CORROSION/ WEAR LIMITS C077
When is a pump casing too thin for safe operation? The loss of material can be caused by wear, corrosion or a combination of both. This question often comes up when stuffing boxes are bored out to give a mechanical seal more axial room.
The obvious and only correct answer is to check with the manufacturer of your hardware. The type of metal, thickness, operating temperatures and pressures are all-important variables.
A rule of thumb used by the Hydraulic Institute is:
- Replace pump casings when the wear or corrosion exceeds .013 inches (3mm)
- Shaft sleeves can be as low as 0.010 inch (0.25 mm) depending on the metal. If shaft sleeves are being used to provide corrosion resistance, they need more bulk to prevent corrosive fluid penetrating through to the solid shaft.
- When 70% of the allowance is gone, the equipment should be shut down for inspection and evaluation.
How do you check how much metal thickness you have left? You have several choices:
- Visual is the most common and considered the most reliable because it can pick up pitting, crevice corrosion and cracking.
- Electrical resistance (ER)
- Linear polarization resistance (LPR)
- Ultra sound testing
- Radio isotope devices
- Unfortunately Stress Corrosion, especially chloride stress corrosion problems with the 300 series of stainless steel, can sometimes occur without any visible signs.
Corrosion is accelerated by heat. This is especially important with acid pumps, where shaft deflection can cause the shaft or sleeve to rub against the breakdown bushing in the bottom of the stuffing box and/or closed impeller wear rings