Pump System Optimization: The Key to Improving Plant Reliability

Gary Patterson, DSI/Dynamatic
Tags: maintenance and reliability

Most pumping systems operate at less than 100 horsepower (hp). Using a typical 75-hp pumping system as an example of life-cycle cost, the graph below outlines the costs related to maintain and operate equipment.

In this example, the total life-cycle cost is approximately $757,145. Eighty-three percent of the life-cycle cost is operation. Electrical consumption is included in the cost of both operation and maintenance. These numbers are based on a very conservative 20-year life cycle.

Pumping system performance, efficiency and reliability are highly influenced by the system they supply. Improving individual pump efficiency does little to reduce pump mean time between repairs/failures (MTBR/MTBF). The focus must be on the entire system.

Pump system reliability and performance are affected by many factors, such as hydraulic and system conditions (valves, piping, elevation and installation); operational sequencing (variable-frequency drive operation, throttling and multiple operation); and pump efficiency (impeller modifications and wear).

In addition, the lack of pump system standards impacts performance and reliability. Currently, no standards exist to guide system design. Therefore, engineering contractors and owners/operators can choose (or ignore) how to calculate system hydraulics. The bottom line is that the specified pump operating point is not subject to standards.

As a result, many existing systems are unreliable and inefficient, leading to costly downtime, high maintenance costs and lost productivity. A reputable source that evaluated 1,690 pumps at 20 process plants found:

  • The average pumping efficiency is below 40 percent.
  • More than 10 percent of pumps run below 10 percent efficiency.
  • Major factors affecting pump efficiency include throttled valves and pump oversizing.
  • Seal leakage causes the highest downtime and cost.

Imagine the impact on life-cycle costs, reliability and efficiency. What can the end user/owner-operator do to correct existing systems and ensure that future systems are efficient and reliable? Assess existing pumping systems, identify sources of inefficiency/reliability and implement corrective measures. For new systems, write specifications to ensure system optimization in the design phase.

System Assessment/Analysis

Your ability to determine pump performance/reliability is highly dependent on your knowledge of pump rotodynamics. Centrifugal pumps should be selected and normally operated at or near the manufacturer's design-rated conditions of head and flow. Any pump operating at excess capacity (i.e., at a flow significantly greater than the best efficiency point and at a lower head) will surge and vibrate, creating potential bearing and shaft seal problems as well as requiring excessive power.

When operating at reduced capacity (i.e., at a flow significantly less than the best efficiency point and at a higher head), the fixed vane angles will cause eddy flows within the impeller, casing and between the wear rings. In addition, the radial thrust on the rotor will increase, causing higher shaft stresses, increased shaft deflection, and potential bearing and mechanical seal problems while radial vibration and shaft axial movement will increase.

Pump Operation

To comprehend the damage that occurs in a rotodynamic pump, one must understand the basic operation principles of rotor centralization to the stationary components. Known as the Lomakin effect, rotor centralization is defined as support force that occurs in pumps at annular seals, such as wear rings (see image below), due to the action of Bernoulli's effect during the normal leakage process. However, this support force only occurs when the pump is operating at or near its design-rated condition of head and flow.

The Lomakin effect can sometimes be confusing because it encompasses two separate phenomena that occur at the wear rings/annular seals: damping and stiffness. Damping does not directly prevent shaft deflection but minimizes rotor response to excitation forces much in the same way that shock absorbers result in a smooth ride in a car. Reduced clearance increases damping and results in a more stable rotor.

Perhaps most important, the stiffness and damping are located at the impeller where the pump has no bearing support. This strategic location gives the Lomakin effect a great deal of power in minimizing shaft deflection and, ultimately, reliability.

Operators should replace pump wear rings when the new clearance reaches twice the original value to maintain sufficient rotor stability and efficiency.

Specific Speed: Is the Pump Properly Specified for the System?

Specific speed describes the geometry (shape) of a pump impeller. Defined as "the speed of an ideal pump geometrically like the actual pump," the specific speed, when running at this rate, will raise a unit of volume in a unit of time through a unit of head.

As noted in the image above, the steepness of the head/capacity curve increases as specific speed increases. At low specific speed, power consumption is lowest at shutoff and rises as flow increases. This means that the motor could be overloaded at the higher flow rates unless this was considered at the time of purchase. At medium specific speed, the power curve peaks at approximately the best efficiency point (BEP). This is a non-overloading feature where the pump can work safely over most of the fluid range with a motor speed to meet the BEP requirement.

High-specific-speed pumps have a falling power curve with maximum power occurring at minimum flow. These pumps should never start with the discharge valve shut. If throttling is required, a motor of greater power will be necessary. As a rule of thumb, lower specific speeds produce flatter curves, while higher specific speeds produce steeper ones. It should be noted that the efficiency and power consumption were calculated at the BEP.

In practice, pumps operate in a throttled condition because they were oversized at the time of purchase. Lower-specific-speed pumps may have a lower efficiency at their BEP but will have lower power consumption at reduced flow than many of the higher-specific-speed designs.

Do I Have Adequate NPSHa?

Net positive suction head (NPSH) is the total suction head in feet of the liquid being pumped (at the centerline of the impeller eye) less the absolute vapor pressure of the liquid being pumped. Net positive suction head available (NPSHa) is also known as plant NPSH. A rule of thumb for ambient temperature is that clear water is 25 percent more NPSHa than net positive suction head required (NPSHr). The pump OEM determines NPSHr during testing.

Suction specific speed is another calculation performed when evaluating a pump system to determine if it is the correct pump for the application. Specific suction speed is typically used by the pump designer/specifier to determine which pump geometry (radial, mixed flow or axial) to use for maximum efficiency and prevent cavitation, as well as to estimate the pump safe operating range. To avoid cavitation, the specific suction speed should be less than 8,500 (calculated with U.S. gallons per minute).

Is the Pump Cavitating?

Cavitation occurs when the pressure of the liquid drops below its vapor pressure, forming vapor pockets. When the pressure of the liquid is later increased above its vapor pressure, the vapor pockets will collapse when they pass into the higher regions of pressure, causing noise (audible cavitation), vibration and damage to many of the pump components. These cavities form at the low pressure or suction side of the pump, causing several occurrences simultaneously, including a loss of capacity, inability of the pump to build the same head (pressure) and a drop in pump efficiency.

The cavities form for several reasons. It’s common practice to lump them into the general classification of cavitation. You must understand why they occur and how to fix them. They include:

  • Vaporization air ingestion (not true cavitation but with similar characteristics)
  • Internal recirculation
  • Flow turbulence
  • The vane passing syndrome: The OD of the impeller passes too close to the pump cutwater. The velocity of the liquid increases as it flows through this small passage, lowering the fluid pressure and causing local vaporization.

Is the Piping Properly Configured?

The pump piping functions in providing a conduit for the flow of liquid to and from a pump while not adversely affecting the performance or reliability of the pump. In addition, a well-designed piping system will usually be more energy-efficient than a poorly designed system. The function of suction piping is to provide a uniform velocity profile approaching the pump inlet (suction) connection with sufficient pressure to avoid damaging cavitation in the pump.

Pump Component Failures/Troubleshooting

Besides a lack of pump system standards, many reliability issues are associated with poor installation practices. The pump system specification should address the installation procedure. In addition, component failure can be attributable to hydraulic instability in the pump due to poor installation practice as shown in the image below.

Typical Failure Modes of a Centrifugal Pump

 

Potential Root Cause of Damaging Forces

 

  

Vertical Pump Installation and Damaging Forces

Vertical Pump Rotor Stability

When a vertical pump is energized and stabilized at the BEP, the wet end will center itself in the stationary components (true with any rotodynamic pump). This phenomenon is known as the Lomakin effect, as previously discussed. In the case of a vertical pump, if the entire pump/motor base plate isn't perfectly plumb and level, the rotating components (above the wet end) may contact the stationary components. Points 1 and 2 in the image above identify critical areas that must comply with HI standards. In addition, the suction and discharge flange loading points 3 and 4 must comply with HI standards.

Conclusion

Many pumping systems are not well-designed or controlled. Your ability to determine pump performance/reliability is highly dependent on your knowledge of pump rotodynamics. Centrifugal pumps should be selected and normally operated at or near the manufacturer's design-rated conditions of head and flow.

Pumping system performance, efficiency and reliability are highly influenced by the system they supply. Component failures are often caused by system problems, system design, specifications, installation and operation. Operators need to understand the symptoms that occur in the system when it operates away from the BEP. Remember, efficiency and reliability go together. Take a systems approach when evaluating your repair and maintenance options.

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