Energy Conservation Week- 2004 16th Dec. - 22nd Dec

Energy conservation opportunity in centrifugal pumps and pumping systems By: S.K. Rastogi

Pump Type Pumps can be classified according to their basic principle as:- Pumps Displacement Dynamic Centrifugal Reciprocating Special Effect Rotary 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 the temperature 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 BEP: Best efficiency point NPSH(M) NPSH(M) NPSH Discharge flow in gpm BEP: The point on head - capacity curve that align with highest point on the efficiency curve.

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. Static head Static Head Flow

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. Friction Head Flow

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. Friction Head Friction Head Friction Head Friction Head Static Head Static Head Flow Flow

Pump operating point The operating point will always be the intersection point of System curve and Head-flow curve. Head vs Flow Operating point Head System Curve Flow

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): Hydraulic Power (kW) Head (ft) X Capacity (gpm) 5308 = Hydraulic Power (kW) Head (meter) X Capacity (m³/h) 360 = Head (meter) X Capacity (lps) 100 Hydraulic Power (kW) = 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 Energy Subtracting 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 Suppose a pump delivers 100 m³/h at 1750 pump rpm. What will be the capacity at 3500 rpm? Solution: 100 / Q2 = 1750/3500 Q2 = 200 GPM

Affinity law: Head vs Speed The head developed (H) will be proportional to the square of the pump speed, so that H1 H2 N1² N2² =

Affinity law: Head vs Speed [Example] Problem: A pump is developing 30 meter head at 1750 rpm of pump, what will be the head at 3500 rpm of pump. H1 H2 N1² N2² = Therefore 30/H2 = (1750/3500)² H2 = 30X(3500/1750)² H2 =120 meter

Affinity law: Power vs Speed The Power consumed (BHP) will be proportional to the cube of the pump speed, so that BHP1 BHP2 N13 N2³ =

Affinity law: Power vs Speed [Example] Problem: A pump is consuming 30 kW power at 2000 rpm of pump, what will be the power at 4000 rpm of pump. BHP1 BHP2 N13 N23 = 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 / D2 Example: 100 / Q2 = 80/60 Q2 = 75 GPM Head: H1/H2 = (D1/D2)² Example: 100 /H2 = (80 / 60)² H2 = 56.25 Ft Horsepower (BHP): BHP1 / BHP2 = (D1 / D2) ³ Example: 5/BHP2 = (80 / 60)³ BHP2 = 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

Situation Before Impeller Trimming Flow control for permanent flow reduction Head Flow Situation Before Impeller Trimming

Situation After Impeller Trimming Flow control for permanent flow reduction Head Flow Situation After Impeller Trimming

Flow control strategies-Parallel pump operation

Flow control strategies-Parallel pump operation Head Flow

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 Head Flow

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.

Thank You

Specific speed