Best practice no. 25

March 2024

Number 25

BEST PRACTICE

Understanding NPSH in Pumps

one of the most widely used and least understood terms associated with pumps is net positive suction head (NPSH). Comprehending the physical significance of NPSH is essential in making a pump installation successful. Moreover, thorough consideration of factors affecting NPSH is critical in minimizing pumping problems. It is probable that more pump problems result from incorrectly determined NPSH than from any other cause. NPSH analysis requires a basic knowledge of vapor pressures of liquids. The relationships between pressure, temperature, and the resultant vapor pressure (pressure at which a liquid and its vapor exist in equilibrium at any given temperature) affect pumping characteristics of all liquids. The ability of a pump to handle a hot liquid decreases as liquid temperature increases, provided other factors remain constant. Vapor pressure of a liquid increases with rising temperature and in effect "opposes" atmospheric pressure that tends to force liquid into the pump suction. Every liquid has a definite temperature and pressure at which it will boil. The boiling point liquid is the temperature at which vapor pressure equals external pressure, which, in an open system, is always equal to atmospheric pressure (the weight of the atmosphere) and, thus, varies inversely with altitude, Table 1. Water boils at 212°F at sea level (atmospheric pressure is 14.7 psia). Therefore, in an open system, the vapor pressure for 212°F water is 14.7 psia. If atmospheric pressure reduced to 6.87 psia, water will boil at 176 °F. When pressure on any liquid is lowered, the temperature required to reach the boiling point will also be lower. Conversely, an increase in pressure on the liquid elevates the boiling point temperature, Table 2. The term "suction lift" is misleading. No pump whether reciprocating, gear, turbine, vane, or centrifugal can "lift" any liquid. Liquid must be forced into a pump before the pump will function. The force (energy) causing flow in an open reservoir system comes from the pressure of the atmosphere and/or from static head. Any pump operating under a "suction lift "condition (with no positive static suction head) operates with less than atmospheric pressure to induce flow .

Vapor Pressure is the pressure at which a liquid and its vapor co-exist in equilibrium at a given temperature. The vapor pressure of a liquid increases with rising temperature and in effect "opposes" atmospheric pressure the positive force that tends cause liquid flow into the pump suction.

NPSH is the pressure, usually expressed in feet of liquid, required to induce fluid flow through the suction line into the impeller of the pump. Available net positive suction head ( NPSHA ) or pressure is the static suction head h , minus the friction loss h f in the suction system plus atmospheric pressure h a (because a vacuum is negative pressure) existing in the suction supply line minus the vapor pressure h vap of the liquid at pumping temperature, or :

Vapor Pressure Curve

Critical Point

Gas

Liquid

Liquid and Vapor

NPSHA = h s – h f + h a - h vap

h s = static suction head resulting from elevation of liquid relative to pump centerline, feet of head. (If liquid level is above pump centerline, h s is positive. If liquid level is below pump centerline h s is negative. Negative h s is commonly denoted as a "suction lift" condition).

Gas

Vapor Pressure

Vapor

h f = friction loss in suction pipe, feet of head.

Temperature

h a = absolute pressure on surface of liquid reservoir supply to pump suction, feet of head. (If the system is open, h a equals atmospheric pressure). h vpa = absolute vapor pressure of liquid at pumping temperature, feet of head. (This value can be obtained from vapor pressure tables for liquid pumped).

In a closed system that is not pressurized, ha is equal h vpa since the liquid source from which the pump takes its suction exists at the saturation pressure corresponding to the operating temperature. Vapor pressure of the liquid exactly equals pressure on the surface of the liquid. There is no additional pressure available to force liquid into the pump. The pump centerline must be below the liquid surface level to create sufficient NPSHA inducing flow of liquid into the pump in order to operate under these conditions. NPSHA must overcome the friction loss in the line and the pressure drop between the pump suction flange and the entrance to the impeller vanes. Closed systems may be pressurized to create and absolute pressure greater than the vapor pressure of the liquid being pumped. Pressurized reservoirs are usually required when the receiver cannot be elevated to create sufficient positive suction head. Two values of NPSH are important in pump selection. Required pressure ( NPSHR ) is a characteristic of the pump itself and varies with operating conditions. It is defined as the pressure required to fill the pump on the suction side and overcome internal pump losses. NPSHR represents the minimum required margin between suction head and vapor pressure at a given capacity. Impeller eye diameter, shape and number of impeller vanes, and size and shape of suction passage determine a pump´s NPSHR . The NPSHR value for any pump can be obtained from the pump manufacturer and should be shown on all pump curves. The available pressure ( NPSHA ) is a characteristic of the system in which the pump operates and of the pressure available in the liquid at the suction side of the pump. NPSHA is the difference between existing absolute suction head and vapor pressure at the prevailing temperature of the liquid being pumped . In any pump installation. NPSHA must always exceed NPSHR if cavitation is to be avoided and flow is to be unimpaired. when a gage is located at the suction side of the pump, NPSHA can be computed for an existing installation by using the equation:

Where:

P g = gauge reading, psi

P a = atmospheric pressure, psi

P vpa = absolute vapor pressure of liquid pumping temperature, psi.

V = velocity in suction line, ft/sec.

g = acceleration resulting from gravity (constant at 32,17 ft/sec/sec)

Y = difference in elevation between pump centerline and gauge, ft. (If gauge is above centerline. Y is a positive value, if below Y is negative).

Typical calculations for various pumping system arrangements are provided in the accompanying examples.

Example 1 .- Open system with liquid reservoir above pump

Problem: Fort eh system shown in the drawing, determine net positive suction available (NPSHA) when A. liquid is 68°F water and reservoir is at sea level. B. liquid is 68°F water and reservoir is 6000 ft above sea level. C. liquid is 60°F gasoline and reservoir is at sea level. Friction head for suction piping is: h f = 2 ft . Also, from drawing, static suction head is h s = 10 ft . Solution A : The general equation for these calculations is:

PRESSURE (ENERGY) AT SURFACE OF LIQUID

h vpa = 0.78 FT

h f = 2.0 FT

h a = 33.9 FT

NPSHA

NPSHA = h s – h f + h a - h vap

Given data are: h f = 2 ft and h s = 10 ft from table 1, at sea level, h a = 33,96 ft , and from table 2 , at temperature of 68°F, h vpa = 0.78 ft. Then,

ATMOSPHERIC PRESSURE

ATMOSPHERIC PRESSURE AT SEA LEVEL

LIQUID

NPSHA = 10 – 2 + 33.96 – 0.78 = 41.18 ft

Solution B : Given data are: h f = 2 ft and h s = 10 ft , from table 1 at 6000 ft above sea level, h a = 27.3 ft and from table 2 at temperature of 68°F, h vpa = 0.78 ft . Then:

h s = 10.0 FT

CL PUMP

NPSHA = 10 – 2 + 27.3 – 0.78 = 34.52 ft

Solution C : Given data are: h f = 2 ft and h s = 10 ft , from table 1 at sea level, h a = 33.96 ft. Absolute vapor pressure of gasoline at 60°F cab be obtained from standard vapor pressure tables and is: h vpa =7.7 ft. Accordingly:

NPSHA = 10 – 2 + 33.96 – 7.7 = 34.26 ft

Example 2 .- Open system with liquid reservoir below pump

Problem: for the system shown in the drawing, determine net positive suction head available (NPSHA) when A. Liquid is 68°F water and reservoir is at sea level. B. Liquid is 176°F water and reservoir is at sea level. Friction head for the suction piping is: h f = 2 ft . Also, from drawing, static suction head is: h s = - 10 ft . Solution A : The general equation for these calculations is:

PRESSURE (ENERGY) AT SURFACE OF LIQUID

h a = 33.9 FT

h vpa = 0.78 FT

h f = 2.0 FT

PUMP CL

h s = - 10.0 FT

ATMOSPHERIC PRESSURE

NPSHA = h s – h f + h a - h vap

Given data are: h f = 2 ft and h s = - 10 ft from table 1, at sea level, h a = 33.96 ft , and from table 2 , at temperature of 68°F, h vpa = 0.78 ft . Then,

ATMOSPHERIC PRESSURE AT SEA LEVEL

LIQUID

NPSHA = - 10 – 2 + 33.96 – 0.78 = 21.18 ft

Solution B : Given data are: h f = 2 ft y h s = - 10 ft . From table 1 at sea level , h a = 33.96 ft , and from table 2 , at temperature de 176°F , h vpa = 15.87 ft. Then:

NPSHA = - 10 – 2 + 33.96 – 15.87 = 6.09 ft

Example 3 .- Unpressurized closed system Problem: For the system shown in the drawing, determine net positive suction head available (NPSHA) when liquid is 248 °F water at sea level. Friction head for the suction piping is h f = 2 ft . Also, from drawing, static suction piping is: h s = 10 ft . Solution : The general equation for this calculation is:

h f = 2.0 FT

VAPOR PRESSURE

LIQUID

NPSHA = h s – h f + h a - h vap

h s = 10.0 FT

Given data are: h f = 2 ft and h s = - 10 ft. Because this is a closed system (without inert gas) , h a = h vpa , from table 2 , at temperature of 248°F, h vpa = 66.53 ft = h a ,

NPSHA = 8.0 FT

CL

PUMP

NPSHA = 10 – 2 + 66.53 – 66.53 = 8 ft

PRESSURE (ENERGY) AT SURFACE OF LIQUID

Example 4 .- Open system with gauge at pump suction.

Problem: For the system shown in the drawing, determine neet positive suction head available (NPSHA) when liquid is 68 °F water and reservoir is at sea level. A gauge is attached to the suction side of the pump at: Y = 1 ft above the pump centerline and reads: P g = 2.4 psi . liquid velocity in suction line is: V = 10 fps. Solution : The general equation for this calculation is: Given data are: P g = 2.4 psi and V = 10 fps , and Y = 1 ft. From table 1 , at sea level, P a = 14.7 psi , and from table 2 at temperature of 68° F, P vpa = 0.339 psi , then, NPSHA = 2.31 (2.4 + 14.7 – 0.339) – 100/64.34 +1 = 38.72 + 1.55 + 1 = 41.27 ft Exempla 5 .- System analysis with suction lift . Problem: A pump application requires 300 gpm at 80 ft total discharge head. The pump will operate at 1750 rpm. A 10 hp pump is selected with a Flow capacity of 300 gpm and 80 ft total head capacity (see graph). Determine the maximum allowable suction lift this pump can safely handle when: A. Liquid is at 85°F and pump operates at elevation of 1000 ft above sea level. B. Liquid is at 85°F and pump operates at elevation of 1000 ft above sea level. Safety margin is 10 percent. Solution A : The general equation for these calculations is: Given data are: NPSHR = 16 ft (taken from NPSH curve in graph), h f = 5 ft and , h vpa = 1.38 ft. (from standard reference tables as vapor pressure of 85°F water). From Table 1, at 1000 ft, h a = 32.8 ft. Then, because NPSH available (NPSHA) must equal or exceed NPSH required (NPSHR), substitute the value of NPSHR from the pump performance curve (above) in the equation and solve for h s or maximum suction lift. NPSHA = h s – h f + h a - h vap

h vpa = 0.78 FT

h f = 12.0 FT

h a = 33.9 FT

NPSHA

ATMOSPHERIC PRESSURE AT SEA LEVEL

LIQUID

h s = 20.0 FT

PUMP

CL

REQUIRED NPSH (FROM PUMP CURVE)

18 FT

VAPOR PRESSURE OF 85°F H 2 O (GIVEN) FRICTION LOSSES IN SUCTION PIPING ( GIVEN)

1.38 FT

5 FT

PRESSURE)

h s OR MAX. ALLOWABLE

10.42 FT AVAILABLE NET POSITIVE SUCTION HEAD 32.8 FT OF LIQUID (ATMOSPHERIC

SUCTION LIFT FOR PUMPO AT STATED CONDITIONS

NPSHA = h s – h f + h a - h vap

Hence:

NPSHR = h s – h f + h a - h vap

16 = h s – 5 + 32.8 – 1.38

h s = - 10.42 ft

Solution B. The theorical maximum allowable suction lift h s for this pump at these conditions would be -10.42 ft. A safety margin of 10 percent to allow for variations in suction head flow conditions is recommended. Then: Recommended NPSHR = NPSHR + 0.10 NPSHR = 16 + (0.10 x 16) = 17.6 Substituting this value in the general equation the recommended suction lift:

NPSHR = h s – h f + h a - h vap

17.6 = h s – 5 + 32.8 – 1.38 h s = - 8.82 ft