Pipe Network with common suction Header

[bmw318be]

Hi,

I would like to get suggestion for calculation of acceleration head in NPSH calculation .

This is positive displacement pump, single acting plunger pump.

with a common header pipe configuration with various pipe dia fron tank to pump

1. Tank source is 3" pipe to common header est 49 m
2. Common header upsized to 4" pipe to pump est 41 m
3. Take off line to each of the pump is 2" pipe est 2 m
4. Pump is 2 "

Scenario 1

How the acceleration losses is calculated ?

Tank Source: 2

Pump running: 12, 11 and 10


Acceleration head, Ha, formula used:

Ha = LVNC/gk, where

L=actual suction pipe length (metres)
V=mean flow velocity in suction line (m/s)
N=pump speed (rpm) stroke per min
C=pump constant factor. I used C=0.4 simplex
g=9.81 m/s2
k=liquid compressibility factor (a constant) 1.4

Total length= 93 m

NPHSHa = 9 m

Q1. Would the acceleration losses be calculated in this summary:

Taking good design design the average velocity is 0.5 m/s which is reasonable low.


L=93m
Vave= 0.5 m/s
N=100
C= 0.4

G.k =9.81 x 1.4 =13.7

LVNC = 1860
Ha =1860/13.7= 135 m

Using this fornula,shows losses greater than the length when the average velocity is considered slow 0.5 m/s.

Deriving this formula
In order to meet the min NPSH, the Ha would need to be < 8 m
( max allowable keeping 1 m npsh margin)

In term of velocity, v

V =(Ha . Gk)/ LNC = 8 x 13.7 / 93 x 100 x 0.4 = 0.029 m/s

The system as followed

https://www.dropbox.com/s/ctnrtltf4v23t04/92 Meter Pipe from source Tank 2 to Pump 12.png?dl=0

Question 2. May i know if there is any other method to calculate the acceleration head of the system having the system with upsized common header .

Q3 can we considered that the acceleration head for each of the pump only be calculated in the last pipe section from common header to the take off point since the header would be cosidered as main reservoir.

 

[georgeverghese]

The GPSA lists this same formula, and the example calc illustrated uses US units for the acceleration head calc. So to be consistent, what head do you get with the same US units as in the GPSA calc? Often, when the term g appears in an expression, one has to be careful with the units used.

 

[bmw318be]

Hi Georgeverghese,



The units i am working is is meter. and g is also in m/s .

What is your opinion on the validity of the formula in the long suction.

can we considered that the acceleration head for each of the pump only be calculated in the last pipe section from common header to the take off point since the header would be considered as main reservoir ?

 

[georgeverghese]

As mentioned earlier, use the same units as in the GPSA calc, which is US units (in my copy of the GPSA), and then g in the denominator may be valid.

I dont have the SI units version of the GPSA to be see if g is used in the same calc.

As can be seen in the example calc in the GPSA, the entire suction run should be taken into account, with the acceleration head loss for each dia section summed up to get the total acceleration head loss.

 

[bimr]

I would like to get suggestion for calculation of acceleration head in NPSH calculation.

For what reason are you doing this calculation? Your system is so small that it may be ignored.

 

[bmw318be]

Hi bimr,

I would need to justify if this system with suction line workable for reciprocating pump in parallel scenario.

Theoretically, the value calculated with 93 m would not something that i can prove to ignore.