Articles & Case Studies

Hulldock gets impounding pumps overhaul

Posted: Monday 14th January 2013

Associated British Port’s (ABP) Port of Hull is one of the UK's leading foreign trading ports handling some 10 million tonnes of cargo annually (Fig.1). Its position on the north bank of the River Humber also provides a major geographical advantage for transport links in to and out of the UK. As a result, the port has very strong short-sea trade links with Europe, Scandinavia and the Baltic, in addition to world-wide deep-sea services.

The port’s leading position is maintained by ABP’s constant programme of capital investment in the development of new facilities and services. In the past couple of years, attention has been given to the maintenance and repair of the impounding pumps in the King George Dock, these being critical to maintaining the water level in the dock at key times of the day. King George Dock is capable of handling vessels up to 199m in length, with draughts up to 10.4m and DWT of 34,000 tonnes.

The function of the impounding station is to maintain the water level in the dock basin, as a certain amount is lost every time a vessel enters/departs through the lock. The pumps, which were installed in 1965, run at around high tide to pump river water into the dock. The job of refurbishing the No 1 and No 2 Vickers Armstrong pumps was given to AxFlow’s Huddersfield base, where it has the necessary heavy lifting gear and engineering workshops for pumps of their size and weight.

The first of the two pumps to be refurbished was the Impounding Pump No2, this being an imperial 44-inch vertical, axial flow unit with a flow rate of 63,500 gpm and a head of 11.33 feet (Fig.2). Although the pump had been taken out of service for maintenance over the years, by the end of 2010, its performance was faltering, as was that of the No1 pump.

“The pump had been suffering from vibration and rough running,” reports Mark Redgrove, AxFlow Technical Support Manager. “The pump was removed from the dock in three sections in early 2011 and transported to our Huddersfield workshops. On arrival in the works it was completely stripped down into its component parts to allow full dimensional checks, along with an overall condition report (Fig.3). This revealed that the bottom water lubricated bearing had worn through the shaft sleeve. This in turn had caused excessive run-out of the shaft wearing the impeller vane tips. One in particular was worn by around 15mm from the original diameter.”

Given the gritty and corrosive nature and condition of the water being handled by the pump, it was not unexpected for AxFlow to find corrosion of several of the key components. However, components such as the motor sole plate, Michell Bearing collar, upper shaft, pump casing and guide vane casing were found to be in a relatively good condition. On the down side, the main cause of the pump’s poor operation was the bottom bearing bush/sleeve. Here, the PEEK bush bearing liner was badly worn into the shaft sleeve causing eccentric running and vibration. This in turn had caused the bolts to vibrate and become loose, resulting in the complete bearing assembly dropping and coming into contact with the back of the impeller.

“We found that the wear on the Michell Bearing journal pads was consistent with normal operating conditions, but the thrust collar was in a good condition,” continues Mark Redgrove. “The surfaces of the journal pads had been ‘wiped’ or scored, but the brass backers were in reusable condition, so all that was necessary was to re-metal the pads and finish them in accordance with the new diameter of the Michel Bearing housing and thrust collar.”

The major works to be undertaken were in relation to the bellmouth, impeller, middle section of the shaft, mechanical seal and bottom bearing bush. “The bellmouth was in a poor condition with several holes through the casting around the vortex breaker vanes,” says Mark Redgrove. “The casting had been previously repaired with Belzona, but the cast iron had eroded further around the edges of the repair leaving fairly deep pockets around the flange surface. Realistically there were only three options open to us and in terms of increasing cost these were: fabricate a new item in GRP, repair the existing casting with Belzona, or manufacture pattern equipment and cast a new item in nickel iron. Our recommendation was to go with the GRP option as this was the most cost effective and provided the best lifespan.”

With regard to the impeller, this had previously been repaired by having tips brazed to the vanes. The AxFlow remedy was to re-tip the vanes and then machine them to bring the impeller back to the original size and rebalance the impeller to G2.5. The bottom bearing bush did, however, require considerable attention for once it was stripped down, the housing was approximately 20mm from its correct position. In addition, the outside of the bearing bush had worn, there was wear to the vane casing boss and the bolt holes had elongated. When the severely worn shaft sleeve was removed there was clear evidence of severe wear in this area (Fig.4), as well as where the radially split mechanical seal had been fitted. The successful solution provided by AxFlow was to metal spray the area and machine it back to size.

Although the vane guide housing was in relatively good order, the fit of the bearing bush was loose and the fairwater from the back was missing; a factor that had allowed silt to build up and accelerate wear of the bearing. To remedy this, a new fairwater was made from GRP. Finally, the faces of the radially split seal were worn and needed to be replaced, whereas the metal parts were refurbished by shot-blasting and cleaning.

“The extensive amount of repair and remedial work on this pump brought together most of our engineering capabilities at Huddersfield and demonstrated to ABP that we could offer a choice of solutions to the issues that needed addressing,” comments Mark Redgrove. “In this way, we were able to provide a combination of good value for money and engineering excellence that will ensure that this pump will give many more years of service. Once all the work was completed, the pump was reassembled on site in three sections: the pump sub-assembly, motor sole-plate and Michell Bearing and the motor.”

As the pump assembly bolts into the foundations without any adjustments being possible, all alignments at commissioning were made at the motor sole plate. This comprised: concentricity of coupling/shaft to motor spigot; perpendicularity of coupling/shaft to motor mounting face; and level of the sole plate. On start up running checks made to all joints for leaks and to the mechanical seal for leaks. A temperature check of the Michell Bearings and checks to the vibration levels at specific predetermined points were also made.

“The No 2 pump was refitted and started up in August 2011 and has run very well ever since,” reports Mark Redgrove. “As a result of our work we successfully tendered for the repair work to the No1 pump earlier this year. This pump had been suffering from excessive vibration, but was due to the upper shaft having a 2mm bend. The Michell journal bearing had worn the key and the whole top end of the line shaft at the motor end was running out. The bottom end of the pump was similar to the No.2 pump, although nowhere near as badly worn. We have now resolved this issue and the pump is currently being returned to service.”

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