media and corrosive environments than mechanical seals.
Reduces the stock holding as one size packing can be used in all similar sized pumps.
It is very cost effective in down time as very little time is required to change a gland packing, as little as fifteen minutes.
They are less expensive than mechanical seals.
The whole pump is not required to be opened for changing the gland packing, however in mechanical seals the pump has to be opened up.
The gland packings are not fragile and any amount of mishandling would not destroy them, unlike the mechanical seals.