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
As of now, maintenance strategy looks similar to strategy taken by the medical fraternity in themes, concepts and procedures.
If things go suddenly wrong we just fix the problem as quickly as possible. A person is healthy to the point when the person becomes unhealthy.
That might work fine for simple diseases like harmless flu, infections, wounds and fractures. And it is rather necessary to do so during such infrequent periods of crisis.
But that does not work for more serious diseases or chronic ones.
For such serious and chronic ones either we go for preventive measures like general cleanliness, hygiene, food and restoring normal living conditions or predictive measures through regular check ups that detects problems like high or low blood pressures, diabetes and cancer.
Once detected, we treat the symptoms post haste resorting to either prolonged doses of medication or surgery or both, like in the case of cancer. But unfortunately, the chance of survival or prolonging life of a patient is rather low.
However, it is time we rethink our strategy of maintaining health of a human being or any machine or system.
We may do so by orienting our strategy to understand the dynamics of a disease. By doing so, our approach changes radically. For example. let us take Type 2 diabetes, which is becoming a global epidemic. Acute or chronic stress initiates or triggers the disease (Initiator). Poor or inadequate nutrition or wrong choice of food accelerates the process (Accelerator) whereas taking regular physical exercise retards or slows down the process (Retarder). Worthwhile to mention that the Initiator(s), Accelerator (s) and Retarder (s) get together to produce changes that trigger of unhealthy or undesirable behavior or failure patterns. Such interactions, which I call ‘imperfections‘ between initiator (s), accelerator (s) and retarder (s) change the gene expression which gives rise to a disease, which often has to be treated over the entire lifecycle of a patient or system with a low probability of success.
The present strategy to fight diabetes is to modulate insulin levels through oral medication or injections to keep blood sugar to an acceptable level. It often proves to be a frustrating process for patients to maintain their blood sugar levels in this manner. But more importantly, the present strategy is not geared to reverse Type 2 diabetes or eliminate the disease.
The difference between the two approaches lies in the fact — “respond to the symptom” (high blood sugar) vs “respond to the “imperfection” — the interaction between Initiators, Accelerators and Retarders”. The response to symptom is done through constant monitoring and action based on the condition of the system, without attempting to take care of the inherent imperfections. On the other hand, the response to imperfections involve appropriate and adequate actions around the I, A, R s and monitoring their presence and levels of severity.
So a successful strategy to reverse diabetes would be to eliminate or avoid the initiator (or keep it as low as possible); weaken or eliminate the Accelerator and strengthen or improve the Retarder. A custom made successful strategy might be formulated by careful observation and analysis of the dynamics of the patient.
As a passing note, by following this simple strategy of addressing the “system imperfections“, I could successfully reverse my Type 2 Diabetes, which even doctors considered impossible. Moreover, the consequences of diabetes were also reversed.
Fixing diseases as and when they surface or appear is similar to Breakdown Maintenance strategy, which most industries adopt. Clearly, other than cases where the consequences of a failure is really low, adoption of this strategy is not beneficial in terms of maintenance effort, safety, availability and costs.
As a parallel in engineering, tackling a diseases through preventive measures is like Preventive Maintenance and Total Productive Maintenance — a highly evolved form of Preventive Maintenance. Though such a strategy can prove to be very useful to maintain basic operating conditions, the limitation, as in the case of human beings, is that it does not usually ensure successful ‘mission reliability’ (high chance of survival or prolonging healthy life to the maximum) as demonstrated by Waddington Effect. (You may refer to my posts on Waddington Effect here 1 and here 2)
Similarly, predictive strategy along with its follow up actions in medical science, is similar to Predictive Maintenance, Condition Based Maintenance and Reliability Centered Maintenance in engineering discipline. Though we can successfully avoid or eliminate the consequences of failures; improvement in reliability (extending MTBF — Mean Time Between Failures) or performance is limited to the degree of existing “imperfections” in the system (gene expression of the system), which the above strategies hardly address.
For the purpose of illustration of IAR method, you may like to visit my post on — Application of IAR technique
To summarize, a successful maintenance strategy that aims at zero breakdown and zero safety and performance failures and useful extension of MTBF of any system may be as follows:
- Observe the dynamics of the machine or system. This might be done by observing energy flows or materials movement and its dynamics or vibration patterns or analysis of failure patterns or conducting design audits, etc. Such methods can be employed individually or in combination, which depends on the context.
- Understand the failures or abnormal behavior or performance patterns from equipment history or Review of existing equipment maintenance plan
- Identify the Initiators, Accelerators and Retarders (IARs)
- Formulate a customized comprehensive strategy and detailed maintenance and improvement plan around the identified IARs keeping in mind the action principles of elimination, weakening and strengthening the IARs appropriately. This ensures Reliability of Equipment Usage over the lifecycle of an equipment at the lowest possible costs and efforts. The advantage lies in the fact that once done, REU gives ongoing benefits to a manufacturing plant over years.
- Keep upgrading the maintenance plan, sensors and analysis algorithms based on new evidences and information. This leads to custom built Artificial Intelligence for any system that proves invaluable in the long run.
- Improve the system in small steps that give measureable benefits.By Dibyendu De
Even when sensitized, the steel is adequate for many applications, such as household products (SS utensils that we use on the gas ovens) and even containers for less concentrated nitric acid. However, while sensitized steel is adequate for 75% nitric acid, it could not be used for the 90% solution.
Whenever corrosive attack on 304 grade of SS is the predominant failure mode it might be prevented in several ways. For example, low carbon stainless steel, designated as 304L could be used. Or, addition of niobium during steel making would tie up the carbon as fine, harmless intragranular niobium carbides. Alternatively, we can anneal, especially welded joints on 304 (if possible — size of the furnace often becomes a constraint) at a bright-red heat to dissolve the carbides and then water quench to prevent their re-nucleation. Any of these techniques can be effective, but the additional cost has to taken into account.