"An ounce of prevention is worth a pound of cure" may have been coined by process and plant engineers tired of repairing or replacing pumps. Pumps are often the most under-serviced 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 a pump doesn't operate properly or completely shuts 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 optimize their process for productive operation can start with 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- petrochemicals (see the image below), food/beverage, pharmaceutical, water/waste treatment, etc. -transforming their plants into 24/7 lean operations. The result is pumps in most plants running near capacity to keep up with material through-put objectives and demand. One of the most common hazards to efficient pump operation is irregular material flow, which can result in three negative conditions: (1) flow turbulence, (2) low flows or (3) dry running conditions.
Facilities and plant engineers should control material flow to ensure that pumps operate efficiently. This results in moving stock or product with the least possible expenditure of energy, 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. Cavitation can reduce through-put or even cause quality problems. Losing a bearing or a seal can lead to pump shutdown, possibly process line shutdown and progressively worse conditions.
Monitoring for Irregular Flows
The first step in protecting your process and pump is analyzing the flow. You want to analyze the flow to ensure the media is flowing regularly at the pressure required by the pump with a minimum headloss. Any number of process conditions can cause irregular flow-turbulence, temperature changes, unwanted air ingestion, etc. 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 buildup 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 if you factor in 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 industry pump installation guideline suggests placing at least ten diameters of unobstructed pipe between the point of pump suction and the first elbow or other disturbance. Obstructions and/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 and/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, optimizing 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, flowmeters and other critical process equipment. Tab type flow conditioners have proved successful in these applications. Other flow conditioning 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-deg elbows. The result is a flow stream that enters the pump in such a way that maximizes 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 caused by a low flow or a dry running condition can be achieved with a point flow switch in the process loop. Dual relay flow switches will not only detect a low flow condition, but also alarm on a dry condition. This capability allows the control system or operator to take corrective measures before the bearings of the pumps are overheated and fail.
This dual-function flow switch indicates flow, temperature and/or level sensing in a single device. It can be specified in either insertion or in-line styles for large pipe or small line applications. This single switch monitors 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, including:
- Paddle
- Piston
- Thermal Mass
- Pressure
- Magnetic Reed
Each of these technologies has advantages/disadvantages, depending on the media and the application 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
Here are three preventative, proactive steps to take to avoid early pump replacement:
- When designing new plants or retrofitting old ones, be sure to consider pump requirements. Optimizing 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. Irregular flows caused by turbulence frequently result 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 expensive 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.
Pumps & Systems, February 2008