Heavy Duty Agitator Pumps Reduce Maintenance Time
One the oldest and largest steel makers on the East Coast solved its sump problems by using one of the worlds strongest submersible motor pumps.
The 68" hot strip mill takes 10" thick steel slabs from its caster and heats the slabs. The slabs then roll through a series of four roughing and seven finishing mill stands transforming them into coil stock. Coil stock is used by other manufacturers to make exterior roof decks, wall panels, refrigerator doors and many other products. The hot strip mill operation involves large quantities of sprayed cooling water which flows by gravity to underground pits. Heated water laden with mill scale from the hot slabs is then pumped from the pits to other plant locations.
Abrasive scale in water is hard on any pump, but especially on long shafted suspended vertical wet pit pumps with bearings. Prior to August 1994, MTBF (mean time between failure) was measured in months. Eight foot deep sumps with seven foot long suspended pumps with submerged bearings required continuous overhaul. Extracting the long pumps from confined areas under the mill was a major maintenance headache. Repairs and parts were expensive and time consuming.The pumps were modified with rubber cutlass bearings and white iron metallurgy. This improved MTBF, but only moderately.
Settled scale would pile up around the pump suction, not only starving the pumps, but decreasing the sump liquid retention volume. Scale build up resulted in increased pump run time due to the loss of sump volume. Expensive scored bearings had to be replaced frequently.
Agitator Pumps Installed
During the Summer of 1994, hot strip mill supervisors made the decision to go with a submersible agitator pump.
Their goal was to stir the settled scale and eliminate the problems associated with scale build up in the sumps. Because the submersible units are close coupled with thick shafts and large bearings,and have the shortest shaft ovrerhang in the industry, frequent overhauls are no longer necessary.
The first pump was installed in the South sump in August 1994 and is still running after 7 years with no maintenance or down time. Three other units were installed soon thereafter and they too are still in operation. Because of the success of these installations, supervisors expect to install additional units in new applications.
Model DP10H: 10 HP, 1800 RPM, 1.35 service factor motor with thermal overload protection; designed for 180 GPM @68' TH; 4" discharge, freestanding on combination strainer and base; 28 % hi-chrome impeller, agitator, and front/back wear plates, with balance of pump construction in heavy cast iron. The pump is fitted with pre-lubricated heavy duty bearings and progressive failure slurry lip seals with mechanical seals as optional. Pump pricing is competitive with long shafted vertical wet pit pumps.
The plant pump repair shop reports that these pumps have required very little maintenance in the seven plus years of service. They continue to enjoy the reputation as some of the best pumps the steel mill has ever used.
Recently two more pumps,a 30 HP and a 50 HP agitator pump, have been purchased. Both pumps are used in the wastewater transfer of solids laden cooling water from the hot strip mill.