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Pump engineering skills help resolve sink hole drama

When a huge sink hole opened up overnight outside a supermarket in Texas, the race was on to replace a destroyed pumping station in just three weeks

The sink hole opened up overnight after torrential rainThe sink hole opened up overnight after torrential rain

by Bryan Orchard

When storms and torrential rain hit Granbury, Texas back in May 2015, residents were alarmed to find as the rain subsided the following morning a large sink hole in the lakeside car park outside Brookshire’s supermarket.

Measuring over 90 feet wide and 30ft deep, the hole was caused by the ground becoming saturated, after an old 7ft diameter storm drain had collapsed following several inches of heavy rain. This was accompanied by a landslip as the ground was washed away down the adjacent hillside. The water build-up and leak had literally washed away the earth supporting the City’s No.4 effluent lift station, creating the large crater.
Such was the damage that Granbury City Council declared it a local disaster; they sealed off the area and brought in specialist engineering crews to stop wastewater flowing from the broken pipes into the nearby Lake Granbury.

The pressing concern then was to formulate a programme to replace the damaged lift station.
With this being a highly specialised area of environmental engineering, Granbury City Council called on the resources of Abilene-based Enprotec/Hibbs & Todd (eHT), and Pump Solutions of Dallas/Fort Worth.
Scott D. Hay, eHT Vice-President, says: “The City took the lead in stopping up all the water and sewage lines and bringing in a by-pass pump at the start of the emergency. It was a rapid and successful response on behalf of the City. Our Granbury City office was then called to the site to get a handle on the environmental issues and stabilize the situation for the long term.”

Rebuilding the pump station

It was only when eHT commenced working on the site that more information about the original lift station was forthcoming. A major issue was that the installation was pumping into a 12” force main and that the pumps did not come even close to providing adequate velocity to maintain a flow that would scour and clean the main to prevent it from being plugged by waste and solids. The prime concern related to the integrity of the force main, particularly in respect as to whether it would it be compromised in its ability to perform a full 12-inch flow. As a result a decision was made to address this issue by installing new pumps capable of delivering an increased flow rate at new head conditions; the urgency of the situation meant eHT was given a brief that would require the company to deliver a fully operational lift station within a three week time frame.

“I saw things happening that took place in time frames that I had never experienced before,” continues Hay. “A major part of this was Pumps Solutions’ ability to deliver two KSB KRT-F submersible pumps, along with the wet well made by US Composite Pipe, in a very short space of time.”

Pump Solutions worked closely with KSB USA to identify the right pumps and types of impellers for the job. The preferred design of impeller was a vortex impeller specified by Hay. In his opinion, the vortex was the best type for the application, even though there had never been a problem with grit, solids or ragging. Being such a high profile project Pumps Solutions and eHT wanted to put in the best possible option for current and future demands on the system. In fact, such was the speed of the project no actual specification was developed. The sole objective was to get the pump station up and running within the City’s exacting deadline.

“Because of the time frame, we had a lot of hoops to jump through,” says Pump Solutions’ Charles Norman. “Our solution was based on the existing conditions and the technical brief given in respect to flow, hydraulic duty points and head, and it was our job to deliver a lift station complete with pumps, pre-cast wet well with access cover and the necessary control panels for the station.” The pumps that were finally selected were two 24hp KSB KRT-F models running at 1160rpm, with a design point of 975 US gpm at 30ft TDH for a velocity of 3ft/sec. and fitted with 105/8 inch (270mm) diameter vortex impellers.

Equipment sourcing

“Having to source the vortex impeller pumps, wet well, piping, wet well access hatch and lift station control panels within three weeks was a very tall order,” explains Norman. “Normally a lift station of this size and specification would take up to 12-14 weeks to complete. The advantage of working with KSB is that they have wet well pumps where the impellers can be swapped out. Their inventory revealed two K-type multi-vane impeller pumps in their warehouse which we could swap to F-type vortex impellers. What’s more, the hydraulics and motors were a good fit for the job, so we had a potential solution.”

However, the vortex impellers in stock were of the wrong dimensions, and calls to other KSB distributors around the USA drew a blank. KSB’s application engineers proposed another solution, which was to use one of their European castings suppliers to manufacture two new impellers and have them air-freighted to KSB’s plant in Henrico VA for pump assembling and testing. This was achieved in two weeks, giving KSB one week to do the assembly and testing.

Sourcing the wet well access hatch and control panels also posed a challenge, but fortunately, Pump Solutions’ regular hatch supplier, US Fabrications in Florida, had a unit of the right dimensions left over from another project.

The final element was getting the control panels manufactured by Quality Controls & Integration in New Prague, Minnesota and supplied to site, a process that normally takes 6-8 weeks. The supplier was able to accelerate the process, providing state-of-the-art controls including a touch screen panel linked to a submersible transducer in the wet well, enabling rapid data acquisition and monitoring of the water level in the wet well, raising alarms to indicate any changes taking place and monitoring pump performance.

Summary

The stability of the new lift station had to be given major consideration, and for this reason it was not built on the original site. A decision was made to move the lift station further away from the area where the land collapsed and the underground drain. New ground works had to be put in place, with some rerouting of the pipes and associated infrastructure.

This disaster has had a positive outcome for the City of Granbury, in that it now has a robust and efficient lift station that has increased pumping capacity to accommodate future growth, and ensures the safe transfer of wastewater and effluent away from the Brookshire’s parking lot and Lake Granbury

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