where Q = flow rate, gpm (m3/h) N = pump speed, rpm H = pump total head, ft (m)

The required pump total head at design conditions is

where H = required pump total head at design conditions, ft (m) Hd = design system pressure at point of control, ft (m) Hs = minimum design suction pressure, ft (m) Hf = sum of all losses between Hd and Hs at design flow, ft (m)

At flow rates less than design conditions and suction pressures higher than the minimum design pressure, the pump head requirement is reduced by the change in pipe fric-tional losses Hf and the additional suction pressure Hs available. Also, because of the rising characteristic of the centrifugal pump performance curve as the flow decreases, an excess head is developed by the pump. All of these changes will alter the required pump total head and the pump speed from the design (maximum) values.

For a single operating pump unit, design maximum speed will be required at design minimum suction head and design maximum flow. Minimum speed will be required at maximum suction head and minimum flow. The procedure required to determine the speed range of the pump driver requires construction of the system-head curve, the pump head-capacity curve (see Section 8.1), and the affinity curve. Rearranging the affinity equations (Eqs. 1 and 2), the speed changes are calculated:

where the subscripts 1 and 2 represent the higher and lower speed values, respectively, for pump conditions at the same specific speed (see Subsection 2.3.1) or along the same affinity line.

The following examples illustrate how changes in flow rate, pipe frictional losses, and suction head affect pump speed.

example 1 With no change in suction pressure—first (design) conditions: Hd = 193 ft (58.8 m) Hs = 50 ft (15.2 m) Hf = 7 ft (2.1 m) Q = 190 gpm (43.1 m3/h) N = 3500 rpm

H = (193 - 50) + 7 = 150 ft (45.7 m) (Eq. 3) Second conditions:

Hd = 193 ft (58.8 m) Hs = 50 ft (15.2 m) Hf = 1.9 ft (0.58 m) Q = 100 gpm (22.7 m3/h) N = to be calculated

The 3500-rpm pump head-capacity curve, the system-head curves, and the affinity (square) curves are shown in Figure 1. The operating points are lettered. Point A is for maximum flow at minimum suction head (design conditions). Point B is for lower flow at minimum suction head at a speed to be determined. The reduced piping losses between head points Hd and Hs are represented by the system-head curves.

To determine the approximate speed at point B, it is necessary to use trial and error because the flow and head ratios in Eqs. 4 and 5 are not known. The following procedure may be used to estimate the speed (Figure 1):

1. Draw an affinity (square) curve passing through the zero-flow/zero-head point and the lower operating point (point B) and intersecting at head-capacity curve of known speed (point C).

FIGURE 1 System and pump curves to determine speed reduction for the 2-in (51-mm) pump in Examples 1 and 2.

FIGURE 1 System and pump curves to determine speed reduction for the 2-in (51-mm) pump in Examples 1 and 2.

2. Read the probable flow rate at point C, the point of intersection between the affinity curve and the pump curve of known speed.

3. Calculate the head relative to the flow rate read at point C, based on the square curve relationship:

where subscripts 1 and 2 represent the values at the 3500-rpm and lower speeds (points C & B), respectively.

4. Compare the head calculated in step 3 with the head of the pump curve (point C) of known speed at the probable flow rate of step 2. If the values differ significantly, try another flow rate and repeat steps 3 and 4.

5. Using the accepted head from step 4, calculate the speed at the operating point (point B) using Eq. 4 or 5. The equation not used may then be used for verification.

For this example, the estimated flow rate at point A (step 2) is 104 gpm (23.6 m3/h) and the calculated head at the same point (steps 3 and 4) is

The speed at the operating point (point B) is


example 2 With change in suction pressure—first conditions: same as Example 1. Second conditions: same as Example 1, except Hs = 110 ft ( 33.5 m)

Operating point B' and other curves related to this example are shown in Figure 1. Again by trial and error, the flow rate and pump head at the intersection of the affinity curve and the 3500-rpm pump curve (point A') are 134 gpm (30.4 m3/h) and 152.4 ft (46.45 m). The speed at the operating point is


The speed change is

These examples indicate that the speed of the pump is not significantly affected unless there is an appreciable change in suction pressure.

For optimum performance, it should be confirmed, after the speed has been determined, that the most often occurring flow demand range is ideally near the pump's maximum efficiency.

Generally, the design point at maximum speed should be selected to the right of the pump's best efficiency point. The operating flow range at different speeds should be within the hydraulically and mechanically stable ranges of the pump. The pump shaft power at any specified speed is

in USCS units hp = 3Q60E

where H = pump total head, ft (m) Q = flow rate, gpm (m3/h) sp. gr. = specific gravity of fluid

E = pump efficiency, % (expressed in decimal equivalent)

H, Q, and E are values from the pump performance curve for the specified speed and impeller size.

The motor power is the same as the shaft power for pumps driven directly by the motor. For pumps driven through intermediate variable-speed couplings, the motor power must also include the drive slip and fixed loses.

When the required design flow exceeds the capacity of a single pump, several pumps, including possibly one constant-speed unit, can be arranged to operate in parallel. Two methods of staging the pumps are usually used:

1. The first pump is operated in the variable-speed mode until its maximum speed is reached; the second pump, also a variable-speed unit, is energized. Now both pumps are operating in parallel at the same reduced speed. This sequence is repeated for the other pumps.

2. The first pump is operated in the variable-speed mode until its maximum speed is reached and is then locked in at this speed to operate as a constant-speed pump. The second pump is energized and operates in the variable-speed mode. The sequence is repeated for the other pumps. The speed variation of the second pump is relatively small because it must develop the same head as the first pump operating at maximum speed. Because the flow rate through the second pump is less, its speed is affected by the increase in total head available from the rise in the pump performance curve.

Which method of sequencing is selected depends on economics, equipment redundancy, and other considerations. For example, with the first method, both pumps must be furnished with variable-speed drive units. With the second method, only one variable-speed drive unit is required if it is an electronic type because the first pump is locked in by separate electric means.

The sole function of the pumps is to maintain constant pressure at the control point; therefore, the controller selected must be pressure-actuated. The type (follower signal) of controller chosen depends on the type of speed change signal acceptable by the variablespeed control unit. For electronic motor speed controls, such as variable-voltage and variable-frequency units, typical follower signals are low-voltage dc, milliamp dc, 135-ohm potentiometer, and pneumatic.

Tankless Constant-Speed Multiple-Pump System The major components of this system (Figure 2) are

FIGURE 2 Constant-speed multiple-pump pressure booster systems: (a) horizontal end-suction volute pumps (ITT Bell and Gossett), (b) vertical end-suction volute pumps (ITT Bell and Gossett), (c) vertical turbine pumps with limited storage (SynchroFlo)
FIGURE 3 Constant-speed multiple-pump pressure booster schematic (ITT Bell and Gossett)
FIGURE 4 Typical pressure-reducing and check valve to maintain constant system pressure and prevent backflow (Cla-Val)

1. One or more pumps, with two or three most common

2. A combination pressure-reducing and check valve (PRCV) for each pump (parallel-piped PRCVs are used with larger sizes; separate pressure-reducing valves and check valves may be used)

3. An automatic sequencing control panel

4. When factory assembled, a steel frame for the entire unit piping arrangement and flow path A schematic of the piping arrangement and flow path in a constant-speed multiple-pump system is shown in Figure 3. Supply water under fluctuating pressure enters the suction header and flows into the pump, where it is boosted to a higher pressure. This varying high-pressure water enters the PRCV (Figure 4), and the pressure is reduced to the constant pressure desired over the design flow range. Flow reversals through the idle and parallel pump circuits are prevented by the checking feature of the PRCV, which also dampens the pressure fluctuations caused by sudden flow changes.

When only minor changes in supply water pressure are anticipated, such as from a nonpressurized tank, and the pump head-capacity curve is relatively flat, silent check valves without pressure-reducing feature may be used. This will result in a slight increase in discharge pressure above the desired design constant pressure. When pumps of different sizes are used, care should be exercised to avoid having the higher-head pump force the lower-head pump to shut off or to operate at less than minimum design pump flow. A decrease in flow from a centrifugal pump must be accompanied by an increase in pump total head as required by the pump head-capacity curve. If check valves only are used, it is preferable to have all pumps and valves identical to avoid unbalanced flows.

Operating the pumps at shutoff (no flow) will cause the water temperature in the pump castings to rise (Subsection 2.3.1). To keep the temperature in the pumps within a safe limit, the heated water is relieved through the thermal relief valve. This valve may be either a self-actuated (thermostatic) type or a solenoid valve actuated by a temperature controller.

pump control panel One of the advantages of a factory-assembled pressure booster package is the prewired, pretested, pretubed, mounted control panel that requires a minimum of field connections. In addition to providing for the proper sequencing of the pumps, the control panel should contain electrical interlocks for the operating and safety controls and circuit connections for remote control units.

Standard items and optional equipment vary considerably from one manufacturer to another. The components usually included with a standard panel are listed:

Steel enclosure Starters

Control transformer Sequencing controllers

Control circuit protector Pump failure interlocks

Selector switches Minimum-run timers

Low-suction pressure control Time delays

Optional features that may be available:

Power supply fused disconnects or circuit breakers Low system pressure control

Most factory-wired panels conform to one or more of the consumer safety agencies, such as Underwriters' Laboratories (UL), National Electrical Code (NFPA/NEC), and Canadian Standards Association (CSA) and are furnished with a label so indicating.

pump control sequence A typical elementary wiring diagram provides the following sequences of events. With both pump selector switches in the auto mode and all safety and operating controls in run status, the lead pump starter is energized, starting pump 1. As the system water demand increases, a staging control switch, which senses motor current, flow, or system pressure, starts pump 2. Pump 2 continues to operate until the decreasing water demand causes the staging control switch to open, stopping pump 2. If the circuit is provided with a minimum-run timer, pump 2 will continue to run for the set time period regardless of the staging switch status. This timer prevents pump 2 from short-cycling during rapidly fluctuating demand periods.

For test purposes and emergency operation, both pumps may be operated by placing the selector switches in the hand mode. In this position, most of the safety and operating controls are bypassed.

Should pump starter 1 fail to operate because of an overload heater relay trip or starter malfunction, a failure interlock switch automatically starts pump 2.

Pump 1 will run continuously unless the circuit is provided with shutdown features, such as high-suction pressure control and/or low-flow shutdown control.

Pilot lights

Enclosure door interlock High water temperature control High system pressure control Pump alternation Program time switch Elapsed time meters

Low water level control Emergency power switchover Unit failure alarm Low-flow shutdown Miscellaneous enclosure types Power economizer circuit Additional pilot lights

Many units are specified for manual or automatic alternation of the equally sized pumps. This feature is intended to equalize the wear among the pumps and associated components. Alternation of any pump designated for standby duty (emergency use) may not be prudent. The standby pump should be preserved until needed, similar to an emergency power generator.

Limited-Storage Constant-Speed Multiple-Pump System The most notable shortcoming of the tankless multiple-pump system is the need for a continuously running pump, even at zero water usage. This drawback is remedied by the addition of a pressurized storage tank connected to the high-pressure side of the piping system with low-flow shutdown controls to stop the pumps.

The limited-storage system is not a scaled-down version of the hydropneumatic system. It differs in three important aspects:

1. The primary function of the tank is water storage.

2. Tank pressure is developed by the booster pump.

3. Air and water in the tank are separated by a flexible barrier (diaphragm, bladder) to eliminate direct interaction between the gas and water in the tank. The barrier also prevents the gas from escaping when the tank is emptied of water.

sequence OF operation control circuitry During periods of normal water usage, the operating sequence is that of the tankless unit. As the water flow approaches zero, a low-flow sensing device stops the pumps. The tank provides the water needs during the shutdown period until the water pressure diminishes to the minimum allowable value. At this time, a pressure switch starts the lead pump to restore the pressure and recharge the tank with water.

The low-flow device may be a flow switch, a pressure switch, or a temperature-actuated switch and must be capable of switching at flows below 5 gpm (1.14 m3/h).

The tank should be sized for sufficient drawdown volume (water available from tank during shutdown) to avoid excessive pump cycling.

RELATIONSHIPS BETWEEN TANK SIZE, DRAWDOWN VOLUME, AND PRESSURE The approximate formula for sizing prepressurized diaphragm tanks at constant temperature is ve

Ve = change of gas volume in tank, ft3 (m3) Pf = final gas pressure lb/in2 (kPa) abs P0 = initial gas pressure, lb/in2 (kPa) abs The corresponding relationships of water volume and pressures in the tank are

• Ve is equal to the drawdown volume.

• Pf is the water pressure at the end of the drawdown cycle, the minimum allowable system pressure.

• P0 is the water pressure at the beginning of the drawdown cycle or at the termination of the charging cycle.

tank location Because both Pf and P0 are influenced by the static head above the tank and the available charging pressure, the location of the tank and point of connection to the system should be carefully selected. Connecting the tank at A in Figure 5 provides the highest available tank charging pressure because this pressure is not affected by the reduction through the PRVs. However, there are several disadvantages to this point of connection:

FIGURE 5 Relative effect of tank location on tank size and pressure for equal draw-down volumes (ITT Bell and Gossett)

1. The highest static pressure is applied to the tank because the pumps are usually located on the lowest floor of the building.

2. The tank may be subjected to high working pressure because any increase in suction pressure above the minimum design pressure is additive to the pump head.

3. The tank and all alternated lead pumps must be interconnected.

4. A silent check valve must be placed between the tank connection point and the pump discharge nozzle.

Connecting the tank in B in Figure 5, downstream of the PRVs, will eliminate disadvantages 2, 3, and 4. The available charging pressure is less, and therefore a larger tank is required. However, tank location is not critical.

Locating the tank on the upper elevation of the building, such as at C, reduces the static head pressure on it; consequently, a smaller tank will suffice. The maximum design working pressure is also lower, and therefore it may be possible to use a tank with a lower pressure rating at less cost.

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

Get My Free Ebook

Post a comment