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Previous page: Upgrading pump monitoring for process efficiency

Eliminating irregular flows

For proper and efficient operation, pumps require a stable upstream flow profile in the pipeline before liquid enters the pump. Irregular flows often result in cavitation, a condition where cavities form in the liquid at the point of pump suction. A commonly cited industry pump installation guideline suggests that at least 10 diameters of unobstructed pipe should be placed between the point of pump suction and the first elbow or other disturbance. Obstructions and/or corrosion in a pipe can change the velocity of the media and affect its pressure as well.

When plant real estate restrictions result in the placement of elbows, valves or other equipment that are too close to a pump, these other devices can create swirl and velocity profile distortion in the pipeline (as well as pressure changes). These disturbances can result in excess noise and cavitation, resulting in reduced bearing and/or seal life.

An inline or elbow flow conditioner can be installed upstream from your pump to ensure an optimal flow profile for efficient operation. 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.

Creating a relatively more benign operating environment helps increase pump life. The 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.

When faced with less than ideal piping configurations, an inline or elbow flow conditioner should eliminate all upstream straight run requirements for pumps, compressors, flow meters and other critical process equipment. Tab type flow conditioners, such as the Vortab flow conditioner, have proved successful in these applications. Other flow condition-ing technology choices, including tube bundles, honeycombs, and perforated plates, may also be considered depending upon the specifics of the application.

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 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

Eliminating the damage caused by a low flow or a dry running condition can be achieved by installing a point flow switch in the process loop. Combination point flow switches will detect not only a low flow situation, but also a dry running condition.

This detection capability allows the control system or operator to take corrective measures before the bearings of the pumps are overheated and fail.

There are many types of point flow switches available. The FCI FlexSwitch® FLT Series, with its no moving parts design, offers a scheme for pump protection with its dual alarm capability. With alarm one, the switch will detect a low-flow situation anywhere between 0.01 and 3 ft per second (FPS or .003 to 0.9 m per second MPS). This low flow alarm can be regarded as a pre-warning signal for the control system or operator. The system or operator can then decide to keep the pump running or to shut it down.

If an alarm two occurs because the feed line to the pump is running dry, this would be an emergency signal to shut down the pump immediately because the bearings now see gas instead of a liquid as a heat transfer media. In such situations, the temperature of the bearings may rise very fast. Using a flow switch prevents permanent damage to the pump's bearings that would require an overhaul of the pump before more damage occurs.

The FLT flow switch is a dual-function instrument that indicates both flow and temperature, and/or level sensing in a single device. It can be specified in either insertion or in-line styles for pipe or tube installation. With the FLT, a single switch monitors your direct variable interest, flow, and temperature simultaneously. The dual 6A relay outputs are standard and are assignable to flow, level or temperature.

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Next page: Choosing a flow switch

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