SURVEY PUMPING SYSTEMS Put together a survey of the type, size and brand of pump. The purpose doing the survey is to have a list of the pumps on the site and also to ensure that pumps that are oversized or not suitable are identified. An example of what the pump survey could look like is shown below:

#	Size (HP)	Type	Brand	Operating Speed (RPM)	Flow rate (lb/min)	Pressure (PSI)	Purpose of pump	Comments for Improvement
1	500	Centrifugal	ABC	2000	20	200	Stock prep to wet end	Fix seal, oversized for use
2	250	Centrifugal	DEF	700	10	150	Hot water circulation	ASD required

 When performing the pumping system survey, look out for the following inefficiencies: 

• Awareness of pumps with high maintenance requirements
• Oversize pumps that operate in a throttled condition
• Cavitating or badly worn pumps
• Pumping systems with large flow rate or pressure variations
• Pumping systems with bypass flow
• Throttled control valve to provide fixed or variable flow rates
• Noisy pumps or valves
• Clogged pipelines or pumps
• Wear on pump impellers and casings that increase clearances between fixed and moving parts
• Excessive wear on rings and bearing
• Improper packing adjustments that causes binding on the pump shaft
• Multiple pump systems where excessive capacity is bypassed or excess pressure is provided
• Changes from initial design conditions. Distribution system cross-connections, parallel main lines, or changes in the pipe diameter or material may change the original system curve
• Low-flow rate, high pressure end use applications. An entire pumping system may be operated at high pressure to meet the requirements of a single end use. A booster or dedicated pump may allow system operating pressure to be reduced

PUMPS OPERATING IN PARALLEL Often some pumping systems may have a lead a spare pump, when the demand is high the spare pump kicks in. Even though the flow rate increases, operating pumps in parallel increases the fluid friction losses which result in higher discharge pressure, in which it reduces the flow rate provided by each pump.

Parallel pump configurations are best suited to a static head dominated system and not advised for friction dominated systems.

OVERSIZED PUMPS Pumps are often oversized to accommodate peak demands, gradual increases from flow resistance over time (due to fouling) and anticipated future capacity expansions. Investigate into oversized and throttled pumps around the plant and determine whether or not impeller replacement or trimming is suitable for the pump.

Trimming Pumps  Trimming is to reduce the diameter of the impeller, the impeller diameter must only be reduced to 75% of the maximum impeller size and not any less. Excessive trimming increases the clearance between the impeller and the fixed pump casing and this causes an increase in internal flow recirculation and results in head lass and lowers pump efficiency.

 NOTE: Consult with the pump manufacturer before performing trimming on the impeller.

 Trimming Calculations

 

Where: 
Q         = Pump flow rate (gallons/minute)
H         = Head (feet)
BHP    = Brake Horse Power (hp)
D         = Impeller diameter (inches)
1          = Denotes original design
2          = Denotes trimmed

Example An oversized centrifugal pump has a 16 inch diameter impeller and is throttled to provide a water flow rate of 4000 gal/min. The pumping system operates 8000 hours a year with a head of 180 ft, BHP of 160 HP and pumping efficiency (h) of 80%. Assume the pumping efficiency remains unchanged. The pump curve indicates that the trimmed impeller can supply a flow rate of 3000 gal/min with a head of 125 ft. Work out the amount of energy saved through the impeller trimming:

D₂ = 14.1 inches

 

By installing a 14.1 inch trimmed impeller the amount of energy saved can be calculated the following way:

 

PREVENTATIVE MAINTENANCE One way to ensure the health of the pumps is kept in shape is to have a planned maintenance schedule in place. This is will detect pump failures and inefficiencies before they become troublesome. Typically, the areas to maintain on regular basis are listed in the table below:

#	Item	Maintenance Description
1	General	Pay special attention to coupling alignment, adequate lubrication, seal maintenance and replacement.
2	Vibration Analysis	Organize to have vibration amplitude and frequency tests to be performed on a regular basis. Monitor the changes that these tests detect, the trend will potentially detect pending bearing, voltage imbalances, mechanical imbalances, impeller erosion, and coupling problems.
3	Motor Current Signature Analysis	This test may detect insulation deterioration, rotor bar damage, electrical system unbalances and harmonics. Trends of the tests should be kept to analyze the issues, trended data can be interpret the issues more accurately.
4	Lubrication Oil Analysis	This only applies to large oil lubricated pumps. Analysis of the oil may detect signs of bearing and seal issues. It will also tell whether the oil change interval is frequent enough.
5	Thermal Analysis	Thermal analysis enables the site to predict pending pump failures from bearing misalignment, over lubricating and incorrect wiring.

ADJUSTABLE SPEED DRIVES Often in industry, pumps are oversized and maybe installed to accommodate different operating conditions. A throttling valve is usually used when the process requirements are less than the pump’s flow capacity.  

ASDs are suitable to be applied to centrifugal pumps with varying demands for flow and are especially feasible for varying flows or pump requirements and when the pump operates for long hours. 

The purpose of the Adjustable Speed Drives (ASDs) is to control the flow by varying the speed of the pump. Small decreases in speed or flow can significantly reduce energy use.  In centrifugal applications with no static lift, small decreases in flow can significantly reduce energy use. In addition to energy savings, ASDs provide speed control and soft starting capability (soft starts prolong the life of the pump and reduce the stress on the pump during operation).

SOURCE: 

1. http://www1.eere.energy.gov/industry/bestpractices/motors.html