Take care of your pumps and they’ll take care of you
The old saying “an ounce of prevention is worth a pound of care” may have been coined by process and plant engineers tired of repairing or replacing pumps. Pumps are often the most underserviced pieces of equipment in process automation when it comes to maintenance and prevention best practices.
Unfortunately, nothing moves without the humble pump and a process becomes inefficient when they don’t operate properly or completely shut down. Many times the pump manufacturer is seen to be the problem when, in fact, the process or the surrounding equipment configuration is the cause.
Engineers and technicians looking to optimise their process for productive operation can start with the pump, and protecting the pump against common hazards. Pump protection improves end product or batch quality, reduces material costs, eliminates waste and lowers maintenance costs. Taking good care of your pump delivers a positive payback. Here are some simple strategies that can be employed - starting with an analysis of process media flow rates.
Protecting your process, 24/7
Today’s highly competitive global market finds demanding process industries such as petrochemicals, food and beverage, pharmaceutical and water and waste treatment, among others, transforming their plants into 24/7 lean operations. The result is that the pumps in most plants are running near capacity to keep up with material throughput objectives and demand. One of the most common hazards to efficient pump operation is irregular material flow, which can result in three negative conditions: flow turbulence, low flows or dry running conditions.
A key process-protection step taken by facilities and plant engineers is the controlling of material flow to ensure that pumps operate efficiently. This results in moving stock or product with the least possible expenditure of energy and at the same time reducing maintenance requirements and extending the life of the pump. Failing to control material flow effectively can lead to some unwanted conditions, such as cavitation, pump bearing failure or seal failure. The first problem, cavitation, can reduce throughput, or even cause quality problems. Losing a bearing or a seal can lead to pump shutdown, and possibly process line shutdown - and the unfavourable conditions could get worse the further you take this type of scenario.
Monitoring for irregular flows
The first step in protecting your process and pump starts with analysing the flow. You want to analyse the flow to ensure the media is flowing regularly at the pressure required by the pump with a minimum head loss. Any number of process conditions can cause irregular flow, such as turbulence, temperature changes, unwanted air ingestion, etc. The problems of irregular flows and turbulence, in particular, can be especially challenging to solve because eliminating the root causes are often difficult to impossible - so you need a workaround strategy.
The chief culprit when it comes to damaged pumps is the build-up of heat from low flow or dry running conditions, which occur when liquid flow dramatically slows down or stops flowing altogether through the line or the pump. When the liquid isn’t there to provide cooling, the heat can destroy a pump’s bearings or seals. If repair is even possible, it is going to be very expensive due to repair or replacement costs and downtime.
Eliminating irregular flows
Pumps require a stable upstream flow profile in the pipeline before liquid enters the pump for proper and efficient operation. Irregular flows often result in cavitation, a condition where cavities form in the liquid at the point of pump suction. One often-cited industry pump installation guideline suggests at least 10 diameters of unobstructed pipe be placed between the point of pump suction and the first elbow or other disturbance. Obstructions or corrosion in a pipe can change the velocity and flow profile of the media and affect its pressure as well.
In most cases, plant real estate limitations result in the placement of elbows, valves or other equipment that are too close to a pump, and these devices can create swirl and velocity profile distortion in the pipeline (as well as pressure changes). Such disturbances can result in excess noise and cavitation, resulting in reduced bearing or seal life.
A good solution to ensure an optimal flow profile for efficient operation is to install an inline or elbow flow conditioner upstream from your pump. Isolating the effects of velocity profile distortions, turbulence, swirl and other flow anomalies in your pipeline will result in a repeatable, symmetric and swirl-free velocity profile with minimal pressure loss.
To increase a pump’s life, start with a more stable operating environment. A conditioned flow stream enters the pump’s impeller in a uniform and equally distributed pattern, optimising pump efficiency and extending bearing life while at the same time decreasing noise and cavitation.
If there is no choice other than to deal with less-than-ideal piping configurations, an inline or elbow flow conditioner will eliminate all upstream straight-run requirements for pumps, compressors, flow meters and other critical process equipment (Figure 1). Tab-type flow conditioners, such as the Vortab flow conditioner, have proved successful in these applications. Other flow conditioning technology choices, including tube bundles, honeycombs and perforated plates, may also be considered depending upon the pressure drop limitations.
The inline or elbow flow conditioner’s profile conditioning tabs produce rapid cross-stream mixing, forcing higher-velocity regions to mix with lower-velocity regions. The shape of the resultant velocity profile is ‘flat’ and repeatable regardless of the close-coupled upstream flow disturbances.
Incorporating anti-swirl mechanisms into the design of the flow conditioner eliminates the swirl condition typically seen exiting 90° elbows. The result is a flow stream that enters the pump in such a way that it maximises the efficiency of its operation and reduces stress. In addition, the tapered design of the anti-swirl and profile conditioning tabs make them immune to fouling or clogging.
Pump flow monitoring
Avoiding the damage that is caused by a low-flow or a dry running condition can be achieved by installing a point flow switch in the process loop. Dual relay flow switches will detect not only a low-flow condition, but also alarm on a dry condition too. This capability allows the control system or operator to take corrective measures before the bearings of the pumps are overheated and fail.
Many types of point flow switches with dual alarm levels are available (Figure 2). At the first alarm level, the switch will detect a low-flow situation anywhere between .003 to .9 m/s. This low flow alarm can be regarded as a pre-warning signal for the control system or operator. The second alarm can be set at a no-flow condition. The system or operator can then decide to keep the pump running or to shut it down.
A dual-function flow switch such as the FCI FlexSwitch of Figure 2 indicates both flow and temperature and/or level sensing in a single device. It can be specified in either insertion or inline styles for large pipe or small line applications. This single switch monitors your direct variable of interest, flow and temperature simultaneously.
Choosing a flow switch
When evaluating a flow switch for pump protection or any application, the first step is choosing the appropriate flow technology. There are multiple flow-switch sensing technologies available, and the major ones now include:
- Thermal mass
- Magnetic reed
Each of these technologies has their advantages and disadvantages, depending on the media and your application’s requirements. Some may be the only choice in certain media for your application. By looking at these factors, as well as your plant’s layout, environmental conditions, maintenance schedules, energy cost and ROI, you will quickly be able to narrow the field to one or two best choices.
Don’t fall into the trap of early pump replacement or repair by ignoring best installation and maintenance pump practices. Here are three preventive proactive steps to take to avoid early pump replacement:
- When designing new plants or retrofitting old ones, be sure to consider pump requirements. Optimising your process with your pumps in mind offers a wide range of benefits: higher capacity, improved quality, lower energy costs, reduced maintenance and increased equipment (pump) life.
- Consider inserting a flow conditioner to eliminate turbulent flow problems. One of the most common pump problems is irregular flows caused by turbulence that frequently results when the minimum pipe straight runs required between the point of pump suction and elbows, valves or other equipment are either ignored or pushed to the limits. Inserting a flow conditioner frequently eliminates turbulent flow problems.
- Another key safeguard is to protect your pump from accidental low-flow or dry running conditions, which can lead to bearing or seal loss requiring expense repairs. Inserting a dual alarm flow switch in your process loop not only protects the pump from damage, but will alert you to a potential problem and let you be proactive in evaluating the necessity of pump shutdown.
by Jim DeLee, senior member of the technical staff of Fluid Components International
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