The difference between inlet pressure and the lowest pressure level inside the pump is called NPSH: Net Positive Suction Head. NPSH is therefore an expression of the pressure loss that takes place inside the first part of the pump housing.The pressure inside a pump varies from the inlet on the suction side to the discharge port on the discharge side. In the first part of the pump, the pressure decreases before it increases on the discharge side to a level higher than the intake pressure.

The difference between inlet pressure and the lowest pressure level inside the pump is called NPSH: Net Positive Suction Head. NPSH is therefore an expression of the pressure loss that takes place inside the first part of the pump housing. The size of the NPSH is shown in the figure to the right.

NPSH will cause the lowest pressure inside the pump to decrease below the evaporation pressure of the pumped liquid, if the inlet pressure is too low. Consequentially, cavitation occurs in the pump, causing noise and leading to breakdowns.

NPSHR (Net Positive Suction Head Required) is provided in the data material for all pumps. NPSHR indicates the lowest inlet pressure required by the specific pump at a given flow to avoid cavitation.1. Suction pressure

2. Pressure line

3. Atmospheric pressure

4. Pump inlet

5. Evaporation pressure

6. Pump discharge

7. Vacuum

8. NPSH

9. NPSHR

Vapor bubbles form in a pump inlet whenever the local absolute pressure of the liquid falls below its vapor pressure. These bubbles collapse rapidly and violently when the local absolute pressure increases due to kinetic forces being imparted by the impeller. Cavitation is the rapid formation and collapse of these vapor bubbles.

Collapsing cavitation bubbles cause noise, vibration, and erosion of material from the impeller. Pump service life is shortened significantly when cavitation occurs. The severity of the effects of cavitation varies as a function of a machine's horsepower. Fig. 1 shows a photograph of a cavitation bubble implosion. Fig 2 shows an impeller that has been severely damaged by cavitation. Fig 3 is a diagrammatic view of the cavitation bubble implosion sequence.

Fig 3: Cavitation bubble implosion onto a solid surface, arrows indicate fluid pressure.

For any given flow rate, every pump has an absolute suction head at which cavitation will occur. This suction head is referred to as the Net Positive Suction Head Required (NPSHR). Head is always expressed in feet or meters to make it independent of any specific fluid. The absolute suction head available at the pump inlet is termed the Net Positive Suction Head available (NPSHA). To avoid cavitation, the available NPSH must be greater than the required NPSH.

NPSHA is determined by subtracting the absolute vapor pressure of the fluid pumped from the total suction head available. Total suction head is the static head (suction gage pressure) corrected to the impeller centerline (or impeller inlet if vertical), plus the velocity head (found in most pipe friction tables), plus atmospheric pressure. All values should be expressed in feet of liquid.

NPSHR is determined by hydraulic testing and is available from the pump manufacturer. Pump manufacturers perform a series of 'breakdown' tests to determine the NPSHR. The pump is operated at a constant flow rate while the NPSHA is steadily decreased. A sudden drop in the total output head is evidence of cavitation. Industry standards establish that a 3% drop in total head as point where the NPSHR reading is taken.

It is important to note that an actual test curve showing NPSHR test results reflects a pump that is cavitating.

To operate cavitation free, pumps need a margin of additional NPSH above the test values. The amount of margin depends on the suction energy of the pump. Suction energy reflects energy available for cavitation damage, and it is a function of the suction specific speed (S) of the pump.

Chart 1 provides some basic guidelines on determining whether a pump falls under high or low suction energy.

Table 1 reflects the recommended margin that should be maintained between the NPSHA and the NPSHR.Chart 1

TABLE 1NPSH MARGIN RATIO GUIDELINES (NPSHA/NPSHR)

Market

Low

High

VeryHigh

Petroleum

1.1a1.3c

Chemical

1.1a1.3c

Electric Power

1.1a1.5c2.0cNuclear Power

1.5b2.0c2.5cWaterwaste water

1.1a1.3c2.0cGeneral Industry

1.1a1.2c

Pulp and Paper

1.1a1.3c

Building Trades

1.1a1.3c

Cooling Towers

1.3b1.5c2.0cSlurry

1.1a

Pipeline

1.3b1.7c2.0cWater Flood

1.2b1.5c2.0c

a. - or 2 feet whichever is greater.

b. - or 3 feet whichever is greater.

c. - or 5 feet whichever is greater.

Note: Note: Vertical turbine pumps often use a NPSH margin of

Getting Fluid to the Pump?

[ Now superseded byPipe Flow Expert ]

What about pump suction? Pumps do not suck!

It is a common belief that pumps provide the energy to lift fluid to the pump inlet. This is not true.

The pump simply moves fluid from the immediate inlet pipework and discharges this fluid against the outlet pressure in the discharge system. This action creates a local suction effect, which allows the external forces acting on the fluid intake system to push the remaining fluid in the intake system towards the pump inlet. This alternative (actual) view of what is happening within the pipework system leading to the pump inlet will help in understanding the limitations introduced by bad pipework system design.

If the inlet system arrangement does not provide enough energy to move the required flow rate to the pump inlet, the pump will be starved of fluid and the required flow rate will not be delivered.

Getting fluid to the pump

The air pressure on the fluid surface is the usual energy source used to push the fluid into the pump.

A supply container positioned above the pump inlet will increase the available energy.

A supply container positioned below the pump inlet will reduce the available energy.

Resistance to fluid flow

Fluids in motion are subjected to various resistances, which are due to friction. Friction may occur between the fluid & the pipe work, but friction also occurs within the fluid as sliding between adjacent layers of fluid takes place. The friction within the fluid is due to the fluids viscosity.

When fluids have a high viscosity, the speed of flow tends to be low, and resistance to flow becomes almost totally dependant on the viscosity of the fluid. This condition is known as Laminar flow.

Will the required flow rate actually reach the pump inlet?

The energy losses in the pipework system must be calculated. This energy loss must be subtracted from the available energy to obtain the condition at the entrance to a pump.

The inlet condition is commonly referred to as the suction condition - Leading to the idea that pumps suck.

If the theoretical pump inlet pressure is too low the system will operate at some lower flow rate or the pump may not operate at all.

Boiling fluid (Cavitation)

Many fluids will boil at ambient temperature if the pressure is reduced below a particular level. This pressure is referred to as the Vapour pressure of the fluid.

If the pump inlet pressure falls below the vapour pressure of the fluid, gas bubbles will form in the fluid. These bubbles will be moved through the pump. The bubbles will collapse when the fluid pressure is raised on the discharge side of the pump. The effect of this is to reduce the flow of delivered fluid. In some systems the effect can cause dramatic vibrations, and may result in damage to the system and the pump.

Increasing the pressure at the pump inlet

Small pipe sizes will result in high pipework energy losses. Increasing the pipework size will help to reduce this energy loss.

In the case of high viscosity fluids, increasing the pipework size may not have the desired result. Also, commercial considerations may limit the size of the pipe that can be used.

Under these circumstances, the easiest solution is to raise the position of the supply container, to increase the positive head available, thus more force will be available to push the fluid through the pipework.

If is not practical to raise the supply container, it may be necessary to enclose the supply and introduce some positive pressure above atmospheric onto the fluid surface.

Look out for sealed supply containers where the force moving the fluid will reduce as the container is emptied.

Suction units

Suction conditions can be described in many different ways.

Normal atmospheric pressure (about 1000 mBar, 14.5 psi.g) will support a water column of 10.2 metres (33.45 ft) high.

If the fluid column was Mercury the column height would be 750 mm (29.52 inches)

Net Positive Suction Head Check

Pump inlet loss N.P.S.H.r

An energy loss occurs during fluid entry into most pumps. This loss is described as N.P.S.H.r (Net Positive Suction Head requirement).

The N.P.S.H.r is determined by the pump manufacturer. The N.P.S.H.r is usually plotted on pump performance curves. The N.P.S.H.r is expressed in metres head ( or ft head) of fluid.

The value of N.P.S.H.r will be dependent on many factors including flow rate, Impellor design, inlet type, pump speed etc.

Boiling fluid (Cavitation) (repeated from last section)

Many fluids will boil at ambient temperature if the pressure is reduced below a particular level. This pressure is referred to as the Vapour pressure of the fluid.

If the pump inlet pressure falls below the vapour pressure of the fluid, gas bubbles will form in the fluid. These bubbles will be moved through the pump. The bubbles will collapse when the fluid pressure is raised on the discharge side of the pump. The effect of this is to reduce the flow of delivered fluid. In some systems the effect can cause dramatic vibrations, and may result in damage to the system and the pump.

Minimum pressure at the pump inlet and N.P.S.H.a

The minimum pressure at the pump inlet minus the Vapour pressure of the fluid is usually known as the Net Positive Suction Head available (N.P.S.H.a)

This must not be confused with the N.P.S.H.r published by the pump manufacturer.