As a producer of specialty chemicals, Nobian manufactures and moves large quantities of chlorine. To allow delivery to customers, this key ingredient for many industrial and chemical processes is often liquefied. An important enabler of this process at Nobian’s Rotterdam plant is a four-stage chlorine compressor and its back-up spare. This means that when the main compressor needs attention from the maintenance team after prolonged service with this materially corrosive element, it is swapped out for repairs and the spare takes its place.
To make sure it had its spare ready, Nobian contacted Sulzer in the Netherlands to perform a complete overhaul. With its turnkey service for the maintenance and upgrade of compressors and other rotating equipment, the local team was ready to take this on.
The results of the compressor inspection showed that the casing could be re-used. This would be achieved by welding some erosion marks and machining the entire split line to remove corrosion, with the entire casing to be re-bored afterwards. Furthermore, the new diaphragms from the compressor OEM did not meet Nobian’s specifications, but the used items were found to be in reasonable condition.
Considering this, the final project scope encompassed machining the casing, the reuse of the existing diaphragms and the replacement of all other internals. These included the rotor and impellers, internal labyrinth seals, bearings, oil seals, the variable inlet guide vanes (VIGV) assembly and stuffing boxes.
Consequently, Sulzer in the Netherlands subcontracted the compressor casing machining, installed the used diaphragms and replaced all other internals with new items supplied by Nobian. However, during a trial fit of the rotor from the OEM, it appeared that its balance drum was wrongly installed, being twisted at 180°. Consequently, the rotor was shipped by Nobian back to the OEM, with final reassembly put on hold.
As this was the spare compressor, Nobian opted to wait for this rotor to be repaired. When it did finally appear, Sulzer’s Venlo Service Center resumed the project to finish the compressor overhaul. However, the refurbished rotor was delivered with some minor nonconformities such as rust and broken, incorrectly-secured labyrinth seals.
“It was clear just from looking at it that this rotor had problems too,” says Richard Quaden, project manager at Sulzer Turbo Services Venlo. “We spotted minor defects all over it. The rotor was provided with a high-speed balancing report from the OEM, but we and Nobian doubted the accuracy of this. We carried out a low-speed balancing and confirmed our suspicions that the rotor did not meet the corresponding specifications.”
As a consolation, the balance drum was now in the correct position, and in terms of dimensions at least, the rotor was fit for purpose. Sulzer commenced with machining the shims to the correct thickness to achieve the right axial clearances for the rotor. Since the labyrinth seals might be replaced by the OEM, it was decided to not machine the stuffing box to the required internal diameter until the refurbished rotor was inspected and the outer diameter of the labyrinth seals were measured again. Despite this progress, the faulty rotor still presented positioning problems.
Why rotor position is important
The positioning of the rotor is absolutely vital in any compressor. As the split line of casing had been machined to remove the corrosion, the internal packaging of the unit had changed. To compensate for this, the internal bores of the casing had been modified to ensure that the central shaft would be properly aligned. However, the faulty rotor could potentially make contact with other elements in the compressor such as the labyrinth seals or stuffing boxes - regardless of the other modifications.
“The tolerances for the radial clearances in the compressor are between 0.03 – 0.04mm and 0.2 - 0.27mm for the axial clearances. With a faulty rotor, spinning at high speed, a rotating part can easily hit another section of the compressor. It is important to make sure all clearances are met. Otherwise, you could potentially cause considerable damage or a leakage of gas,” says Richard.
Sulzer invited Nobian to the Venlo Service Center so that it could share its unwelcome discoveries regarding the rotor. The team presented its inspection results and comprehensive 3D scans of the compressor casing and rotor, conforming that there was a positional offset between the centreline of the casing and rotor shaft. Because of that, the internals would need to be machined to compensate for this offset. Representatives from both businesses sat down to review the measurements and Nobian agreed with Sulzer’s diagnosis.
Both parties confirmed that the replacement rotor sent by the original equipment manufacturer (OEM) had to be returned again, so the project was put on hold for a second time until it was repaired. Nobian was invited to return to Sulzer for the unboxing and inspection of the rotor when it arrived.
Better late than never, the refurbished rotor arrived at the Venlo Service Center to be welcomed by experts from both businesses. A close inspection and a 3D scan confirmed the rotor was finally fit for purpose and would sit in the right position.
To complete the overhaul, the last stuffing box was machined within tolerances. The rotor was installed and the entire unit reassembled, all witnessed by Nobian. Using its engineering expertise and identifying the nonconformities on the refurbished rotor, the team at the Venlo Service Center had ensured that the spare compressor was brought back to optimum condition.