Looking for trouble
Staying ahead of pump problems.
Pumps are critical to plant operations. A breakdown could halt production and result in costly downtime, repair and/or replacement. Trouble will come at the most inconvenient time, which is why it’s important to stay ahead of potential problems.
Each pump has its own sound and vibration patterns. Once “normal” is established, pay attention to day-to-day changes in these patterns – they could mean trouble. Increased or erratic vibration of the driver is often the first symptom of an impending breakdown. Other changes include reduced speed, decreased flow rate, excessive leakage and strange noises.
Failure causes such as unevenly worn parts, bent shafts, loose impellers and signs of corrosion or abrasion become evident when a pump is pulled and disassembled.
Michael Dufresne, customer support services at Sulzer Pumps (Canada) Inc. in Toronto, provided six troubleshooting guidelines at a recent Education Day of the Hamilton Section of the Society of Tribologists and Lubrication Engineers (STLE).
1. Selection. Review the ISO 13709 (API 610) standard for hydraulic selection criteria, which states: the rated duty flow should be within 80% to 110% of best efficiency flow; pumps should have a preferred operating region of 70% to 120% of best efficiency flow; pumps should be capable of at least 5% head increase at rated condition; and head rise to closed valve (shutoff) should be at least 110% of rated head.
2. System curves. There’s always friction loss in a system from length of piping, valves, strainers and reducers. An increase in flow results in more friction loss, which is proportional to the square of flow.
The net positive suction head (NPSH) available at site (measured in feet of liquid and defined as the total head available in absolute terms above vapour pressure) must always be greater than the NPSH for the pump. Every liquid has a unique vapour pressure, which varies with temperature. The required NPSH can’t be calculated but can be estimated with values determined by testing. Impellers designed for low NPSH applications usually have high suction-specific speeds and are prone to operational problems at low flows.
Pumps have “comfort zone” within characteristic curves where the pump is good for continuous operation. Comfort zone is related to API 610 preferred flow bands.
3. Cavitation. It occurs when the pressure of a liquid falls below its own vapour pressure, causing noise and vibration when bubbles move into the higher pressure side of the impeller.
Suction-specific speed is a rating number indicating the relative ability of centrifugal pumps to operate under conditions of low available NPSH. Industry standards recommend pumps be in the 10,000 to 11,000 range.
4. Backflow and vibration. Flow spikes from the impeller may impact adjacent stationary elements, such as anti-swirl ribs or flow straighteners, producing vibration at vane pass frequency.
The swirling liquid ring slows down with increasing distance from the pump inlet. When the ring has an impact on stationary elements farther from the pump inlet, it produces vibration at a lower frequency. So-called vane pass occurs when the interaction between the impeller vane and volute lip generates pressure pulsations or waves at vane passing frequency. Staggered discharge vanes reduce pass pulsation.
Pump vibration has many causes, the most common being unbalance. Others include shafts, misalignment, oil whip or foundation failures.
5. Pump performance modifications. Underfiling the impeller discharge vanes increases pump performance by increasing the impeller outlet area.
Overfiling suction vanes or cutting them back lowers the required NPSH. Overfiling impeller discharge vanes by removing material from the pressure side thins the blade at the outlet to roughly one-third of its original width, which doesn’t impact discharge performance.
Cutting back volute lips on low- and medium-specific speed pumps moves the peak efficiency and head to the right by a factor of the volute area change.
6. Lubrication. The two most popular oils used for pump bearings are pure and refined mineral oils; and synthetic oils for high temperatures.
Various additives increase lube performance. Anti-oxidants improve oxidation stability to decrease corrosion and prevent the oil from becoming more viscous and there are additives that prevent foaming, which reduces the load-carrying capability of the lubricant. Film stiffeners reduce wear from metallic contact by forming a surface layer with a tension greater than the lubricant. Organic zinc compounds prevent direct contact between the ball and the races. Active EP additives form a chemical combination with the bearing metal to reduce friction. And solid additives, such as molybdenum disulfide, improve lubrication qualities.
The oil level should be halfway through the bottom ball when the pump is at rest.
Most pumps have a facility for cooling the oil when it gets too hot. Never attempt to cool a bearing by cooling the housing – steel will expand or contract.
Heat decreases oil’s velocity, creating even more heat as the lubricant loses its ability to support the load. The interference fit conducts heat away from the bearing and onto the shaft. Ensure there are no knurled surfaces or polymers used to build up the shaft to the proper dimension.
Effective troubleshooting is about knowing your pump. Become familiar with its proper performance, closely monitor operation and correct problems quickly to avoid costly interruptions to production.
Steve Gahbauer is an engineer, a Toronto-based business writer and a regular contributing editor to PLANT.
This article appears in the July/August 2015 issue of PLANT.