Mechanical Engineering - Hydraulic Machines - Discussion

1. 

Power required to drive a centrifugal pump is directly proportional to __________ of its impeller.

[A]. diameter
[B]. square of diameter
[C]. cube of diameter
[D]. fourth power of diameter

Answer: Option D

Explanation:

No answer description available for this question.

Bibin said: (Jun 1, 2013)  
As per affinity law break horse power is directly proportional to cube of diameter or cube of rpm.

Safy said: (Aug 10, 2013)  
According to the affinity laws horsepower is directly proportional to the fifth power of impeller diameter or cube of rpm, so this answer is not right.

Yogendra Maurya said: (May 22, 2014)  
POWER is proportional to Q.H=d^2.V^2=d^2.(pi.dN)^2=which is proportional to d^4.

Mohan said: (Aug 11, 2014)  
Yes this answer is right but its against affinity laws, also discharge varies to square of this which is also against affinity law.

Afrasiyab said: (May 26, 2015)  
Power is proportional to cube of diameter.

Radhika said: (Aug 1, 2015)  
Cube of diameter. As per affinity laws option C is correct.

Amal said: (Jan 6, 2016)  
@Safy.

I think its d^5.

Because p/(N^3*D^5) = Constant.

Akshat said: (Jan 6, 2016)  
Answer D is correct, if there is a difference between the velocity triangles and if not then answer C is correct.

Jaimin said: (Jan 10, 2016)  
I think D^5 is right answer.

Because from similarity law come the equation of power coefficient P/(N^3 D^5) that's it.

Manthesh said: (Apr 13, 2016)  
I think C is the correct answer.

Ram said: (May 18, 2016)  
Power = density * acceleration due to gravity * discharge * head loss.

Where Q = A * V = area * velocity.

Area= (pi/4) *d^2.

Velocity= (pi * d * N/60).

Head loss= (friction * length * velocity^2)/(2 * g * d).

= (f * length * (pi * d * N/60) ^2)/(2 * g * d).

Assume f, length and N are constant.

Head loss proportional to diameter.

Power = constant * d^4.

Anurag Adhikary said: (Jul 2, 2016)  
There are so much of answers if we look into this from different laws and formulas. What answer should we choose in the exams?

Is someone there to give the most appropriate answer?

Anonymous said: (Sep 24, 2016)  
I think C is the right answer.

Prashant said: (Oct 12, 2016)  
The relationship can be explained by the Affinity laws as described below:

Law 1. With impeller diameter (D) held constant:

Law 1a. Flow is proportional to shaft speed:
(Q1/Q2) = (N1/N2).

Law 1b. Pressure or Head is proportional to the square of shaft speed:
(H1/H2) = (N1/N2)^2.

Law 1c. Power is proportional to the cube of shaft speed:
(P1/P2) = (N1/N2)^3.

Law 2. With shaft speed (N) held constant:

Law 2a. Flow is proportional to impeller diameter:
(Q1/Q2) = (D1/D2).

Law 2b. Pressure or Head is proportional to square of impeller diameter:
(H1/H2) = (D1/D2)^2.

Law 2c. Power is proportional to the cube of impeller diameter:
(P1/P2) = (D1/D2)^3.

where
Q is the volumetric flow rate (e.g. CFM, GPM or L/s),
D is the impeller diameter (e.g. in or mm),
N is the shaft rotational speed (e.g. rpm),
H is the pressure or head developed by the fan/pump (e.g. psi or Pascal), and
P is the shaft power (e.g. W).

Krishan Kaushik said: (Oct 22, 2016)  
According to affinity law, discharge is proportional to rpm, head is proportional to the square of the head, &power is proportional to the cube of rpm. On the other hand, discharge is proportional to shaft diameter, head is proportional to the square of diameter & power is proportional to the cube of shaft diameter.

Subrata said: (Oct 26, 2016)  
Not right.

The correct answer Will be d^5.

Mahi Ss said: (Nov 4, 2016)  
The right answer is P proportional to D^5.

Ravi said: (Nov 17, 2016)  
It is 1/4 of the diameter.

Sreejith said: (Feb 11, 2017)  
Power is proportional to fifth power of diameter.

Dipanjan said: (Apr 8, 2017)  
I agree @Sreeejith.

Indrajeet said: (May 29, 2017)  
According to affinity law of pump.

Power is directly proportional to the fifth Power of diameter.

Vinit said: (Jun 15, 2017)  
The Correct answer will be d^5.

Jigar K Patel said: (Jun 16, 2017)  
Correct Ans is C.

Law 2c Power is proportional to the cube of impeller diameter (assuming constant shaft speed):

{\displaystyle {P_{1} \over P_{2}}={\left({D_{1} \over D_{2}}\right)^{3}}} {P_{1} \over P_{2}}={\left({D_{1} \over D_{2}}\right)^{3}}.

Pabitra said: (Sep 19, 2017)  
Fifth power of diameter will be the correct answer.

Manish said: (Sep 23, 2017)  
Note that there are two sets of affinity laws:.

Affinity laws for a specific centrifugal pump - to approximate head, capacity and power curves for different motor speeds and /or different diameter of impellers.

Affinity laws for a family of geometrically similar centrifugal pumps - to approximate head, capacity and power curves for different motor speeds and /or different diameter of impellers.

Sharan said: (Oct 22, 2017)  
Ans: C is correct.

i.e d^3--p=TW=(Tou*pi*d^3/16*T)*(v/r).

Pushpender said: (Feb 23, 2018)  
Affinity laws applied to axial and radial flows pumps and turbines. Its a tangent flow ; impulse turbine so I think 4th power is correct.

Sandeep said: (Mar 20, 2018)  
d^4 will be the correct answer. Since the question is asked as directly proportional the tangent flow of an impulse turbine always takes 4th power to diameter.

Sullahe Sanu said: (Mar 20, 2018)  
d^4 is the answer.

Deepak Rathva said: (Jun 22, 2018)  
The Correct answer is D^5.

Ref: R.K.Bansal.

Raj said: (Jul 15, 2019)  
Given answer D^4 is right.

Evans Siripi said: (Aug 5, 2019)  
I think the answer is D.

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