Inverter drives fitted with motion control cards maintain smooth and
precise operation of a vessel closure lifting and traversing actuator
system, built in Scotland for a prestigious new pressure testing centre
in Houston
Houston's largest and most advanced hydrostatic test centre was recently
opened at Balmoral Group International’s Rankin Road premises. The test
centre has several pressure vessels, ranging from 800mm diameter by
2,200mm in length, all the way up to 2,500mm diameter by 8,500mm in
length, and carries out simultaneous, fully instrumented pressure and
water ingress tests at pressures up to 350bar. The largest pressure
vessel weighs in at 140 tonnes and has a 35-tonne lid, which has to be
removed for maintenance and the loading of test pieces. During these
operations, the lid is raised 1,045mm and moved 6m to the side where it
is lowered on a rest, the procedure being reversed for lid replacement.
In order to achieve acceptable levels of productivity, the rates of lift
and traverse are set at 95mm/min and 2.4m/min respectively.
The special vehicle and control system required for lifting and moving
this heavy lid was designed, manufactured and tested at Power Jacks’
factory in Fraserburgh, Scotland. It comprises a gantry type framework
mounted on four two-wheel units, located in each corner. One wheel unit
on each side is driven to provide the traversing linear motion, while a
four-actuator system on top of the framework lifts and lowers the lid on
and off the vessel. The vehicle control system is mounted on one side of
the framework and travels with the vehicle, with a remote operator
console located beside the pressure vessel.
Construction
As the vehicle had to be built, tested and dismantled for shipping, the
framework was designed in two sections, each section of the frame
representing one side of the vehicle. The sections are bolted together on
two main flanges at each end of the vehicle, the frame being located on
four wheel-assemblies, each with two wheels mounted on support bearings
in a self-contained structure. This assembly was bolted to the framework
for initial location, and then welded in place for final positioning. The
two front pairs of wheel units have drive sprockets that attach to a
motorised reduction gearbox, mounted on the framework. As it was not
possible to mechanically link the two motorised gearboxes for the vehicle
drive, they are electronically synchronised.
The lid lifting system comprises four actuators, mechanically linked in a
’U’ configuration with a main drive unit at one end. These are special
units based on Power Jacks’ 300kN metric machine screw model, and are of
the inverted type with wear monitoring for the gear sets. The specially
designed lifting screws have conical locating ends, which ensure that the
screws are locked in the framework when the lid is lifted to its full
height, thus preventing any swaying on transfer to and from the vessel. A
motorised reduction gearbox, with bevel gearboxes to direct the drive
line, provides drive to each of the four actuators. The motor unit also
includes an integral brake for secure load holding while parked.
For positioning purposes, a rotary cam limit switch is connected to the
actuator system, providing emergency over-travel limits and normal system
stop limits. The actuator system has two mechanical safety features: it
is self-locking and therefore will not back-drive, even in the unlikely
event of motor brake failure, and the wear monitors on each actuator
provide an early indication of any gear deterioration.
Control system
The main control system drives the vehicle and actuator, and is based on
advanced inverter controls. The vehicle drive consists of two inverters
with integral motion control cards linked together to provide precise,
synchronised speed and position movement for both vehicle motor