Prem BabooSr. Manager (Prod)National Fertilizers Ltd. IndiaF.I.E., Institution of Engineers (India)Technical Advisor & an Expert for www.ureaknowhow.com

Pump Type• Pumps can be classified according to their basic

principle as:-Pumps

Dynamic Displacement

Centrifugal Rotary ReciprocatingSpecial Effect

When different design can be used the centrifugal pump is generally the most economical followed by rotary & reciprocating pump.

Why prefer centrifugal pump

• Although the positive displacement is more efficient than centrifugal pump, the benefit of higher efficiency tends to be offset by increased maintenance cost.

• That is the reason for incorporating Ammonia feed pump of reciprocating type with centrifugal type in recent Urea plant .

A Brief introduction to centrifugal pump

Mechanism:Rotation of the impeller imparts energy to the liquid causing it to exit the impeller’s vane at a greater velocity than it possessed when entered.

The liquid that exits the impeller is collected in the casing (volute/diffuser) where its velocity is converted to pressure.

A Brief introduction to centrifugal pump

•Do you know: Centrifugal pump was developed in the late 1600’s. Its wide spread use, however has occurred only in last seventy five years.

Theory: •In operation, a centrifugal pump “slings” liquid out of the impeller via centrifugal force.•The flow and head developed in centrifugal pumps depend on peripheral velocity of its impeller.

V ²=2gh

Performance Parameter of Pump• Head: The quantity used to express the energy

content of the liquid per unit weight of the liquid. Head is expressed in m of liquid.

• Capacity/Flow: Discharge delivered by the pump in a unit time. It is expressed in m³/h or Lps or gpm.

• Hydraulic power: Theoretical power delivered to the liquid by pump. [Mass flow(kg/s)*g (9.81m/s²)*H(m)] Watt

• BHP: Power delivered to the Pump shaft. [Hydraulic Power/Pump efficiency]

• Pump Efficiency: The ratio of energy delivered by the pump to the energy supplied to the pump shaft.[Hydraulic power/BHP]

• NPSH: Net positive suction head. [m]

NPSH [Net positive suction head]• NPSH: The value by which the pressure in the pump suction

exceeds the liquid vapour pressure. It is expressed as head of liquid. NPSH is an analysis of energy condition on the suction side of the pump to determine if the liquid will vaporize at lowest pressure point in the pump.

• The value of NPSH needed at pump suction to avoid cavitation in the pump is known as NPSHR.

• NPSHR is a characteristic of pump design. As the liquid passes from the pump suction to the eye of impeller the velocity increases and pressure decreases. There are also pressure loss due to shock and turbulence as liquid strike the impeller.

• A lower speed pump requires lower NPSH.• A double suction pump requires 2/3rd as much NPSH as

compared to similar rated single suction pump.• NPSH required increases with increase in flow.

NPSH Calculation

NPSH: Hs (m of fluid) = -ΔP (suction piping) + (v1²-vs²)/g+ (z1-zs+H1)

Cavitation:

Cavitation begins as the formation of vapor bubbles at the impeller eye due to low pressure. The bubbles form at the position of lowest pressure at the pump inlet (see Figure 1), which is just prior to the fluid being acted upon by the impeller vanes, they are then rapidly compressed.

Cavitation:The compression of the vapor bubbles produces a small shock wave that impacts the impeller surface and pits away at the metal creating over time large eroded areas and subsequent failure. The sound of cavitation is very characteristic and resembles the sound of gravel in a concrete mixer.

Vapor pressure and cavitation

•There are two ways to boil a liquid. One way is to increase the temperature while keeping the pressure constant until thetemperature is high enough to produce vapor bubbles.•The other way to boil a liquid is to lower the pressure. If you keep the temperature constant and lower the pressure the liquid will also boil.

Centrifugal pump: Characteristic curve

Discharge flow in gpm

BEP: Best efficiency point

BEP: The point on head - capacity curve that align with highest point on the efficiency curve.

NPSH

NPSH(M)NPSH(M)

Centrifugal Pump characteristic

• Capacity of the centrifugal pump decreases as discharge pressure increases.

• It is important to select a centrifugal pump that is design to do a particular job.

• The pump generate the same head of liquid whatever be the density of liquid being pumped.

• Even a small improvement in pump efficiency could yield very significant saving of electricity as it is least efficient of the component that comprises a pumping system.

Centrifugal pump performance curve

Performance curve for family of pumps

Performance curve for different impeller

Multiple speed performance curve

System Characteristic• The objective of the pump is to transfer or circulate

the liquid.• A pressure is needed to make the liquid flow at

required rate and overcome head losses in the system. Losses are of two type- 1. Static 2. Friction

• Static head is simply difference of height of supply and destination reservoir.

Flow

Sta

tic

Hea

dStatic head

System Characteristic-frictional head• Friction head is the friction loss due to flow of liquid through

pipe, valves, fitting, equipments.• Frictional losses are proportional to square of the flow rate.• For given flow rate friction loss can be reduced by increasing

diameter of pipe, using improved fitting etc..• A close loop circulating system without a surface open to

atmospheric pressure, would exhibit only frictional losses and would have a system resistance curve as below.

Flow

Fri

ctio

n H

ead

System Head vs flow- typical system• Most system are a combination of frictional head and

static head however the ratio of two head may vary from system to system.

Flow

Fri

ctio

n H

ead

Fri

ctio

n H

ead

Flow

Static Head

Friction Head

Friction Head

Static Head

Pump operating pointH

ead

Head vs Flow

System Curve

Operating point

Flow

•The operating point will always be the intersection point of System curve and Head-flow curve.

Pump Operating Point• If the actual system curve is different in reality as

compared to calculated, the pump will operate at a flow and head different to expected.

• An error in system curve calculation may lead to selection of a centrifugal pump which will have efficiency less than expected.

• Ideally, the operating point should correspond to the flow rate at the pump’s Best Efficiency Point (BEP).

• In many applications, some margin in the pump capacity may be needed to accommodate transient changes.

• However, it is generally desirable to limit over-sizing to no more than 15-20%.

• Adding too much safety margin may lead to inefficient pump selection in actual operation.

Over designed pump

Effect on system curve with throttling

Power Requirement for Pump [Mgh]You can use any of the following formulas to make your calculations (for water only):

Head (ft) X Capacity (gpm)

5308

Hydraulic Power (kW)

=

Hydraulic Power (kW)

Head (meter) X Capacity (m³/h)

360

Hydraulic Power (kW)

Head (meter) X Capacity (lps)

100

=

=

For fluids other than water multiply with sp. Gravity of fluid to calculated power

Estimation of energy loss in oversized pump

Back

Power calculations

Assume that we need to pump 68 m3/hr. to a 47 meter head with a pump that is 60% efficient at that point.

Liquid Power = 68 x 47 / 360 = 8.9 KW

Shaft Power = 8.9 / 0.60 = 14.8 Kw

Using oversized pump

As shown in the drawing, we should be using impeller "E" to do this, but we have an oversized pump so we are using the larger impeller "A" with the pump discharge valve throttled back to 68 cubic meters per hour, giving us an actual head of 76 meters.

Now our hydraulic power will be =68 x 76 / 360

= 14.3 Kilowatts

and Pump input power =14.3 / 0.50 (efficiency) = 28.6 Kilowatts

required to do this.

Loss in EnergySubtracting the amount of kilowatts we should have been using from the actual power used gives us extra power used =28.6 -14.8

= 13.8 extra kilowatts [ being used to pump against the throttled discharge valve]

Extra energy used =8760 hrs/yr x 13.8 = 120,880kw.= Rs. 4,80,000/annum

In this example the extra cost of the electricity could almost equal the cost of purchasing the pump.

Flow versus speed

If the speed of the impeller is increased from N1 to N2 rpm, the flow rate will increase from Q1to Q2 as per the given formula:

Q1

Q2=

N1

N2

Example: Affinity law

•The affinity law for a centrifugal pump with the impeller diameter held constant and the speed changed:

Flow:Q1 / Q2 = N1 /N2

Solution: 100 / Q2 = 1750/3500Q2 = 200 GPM

Suppose a pump delivers 100 m³/h at 1750 pump rpm. What will be the capacity at 3500 rpm?

Affinity law: Head vs Speed

H1

H2

=N1²

N2²

The head developed (H) will be proportional to the square of the pump speed, so that

Affinity law: Head vs Speed [Example]

H1

H2

=N1²

N2²

Problem:

A pump is developing 30 meter head at 1750 rpm of pump, what will be the head at 3500 rpm of pump.

Therefore 30/H2 = (1750/3500)²

⇒H2 = 30X(3500/1750)²

⇒H2 =120 meter

Affinity law: Power vs Speed

BHP1

BHP2

=N13

N2³

The Power consumed (BHP) will be proportional to the cube of the pump speed, so that

Affinity law: Power vs Speed [Example]

BHP1

BHP2

=N1

3

N23

Problem: A pump is consuming 30 kW power at 2000 rpm of pump, what will be the power at 4000 rpm of pump.

Therefore 30/BHP2 = (2000/4000)³

⇒BHP2 = 30X(4000/2000)³

⇒BHP2 =240 kW

Effect of speed variation

The affinity law for a centrifugal pump- for change in impeller diameter

•with the speed held constant and the impeller diameter (D) changed:Flow: Q1 / Q2 = D1 / D2Example: 100 / Q2 = 80/60Q2 = 75 GPMHead: H1/H2 = (D1/D2)²Example: 100 /H2 = (80 / 60)²H2 = 56.25 FtHorsepower (BHP): BHP1 / BHP2 = (D1 / D2) ³Example: 5/BHP2 = (80 / 60)³BHP2 = 2.1

Effect of changing impeller diameter

Solution of over designed pump

•Reduce the speed / Trim the impeller•Blue pump curve shows either of these option

Flow control strategies-by varying speed for system with friction loss

Flow control strategies-by varying speed for system with high static head

Flow control for permanent flow reduction

Flow

He

ad

Situation Before Impeller Trimming

Flow

He

ad

Situation After Impeller Trimming

Flow control for permanent flow reduction

Flow control strategies-Parallel pump operation

Flow

He

ad

Flow control strategies-Parallel pump operation

Flow control with control valve

Flow Control Strategies• By-pass control: The pump runs continuously at

almost maximum load with a permanent bypass line attached to outlet. When lower flow is required the surplus liquid is passed.– This system is even less efficient than throttling control.

• Start-stop control: This is a effective way to minimize energy consumption where intermittent flow are acceptable.– e.g. Pumps in Raw water reservoir– Pumps for sanitary water– Pumps for fire water** Note: Frequency of start/stop cycle should be

within the motor design criteria.

Variable speed drives

Flow

He

ad

Best practices in pumping system• Ensure adequate NPSH at site of installation • Ensure availability of basic instruments at pumps like

pressure gauges, flow meters. • Operate pumps near best efficiency point. • Modify pumping system and pumps losses to

minimize throttling. • Adapt to wide load variation with variable speed drives

or sequenced control of multiple units. • Stop running multiple pumps - add an auto-start for an

on-line spare or add a booster pump in a problem area.

• Use booster pumps for small loads requiring higher pressures.

Best practices in pumping system• Increase fluid temperature differentials to reduce

pumping rates in case of heat exchangers.• Repair seals and packing to minimize water loss by

dripping.• Balance the system to minimize flows and reduce

pump power requirements.• Use siphon effect to advantage: Avoid pumping head

with a free-fall return. • Conduct water balance to minimise water

consumption.• In multiple pump operations, judiciously combine the

operation of pumps and avoid throttling.

Best practices in pumping system• Provide booster pump for few areas of higher

head.

• Replace old pumps by energy efficient pumps.

• In the case of over designed pump, provide variable speed drive, or downsize / replace impeller or replace with correct sized pump for efficient operation.

• Optimize number of stages in multi-stage pump in case of head margins.

• Reduce system resistance by pressure drop assessment and pipe size optimization.

Specific speed