rotarypump

Reliability best practices for rotary positive displacement pump users

If you’re responsible for any process using rotary positive displacement (PD) pumps, and if you’re interested in maximizing reliability, then read on. Pumping principles within this category include internal gear, external gear, vane, progressive cavity (PC), three-screw, peristaltic and timed-rotor pumps including lobe, circumferential piston and twin screw, plus a few lesser known variants.
 

While there are many differences between these technologies, there are also some commonalities that lead to best practices for improving reliability and minimizing maintenance.

By John H. Hall, Senior Product Manager, Viking Pump Inc.

Some of these best practices may increase initial costs, but usually pay for themselves many times over by reducing unplanned downtime. Pump basics, like proper foundation and installation, pipe supports, shaft alignment, and short suction line, are assumed. In no particular order, consider these eight tips specific to rotary PD pumps:

1. Strain it out
Rotary PD process pumps are characterized by very close clearances between rotating elements (gears, rotors, screws) and stationary elements (casing, head, stator), to minimize internal liquid slip from the high pressure (discharge) side of the pump to the low pressure (suction) side. When foreign objects like rocks, welding rods, bolts and rags are sucked into a rotary PD pump, it can suffer excessive wear or catastrophic failure if a rotating member is driven against a second rotary member or a stationary member. Unless you are using a pump designed to handle larger solids, like some of the PC, lobe and peristaltic pumps, the best insurance you can buy is a strainer located upstream of the pump.
Anyone who has ever emptied a strainer basket can attest to the fact that there are bad things flowing through even the cleanest systems!

2. Relieve pressure
All rotary PD pumps are capable of developing almost infinite pressure (to the point of something failing) when the discharge line is restricted by an obstruction or closing a downstream valve. To protect the pump, drive and downstream piping system, some form of pressure relief is required. Many PD pumps offer integral pressure relief valves that will bypass from the high pressure to the low pressure side of the pump when the developed pressure exceeds a predetermined limit. However, heat builds up if the flow is bypassed for more than a short time, therefore, for flammable
liquids consider a return-to-tank or external pressure relief valve, rupture disk or other pressure relief device, either in place of or in addition to an integral relief valve. Many rotary pumps operate equally well in either direction, so if the flow is to be reversed, make sure that pressure relief is provided on both sides of the pump.

3. Slow it down
Reducing rotational speed on any equipment will increase the life of bearings and seals. It also helps reduce wear on parts that are contacting during operation, especially when pumping abrasive or nonlubricating liquids. While a PD pump may be capable of operation at motor speeds, using a larger displacement pump at slower speeds is going to result in less maintenance and longer life. For higher viscosities, slower speeds are a must to enable completely filling the voids created on the suction side of the pump. Selecting a PD pump to operate at slower-than-rated speeds gives you room to increase speed as the pump wears over time to maintain the design flow rate, or to increase capacity as processes change, in addition to the benefit of reduced wear.

4. Standardize
The flow rate of a PD pump is directly proportional to speed and they can operate anywhere on the curve. This enables one of the key benefits – standardization. For example, a US roofing manufacturer had fifteen different gear pumps, each sized for its design conditions. This resulted in lowest initial cost, because smaller pumps were used on lower flows, and larger pumps on higher flows. But a pump with unplanned downtime could shut down production if the needed spares were not on the shelf. So, they recently implemented a strategy to standardize on one size and model of gear pump that would handle all of the applications throughout the plant. This meant higher retrofit costs because they were using larger displacement pumps than before, along with remounting drives and adapting piping. However, the
maintenance shop can now stock one complete spare pump that will change out for any pump in the plant, and has reduced the overall number of spares.

5. Monitor
Every pump, regardless of type, should have suction and discharge gauges or transducers to enable an operator to identify potential issues before they become failures. Suction side gauges can identify blocked suction strainers, partially-closed valves or other impediments (high vacuum reading), as well as air leaks into suction piping or empty source tank (low vacuum reading). Discharge pressure gauges can identify overpressure situations due to blockages.
The difference between the two readings tells you differential pressure, which lets you ensure that the pump is operating within its rated pressure range. There are some issues unique to PD pumps that have been diagnosed using gauges. For example, one edible oil processor noticed extreme spikes on their discharge pressure gauges every time the pumps were started, well beyond what was expected for momentary acceleration surges. The pumps and downstream
valves were controlled by the plant automation system, and they discovered that while the PD pumps started flowing instantaneously, the downstream valve actuators took several seconds for the valves to fully open. A minor programming change solved that problem, which wouldn’t have been identified without operators looking at gauges.

To read tip 6, 7 and 8, please contact the editor, Deirdre Morgan, to receive the full article. 
 

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