Although the cost of a component inside a hydraulic pump, such as a rolling element bearing, rotor or fastener, is often very small compared to the total cost of the pump. The cost of production downtime and any consequential losses as a result of a component failure, are often significant.
For a processing plant, the typical cost of production downtime can equate to hundreds of thousands of pounds per day. Lost production in a paper mill, for example, could equate to around £15,000 per hour. Total maintenance costs for a typical food or beverage manufacturer are around 15–20% of total costs. Of course, every manufacturing company has a maintenance department to deal with problems such as these, but often, because of time and resource constraints, maintenance becomes reactive, problems around the plant are dealt with as they occur, with no predictive maintenance systems, little preventive maintenance and possibly no maintenance strategy at all.
Technology safeguards
But there are numerous technology safeguards out there that, when compared to the cost of lost production, are relatively inexpensive. They use the latest condition monitoring technology and predictive maintenance systems, including acoustic emissions monitoring, vibration monitoring and thermography, to protect plant and machines.
As Ian Taylor, Business Development Engineer Plant Condition Monitoring at CNES states: “At Corus and for external customers, CNES' Plant Condition Monitoring team monitors hydraulic pumps, fans, compressors and blowers, using every technique available, including patrol monitoring, fixed and portable CM systems. Inside Corus, we monitor all pumps, some every week, but each pump at least once per month. Where the pump is critical to our production process, such as the hood cooling pumps on our Basic Oxygen Steelmaking plant, we have installed fixed CM systems to monitor the plant 24/7.”
Condition monitoring prevents maintenance teams replacing components unnecessarily and introducing possible new and unrelated problems. As Taylor advises: “Maintenance teams should be using condition monitoring technology to predict when failures are likely to occur and plan replacement during production shutdowns. In too many companies, components are replaced on a time basis rather than on a condition basis because maintenance considers this to be the safest option. The problem is that this then introduces a further risk, because whenever there's human intervention, problems can occur.”
According to Taylor, when it comes to monitoring the condition of pumps and other hydraulic systems, a number of techniques are available to the engineer. These can be used individually, or it may be necessary to use some or even all in combination to protect the company's valuable assets.
“As most pumps run at a steady load and speed, patrol monitoring with vibration analysis equipment is usually the most effective condition monitoring technique,” explains Taylor. “However, it really does depend on the pump design. Acoustic emissions monitoring may be more effective if the pump speed is less than 80 rpm or if the maintenance technician wants to monitor the condition of plain bearings inside the pump or motor.”
Vibration monitoring can identify a number of potential pump problems, including misalignment or coupling issues; mechanical looseness inside the pump or from the baseplate, including loose joints or fasteners, through to the condition monitoring of rolling element bearings; cavitation issues; erosion of rotors identified as imbalance.
Steel outline
Corus Northern Engineering Services (CNES) is an engineering support business with bases in Teesside, Scunthorpe and Rotherham and Workington. Part of the Corus Group, the company utilizes a ‘toolbox’ of techniques to help customers of all sizes and from every industry sector, maintain their output, efficiency and profitability, by providing a detailed and accurate assessment of their plant assets. CNES uses a range of non-intrusive techniques, including acoustic monitoring; thermographic imaging; vibration monitoring; laser alignment; lubricant technology; and remote visual inspection. CNES also offers structural design, project management, training, electrical and process control, as well as machining, fitting and weld reclamation services.
Corus is Europe's second largest steel producer with revenues of £9.7 billion and crude steel production of 18.3 million tonnes in 2006, primarily in the UK and the Netherlands. Combining international expertise with local customer service, the Corus brand represents quality and strength, providing innovative solutions to the construction, automotive, packaging, mechanical engineering and other markets worldwide.
Pumping of heavy, viscous fluids such as foodstuffs or sludge, for example, can cause damage to rotors, which in turn could result in the pump going out of balance. Similarly, any rotor deterioration caused by the pumping of corrosive liquids, can also lead to an out of balance pump. Wear of gear teeth on gear pumps can also be monitored effectively by vibration analysis systems.
Standby pumps can also be monitored using vibration monitoring techniques. For example, a maintenance team may decide to operate two identical pumps, side-by-side, one being duty, the other standby. However, to try to cut costs, companies often purchase pumps on a single, common bedplate. When the duty pump fails, simply switch to the standby pump, which is common practice in many industries. Well that is the concept.
However, what happens in reality is that the constant vibrations from the duty pump can cause bearing problems on the standby pump, referred to as ‘false brinelling’. Once the standby is switched on, it very quickly fails, resulting in two pumps out of service. To prevent this occurring, the two pumps should be switched over regularly on an 80–20 or 70–30 duty/standby ratio.
For hydraulic systems that rotate at less than 80 rpm and operate under fluctuating load conditions, or only move through a part revolution, it is more difficult to collect meaningful data from methods such as vibration monitoring. For this reason, CNES has developed its own, patented acoustic online condition monitoring system, Aquilla AE Pro.
High sensitivity
Acoustic emission monitoring equipment has a high sensitivity to machine faults but is also immune to audible noise and low frequency background vibration. “The problem,” warns Taylor, “is that many engineers are not fully aware of how acoustic emission monitoring systems can help them reduce plant maintenance costs and improve machine availability. Also, many companies simply do not possess the necessary skills in-house to interpret the data from acoustics emissions monitoring so they continue to use vibration monitoring or other devices.”
Monitoring acoustic emissions is certainly not a new method of monitoring high capital plant and machinery. The technique has been around since the early 1990s, but, according to Taylor, more manufacturing companies should be using it. Acoustic emissions are the high frequency stress waves generated by the rapid release of strain energy that occurs within a material during crack growth, plastic deformation or phase transformation. Acoustic emission monitoring systems use surface-mounted transducers to detect these stress waves, which lie within the 25 kHz to 1MHz frequency range.
Correct selection of lubricant or hydraulic fluid can also significantly reduce component wear and associated energy costs. Effectiveness of current lubricants can be determined by analysing the level of degradation and debris present. This facilitates correct lubricant selection and oil change periods.
