MRO Magazine

How to get optimal performance from pumps

Knowing how to maintain pumps and their components while dealing with malfunctions effectively goes a long way to improving pump performance, increasing reliability and saving some money.


Machinery and Equipment Maintenance

December 1, 2011
By STEVE GAHBAUER

Knowing how to maintain pumps and their components while dealing with malfunctions effectively goes a long way to improving pump performance, increasing reliability and saving some money.

Here are some nuggets of pump wisdom gleaned from presentations at a recent Fluid Prime Movers seminar in Toronto organized by the Society of Tribologists and Lubrication Engineers (STLE).

Pump failure modes

When a centrifugal pump does not reach design flow rate or design head, or operates for a short period but then loses prime, it probably has insufficient net positive suction head or entrained air.

Advertisement

The likely cause of a pump exhibiting similar symptoms, but also showing decreased power consumption with no discharge or flow while running, is a greater than anticipated system head. Other causes include the following: the direction of rotation is reversed, the pump speed is too low, or the impeller is too small. Also check for a plugged impeller, suction line or casing. Improper priming could be the cause of decreased power consumption.

There could be damaged bushings, thrust bearings or impeller rubbing when a pump runs with excessive noise from the wet end and exhibits an increase in power consumption. For noise in magnetic drive pumps, check for rubbing of the inner magnet against the shell. Excessive noise could also be caused by abnormal fluid rotation due to complex suction piping, which should always be straight.

There are several possible causes for increased power consumption and excessive noise from the power end, including: bearing contamination appearing on the raceways as scoring, scratching or rusting; brinelling of bearings caused by incorrectly applied forces during assembly, identifiable by indentation on the raceway; thrust overload on bearings; misalignment; a bearing out of square with the centreline; or a bent shaft.

Pump selection criteria

When selecting positive displacement pumps, look at the following criteria:

• capacity (constant or variable)

• viscosity (high or low)

• pressure (discharge and suction)

• solids and abrasives (size, hardness and amount)

• air and/or gas (volume, entrained, or slugs)

• shearing properties (shear thinning, Newtonian, etc.)

• temperature (high, low, ambient)

• whether a variable-speed drive pump is necessary for total process control.

Also consider pump material (soft, hard, coated, corrosion resistant); proper sealing (packing, seal type, magnetic drive); lubrication (product, grease, oil, oil mist or external system); relief valves in the system; and instrumentation for monitoring what’s going on inside the pump (pressure, speed, temperature, vibration).

Care for pump components

Bearings and seals need special attention. Pump bearings support the shaft, permit the shaft to rotate freely, keep lateral deflections to a minimum and maintain proper clearances between the impeller and the housing.

Bearing types are either fixed side or free side. Bearing cages keep rolling elements evenly spaced around the bearing and reduce friction by preventing rolling elements from coming into contact with each other. Cage materials can be steel or brass. Metallic cages are most popular, however polyamide cages are quieter, but they’re more sensitive to poor lubrication or excessive temperatures.

Pump seals

Mechanical seals keep the fluid contained and prevent it from leaking along the rotating pump shaft. The seal faces, which rotate or are stationary and are angular to the shaft, are pressed on to the seat by axial forces.

The main components of a seal arrangement are a rotating or stationary seal face, a dynamic or stationary O-ring, and a spring. Static or hydraulic forces close the seal and the product or barrier medium serves as a lubricating and cooling agent.

Seals must operate within their design specifications, but leakage may occur before or after the seal’s designed life expectancy. Seal performance can be improved by changing the design and type, or the material it’s made of. It’s also possible to improve installation, operating procedures and maintenance activities. But when seals fail – and they often do – look for and remedy the causes immediately.

Reasons for seal failures include: the wrong seal type for the application; wrong handling, installation or storage, which may cause a loss of elasticity; improper start-up or shutdown; improper operation of the pump; media contamination; shaft deflection or run-out; or worn seal faces.

Uneven wear patterns mean that the shaft is out of alignment or the seal face was not properly aligned. Out-of-alignment shafts also cause an eccentric wear pattern.

Radial cracks and heat checking indicate poor lubrication and overheating of contact faces. These conditions also cause blistering and pitting at the carbon faces. Edge chipping on the contact face is a sign of incorrect handling, or shaft deflections beyond manufacturer’s specification, and possibly of bearing damage.

All mechanical seal faces leak, albeit at a very slow rate. For some high-temperature products, such leakage oxidizes or hangs up the seal face, while dissolved solids crystallize on the atmospheric side. A frequent liquid or gas quench is a very effective remedy for these conditions.

Steve Gahbauer is an engineer and a Toronto-based freelance writer.

 

Seven pump installation tips

Proper installation practices, with attention to foundation, piping arrangement and shaft alignment, mean the difference between a pump that gives many years of trouble-free service and one that requires frequent and repetitive maintenance. Follow these tips:

1. Select the right pump for the job. Consider variable-speed technology for multiple operating conditions, instead of throttling. Trim the impeller to about 75%.

2. Balance the impeller after trimming. Couplings should be the elastomer type for soft start and balanced to the AGMA 8 standard, or AGMA 10 for pumps over 75 hp. Offset the pump and motor shaft/coupling hub keyways 180 degrees and cut keys to one-half unfilled keyway length.

3. Install the pump, base and motor level and free of soft foot. Provide at least 10 diameters of straight pipe from the suction inlet. Grout the baseplate with low-shrink cement or, better still, epoxy grout. Ensure pipe fasteners don’t exert a force on the piping.

4. Align properly. Use a laser or reverse dial indicator technology for correct alignment. Align before and after the pipe/pump bolt-up. Keep tolerances as small as practicable.

5. Lubricate properly. Change the lubricant at recommended intervals, protect lubricant from contamination, use synthetic lubes and sealed or vapour-block bearings, and maintain a mechanical seal flush environment.

6. Start-up properly. For safe and reliable operation, develop suitable start-up procedures and checklists, avoid rapid closing of process valves, and use a power monitor for minimum/maximum flow protection. Never run a pump dry. Operate spare pumps every three months.

7. Maintain tolerances. Use cartridge
seals designed to reduce fretting corrosion and maintain critical impeller setting tolerances.


Print this page




Leave a Reply

Your email address will not be published. Required fields are marked *

*