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Net Positive Suction Head

Discflo Net Positive Suction Head Available The Net Positive Suction Head Available (NPSHa) is the most important element in the design of an effective pumping system. The NPSHa is calculated as the total suction head (absolute being measured from the pump centerline) less the absolute vapor pressure, as shown in Fig 1.4. The value must always be positive and can be found from the following equations.

For positive (flooded) suction:
NPSH_a = H_st + H_a - H_vpa - H_fs
For suction lift:
NPSH_a = - H_st + H_a - H_vpa - H_fs
Where:
H_st = static head (feet or meters) that liquid supply level is above or below pump centerline.
H_a = absolute pressure (in feet or meters of the liquid being pumped) on the surface of the liquid supply level; this is barometric pressure if suction is from an open tank or sump, or the absolute pressure exist-ing in a closed tank like a condenser hotwell or deareator.
H_vpa = head (in feet or meters) corresponding to the lvapor pressure of the liquid at the temperature being pumped.
H_fs = all suction line losses (in feet or meters) including entrance losses and friction losses through pipe, valves and fittings etc.

The other value to consider is the Net Positive Suction Head required (NPSHr), which is determined by the pump manufacturer. For the Discflo pump, it depends on several factors, including size of inlet to the Discpac, disc design, pump flow, operating speed, nature of liquid being pumped, etc. This reduction in head takes place between the suction flange of the pump and the low pressure point, i.e. the eye of the Discpac.

Friction, turbulence and entrance losses prior to the pump all play key roles in determining the NPSHr. We know that for the Discflo pump, the more laminar the flow, the lower the NPSHr.

CALCULATING NPSHa: There are six typical pump installations for which the available NPSH should always be calculated. These are the following: 1) when the pump is installed above the liquid level; 2) when the pump takes suction from a tank under vacuum; 3) when the liquid has a high vapor pressure; 4) when the suction line is long; 5) when a high specific gravity fluid is being pumped; and 6) when the fluid is hot.
From above,
NPSH_a = H_st + H_a - H_vpa - H_fs

EXAMPLE 1A: Calculating Suction lift (in US units). The pump is handling 100 GPM of water at 60°F. The suction lift is 15-ft.
The suction line consists of 25-ft of 2-in steel pipe with one 90
degree flanged elbow and one 2-in foot valve. The
barometric (atmospheric) pressure is 29.96-in of mercury.
The available NPSH at 100 GPM is found as follows:

Step 1: H_a = atm. pressure in feet = inches of mercury × 1.13
``````````````````````specific gravity ```````specific gravity
Step 1: H_a= 29.96 × 1.13
````````````````````````1.0
Step 1: H_a = 33.9 ft, absolute
Step 2: H_st = -15.0 ft
Step 3: Head loss per 100 ft = 17.4 from friction loss table
Step 3: H_fs (for the pipe) = 4.4 ft
Step 3: H_fs (for the valve) = 1.1 ft
Step 3: H_fs (for the elbow) = 0.5 ft
Step 3: At 100 GPM, H_fs = 6.0 ft
Step 4: H_vpa = 0.6 ft from vapor pressure table
Step 5: NPSH_a = H_a + H_st - H_fs - H_vpa
`````````````````````= 33.9 + (-15.0) - 6.0 - 0.6
`````````````````````= 12.3 ft of liquid at 100 GPM

EXAMPLE 1B: Calculating Suction lift (in metric units).
The pump is handling 23 m3/h of water at 15°C. The suction lift is 5m. The suction line consists of 8m of 50mm steel pipe
with one 90 degree flanged elbow and one 50mm foot valve.
The atmospheric pressure is 101 kPa (1 Bar).
The available NPSH at 23 m3/h is found as follows:

Step 1: H_a = atm. pressure in meters
``````````````````````specific gravity
Step 1: H_a = 101 x 0.102
``````````````````````1.0
Step 1: H_a = 10.3m, absolute
Step 2: H_st = -5.0m
Step 3: Head loss per 30 m = 17.4 from friction loss table
Step 3: H_fs (for the pipe) = 1.3m
Step 3: H_fs (for the valve) = 0.3m
Step 3: H_fs (for the elbow) = 0.2m
Step 3: At 30 m3/h, H_fs = 1.8m
Step 4: H_vpa = 0.2m from vapor pressure table
Step 5: NPSH_a = H_a - H_st - H_fs - H_vpa
`````````````````````= 10.3 + (-5.0) - 1.8 - 0.2
`````````````````````= 3.3 m of liquid at 30 m3/h

EXAMPLE 2A: Flooded suction and boiling liquid (in US
units). It is required that the above pump handle 800 GPM of
water at 160°F. The liquid in the suction tank is at its boiling
point, that is to say the pressure on the liquid equals the vapor
of the liquid. The 4-in suction line consists of 10-ft of pipe with
one 90 degree flanged elbow and one flanged gate valve.

Step 1: H_a = H_vpa = 11.0 ft, absolute, from the vapor pressure table
Step 2: H_st = +5.0 ft
Step 3: Head loss per 100 ft = 32.4 from friction loss table
Step 3: H_fs (for the pipe) = 3.2 ft
Step 3: H_fs (for the valve) = 1.1 ft
Step 3: H_fs (for the elbow) = 0.5 ft
Step 3: H_fs (for the pipe inlet) = 3.2 ft
Step 3: At 800 GPM, h_fs = 8.0 ft
Step 4: H_vpa = H_st= 11.0 ft, absolute
Step 5: NPSH_a= H_a + H_st - H_fs - H_vpa
`````````````````````= 11.0 + 5.0 - 8.0 - 11.0
`````````````````````= -3.0 ft of liquid at 800 GPM
Pump cannot operate at this flow rate.

EXAMPLE 2B: Flooded suction and boiling liquid (in metric
units). It is required that the pump handle 250 m3/h of
water at 71°C. The liquid in the suction tank is at its boiling
point, that is to say that the pressure on the liquid equals the
vapor of the liquid. The 100mm suction line consists of 3m of
pipe with one 90° flanged elbow and one flanged gate valve.

Step 1: H_a= H_vpa = 3.4m, absolute, from the vapor pressure table
Step 2: H_s = +1.5m
Step 3: Head loss per 30m = 32.4 from friction loss table
Step 3: H_fs (for the pipe) = 1.0m
Step 3: H_fs (for the valve) = 0.3m
Step 3: H_fs(for the elbow) = 0.1m
Step 3: H_fs(for the pipe inlet) = 1.0m
Step 3: At 250 m3/h, H_fs = 2.4m
Step 4: H_vpa = H_st = 3.4m, absolute
Step 5: NPSH_a = H_a + H_st - H_fs - H_vpa
`````````````````````= 3.4 + 1.5 - 2.4 - 3.4
`````````````````````= -0.9 m of liquid at 250 m3/h
Pump cannot operate at this flow rate.