Download Minimum Safe Flow for a Centrifugal Pump

if you say minimum recycle line shall approx be 25 to 30 % of the forward flow line but i have seen in one P&ID as under :

discharge line ( forward line ) for pump is 3 inches

minimum recycle flow line is  also 3 inches ( same as forward flow line) 

my inderstanding is that is must be lesser than forwad flow line ( discharge ) of pump.

2ndly there is SDV on the combine header of the dieschage lines of parralel pumps . if SDV shuts down , the maximum flow will flow back through reclyle line of each pump therefore minimum recycle line is same as forward line ( discharge ) of pump .

please clarifiy ? i will be thankful for your advise 

 

Sir,

 

Please reply with answers to above queries

Ayush,

 

I had already answered your question in the main blog entry. I will repeat my answer here:

 

Your statement is incorrect. The pump efficiency will be low but it cannot be zero. The pump is still running against a blocked discharge and the mechanical energy from the driver of the pump is being lost by generating heat in the pump casing instead of overcoming the head as change in the potential energy of the liquid being pumped

This can be proven through the power equation for any pump:

P = Q*H*ρ*g / 3600*η*1000

where:

P = power, kW
Q = volumetric flow rate, m3/h
H = head, m
ρ = density, kg/m3
g = 9.81 m/s2 (grav. constant)
η = efficiency, %

If η becomes zero the power required at shut-off would be infinity which is not correct. Hope it is clear now that efficiency is not zero at shut-off.

Regards,
Ankur.

 

But this equation also shows that if the flow is zero, power is zero, so this equation cannot be valid at zero flow!

Thank you so much for your effort. It is very useful spreadsheet. Just I need some clarification.

Thank you again!