Stuffing Box (Piston rod gland box) is a unique feature of 2-stroke marine diesel engine.
In 2-stroke marine diesel engine, piston rod is used which is large and needs separate scavenge space or under-piston space and crankcase space. This is achieved by this stuffing box (Piston rod gland box).
It is nothing but a seal between crankcase and under-piston space (scavenge space).
It prevents the dirtying of the engine bearing oil by combustion residue.
It seals the charge air pressure in the piston underside.
It scrapes the engine bearing oil from the piston rod.
It is an assembly of different type of rings and generally, these rings are divided into 3 groups, each consists of segmented rings:
Upper gland group: It scrapes the dirty oil, contaminated by coke residue from the piston rod. This dirty oil is drained along the slanting bottom of the cylinder jacket and flows into a collector main pipe. Sealing ring stops the charge air escaping out of the under-piston space. All upper gland rings are held together by locks and garter springs which press them against the piston.
Middle scrapper group: The dirty oil carrying oil mixture scraped off by the first two oil scraper rings, is drained off via the free return line. This oil may not be re-used in the engine without thorough treatment.
Lower scrapper group: Lower groups of scraper rings scrape bearing oil off the piston rod leaving only a thin oil lubricating film. The scraped-off oil is drained through slots in the oil scraper rings and through bores in the ring holders back into the crankcase.
General arrangement: (Scrapper rings + Sealing rings + Scrapper rings).
Upper scrapper rings prevent carbon residue material to go inside, Sealing rings stops the charge air to go inside or leak to crankcase and Lower scrapper rings prevents oil to go upward i.e. in under-piston or scavenge space.
During each piston overhaul the piston rod gland box (stuffing box) must also be dismantled and inspected and excessive damaged parts should be replaced. It is very important that the piston rod glands are in perfect condition when assembling them.
Piston rod gland box has to be removed as a whole from the engine, after the piston has been pulled out. Loose and remove the screws and jack-up the assembly. Now use the crane for lifting and removing from the system. (Stuffing Box can be overhauled without removing piston rod assembly also but it needs experienced engineer).
For cleaning, immerse the complete gland assembly into a basin filled with cleaning agent and then prepare for dismantling. After dismantling, measure the wear of different rings, if it’s not admissible then replace.
Always keep the rings clean and separate for assembling the gland box properly.
Poor sealing due to deteriorated rings, badly aligned ring joints, sticky rings, closed butt joints, weak springs, excessive axial clearance or scoring or wearing of the piston rod.
Consequences of stuffing box not performing properly is a loss of crankcase oil, higher costs, contamination of crankcase with scavenge deposits and unburnt cylinder oil.
Indications of poor stuffing box gland sealing:
- Crankcase oil contamination test giving poor results
- A case of no oil replenishment
- Increasing TBN or viscosity
- Reduced piston cooling effect
- Poor lubrication