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NPSH

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NPSH

What does it mean NPSH?

(Net Positive Suction Head)

اقرء المقالة باللغة العربية

net positive suction head
net, positive, suction, head,

Before the binning we have to illustrate couple things so we`ll be clear about everything:

Head= is the pressure expression

Hs= the head at the suction side

Hv= the head vapor(for the liquid)

Vs= velocity at the suction side

g= gravity acceleration (9.8 M/s^2,  or 386.1 In/s^2)

Pv= vapor pressure ( for the liquid)

Y= weight in (N/m^3 or Ib/ft^3)

Now let`s simplify the theory for the NPSH,

What is it matter for?

How we can generate it ?

Can we characterize anything out of it?

net positive suction head
NPSH, experimental, chart

lets start with the NPSH; it is the difference between the Head at suction side and the vapor head (pressure) of the liquid itself, that`s why we have to avoid making the suction head less the vapor pressure `cause that going to :

cavitation damage
cavitation, damage

  • decrease the efficiency

  • cavitation

  • damage

We can express the formula of the Head at suction side:

suction head
NPSH, suction, head

Hs=Ps/Yliquid+Vs^2/2g

And about Hv= Pv/Yvapor

So the final formula of the NPSH(m,in)= Ps/Yliquid+Vs^2/2g-Pv/Yvapor

But put in mind that all conditions above based on the fixed temperature. for instant water boiling point at 20C(when absolute pressure =2.3KB/M^2) ,and 100C at (101.3KN/M^2).

Further than that,let`s speak about NPSHa or available NPSH, for pumping a liquid from the tank to another tank

NPSH formula
NPDH Available formula

 

h0=hs+hl

where h0=Patm/Y

hs=Ps/Y+Vs^2/2g

hl= head loss between tank and implere end

Patm/Y=Ps/Y+Vs^2/2g+hl+he

where the (he) elevation head

NPSH=Patm/Y-hl-hv-he

Now if the pump above tank level the NPSH will be decreased(he)positive. be notified sometimes when the NPSH reach Zero, the fluid  starts evaporate.

But if the pump below the tank the NPSH will increase.

To increase or decrease the NPHS it is clear what to do now as long as the head loss are fixed it is easy to optimize NPSH according to the physical or system requirements.

Now it is so clear why the pump of the water going to be different from the oil,fuel, air pumps,regardless the technical detail  if we have the pump on a plane surface, dealing with two different levels situation, or bank of pumps.

From the maintenance point of view ; what if we need to isolate one off the service, or just making a lubrication

To be continued

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مهندس كهرباء عراقي من بغداد عَمل في مجال النفط والطاقه المتجدده لاكثر من 12 سنه تحديدا في مجال المعدات النفطية من مبادلات الحرارة، الضواغط، العوازل النفطية والمضخات بالاضافه على وحدات التكييف والتهويه ومجالات الطاقة الشمسية والHMI

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