This is a HFO (Heavy Fuel Oil) screw pump used in Power Plant for running boilers. There was a catastrophic failure of the pump. Though this pump was regularly monitored by vibration (in velocity mode — mm/sec) it didn’t give any indication of the impending failure.
The screws of the pump rubbed against each other and the case hardened layers of both screws were crushed. The force was so great that the body of the pump also cracked. Evidence of corrosion was also noticed.
What caused it?
For want of HFO oil, the plant personnel were forced to pump LDO (Light Diesel Oil) through this HFO pump for the past one year.
Hence the I, A, R factors that contributed to this catastrophic failure are the following:
Initiator(s) — I — factor(s), which triggers the problem — low viscosity of LDO compared to that of HFO was the significant ‘initiator’ in this case. While viscosity of LDO ranges from 2.5 to 5 cSt, the viscosity of HFO varies between 30 to 50 cSt (depending on the additives used). Use of lower viscosity oil ensured metal to metal contact thereby increasing Hertz stress that led to collapse of the hardened layer of the screws.
Accelerator(s) — A — factor(s), which accelerates the process of failure — a) Indian HFO does not contain friction modifiers such as vanadium and magnesium. Their absence causes higher friction between the screws (approximately 70 times increase in friction), which accelerates the wear process. b) Moreover, presence of vanadium and magnesium additives in HFO and LDO acts as anti-corrosive agents. Notice that the failure happened a year after the management decided to pump LDO rather than HFO through the HFO pump — enough time for corrosion to take effect. So, we may say that there are at least two factors that accelerated the failure process. There are other effects too on system performance, which we shall discuss in a moment (refer “Note”).
Retarder(s) — R — factors that slow down the failure process — a) surface finish of the screws b) right clearance of the bearings c) presence of chromium in the screws.
Surface finish plays a very important role in reduction of metal to metal friction and also allows fluid film development. Ideally the surface finish should be between 3 to 6 microns CLA (Centre Line Average) for best effect. This can be introduced as a specification of the MOC (Material of Construction).
Similarly, excessive clearance in bearings would modify the hertz stress zone or profile — both in width and depth, which would cause shear of the hard layer (depth of which depends on the type of hardening and the type of steel used) and the soft layer (core material). Depth and type of hardening might also be specified in the MOC to prevent failures and extend life of the equipment. Presence of chromium in the metal would help formation of Vanadium – Oxygen – Chromium bond which would effectively enhance the life by providing better lubricating property which in turn would ensure a high level of reliability of the equipment.
Hence, once the I, A and R s are identified appropriate measures can be taken to modify maintenance plan, MOC etc to ensure long life of the equipment without negative safety consequences (heart of reliability improvement).
- Specify addition of Vanadium and Magnesium in the HFO during supply or these may be added at site after receipt of supply. (Material specification during purchase)
- Ensure the right viscosity of oil to be pumps through HFO pumps. (Monitor viscosity of the supply oil — not higher than 50 cSt and not lesser than 30 cSt)
- Specify surface roughness of the screws — 3 to 6 microns (CLA).
- Specify depth of hardness of the screws (below 580 microns so that the interface between the hard layer and the soft core remains unaffected by the Hertz stress) during procurement and supply. Preferable type of hardening of the screws would be nitriding.
- Specify chromium percentage in the screws (during purchase).
- Monitor bearing clearance on a regular basis and change as needed (by vibration analysis based on velocity and acceleration parameters).
- Monitor the body temperature of the pump to notice adverse frictional effects
- Monitor growth of incipient failures in the screws by vibration monitoring (acceleration and displacement parameters)
1. (Effect of IAR on system performance — i.e. the boiler – superheater – pipes):
Problems of high temperature corrosion and brittle deposits drastically impair the performance of high-capacity steam boiler of Power Plants, using HFO. Research* shows that heavy fuel oil (HFO) can be suitably burned in high capacity boilers. However, if HFO is chemically treated with an anticorrosive additives like Vanadium and Magnesium, it diminishes high temperature corrosion that affect some operational parameters such as the pressure in furnace and pressure drop in superheaters and pipe metal temperature, among others like atomization and combustion processes. Therefore, inclusion of right additives like Vanadium and Magnesium have been found to diminish high-temperature corrosion and improved system performance. It therefore makes sense to monitor these parameters, which can provide direct information on the degree of fouling, as well as of the effectiveness of the treatment during normal boiler operating conditions.
- Two approaches to improving Plant Reliability:
- Rethinking Maintenance Strategy:
- Applying IAR Technique:
By Dibyendu De