Preventing Cavitation

7 Jun

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.  For example, 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 space above the liquid increases suction pressure.

It is 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. Recall from the previous module on heat exchangers that large steam condensers usually subcool the condensate to less than the saturation temperature, called condensate depression, to prevent cavitation in the condensate pumps.

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 upon 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.   However, since a pump’s flow rate is most often dictated by the needs of the system on which it is connected, only limited adjustments can be made without starting additional parallel pumps, if available.

The net positive suction head required to prevent cavitation is determined through testing by the pump manufacturer and depends upon 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.

Centri fugal Pum p Characteristic Curves

For a given centrifugal pump operating at a constant speed, the flow rate through the pump is dependent upon the differential pressure or head developed by the pump.  The lower the pump head, the higher the flow rate.  A vendor manual for a specific pump usually contains a curve of pump flow rate versus pump head called a pump characteristic curve. After a pump is installed in a system, it is usually tested to ensure that the flow rate and head of the pump are within the required specifications.  A typical centrifugal pump characteristic curve is shown in Figure 11.

There are several terms associated with the pump characteristic curve that must be defined. Shutoff head is the maximum head that can be developed by a centrifugal pump operating at a set speed.   Pump  runout is the maximum flow that can be developed by a centrifugal pump without damaging the pump.  Centrifugal pumps must be designed and operated to be protected from the conditions of pump runout or operating at shutoff head.  Additional information may be found in the handbook on Thermodynamics, Heat Transfer, and Fluid Flow.

Centri fugal Pump Protection

A centrifugal pump is dead-headed when it is operated with no flow through it, for example, with a closed discharge valve or against a seated check valve.  If the discharge valve is closed and there is no other flow path available to the pump, the impeller will churn the same volume of water as it rotates in the pump casing. This will increase the temperature of the liquid (due to friction) in the pump casing to the point that it will flash to vapor.  The vapor can interrupt the cooling flow to the pump’s packing and bearings, causing excessive wear and heat.  If the pump is run in this condition for a significant amount of time, it will become damaged.

When a centrifugal pump is installed in a system such that it may be subjected to periodic shutoff head conditions, it is necessary to provide some means of pump protection.   One method for protecting the pump from running dead-headed is to provide a recirculation line from the pump discharge  line  upstream  of  the  discharge  valve,  back  to  the  pump’s  supply  source.The recirculation line should be sized to allow enough flow through the pump to prevent overheating and damage to the pump.  Protection may also be accomplished by use of an automatic flow control device.

Centrifugal pumps must also be protected from runout.  Runout can lead to cavitation and can also cause overheating of the pump’s motor due to excessive currents.  One method for ensuring that there is always adequate flow resistance at the pump discharge to prevent excessive flow through the pump is to place an orifice or a throttle valve immediately downstream of the pump discharge.  Properly designed piping systems are very important to protect from runout.

Gas Binding

Gas binding of a centrifugal pump is a condition where the pump casing is filled with gases or vapors to the point where the impeller is no longer able to contact enough fluid to function correctly.  The impeller spins in the gas bubble, but is unable to force liquid through the pump. This can lead to cooling problems for the pump’s packing and bearings.

Centrifugal pumps are designed so that their pump casings are completely filled with liquid during pump operation.  Most centrifugal pumps can still operate when a small amount of gas accumulates in the pump casing, but pumps in systems containing dissolved gases that are not designed to be self-venting should be periodically vented manually to ensure that gases do not build up in the pump casing.

Priming Centri fugal Pumps

Most centrifugal pumps are not self-priming. In other words, the pump casing must be filled with liquid before the pump is started, or the pump will not be able to function.  If the pump casing becomes filled with vapors or gases, the pump impeller becomes gas-bound and incapable of pumping.  To ensure that a centrifugal pump remains primed and does not become gas-bound, most centrifugal pumps are located below the level of the source from which the pump is to take its suction.         The same effect can be gained by supplying liquid to the pump suction under pressure supplied by another pump placed in the suction line.

Summary

The important information in this chapter is summarized below.

Centri fugal Pump Operation Summary

There are three indications that a centrifugal pump is cavitating.

Noise

Fluctuating discharge pressure and flow

Fluctuating pump motor current

Steps that can be taken to stop pump cavitation include: Increase the pressure at the suction of the pump. Reduce the temperature of the liquid being pumped. Reduce head losses in the pump suction piping. Reduce the flow rate through the pump.

Reduce the speed of the pump impeller.

Three effects of pump cavitation are: Degraded pump performance Excessive pump vibration

Damage to pump impeller, bearings, wearing rings, and seals

To avoid pump cavitation, the net positive suction head available must be greater than the net positive suction head required.

Net positive suction head available is  the difference between the pump suction pressure and the saturation pressure for the liquid being pumped.

Centrifugal Pump Operation Summary ( Cont.)

Pump runout is the maximum flow that can be developed by a centrifugal pump without damaging the pump.

The greater the head against which a centrifugal pump operates, the lower the flow rate through the pump.  The relationship between pump flow rate and head is illustrated by the characteristic curve for the pump.

Centrifugal pumps are protected from dead-heading by providing a recirculation from the pump discharge back to the supply source of the pump.

Centrifugal pumps are protected from runout by placing an orifice or throttle valve immediately downstream of the pump discharge and through proper piping system design.

Cavitation is the process of the formation and subsequent collapse of vapor bubbles in a pump.

Gas binding of a centrifugal pump is a condition where the pump casing is filled with gases or vapors to the point where the impeller is no longer able to contact enough fluid to function correctly.

Shutoff head is the maximum head that can be developed by a centrifugal pump operating at a set speed.

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