HYDRAULIC PUMPS

 

HYDRAULIC PUMPS

Definition: - It is a hydraulic machine which converts mechanical energy into hydraulic energy (i.e. Pr. Energy).

Classification:

 Rotodynamic :

Continuously pump fluid by rotating impeller.

a.   Centrifugal pump –

• Use centrifugal force for pumping fluid.
• Low specific Speed pump (10-80).
• Fluid entry- Axial, Fluid exit- radial
• Delivery valve is closed during start. 

b.   Axial flow pump –

• High discharge and low pressure pump.
• It is High specific speed pump (160-450).
• Fluid entry is Axial ad Fluid exit is nearly axial.
• Delivery valve is open during start. 

c.   Mixed flow pump –

• It is High specific speed pump(80-160). 

    Positive Displacement Pump:

A fixed amount of fluid is first tapping, then tapped volume is forcefully discharge. 

a.   Reciprocating –

• It is a small discharge and high head pump.
• Example: Piston plunger pump. 

b.   Rotary –

Example: Vane pump, Screw pump, Gear pump, Lobe pump etc.

 

Centrifugal Pump: -

In which pump, conversion of mechanical energy to hydraulic energy is done by means of centrifugal force is called centrifugal pump.

• Principle- Forced vortex flow.
• Reverse of Inward flow Reaction turbine.
• Fluids enter in the pump axially and exit radially.

Fig.1 | Main components of centrifugal pump

Main components of Centrifugal Pump:

1.   Impeller – It is the rotating part of the pump mounted with no of vanes.

It is three types- Open, semi-open and enclosed impeller.

Types of vanes-

• Backward vane- ϕ < 90⁰
• Radial vane- ϕ = 90⁰
• Forward vane- ϕ > 90⁰

For centrifugal pump, efficiency is maximum when blades are bent backward.  

2.   Casing –

There are three types of casing used in centrifugal pump – Volute casing, Vortex casing and Volute casing with guide blades.

3.   Suction pipe with a foot valve and strainer

4.   Delivery Pipe

 

Work done by Centrifugal Pump on impeller on water:

For pump work done per sec per unit weight of water

=1/g (V_w2u₂ – V_w1u₁)

∵ For centrifugal V_w1 = 0,

∴ Work done per sec per unit weight of water for centrifugal pump

=1/g (V_w2u₂)

 

Head, Efficiency and Discharge:

Suction Head- Suction lift

Delivery Head- Delivery lift

Static Head- Static suction lift + Static delivery lift.

Manometric Head-

It is the head against which pump has to actually work.

1^st Method -

H_m = [(Head imparted by water to impeller) – (Losses in pump)]

      = [(Vw2u2)/g] - h_L

2^nd Method –

H_m = [(Total Head at the outlet of pump) – (Total Head at the inlet of pump)]

      = [(P_o/ρg + V_o²/2g + Z_o) - (P_i/ρg + V_i²/2g + Z_i)]

      = [(h_d + V_d²/2g + Z_o) - (h_s + V_s²/2g + Z_i)]

Where, h_d = Delivery Head, h_s = Suction Head.

3^rd Method –

H_m = (h_s + h_d + h_fs + h_fd + V_d²/2g)

 

Manometric Efficiencies-

η_manometric = (Manometric Head)/ (Head imparted to water by impeller)

                =gH_m/ (V_w2u₂)

 

Mechanical Efficiencies-

η_m = Impeller Power/ Shaft power

 

Overalll Efficiencies-

η_o = Water Power/ Shaft power

      = wH_m/ (S.P.x1000)

      = η_manometric x η_m

  

Discharge-

Q = πDbV_f

Where, D= Diameter of impeller

      b=Width of impeller at inlet

      V_f=Velocity of flow at inlet

 

Multistage Centrifugal Pumps:

To increase head and discharge, multi-staging of pump is done.

a.   Pumps in series – To obtain high head.

Q₁ = Q₂ = Q₃

H = H₁ + H₂ + H₃ 

b.   Pumps in Parallel – To obtain high discharge.

Q = Q₁ + Q₂ + Q₃

 H₁ = H₂ = H₃

 

Minimum speed for starting centrifugal pump:

 N = [(120x η_manoxV_w2xD₂)/ π(D₂² – D₁²) 

Specific speed for centrifugal pump: -It is defined as the speed of imaginary pump, identical with the given pump, which will discharge 1 cubic meter of water against 1m head.

N_s = (N√Q)/ (H_m) ^3/4

 

Speed ratio: -     ϕ =u/ v

Flow ratio: -        K_f = V_f/ √ (2gH_m)

 

Priming of centrifugal pump:-

It is an operation in which liquid is completely filled in the chamber of pump so that air or gas or vapor from the portion of pump is driven out.

If the pump casing is filled with air or vapor instead of liquid, the pump can’t suck liquid through suction pipe because pressure difference is very less. For this, before starting centrifugal pump, priming must be done.

 

Net Positive Suction Head (NPSH):

It is the total head required to make the liquid flow through suction pipe and pump impeller.

If suction pressure is less than the vapor pressure of the liquid, cavitation will be formed.

 

Model testing (Geometrically similar pump)

(N_s)_m = (N_s)_p

(√ H_m/DN)_m = (√ H_m/DN)_p

(Q/ND³)_m = (Q/ND³)_p

(P/N³D⁵)_m = (P/N³D⁵)_p