Building Water Supply System: PUMPS

In buildings, pumps are used to circulate or pump water in domestic hot and cold water systems, hydronic (hot water) and chilled water distribution systems, feed water systems for boilers and chillers, condensate return systems, well systems, wastewater treatment systems, sump installations, and other process piping systems.

Types of Pumps

There are basically two types of pumps used in building systems: positive displacement pumps and centrifugal pumps.

Types of pumps are discussed in the sections that follow. Centrifugal pumps are most common in building systems and are discussed in more detail.

Positive Displacement Pumps---A positive displacement pump has an expanding cavity on the suction side of the pump and a decreasing cavity on the discharge side. An example of a positive displacement pump is the human heart. Liquid is allowed to flow into the pump as the cavity on the suction side expands. The liquid is forced out of the discharge as the cavity collapses. This principle applies to all types of positive displacement pumps including piston, rotary lobe, gear within a gear, diaphragm, or screw pumps.

Centrifugal Pumps

A centrifugal pump is made up of an outer casing that has a rotating wheel-like component called an impeller inside a stationary cavity created by the casing and called the volute. Fluid is drawn into the inlet port at the center of the rotating impeller. As the impeller spins, vanes on the impeller force the fluid to rotate, thrusting it outward radially and thus moving the fluid from inlet to outlet under its own momentum. As it does this, it creates a vacuum, which draws more fluid into the inlet.

Manufacturers will offer different size impellers that fit inside a particular casing to offer many different pump capabilities. Thus, the impeller can be changed out to change pump flow and pressure characteristics. This reduces manufacturing and storage costs associated with pump casings. However, a pump's efficiency typically decreases as impeller size is decreased for a particular pump casing.

The number of impellers determines the number of stages of a centrifugal pump. A single-stage pump has just one impeller and is better for low head service. A two-stage pump has two impellers mounted in series for medium head service.

A multistage pump has three or more impellers mounted in series for high head service such as in deep well pumps.

Centrifugal pumps can operate at different speeds. The faster the impeller rotates, the faster the fluid movement and the stronger its force.

Pump Drives

Pumps are driven by a drive, usually an electric motor in building plumbing systems. A fuel-powered engine can also drive a pump, but this type of drive is typically reserved for temporary or emergency applications. The pump drive may be close coupled such that it’s on the same shaft as the pump impeller; that is, the drive and pump are directly connected. This design approach is less costly and is acceptable on small pumps where vibration and noise are not problems. Flexible coupled pump drive configurations minimize vibration and wear between the pump and pump drive, especially for large pumps. Pumps used in residential and small commercial installations are typically small enough that they can be close coupled. Typically large pumps used in building plumbing and heating and cooling systems need to be flexible coupled.

The speed of a pump is measured in revolutions per minute (rpm). Constant speed drives are drives designed to operate at a specific speed (e.g., 1750, 3500 rpm). These drives are usually sized to handle the largest loads so they are typically oversized for normal operating loads. They are economical in terms of initial cost but not economical in lifetime operating cost.

There are two types of speed control in pumps: multi speed drives and variable speed drives. Multispeed drives have separate speed settings (e.g., high, medium, and low) so they can be adjusted to control speed and, thus, pump flow rate.

Variable-speed drives provide speed control over a continuous range. Variable speed drives control pump speed by changing the speed of the driver and thus flow rate. These drives are useful in meeting the needs of installations with highly variable loads. In such cases, the use of multiple pumps (e.g., two or more pumps), multispeed drives, or variable-speed drives often improve system performance over the range of operating conditions. They are the most expensive type of drive in initial cost but the least costly to operate.

Pump-drive installations need to be secured well for vibration and sound control. A heavy foundation or base is required for large pumps. The weight of a foundation should be 3 to 5 times the combined weight of the pump and drive. Isolation, by vibration damping, is another solution for vibration and sound control.

Pumping Configurations---For some pumping installations it may be necessary to use multiple pumps to meet design requirements. In systems with highly variable loads, pumps that are sized to handle the largest loads may be oversized for normal operating loads. In such cases, the use of multiple pumps improves system performance over the range of operating conditions. Multiple pumps can be staged so both pumps operate only during periods of full demand. A single pump operates when demand is low. Multiple pumps can be con figured in parallel or series.

Pumping in Parallel

Parallel pumping entails installing two pumps side by side in a piping system. When installing two identical pumps in parallel, the combined flow rate will be less than double. Pumps with different flow rates can be installed in parallel and con figured such that the small pump operates during periods of low to average demand while the larger pump operates during periods of high demand. One major benefit of parallel pumping is the high level of standby capacity provided by single-pump operation. When one pump is out of operation for maintenance or repair, the other pump continues to pump water through the installation.

Pumping in Series

Series pumping involves installing two pumps one in line with the other in a single pipe in a piping system. Pumps in series double the head at the same flow condition point. One pump discharge is piped into the suction of the second pump, producing twice the head capability of each pump separately. The second pump must be capable of operating at the higher suction pressure that is produced by pump number one. This mode of operation is a very cost-effective way of overcoming high discharge heads when the flow requirement remains the same.

Pump Performance The fundamental performance considerations of centrifugal pumps are capacity, total dynamic head, brake horsepower, pump efficiency, and pump speed. These characteristics and related terms are discussed in the following sections.

Pump Capacity

Pump capacity (Q_pump) is the flow rate at which liquid is moved or pushed by a pump to the desired point in the system. It’s commonly measured in either gallons per minute (gpm), liters per minute (L/min), or cubic meters per hour (m3 /hr). Pump capacity depends on pressure, temperature, and viscosity and specific gravity of the liquid being pumped; on the size and speed of the impeller; and the size and shape of the cavities between the impeller vanes.

For a pump with a specific impeller design and that is running at a certain speed, the basic factors that affect flow rate are the pressures at the pump inlet and outlet (i.e., pressure difference between the pump inlet and outlet). The larger the pressure difference a specific pump must overcome, the lower the flow rate of the liquid. Furthermore, the bigger the pressure difference a specific pump must overcome, the greater the horse power required to drive the pump. Conversely, a small pressure difference means a higher flow rate and a smaller horsepower requirement.

A centrifugal pump is not limited to a single flow rate at a given speed. Its flow rate depends on the amount of head it encounters in the piping, so it operates at the point on the performance curve where its total dynamic head matches the resistance in the piping (i.e., the sum of the static head and friction head). As a general rule, an increase in flow rate causes a decrease in the total dynamic head the pump produces. Conversely, a decrease in flow rate increases the total dynamic head the pump produces.

Total Dynamic Head

Total dynamic head (?PTDH) is a pressure difference, expressed in feet (meters), that represents the measurement of the height of a liquid column that the pump can generate from the kinetic energy imparted to the liquid. An illustration to clarify this unit of measure would be a pump shooting a stream of water straight up into the air; the maximum height attained by the water stream would be the total dynamic head produced by the pump.

To pump water at a specific flow rate, the total dynamic head generated by a pump must overcome the head of the system through which the pump is pushing water. Thus, total dynamic head developed by a pump must overcome a combination of the static head and friction head of the piping system:

total dynamic head (?PTDH) _ static head (? P_static) _ friction head (? P_friction)

Static head (? P_static) is the actual vertical distance measured from the water level in the reservoir from which the pump draws the fluid to the highest point in the discharge piping.

Friction head (? P_friction) is the additional head created in the discharge system from resistance to flow within its piping components (e.g., friction from pipes and fittings). Computation of static head and friction head was introduced previously in Section 3.

Head is a term that can have units of a length (feet or meters) and pressure (e.g., psi, Pa). The principal reason for using head in units of height (instead of units of pressure) to express a centrifugal pump's pumping capability is that the pressure from a pump will change if the specific gravity (weight) of the liquid changes, but head won’t change. Because most centrifugal pumps can move various fluids, with different specific gravities, it’s simpler to express the pump's head in feet (or meters). In systems with water, this change in specific gravity is generally not a consideration because most pump performance criteria is based on the specific weight of water.

Power and Hydraulic Efficiency

The work performed by a pump is related to the total dynamic head and weight of the liquid pumped in a specific time period.

Brake horsepower (BHP) is the actual horsepower delivered to the pump shaft under stated operating conditions of the pump where horsepower is 550 foot/pounds per second. Pump output is the water horsepower (WHP) delivered by the pump. The brake horsepower of a pump is greater than the water horse power because of the mechanical and hydraulic losses incurred in the pump. These are determined by manufacturer's testing of the pump under a specific flow rate and head.

The water efficiency (?water) of a pump (sometimes called the hydraulic efficiency) is expressed as a percentage of hydraulic horsepower to BHP over the recommended operating range of the indicated impeller size(s). It describes the change of centrifugal force (expressed as the velocity of the fluid) into pressure energy. The relationship between BHP, WHP, and ?water of a pump is:

Water efficiency of a specific pump/impeller configuration will vary with flow rate and head, but some configurations are better than others at a specific flow rate and head.

A pump operating at high efficiency uses less energy to operate than one operating at low efficiency. The best efficiency point (BEP) is the operating condition at which a pump most efficiently converts shaft power to flow. This is also the point where there is no radial deflection of the shaft caused by un equal hydraulic forces acting on the impeller. Operation of a pump well outside the BEP results in unstable conditions like vibration, erosion, and cavitation, which result in premature bearing and mechanical seal failures. An industry design standard used to approximate the lowest performance level of a pump is an efficiency of 55%. If possible, it’s best to avoid operating any pump at efficiencies of less than 55%.

BHP, the power delivered to the pump shaft under stated operating conditions of the pump, is found by the following expression:

As introduced in Section 12, the specific gravity (s.g.) of a fluid or solid is the ratio of the specific weight of the fluid or solid to the specific weight of water at a temperature of 39°F (4°C), the temperature at which water is most dense. The specific gravity of water is 1.0 at common plumbing system temperatures. The expression above implies that, under specific conditions, a fluid heavier than water (a fluid with a specific gravity greater than one) will require more horsepower to pump. Viscosity of a fluid (i.e., fluid thickness) will also affect BHP; a higher viscosity fluid requires increased BPH.

Net Positive Suction Head Pumps are designed to pump only liquids, not vapors. Vaporization of the liquid being pumped should not occur at any condition of operation. As discussed earlier, vaporization results in cavitation, which is undesirable because it can obstruct the pump, impair performance and flow capacity, and damage the impeller and other pump components. To prevent cavitation, a pump always needs to have a sufficient amount of suction head present to prevent this vaporization at the lowest pressure point in the pump.

The inlet of a pump sucks liquid into the pump. The term used to describe this suction pressure of a pump is net positive suction head (NPSH). It’s expressed in height, as feet or meters. The minimum head required to prevent cavitation with a specific liquid at a specific flow rate is called net positive suction head required (NPSHr). Pump manufacturers determine the NPSHr of a specific impeller design by conducting a test using water as the pumped fluid. NPSHr varies with speed and capacity of a specific pump. It increases as capacity increases because as the velocity of a liquid increases, the pressure (or head) decreases. The NPSHr shown on pump performance curves is for fresh water at 68°F (20°C), which won’t relate directly to another type of fluid or combination of fluids being pumped.

In a pumping installation, the difference between the actual head of the liquid available (as measured at the pump's suction inlet) and the vapor pressure of that liquid is called the net positive suction head available (NPSHa). This is the amount of NPSH available to the pump from the suction line. It’s defined as atmospheric pressure _ gauge pressure _ static pressure _ vapor pressure _ friction loss in the suction piping.

NPSHa is directly related to system design, while NPSHr is directly related to pump design. NPSHr can be roughly thought of as the maximum height that a pump can be located above the reservoir from which it’s pumping (assuming negligible vapor pressure and friction loss in the suction piping). Technically, for a pump to operate properly, the system's NPSHa must always be greater than the pump's NPSHr. So, if the system's NPSHa falls below the pump's NPSHr, then suction pressure is too great and the water being pumped will spontaneously vaporize in the pump casing, causing cavitation.

It’s prudent that a margin of safety be provided between the pump's NPSHr cited by the manufacturer and the system's NPSHa at the operating conditions. It’s normal practice to have at least 2 ft (0.6 m), and in some cases more, of extra NPSHa existing at the suction inlet to avoid cavitation problems under demands outside design conditions. Thus, excessively long runs of suction piping that potentially cause the system's NPSHa to be greater than the pump's NPSHr should be avoided in system design.

Pump Speed

Pump speed (N) is tied to the rotational speed of the drive (motor). It’s expressed in revolutions per minute (rpm).

Pump Affinity Laws Pump affinity laws are scientific relationships that describe changes in pump capacity, total dynamic head, and BHP when a change is made to pump speed, impeller diameter, or both.

The affinity laws are only valid under conditions of constant efficiency. According to affinity laws:

Capacity (Q_pump) changes in direct proportion to impeller diameter (D) ratio, where the subscript 1 refers to the original impeller diameter and subscript 2 refers to the new diameter:

Capacity (Q_pump) changes in direct proportion to the ratio of pump impeller speeds (N), where speed is in rpm, the sub script 1 refers to the original speed, and subscript 2 refers to the new speed:

From these relationships it can be observed that capacity of a pump can be increased by increasing pump impeller diameter or pump speed. Manufacturers make several impeller sizes that fit in one size of pump casing, making it easier to change to another impeller size. Variable-speed drives (i.e., motors) control flow by changing pump speed.

Total dynamic head (?PTDH) changes in direct proportion to the square of impeller diameter (D) ratio, where the subscript 1 refers to the original impeller diameter, and subscript 2 refers to the new diameter:

Total dynamic head (?PTDH) changes in direct proportion to the square of the ratio of pump impeller speeds (N), where the subscript 1 refers to the original speed, and subscript 2 refers to the new speed:

Increasing impeller diameter or speed will increase a pump's total dynamic head. Because of the square function of the expression, even slight changes in impeller size or speed produce large changes in total dynamic head.

Pump Performance Curves

A pump performance curve is a graph that shows the flow rate that a specific pump model and impeller size is capable of pumping over a range of pressure differences. The pump curve also shows pump speed (in rpm) and other information such as pump size and type, and impeller size. The pump performance curve is produced by a pump manufacturer from actual tests performed on a single model of pump. Manufacturers usually incorporate several performance curves on a single chart, with each curve representing a different impeller size.

A chart of pump performance curves for a specific pump model is shown in fgr.23. This chart shows several performance curves, impeller sizes, and drive horsepower ratings.

If a pump is available with only one size of impeller, there will be just a single performance curve on the entire chart. Similarly, if a pump is only available with one drive (motor) size, the chart won’t have separate horsepower lines.

On the chart in fgr.23, pump capacity with the range of flow rates is shown along the bottom of the chart. The left side of the chart indicates the amount of total dynamic head a pump is capable of generating. The bold curved lines represent the performance curves for this pump model for impeller sizes available. The dash-dot lines represent BHP ratings available for this pump. The top right of the chart shows pump speed; in the chart provided, this is 3500 rpm.

Curved lines representing efficiency are also shown on the chart. Efficiency varies with pump impeller size and operating conditions. The BEP is the point on each of the performance curves where the pump operates at highest efficiency. For example, in fgr.23, the BEP for a pump with the 6-in diameter impeller occurs at a pump capacity of 240 gpm and a total dynamic head of 103 ft. All points on a performance curve to the right or left of the BEP have a lower efficiency.

Using Pump Performance Curves

Two variables affect pump performance, drive horsepower, and impeller size: total dynamic head and pump capacity. These hydraulic design conditions are specific to a particular piping installation and are used to determine the required size of a pump.

The first step in selecting pump size is computing the system head, the sum of the static head and friction head of the piping system (as introduced in Section 3.2), and establishing the required flow rate of the system. A pump overcomes system head by producing total dynamic head, so system head relates to total dynamic head. The required system flow rate is tied to the desired pump capacity.

The intersecting point on a pump performance curve where the desired pump capacity and total dynamic head match the installation's requirement is known as the operating point (sometimes called the duty point). The operating point will al ways fall on the pump performance curve of a specific impeller.

Selection of a pump impeller size and drive horsepower are based on this operating point.

In most cases when selecting a pump impeller size, desired pump capacity and total dynamic head needed don’t intersect a performance curve exactly; that is, the intersecting point falls between two impeller curves. In this case, the larger impeller is selected. However, this moves the operating point to the performance curve of the selected impeller. Movement is up and to the right because system head increases with an increase in flow rate. To control the flow rate of a centrifugal pump, it’s normally necessary to further restrict flow through the discharge pipeline with a valve. By partially closing the valve, friction head (resistance to flow) in the system is increased and flow in the discharge piping is reduced to the desired flow rate.

When selecting a centrifugal pump and impeller size, the pump efficiency under hydraulic design conditions (i.e., capacity required and total dynamic head) must be taken into consideration. Theoretically, the BEP is the ideal spot for a pump to operate, so when evaluating several different pumps or impeller diameters for a specific pump model, the pump that best matches the design conditions (i.e., capacity and head) and that comes closest to the BEP is hypothetically the best choice.

Practically, selecting a pump with the flow and head requirements that are slightly to the left of the BEP makes good engineering sense. This offers the capability to make adjustments if necessary to flow and head conditions for the system to operate properly while providing a high level of efficient operation. For optimal performance, it’s desirable that pumps operate within a range of 70 to 115% of BEP.

With small pumps, manufacturers typically match a drive to the pump. For large pumps, the designer must select the driver based on the BHP related to the selected pump model and impeller diameter. Selecting a drive large enough to handle the largest impeller that can fit in the pump casing can eliminate the need for drive replacement if the pump.

The following examples introduce the technique of simple pump selection.

EX.20

Design conditions for a piping system are that the pump must deliver water at a flow rate of 180 gpm and generate 60 ft of total dynamic head to overcome the static head and friction head of the piping system.

a. Based on pump performance curves in fgr.23, size a pump impeller and drive that can meet these conditions.

At the intersecting point of 60 ft of total dynamic head and a flow rate of 180 gpm, the pump performance curve for the 4 3/4-in diameter impeller meets the design conditions exactly (and conveniently). This is the pump's operating point. A 4 3/4-in diameter impeller is selected.

The BHP at the operating point is midway between 3 BHP and 5 BHP. A 4 hp drive (motor) would be adequate and could be selected, if one was available. However, the 4 hp drive would have a tough time operating on the right side of the curve because more brake horsepower is required. A 5 hp drive available through the manufacturer is a better selection because, in the future, additional pump capacity may be needed.

b. Determine water efficiency and net positive suction head required at the design conditions.

Efficiency at this operating point is about 70%.

The NPSHr at this operating point is about 9 ft.

EX.21

Design conditions for a piping system are that the pump must deliver water at a flow rate of 180 gpm and generate 80 ft of total dynamic head to overcome the static head and friction head of the piping system.

a. Based on pump performance curves in fgr.23, size a pump impeller and drive that can meet these conditions.

The intersecting point of 80 ft of total dynamic head and a flow rate of 180 gpm falls between the pump performance curves for the 5-in and 5 1/2-in diameter impellers. The larger 5 1/2-in diameter impeller is selected.

The larger impeller will have a flow rate much higher than the required 180 gpm if left unthrottled. A valve must be in stalled in the discharge piping to increase head and reduce flow. To achieve a flow rate of 180 gpm, the valve will need to increase the head to about 92 ft (the intersecting point of the 180 gpm line and the 5 1/2-in diameter impeller performance curve). This is the pump's operating point.

The BHP required at this operating point is about 6 BHP.

Like in the previous example, the 6 hp pump would have a tough time operating on the right side of the curve because more BHP would be required. A 7 1/2 hp drive available through the manufacturer is a better selection because, in the future, additional pump capacity may be needed.

b. Determine water efficiency and net positive suction head required at the design conditions.

Efficiency at this operating point is about 74.5%.

The NPSHr at this operating point is about 7.3 ft.

EX.22

Design conditions for a renovated piping system are that the pump must deliver water at a flow rate of 275 gpm and generate 33 ft of total dynamic head to overcome the static head and friction head of the piping system. The existing pump has a 5-in diameter impeller and a 5 hp drive. Based on the pump performance curve in fgr.23, evaluate the existing pump's water efficiency.

The operating point of the pump will be the intersecting point of a 275 gpm pump capacity and 33 ft of total dynamic head, which falls directly on the 5-in diameter impeller's performance curve. Water efficiency at this operating point is about 50%. It’s best to avoid operating any pump at efficiencies of less than 55%, so a different model pump should be considered.

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