Process Design Of Pump

Table of Contents

Process Design of Pump, let’s understand some basic things about pumps.

“A pump is a device. It is used in the flow system of liquid. Its purpose is to increase the mechanical energy of the flowing liquid.”

Pumps are classified mainly into two categories.
1. Dynamic Pumps (eg. Centrifugal Pump)
2. Positive Displacement Pumps (eg. Reciprocating Pump, Rotary Pump)

Important Terminologies

Some of the important terminologies used with the pumps are as follows.

Capacity of Pump (q_v)
Total Dynamic Head (H)
1. Total Discharge Head (h_d)
2. Total Suction Head (h_s)
Net Positive Suction Head (NPSH)

Capacity of Pump (q_v)

”Flow rate (q_v) of fluid created by the pump is known as capacity of the pump.”

In SI units, capacity is expressed in m³/h or L/s.

Total Dynamic Head (H)

To calculate the total dynamic head (H) of a pump, first find the total discharge head (h_d). Then subtract the total suction head (h_s) from it.

$H=h_d-h_s$

Where,
H =Total dynamic head, measured in Liquid Column (LC) unit.
h_d =Total Discharge Head, measured in Liquid Column (LC)
h_s =Total Suction Head, measured in Liquid Column (LC)

Total Discharge Head (h_d)

The total Discharge Head (h_d) of the pump can be measured in two conditions. It can be measured before installation of the pump or after installation of the pump.

Before Installation of the Pump

$h_d = h_{sd} + h_{fd}$

Where,
h_sd = Static Discharge Head
h_fd = Frictional Loss in Discharge Line

Static Discharge Head (h_sd ) can be calculated using the following equation.

$h_{sd} = p' \pm z'$

Where,
p′ = Absolute pressure over the free surface of the liquid in the receiver.
Z′ is the vertical distance from the free surface of the liquid in the receiver to the pump’s centerline. The pump is placed horizontally. For a vertical pump, Z′ is the distance between the free surface of the liquid. It extends to the eye of the suction of the impeller.

If the pump is below the level of the free surface of the liquid in the receiver.

$h_{sd} = p' + z'$

If the pump is placed above the free surface of the liquid in the receiver.

$h_{sd} = p' - z'$

After the Installation of the Pump or During Operation

$h_d=h_{gd}+atm\; Pressure + h_{vd}$

Where,
h_gd= Discharge gauge pressure measured by the pressure gauge. If pressure is below atmospheric, a vacuum gauge reading is used for h_gd. The reading uses a negative sign.
h_vd = Velocity head at the point of gauge attachment in the discharge line

Total Suction Head (h_s)

Total Suction Head (h_s) of the pump can also be measured in two conditions. One is before the installation of the pump. The other is after the installation of the pump.

Before Installation of the Pump

$h_s = h_{ss} + h_{fs}$

Where,
h_ss = Static Suction Head
h_fs = Frictional Loss in Suction Line

Static Suction Head (h_ss) can be calculated using the following equation.

$h_{ss} = p \pm z$

Where,
p = Absolute pressure over the free surface of the liquid in the source.
Z = The vertical distance between the free surface of the liquid at the source and the pump centerline. The pump is placed horizontally. For a vertical pump, Z is the distance between the free surface of the liquid. It is also the distance to the eye of the suction of the impeller.

If the pump is going to be installed below the free surface of the liquid

$h_{ss} = p + z$

If the pump is going to be installed above the free surface of the liquid

$h_{ss} = p - z$

After the Installation of the Pump or During Operation

$h_s=h_{gs}+atm\; Pressure + h_{vs}$

Where,
h_gs = Suction gauge pressure
h_vs = Velocity head at the point of gauge attachment

Net Positive Suction Head (NPSH)

The net positive suction head is the total head in the suction line. This includes the velocity head and the pressure head. Subtract the vapor pressure head of the liquid from this total.

When pump installation is designed,

$(NPSH)_A \geq (NPSH)_R$

(NPSH)_A = Net Positive Suction Head Available

(NPSH)_R = Net Positive Suction Head Required

(NPSH)_R is normally specified by the pump supplier

(NPSH)_A should be calculated and specified by the process engineer.

When (NPSH)_A is less than (NPSH)_R, cavitation can occur, and vapor bubbles form in the suction line.

Eventually, these bubbles collapse inside the casing of the pump. The impeller of the pump exerts pressure on them.

Such collapse of bubbles can cause severe erosion and damage to the pump.

It forms minor cavities on the inside surface of the casing and of the impeller. Hence, this phenomenon is called cavitation.

Theoretically, (NPSH)_A should be greater than zero to avoid cavitation.

(NPSH)_R depends on the properties of the liquid, the total liquid head, pump speed, capacity, and impeller design.

(NPSH)_A Calculation

Before Installation of the Pump

$latex (NPSH)_A = h_{ss} – h_{fs} – p_v $

Where,
h_ss = Static suction head, m LC = p ± Z
h_fs = Friction loss in the suction line, m of liquid column (LC)
p_v = Vapour pressure of liquid at suction temperature expressed in m of liquid column (LC)

For existing installation

$(NPSH)_A = atm. pressure + h_{gs} - p_V + h_{vs}$

$(NPSH)_A = + h_{gs} – p_v + h_{vs}$

Where,
h_gs = Suction gauge pressure, m of liquid column (LC)
p_v = Vapour pressure of liquid at suction temperature expressed in m of liquid column (LC)
h_vs = Velocity head at the point of gauge attachment, m of liquid column (LC)

As a general guide, (NPSH)_A should preferably be above 3m for pump capacities up to a flow rate of 100 m³/h. It should be 6m above this capacity.

For a given system, if (NPSH)_A is less than (NPSH)_R, following remedial measures are recommended:

Change the location of the pump to improve (NPSH)_A. In other words, positive suction head (h_ss) be increased.
Provide jacketed cooling in the suction line to decrease the vapor pressure (p_v) of the liquid.
Reduce the operating speed of the pump; thereby specific speed of the pump is reduced and its (NPSH)_R is less.

NPSH Requirement for Liquids Saturated with Dissolved Gases

In many situations, the liquid to be pumped is saturated with gases, which have definite solubilities in the liquid. When a suction system for such a liquid is to be designed for a centrifugal pump, (NPSH)_A calculations are different.

For example, pumping of cooling water which is saturated with air. It includes pumping of condensate from a knock-out drum of a compressor and pumping of solution from an absorber, etc.,

Dissolved gases start desorbing when the pump is started and suction is generated at the pump eyes.

Normally, a pump can tolerate 2 to 3% flashed gases at the pump eye without encountering cavitation.

If the design of the suction system is made to restrict about 2.5 % flash, it is considered safe for the pump operation.

For a liquid saturated with dissolved gases, p_v is replaced by p_va which is called artificial liquid vapor pressure. For evaluation of p_va, following procedure is recommended:

Calculate the molar mass of the gas mixture, dissolved in the liquid.
Calculate mass fraction (w_o) of the dissolved gas mixture.
Calculate pseudo-critical properties of the dissolved gas mixture, if system pressure is high.
Calculate the specific volume of the dissolved gas mixture (V_Ga) at the operating conditions.

Steps (1) to (4) can be avoided. This is true if the solubility of the gas mixture in the liquid, such as air in water, is known. You can find this information in the literature.

Calculate the volume fraction of the dissolved gas (GVP) in a hypotheoretical gas-liquid mixture.

Consider one unit mass of the liquid in which the gas mixture is dissolved.
If GVP is less than or equal to 2.5%, (NPSH)_A can be safely used to calculate (NPSH)_A using vapor pressure (p_v) of the liquid at the operating temperature.
If higher, calculate the volume fraction (a) of the flashed gas mixture. This occurs as the pressure is lowered over the liquid, which is saturated with the dissolved gas mixture. Use the following equation.

$a=\left[\frac{1}{\frac{\left(\frac{p}{p_0}-\frac{p_v}{p_0}\right)^2\left(1-\frac{p_v}{p_0}\right)}{\left(\frac{V_{Ga}}{V_L}\right)\left(\frac{p}{p_0})(1-\frac{p}{p_0}\right)}+1}\right]$

This equation assumes that the dissolved gas mixture follows the ideal gas law, Dalton’s law, and Henry’s law.

Where,
p = liquid pressure at pump eye, kPa
p_v = vapour pressure of liquid at the operating temperature, kPa
p₀ = system pressure, kPa
V_Ga = specific volume of the dissolved gas mixture, m³/kg
V_L = specific volume of the liquid at the operating conditions, m³/kg

Calculate a for different values of p. Draw a graph of ‘a’ vs ‘p’. Read p corresponding to a = 0.025 which is called p_va. Alternately by trial and error, calculate p_va, for a = 0.025.

Use (NPSH)_A and insert p_va in place of p_v and calculate (NPSH)_A.

Power Required for Pumping

The power required for pumping an incompressible fluid is given by the equation:

$P=\frac{H \; q_v \; \rho}{3.67 \times 10^5 \times \eta }$

Where,
P = Power Required, kW
H = Total Dynamic Head, m of liquid column (LC)
q_v = Capacity, m³/h
$\rho$ = Fluid Density, kg/m³
$\eta$ = Efficiency of Pump

Conclusion

In conclusion, understanding the process design of pumps is crucial for ensuring their efficient and reliable operation. Engineers can optimize pump performance by categorizing pumps. They define key terminologies like capacity, total dynamic head, and NPSH, and analyze their calculations. This process prevents issues such as cavitation. Proper design and consideration of operational parameters not only enhance system efficiency but also extend the lifespan of the equipment. Mastery of these basics lays the foundation for advanced pump system design and troubleshooting.

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Non Return Valve

Important Terminologies

Capacity of Pump (q_v)

Total Dynamic Head (H)

Total Discharge Head (h_d)

Total Suction Head (h_s)

Net Positive Suction Head (NPSH)

Power Required for Pumping

Conclusion

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Important Terminologies

Capacity of Pump (qv)

Total Dynamic Head (H)

Total Discharge Head (hd)

Total Suction Head (hs)

Net Positive Suction Head (NPSH)

Power Required for Pumping

Conclusion

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Capacity of Pump (q_v)

Total Discharge Head (h_d)

Total Suction Head (h_s)