Joe Evans, Ph.D

http://www.pumped101.com

SUCTION CONDITIONS

Suction conditions are some of the most important factors affecting centrifugal pump operation. If they are ignored during the design or installation stages of an application, they will probably come back to haunt you.

Suction Lift

A pump cannot pull or "suck" a liquid up its suction pipe because liquids do not exhibit tensile strength. Therefore, they cannot transmit tension or be pulled. When a pump creates a suction, it is simply reducing local pressure by creating a partial vacuum. Atmospheric or some other external pressure acting on the surface of the liquid pushes the liquid up the suction pipe into the pump.

Atmospheric pressure at sea level is called absolute pressure (PSIA) because it is a measurement using absolute zero (a perfect vacuum) as a base. If pressure is measured using atmospheric pressure as a base it is called gauge pressure (PSIG or simply PSI).

Atmospheric pressure, as measured at sea level, is 14.7 PSIA. In feet of head, it is:

Head = PSI X 2.31 / Specific Gravity

For Water it is:

Head = 14.7 X 2.31 / 1.0 = 34 Ft

Thus 34 feet is the theoretical maximum suction lift for a pump pumping cold water at sea level. No pump can attain a suction lift of 34 ft; however, well designed ones can reach 25 ft quite easily.

You will note, from the equation above, that specific gravity can have a major effect on suction lift. For example, the theoretical maximum lift for brine (Specific Gravity = 1.2) at sea level is 28 ft.. The realistic maximum is around 20ft. Remember to always factor in specific gravity if the liquid being pumped is anything but clear, cold (68 degrees F) water.

In addition to pump design and suction piping, there are two physical properties of the liquid being pumped that affect suction lift.

1. Maximum suction lift is dependent upon the pressure applied to the surface of the liquid at the suction source. Maximum suction lift decreases as pressure decreases.
2. Maximum suction lift is dependent upon the vapor pressure of the liquid being pumped. The vapor pressure of a liquid is the pressure necessary to keep the liquid from vaporizing (boiling) at a given temperature. Vapor pressure increases as liquid temperature increases. Maximum suction lift decreases as vapor pressure rises.

It follows then, that the maximum suction lift of a centrifugal pump varies inversely with altitude. Conversely, maximum suction lift will increase as the external pressure on its source increases (for example: a closed pressure vessel). The figure below shows the relationship between altitude and atmospheric pressure.

Figure 11

A pumping application located at an elevation of 5000 feet will experience a reduction in atmospheric pressure of approximately six feet. This will result in a reduction in NPSHA (discussed in the next section) by the same amount. Elevation must be factored into a pumping application if the installation is more than a few hundred feet above sea level.

The maximum suction lift of a liquid varies inversely with the temperature of the liquid. The higher the temperature, the higher the vapor pressure and thus suction lift is decreased. If a centrifugal pump is used to pump a liquid that is too hot the liquid will boil or vaporize in the pump suction. This condition is called cavitation and will be discussed in more detail later.

The figure below shows the relationship between vapor pressure and temperature for clear, cold water.

Figure 12

At a temperature of 70 degrees F, a pressure of only one foot is required to keep water in the liquid state. As its temperature rises, however, more and more pressure is required. At about 210 degrees, a pressure of 34 feet or, sea level atmospheric pressure, is required. As it rises to 212 degrees the water will boil unless some additional pressure is applied. When pumping liquids at elevated temperatures, the liquid’s vapor pressure at that temperature must be included in the NPSHA calculation.

Capacity and Suction Lift

The suction lift of a centrifugal pump also varies inversely with pump capacity. This is illustrated in the figure below. Figure 13 shows how the head - capacity curve falls off quickly at various suction lifts. You will notice that maximum suction lift increases as pump capacity decreases. For this reason, pumps used in high suction lift applications are selected to operate in a range considerably to the left of their peak efficiency.

Figure 13

Net Positive Suction Head (NPSH)

Net Positive Suction Head Required (NPSHR) is a function of a specific pump design. In simple terms it is the pressure, measured at the centerline of the pump suction, necessary for the pump to function satisfactorily at a given flow. Although NPSHR varies with flow, temperature and altitude have no effect.

Net Positive Suction Head Available (NPSHA) is a characteristic of the system in which the pump operates. It depends upon the elevation or pressure of the suction supply, friction in the suction line, elevation of the installation, and the vapor pressure of the liquid being pumped.

Both available and required NPSH vary with the capacity of a given pump and suction system. NPSHA is decreased as the capacity is increased due to the increased friction losses in the suction piping. NPSHR increases approximately as the square of capacity since it is a function of the velocities and friction in the pump inlet. NPSHA can be calculated as follows:

NPSHA = Ha + Hs - Hvp

Where:

Ha = Atmospheric pressure in feet

Hs = Total suction head or lift in feet

Hvp = Vapor pressure in feet

For example, a pump installed at an elevation of 2500 ft and has a suction lift of 13 ft while pumping 50-degree water. What is NPSHA?

NPSHA = Ha + Hs - Hvp

NPSHA = 31 - 13 - .41

NPSHA = 17.59 ft

Often a two-foot safety margin is subtracted from NPSHA to cover unforeseen circumstances. When selecting a pump for the conditions above, the NPSHR as shown on the pump's characteristic curve should be 15.59 ft or less (17.59 - 2).

Working in the opposite direction, we have a pump that requires 8 ft of NPSH at 120GPM. If the pump is installed at an elevation of 5000 ft and is pumping cold water at 60 degrees, what is the maximum suction lift it can attain?

NPSH = Ha + Hs - Hvp

8 + 2 = 28.2 - Hs - .59

Hs = 28.2 - 8 - 2 - .59

Hs = 17.61 ft (Including the 2 ft margin of safety)

The preceding has dealt only with water. The same general principles apply to other liquids; however, vapor pressure must be factored into the equations. For rough approximations where the vapor pressure is unknown, a pump will usually operate satisfactorily if the NPSHA is equal to or greater than that required for water under similar conditions. This method may be used only when the viscosity of the liquid is approximately the same as water.

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About the Author

Joe Evans lived in beautiful Rhododendron Oregon and retired from PumpTech Inc on 12/31/15. Since entering graduate school, a continuing interest has been one of computer control of mechanical and electronic systems. It began with the introduction of the minicomputer, in the late sixties, and continued with the advent of the PC and PLC in the eighties and nineties. He accidentally entered the pump industry in 1986 and has been trapped there since. He is passionate about the sharing of knowledge and its ability to replace memorization with understanding.