by Ian Taylor, Corus Northern Engineering Services

Although the cost of a component inside a hydraulic pump, such as a rolling element bearing, rotor or fastener, is often low compared to the total cost of the pump, the costs 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 costs of hundreds of thousands per day. Lost production in a paper mill, for example, equates to around $27,000 (£15,000) per hour. Total maintenance costs for a typical food or beverage manufacturer are around 15 to 20 percent of total costs. Of course, every manufacturing company has a maintenance department to deal with these problems, but maintenance often becomes reactive because of time and resource constraints. Plant problems are dealt with as they occur, with no predictive maintenance systems, little preventive maintenance and possibly no maintenance strategy at all.

Condition monitoring technology and predictive maintenance systems, including acoustic emissions monitoring, vibration monitoring and thermography, are relatively inexpensive compared to the cost of lost production.

Condition Monitoring

Plant condition monitoring teams can monitor hydraulic pumps, fans, compressors and blowers using patrol monitoring, fixed and portable CM systems. Monitor all pumps, some every week, but each pump at least once per month. Where the pump is critical to production process, such as hood cooling pumps at a basic oxygen steelmaking plant, install fixed CM systems to monitor the plant 24/7.

Condition monitoring prevents maintenance teams from replacing components unnecessarily and introducing possible new and unrelated problems. 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 the safest option. This method introduces further risk, because whenever there is human intervention, problems can occur.

A number of techniques are available to engineers for monitoring the condition of pumps and other hydraulic systems. 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.

Vibration Monitoring

As most pumps run at a steady load and speed, patrol monitoring with vibration analysis equipment is usually the most effective condition monitoring technique. 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.

Pumping of heavy, viscous fluids such as foodstuffs or sludge, for example, can cause damage to rotors, which in turn could result in pump imbalance. Similarly, any rotor deterioration caused by the pumping of corrosive liquids, can also lead to an unbalanced 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. 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 baseplate. When the duty pump fails, they simply switch to the standby pump, which is common practice in many industries.

However, 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 quickly fails, resulting in two pumps out of service. To prevent this, the two pumps should be switched over regularly on an 80-20 or 70-30 duty/standby ratio.

Acoustic Online Condition Monitoring

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, acoustic online condition monitoring systemss exist.

Acoustic emission monitoring equipment has a high sensitivity to machine faults but is also immune to audible noise and low frequency background vibration. Many engineers are not aware how acoustic emission monitoring systems can help reduce plant maintenance costs and improve machine availability. 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. Acoustic emissions are the high frequency stress waves generated by the rapid release of strain energy that occurs within 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 1-MHz frequency range.