Friction head 2

[mech1122]

Hi everyone. I wanna ask regarding friction head, why is it at total discharge head calculation the friction head is added while in total suction head calculation the friction head is subtracted?

 

[petrochemical87]

let
h1 be the total suction(static) head
h2 be the total discharge(static) head
hf1 be the friction head loss in the suction piping
hf2 be the friction head loss in the discharge piping
total head to be developed by the pump= h2+hf2-h1+hf1

 

[Artisi]

Friction head is always added to the head calculation on both discharge side as well as inlet side - are you confusing friction head with static head in the case of the source being higher than the pump centreline.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[mech1122]

Thanks for your reply but please explain regarding the formulas below.

Total Disch Head = hsd + hfd + hvd ± hpd

Where:
hsd = Discharge Static Head
hfd = Discharge Friction Head
hvd = Discharge Velocity Head
hpd = Discharge Pressure Head

Total Suction Head = hss – hfs + hvs ± hps

Where:
hss = Suction Static Head
hfs = Suction Friction Head
hvs = Suction Velocity Head
hps = Suction Pressure Head

Note that (-)hfs is negative while (+)hfd is positive. Why is it?

 

[CaracasEC]

hfs-suction friction head is negative because it will affect the suction head available down to the pump suction flange. That is why we have and NPSHA (Net positive suction head available) to be maintained to avoid cavitation (classical). Any losses upstream of the pump suction flange should be accounted to be sure adequate NPSHA. Hfd is always positive because it is an additional power (head) to be accounted for.

 

[bimr]

It is only because of the reference point in the system where the pressure is calculated. The discharge head measures the pressure AFTER the pump. The suction pressure measures the pressure BEFORE the pump.

System head = total discharge head - total suction head

H = hd - hs

The total discharge head is made from three separate heads:

hd = hsd + hpd + hfd

[ul]
[li]hd = total discharge head[/li]
[li]hsd = discharge static head[/li]
[li]hpd = discharge surface pressure head[/li]
[li]hfd = discharge friction head[/li]
[/ul]

The total suction head also consists of three separate heads

hs = hss + hps - hfs

hs [ul]
[li][/li]
[/ul]= total suction head
hss = suction static head
hps = suction surface pressure head
hfs = suction friction head

http://www.mcnallyinstitute.com/07-html/7-01.html

 

[mech1122]

Thank you guys. I really appreciate your answers.

 

[renderu]

Another simple way to look at it is that on the discharge, the friction force is pushing on the column of water towards the pump (therefore increasing pressure at the pump). And on the suction side, the friction force pushes on the column of water away from the pump (decreases the pressure at the pump).