By: G. Michael Strombach
With an installed capacity of 6,809 megawatts, the Grand
Coulee Dam on located on the Columbia River in the state of Washington is the
largest hydro project in North America.
For a short time following its completion, it was known as
the largest hydro project in the world. Measuring more than a mile long and 550
ft tall, it is the biggest concrete structure ever built, containing nearly 12
million cubic yards of concrete. Lake Roosevelt, which stretches out behind the
dam, is approximately 150 miles long as it extends up the Columbia River to the
Canadian border and to the Spokane River within 37 miles of the city of
Spokane. As such, Lake Roosevelt contains more than five million acre-feet of
active storage capacity.
The Grand Coulee Dam releases, on average, 110,000 cubic
feet of water per second, primarily for generating electricity. Controlling
huge volumes of water requires giant-sized equipment, which can sometimes face
giant-sized problems when problems occur.
Three of the 24 turbines are rated at 805 megawatts and are
some of largest turbines ever built. Making sure they consistently function in
a reliable manner can present significant hurdles for maintenance engineers and
mechanics.
Spotlight on the problem
Mentioned above, three of the turbines, Units G-22, G-23,
and G-24, each are rated at 805 megawatts. Their main turbine shaft is
approximately eight feet in diameter with shaft seals that consist of a set of
braided packing rings, held in place by a gland within the stuffing box. Water
supplies cooling and lubrication for the packing through a standard lantern
ring located in the middle of the packing set. Leakage during operation is to
be expected.
Further, the amount of leakage increases as the packing
wears and ages. This leakage collects in the turbine pit and has to be pumped
out periodically; otherwise it could damage the turbine.
The original design for turbine pit drainage provided for
two 200 gpm pumps, one designated as the lead pump and the other as backup.
These pumps were controlled by a multi-step float switch system. If the leakage
became severe, exceeding the ability of the pumps to keep up, the generator
would have to be to be shut off.
Because it is highly undesirable to shut off the generator,
maintenance personnel placed additional, small, submersible pumps to remove
water from the turbine pit. The discharge from the pumps was sent to gravity
drains or to adjacent units with less leakage.
The amount of packing leakage varies, depending on the unit
load as well as other variables. Thus, the additional pumps are not needed all
the time. When the water level eventually falls below the pump impeller, the
seal in the suction cavity is broken. All cooling and lubrication provided by
the water stops and the pumps overheat and burn up. The problem can be
exacerbated by the fact that the added submersible pumps were not always
visible to operations personnel.
To make matters worse, the water that comes out of the
packing area mixes with water leaking out of wicket gates and other mechanical
seals. As a result, the water often contains grease, rags and oil that
sometimes clogged up the added pumps.
One of the mechanics based at the Grand Coulee Dam suggested
replacing one of the small pumps with a bearingless Vanton cantilever-shaft
pump which was specifically designed to operate under extended dry-run
conditions. Because it incorporated a vortex pump head configuration to handle
foreign objects without clogging, it was put to the test.
The original intent was to use it as a backup pump, which
operators could simply start and leave running when one of the turbine units
experienced severe leakage. To the surprise of the Grand Coulee Dam operators, the
600 gpm Vanton Sump-Gard SGK polypropylene pump quickly drained the entire pit
and handled the contaminated water without any problems and without the risk of
damage associated with dry running conditions.
Because of the positive results, the Grand Coulee Dam
retrofitted all three 805 MW turbines to use the cantilever pumps. There are
two Vanton SGK pumps in each of the turbine pits, connected to a multi-stage
float system that alternates running time. Although each pump is able to
completely drain the pit, they are instrumented with level controls so the idle
pump will kick in should the volume of water in the pit get too high. These are
the specifications for the installed pumps that have been providing
uninterrupted, dependable service for the past two years.
Design specifications
To address the problems faced by the water leakage at the
Grand Coulee Dam, various design specifications were indicated. For example,
engineers requested a rugged, thermoplastic vertical pump with cantilevered
shaft and dry running capability that had no bearings immersed in the pumped
fluid.
The pump required a large diameter alloy steel shaft
isolated from the fluid by a thick-sectioned thermoplastic sleeve and
heavy-duty external ball bearings above cover plate, housed in epoxy-coated
cast iron motor bracket to accommodate NEMA, IEC and European standard motors.
All immersed pump parts and hardware were made of homogeneous thermoplastics so
that no metal or thermoset composites in fluid contact.
Finally, it was required the pump be dynamically balanced
(semi-open or closed) impeller with imbedded, molded-in stainless steel
reinforcing insert and a vortex pump head with recessed impeller to handle
foreign objects without clogging.
About The Author: G. Michael Strombach is a senior mechanical engineer at the Grand Coulee Dam. For further information, phone 908/688-4216.
