We used non-destructive computed tomography (CT) and slurry erosion testing to:
- Identify the cause of failure of a bronze centrifugal impeller from the seawater pump of a military vessel
- Assess the integrity of the impeller material
The challenge: identifying the cause of severe wear on the impeller blades
The damaged impeller
We were asked to apply our failure investigation expertise to examine a military ship’s impeller and identify the cause of failure.
The impeller is a component in one of the ship’s three main high-pressure seawater pumps, which feed a constant supply of high pressure sea water around the ship. It comprises seven blades and five balancing holes and is driven in rotation by a shaft. The liquid between the blades rotates and is subjected to a centrifugal force; it will therefore flow radially from the centre of the impeller to the outer edge, if it is not opposed by a pressure at the outlet.
Applying nC2’s failure investigation and erosion testing expertise
Initial inspection showed evidence of wear:
- There was severe wear around the balancing holes.
- The leading edges of each of the blades displayed thinning and wear.
- The blades and the back sloping surface that blended into the central key way collar were rough with pitting holes.
Advanced CT scanning
A CT scan of the failed impeller
We used CT scanning to examine the damage in detail, creating a 3D volume of the impeller without the suction and discharge flanges.
- By circling around the impeller in virtual space we could examine all the surfaces, enabling us to assess the damage without the need for destructive cutting.
- The 3D volume clearly displayed the extent of pitting damage on the blades.
- By observing the high detailed view of the morphology of the damage, we could identify interesting features revealing clear evidence of severe localised erosion attack.
- Using this non-destructive technique, we were able to determine that erosion was the dominant mechanism that contributed to the failure of the impeller.
Erosion testing to assess material integrity
Post erosion testing comparison showing the mass loss of the bronze impeller at different test conditions, and of the standard stainless steel material
Our erosion expert assessed the integrity of the impeller material using a water-sand (slurry) erosion test. This aimed to establish the influence of silt intake into the impeller during operation.
- The test conditions, such as test speeds, flow rate, slurry concentration and test angle, were selected to mimic the in-service operating conditions of the leading edges of the impeller.
- A total of 14 samples, cut from an actual impeller, were tested. A standard 316 stainless steel material was also tested as a baseline material for comparison.
- The tests showed that the bronze impeller material is generally prone to jet erosion attack. Even without sand particles present in the slurry, the material experienced some erosion at relatively low (13 meters per second (m/s)) and high (22 m/s) jet speed.
- Increasing the jet speed exacerbated the erosion of the bronze impeller material. The erosion was worse when sand particles were present in the slurry jet; this approximately doubled the impingement speed and resulted in a 10-fold rate of material loss over the same test time.
- The standard 316 stainless steel material, tested under similar conditions (22 m/s with sand particles in slurry), outperformed the bronze impeller material by approximately a factor of 12.
- The post-test images of the tested samples, with and without sand in slurry, showed wear on the surface of the samples, which was more significant when sand was present. It was also observed that at the higher speed the patina was completely removed from the surface of the material. The damage was typical of slurry erosion; no evidence of cavitation erosion was seen.
The erosion study of the impeller demonstrated that if silt did enter the pump during operation, it would compromise the impeller material, eroding the surface as shown by the difference in performance of the material with and without sand particle testing.
Bronze sample following erosion testing without sand particles
Bronze sample following erosion testing with sand particles
nC2’s added value
- Expertise in failure investigation and different types of erosion and wear mechanisms to identify and understand the cause
- Access to high-spec CT facilities at the University
- A purpose built erosion test rig and specialist expertise in erosion testing of both bulk material and coated samples
The outcome
The results enabled the client to understand the root cause of the failure of their ship’s impeller and empowered them to make informed decisions about material selection for an improved design.
