Midstream Oil and Gas

Net Positive Suction Head for Centrifugal Pumps

August 13, 2019 Welcome

What Is NPSH in Centrifugal Pumps?

Net Positive Suction Head (NPSH) is a measurement used to determine the ability of a pump to handle liquid without cavitation. Before methods of figuring NPSH are discussed, cavitation should be examined.

What Is Cavitation in Centrifugal Pumps?

The term “cavitation” for pumps refers to a condition within the impeller eye where the pressure falls below the liquid-vapor pressure and the liquid boils. The low-pressure point in the pump suction system is the eye of the impeller and if the pressure there falls below the vapor pressure of the liquid, vapor bubbles form.

As the mixture of liquid and bubbles continues through the pump, the pressure increases, and the bubbles return to the liquid state. The location of bubbles collapsing varies for different impellers and different conditions, but the damage to the impeller is done where the bubbles collapse.

Effects of Cavitation

The effects of cavitation are threefold:

1. Noise
2. Damage to the pump
3. Falling off of pump performance

Number 1 is not a major objection to cavitation, but the characteristic “gravel” noise as the bubbles collapse can be annoying. The other two results of cavitation, however, are both very important.

The degree of damage and fall-off in performance are dependent upon the degree of cavitation. The method of counteracting damage (assuming the cavitation cannot be eliminated) is by the selection of better materials to resist the erosive effect of the millions of little hammer blows as the bubbles collapse.

Effect of Cavitation on Performance

Usually, the most objectionable result of cavitation is its effect on performance. At a given capacity, the pump continues to put out the same head, but the capacity and head are that of a mixture of liquid and vapor, rather than liquid.

For example, Pump 1 is rated 120 GPM against 210 feet. If handling water and cavitating so that the steam bubbles occupy 50% of the volume taken in, the intake capacity would be 120 GPM of mixture, but after the steam bubbles re-condensed the discharge capacity (which is what is measured) would be 60 GPM of water. Similarly, the total head produced by the pump would be 210 feet of light gravity mixture which could be equal to as little as 105 feet of water.

NPSH Calculation for Centrifugal Pumps

NPSHA is a measure of the pressure available over the vapor pressure of the liquid at the suction of the pump. NPSHA is the amount of NPSH available in the system, and NPSHR is the minimum amount of NPSH required by a pump. To eliminate cavitation, the NPSHA must exceed the NPSHR.

The NPSH required by the pump is determined by pump design, shown on pump performance curves. The engineer’s job is to properly calculate the NPSH available, and then select a pump which requires less than that amount to run satisfactorily at the desired capacity.

NPSHA can come from two things:

1. Static head
2. Pressure

The first is obviously from the elevation of the liquid source over the pump elevation. The second is from the pressure acting on the liquid source. The apparent NPSHA from static head and pressure is reduced by friction in the suction line.

How to Calculate NPSH Available (NPSHA)

The formula for calculating NPSHA, therefore, becomes:

NPSHA = h1 + hss – hf – hvp

Where:

• h1 is the absolute pressure over the liquid in feet
• hf is the friction in the suction line in feet
• hss is the static head in feet – this is negative if there is a suction lift
• hvp is the vapor pressure of the liquid at the pump in feet

Regardless of pressures, temperatures and physical arrangements, the same method of figuring NPSHA is always employed and the above formula is always correct. It is only necessary to remember:

ADD THE STATIC HEAD AND PRESSURE OVER THE LIQUID AND SUBTRACT THE SUCTION LINE FRICTION AND VAPOR PRESSURE AT THE PUMP

The important thing to remember in these calculations is that the method of calculating NPSHA and the formula used are always the same. It should be emphasized here that the calculation for NPSHA and the calculation for total pump head must be kept separate.

Some pump curves show both NPSHR and lift lines. These are reciprocals. No mention has been made here of velocity head. It was agreed several years ago by most pump manufacturers that NPSH curves shown on pump performance curves would take velocity head into account so that it need not be considered in figuring NPSHA in the system.

NPSHR does not vary with the specific gravity of the liquid. Looking at the analogy of NPSH being a force which pushes the liquid into the pump, and assuming a pump which requires 8 feet NPSH at a given capacity, it can be shown that 8 feet of water is equal to 3.45 psi, but 8 feet of gasoline is equal to only 2.6 psi. It would appear that the force pushing the liquid into the pump has decreased with the specific gravity. At the same time, however, the mass of the liquid being accelerated has gone down by the same proportion. Hence, one NPSHR curve is good for all specific gravities.

NPSHR Correction – Curves

For some time, manufacturers in the Hydraulic Institute have been lowering NPSHR curves on hydrocarbons under certain conditions, and pump manufacturers have started drawing lower NPSHR curves for high-temperature water than for cold. There is good logic to both corrections.

It must be recognized, however, that the point at which cavitation commences in a pump is the same regardless of the liquid temperature or character. Why, then, the lower curve?

The major reason is the fact that at 70°F, one cubic foot of water will flash into 54,000 cubic feet of vapor, but at 300°F, one cubic foot of water will make only 369 cubic feet of vapor. According to this calculation, a certain amount of 70°F water will have 146 times as much volume as vapor as will the same amount of 300°F water. The greater damage to the head-capacity curve is obvious.

Hydrocarbons tend to act more like hot water than cold. Consequently, they too can logically have a correction applied to the NPSHR curve. There has been some disagreement as to the magnitude of the correction. Today the Hydraulic Institute correction chart is well accepted, but many people feel it is too conservative to be useful.

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