Q - We have a 20-year old pump still in service. During a routine maintenance we did an NDE on the casing and found that, in some sections, the metal is barely 1/32" thicker than its case retiring thickness. We also noticed in the catalogue that some pumps have retiring thickness 1/8" smaller than its normal case thickness, but 1/4" smaller in others.
What is the significance of the case retiring thickness? What is the true retiring thickness of a casing? Is it time for us to retire our old pump and replace it with a new one?
A - A casing will suffer metal loss, or loss of thickness, due to corrosion, erosion, abrasion, and cavitation. A case retiring thickness is the thickness at which the casing should be retired due to significant metal loss. It means that the remaining case metal thickness is marginally good for its design pressure and temperature. The continuous use of the pump can result in catastrophic failure if there would be further loss of metal in the casing.
The casing thickness takes into account factors such as its maximum design pressure and temperature, and casing material. Based on these factors the casing minimum thickness is calculated and a corrosion allowance is added, typically1/8", as mandated by an applicable standard, like ASME, API, etc.
But the casing may be thicker than that because additional thickness may be added for various reasons, such as:
to accommodate a different volute pattern, for pattern change flexibility
to accommodate bosses for fittings and instrumentations
to minimize large variations in casing thickness to avoid the potential for casting deformation due to uneven rate of cooling
In this instance, more amount of metal loss can be tolerated before a casing reaches its real retiring thickness. Similarly, if a pump operates well below its design pressure and temperature, or if the casing is made of high strength material, then a smaller case retiring thickness may become acceptable. But do not make that decision - check with the pump vendor.
When a casing reached its real retiring thickness it is not always necessary to replace the entire pump. Change the casing only to avoid making changes to the baseplate and piping which can be very expensive, unless there is a compelling reasons to change the entire pump such as its being inefficient, or having short MTBF..
If the casing appeared to be losing its metal thickness prematurely consider the feasibility of coating or overlaying the casing, or changing to a different casing metallurgy that is more resistant to corrosion, erosion, abrasion, or cavitation.