Maintenance Engineering -- PUMPS: part 3

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CAVITATION

Many centrifugal pumps are designed in a manner that allows the pump to operate continuously for months or even years. These centrifugal pumps often rely on the liquid that they are pumping to provide cooling and lubrication to the pump bearings and other internal components of the pump. If flow through the pump is stopped while the pump is still operating, the pump will no longer be adequately cooled and the pump can quickly become damaged. Pump damage can also result from pumping a liquid that is close to saturated conditions. This phenomenon, referred to as cavitation, is discussed further in section 5 (Preventing Cavitation). Most centrifugal pumps are not designed to withstand cavitation.

The flow area at the eye of the impeller is usually smaller than either the flow area of the pump suction piping or the flow area through the impeller vanes. When the liquid being pumped enters the eye of a centrifugal pump, the decrease in flow area results in an increase in flow velocity accompanied by a decrease in pressure.

The greater the pump flow rate, the greater the pressure drop between the pump suction and the eye of the impeller. If the pressure drop is large enough, or if the temperature is high enough, the pressure drop may be sufficient to cause the liquid to flash to vapor when the local pressure falls below the saturation pressure for the fluid being pumped. Any vapor bubbles formed by the pressure drop at the eye of the impeller are swept along the impeller vanes by the flow of the fluid. When the bubbles enter a region in which local pressure is greater than saturation pressure farther out the impeller vane, the vapor bubbles abruptly collapse. This process of the formation and subsequent collapse of vapor bubbles in a pump is called cavitation.

Cavitation in a centrifugal pump has a significant effect on performance. It degrades the performance of a pump, resulting in a degraded, fluctuating flow rate and discharge pressure. Cavitation can also be destructive to pump internals.

The formation and collapse of the vapor bubble can create small pits on the impeller vanes. Each individual pit is microscopic in size, but the cumulative effect of millions of these pits formed over a period of hours or days can literally destroy a pump impeller. Cavitation can also cause excessive pump vibration, which could damage pump bearings, wearing rings, and seals.

A small number of centrifugal pumps are designed to operate under conditions in which cavitation is unavoidable. These pumps must be specially designed and maintained to withstand the small amount of cavitation that occurs during their operation.

Noise is one of the indications that a centrifugal pump is cavitating. A cavitating pump can sound like a can of marbles being shaken. Other indications that can be observed from a remote operating station are fluctuating discharge pressure, flow rate, and pump motor current.

---22 Vane pass frequency.

RECIRCULATION

When the discharge flow of a centrifugal pump is throttled by closing the discharge valve slightly, or by installing an orifice plate, the fluid flow through the pump is altered from its original design. This reduces the fluid's velocity as it exits the tips of the impeller vanes; therefore the fluid does not flow as smoothly into the volute and discharge nozzle. This causes the fluid to impinge on the ''cutwater'' and creates a vibration at a frequency equal to the vane pass x rpm.

The resulting amplitude quite often exceeds alert setpoint values, particularly when accompanied by resonance.

Random low-amplitude, wide-frequency vibration is often associated with vane pass frequency, resulting in vibrations similar to cavitation and turbulence, but is usually found at lower frequencies. This can lead to misdiagnosis. Many pump impellers show metal reduction and pitting on the general area at the exit tips of the vanes. This has often been misdiagnosed as cavitation.

It’s very important to note that recirculation is found to happen on the discharge side of the pump, whereas cavitation is found to happen on the suction side of the pump.

To prevent recirculation in pumps, pumps should be operated close to their operational rated capacity, and excessive throttling should be avoided.

When a permanent reduction in capacity is desired, the outside diameter of the pump impeller can be reduced slightly to increase the gap between the impeller tips and the cutwater.

NET POSITIVE SUCTION HEAD

To avoid cavitation in centrifugal pumps, the pressure of the fluid at all points within the pump must remain above saturation pressure. The quantity used to determine whether the pressure of the liquid being pumped is adequate to avoid cavitation is the net positive suction head (NPSH). The net positive suction head available (NPSHA) is the difference between the pressure at the suction of the pump and the saturation pressure for the liquid being pumped. The net positive suction head required (NPSHR) is the minimum net positive suction head necessary to avoid cavitation.

The condition that must exist to avoid cavitation is that the net positive suction head available must be greater than or equal to the net positive suction head required. This requirement can be stated mathematically as shown below.

NPSHA > = NPSHR

A formula for NPSHA can be stated as the following equation:

NPSHA = P_suction _ P_saturation

When a centrifugal pump is taking suction from a tank or other reservoir, the pressure at the suction of the pump is the sum of the absolute pressure at the surface of the liquid in the tank plus the pressure caused by the elevation difference between the surface of liquid in the tank and the pump suction, less the head losses caused by friction in the suction line from the tank to the pump.

NPSHA = Pa = Pst _ hf _ Psat where NPSHA = Net positive suction head available Pa = Absolute pressure on the surface of the liquid.

Pst = Pressure caused by elevation between liquid surface and pump suction hf = Head losses in the pump suction piping P_sat = Saturation pressure of the liquid being pumped.

PREVENTING CAVITATION

If a centrifugal pump is cavitating, several changes in the system design or operation may be necessary to increase the NPSHA above the NPSHR and stop the cavitation. One method for increasing the NPSHA is to increase the pressure at the suction of the pump. If a pump is taking suction from an enclosed tank, either raising the level of the liquid in the tank or increasing the pressure in the gas space above the liquid increases suction pressure.

It’s also possible to increase the NPSHA by decreasing the temperature of the liquid being pumped. Decreasing the temperature of the liquid decreases the saturation pressure, causing NPSHA to increase.

If the head losses in the pump suction piping can be reduced, the NPSHA will be increased. Various methods for reducing head losses include increasing the pipe diameter; reducing the number of elbows, valves, and fittings in the pipe; and decreasing the length of the pipe.

It may also be possible to stop cavitation by reducing the NPSHR for the pump.

The NPSHR is not a constant for a given pump under all conditions but depends on certain factors. Typically, the NPSHR of a pump increases significantly as flow rate through the pump increases. Therefore, reducing the flow rate through a pump by throttling a discharge valve decreases NPSHR. NPSHR is also dependent on pump speed. The faster the impeller of a pump rotates, the greater the NPSHR. Therefore, if the speed of a variable-speed centrifugal pump is reduced, the NPSHR of the pump decreases.

The net positive suction head required to prevent cavitation is determined through testing by the pump manufacturer and depends on factors including type of impeller inlet, impeller design, pump flow rate, impeller rotational speed, and the type of liquid being pumped. The manufacturer typically supplies curves of NPSHR as a function of pump flow rate for a particular liquid (usually water) in the vendor manual for the pump.

TROUBLESHOOTING

Design, installation, and operation are the dominant factors that affect a pump's mode of failure. This section identifies common failures for centrifugal and positive-displacement pumps.

CENTRIFUGAL

Centrifugal pumps are especially sensitive to (1) variations in liquid condition (i.e., viscosity, specific gravity, and temperature); (2) suction variations, such as pressure and availability of a continuous volume of fluid; and (3) variations in demand. ----1 lists common failure modes for centrifugal pumps and their causes.

Mechanical failures may occur for a number of reasons. Some are induced by cavitation, hydraulic instability, or other system-related problems. Others are the direct result of improper maintenance. Maintenance-related problems include improper lubrication, misalignment, imbalance, seal leakage, and a variety of others that periodically affect machine reliability.

Cavitation

Cavitation in a centrifugal pump, which has a significant, negative effect on performance, is the most common failure mode. Cavitation not only degrades a pump's performance, it also greatly accelerates the wear rate of its internal components.

---- Common Failure Modes of Centrifugal Pumps

Causes

There are three causes of cavitation in centrifugal pumps: change of phase, entrained air or gas, and turbulent flow.

Change of Phase

The formation or collapse of vapor bubbles in either the suction piping or inside the pump is one cause of cavitation. This failure mode normally occurs in applications such as boiler feed in which the incoming liquid is at a temperature near its saturation point. In this situation, a slight change in suction pressure can cause the liquid to flash into its gaseous state. In the boiler feed example, the water flashes into steam. The reverse process also can occur. A slight increase in suction pressure can force the entrained vapor to change phase to a liquid.

Cavitation caused by phase change seriously damages the pump's internal components. Visual evidence of operation with phase-change cavitation is an impeller surface finish like an orange peel. Prolonged operation causes small pits or holes on both the impeller shroud and vanes.

Entrained Air/Gas Pumps are designed to handle gas-free liquids. If a centrifugal pump's suction supply contains any appreciable quantity of gas, the pump will cavitate. In the example of cavitation caused by entrainment, the liquid is reasonably stable, unlike with the change of phase described in the preceding section. Nevertheless, the entrained gas has a negative effect on pump performance. While this form of cavitation does not seriously affect the pump's internal components, it severely restricts its output and efficiency.

The primary causes of cavitation that is due to entrained gas include two-phase suction supply, inadequate available net positive suction head (NPSHA), and leakage in the suction-supply system. In some applications, the incoming liquid may contain moderate to high concentrations of air or gas. This may result from aeration or mixing of the liquid prior to reaching the pump or inadequate liquid levels in the supply reservoir. Regardless of the reason, the pump is forced to handle two-phase flow, which was not intended in its design.

Turbulent Flow The effects of turbulent flow (not a true form of cavitation) on pump performance are almost identical to those described for entrained air or gas in the preceding section. Pumps are not designed to handle incoming liquids that don’t have stable, laminar flow patterns. Therefore, if the flow is unstable or turbulent, the symptoms are the same as for cavitation.

Symptoms Noise (e.g., like a can of marbles being shaken) is one indication that a centrifugal pump is cavitating. Other indications are fluctuations of the pres sure gauges, flow rate, and motor current, as well as changes in the vibration profile.

How to Eliminate

Several design or operational changes may be necessary to stop centrifugal-pump cavitation. Increasing the available net positive suction head (NPSHA) above that required (NPSHR) is one way to stop it. The NPSH required to prevent cavitation is determined through testing by the pump manufacturer. It depends on several factors, including type of impeller inlet, impeller design, impeller rotational speed, pump flow rate, and the type of liquid being pumped. The manufacturer typically supplies curves of NPSHR as a function of flow rate for a particular liquid (usually water) in the pump's manual.

One way to increase the NPSHA is to increase the pump's suction pressure. If a pump is fed from an enclosed tank, either raising the level of the liquid in the tank or increasing the pressure in the gas space above the liquid can increase suction pressure.

It also is possible to increase the NPSHA by decreasing the temperature of the liquid being pumped. This decreases the saturation pressure, which increases NPSHA.

If the head losses in the suction piping can be reduced, the NPSHA will be increased. Methods for reducing head losses include increasing the pipe diameter; reducing the number of elbows, valves, and fittings in the pipe; and decreasing the pipe length.

It also may be possible to stop cavitation by reducing the pump's NPSHR, which is not a constant for a given pump under all conditions. Typically, the NPSHR increases significantly as the pump's flow rate increases. Therefore, reducing the flow rate by throttling a discharge valve decreases NPSHR. In addition to flow rate, NPSHR depends on pump speed. The faster the pump's impeller rotates, the greater the NPSHR. Therefore, if the speed of a variable-speed centrifugal pump is reduced, the NPSHR of the pump is decreased.

Variations in Total System Head

Centrifugal-pump performance follows its hydraulic curve (i.e., head versus flow rate). Therefore any variation in the total backpressure of the system causes a change in the pump's flow or output. Because pumps are designed to operate at their BEP, they become more and more unstable as they are forced to operate at any other point because of changes in total system pressure, or head (TSH).

This instability has a direct impact on centrifugal-pump performance, reliability, operating costs, and required maintenance.

Symptoms of Changed Conditions

The symptoms of failure caused by variations in TSH include changes in motor speed and flow rate.

Motor Speed

The brake horsepower of the motor that drives a pump is load dependent. As the pump's operating point deviates from BEP, the amount of horsepower required also changes. This causes a change in the pump's rotating speed, which either increases or decreases depending on the amount of work that the pump must perform.

Flow Rate

The volume of liquid delivered by the pump varies with changes in TSH. An increase in the total system backpressure results in decreased flow, while a backpressure reduction increases the pump's output.

Correcting Problems

The best solution to problems caused by TSH variations is to prevent the variations. While it’s not possible to completely eliminate them, the operating practices for centrifugal pumps should limit operation to an acceptable range of system demand for flow and pressure. If system demand exceeds the pump's capabilities, it may be necessary to change the pump, the system requirements, or both. In many applications, the pump is either too small or too large. In these instances, it’s necessary to replace the pump with one that is properly sized.

For the application in which the TSH is too low and the pump is operating in run-out condition (i.e., maximum flow and minimum discharge pressure), the system demand can be corrected by restricting the discharge flow of the pump.

This approach, called false head, changes the system's head by partially closing a discharge valve to increase the backpressure on the pump. Because the pump must follow its hydraulic curve, this forces the pump's performance back to wards its BEP.

When the TSH is too great, there are two options: replace the pump or lower the system's backpressure by eliminating line resistance caused by elbows, extra valves, etc.

POSITIVE DISPLACEMENT

Positive-displacement pumps are more tolerant of variations in system demands and pressures than centrifugal pumps. However, they are still subject to a variety of common failure modes caused directly or indirectly by the process.

Rotary Type

Rotary-type, positive-displacement pumps share many common failure modes with centrifugal pumps. Both types of pumps are subject to process-induced failures caused by demands that exceed the pump's capabilities. Process induced failures also are caused by operating methods that either result in radical changes in their operating envelope or instability in the process system.

----2 lists common failure modes for rotary-type, positive-displacement pumps. The most common failure modes of these pumps are generally attributed to problems with the suction supply. They must have a constant volume of clean liquid to function properly.

  • Common Failure Modes of Rotary-Type, Positive-Displacement Pumps
  • Common Failure Modes of Reciprocating Positive-Displacement Pumps

RECIPROCATING

----3 lists the common failure modes for reciprocating-type, positive-displacement pumps. Reciprocating pumps can generally withstand more abuse and variations in system demand than any other type. However, they must have a consistent supply of relatively clean liquid to function properly.

The weak links in the reciprocating pump's design are the inlet and discharge valves used to control pumping action. These valves are the most frequent source of failure. In most cases, valve failure is caused by fatigue. The only positive way to prevent or minimize these failures is to ensure that proper maintenance is performed regularly on these components. It’s important to follow the manufacturer's recommendations for valve maintenance and replacement.

Because of the close tolerances between the pistons and the cylinder walls, reciprocating pumps cannot tolerate contaminated liquid in their suction-supply system. Many of the failure modes associated with this type of pump are caused by contamination (e.g., dirt, grit, and other solids) that enters the suction side of the pump. This problem can be prevented by the use of well-maintained inlet strainers or filters.

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