Good

FIGURE 23 Suction pipe header recommendations for dry-pit centrifugal pump

FIGURE 24 Higher tunnel velocities require isolation of pump from direct flow to prevent distortion of close-clearance parts and shaft (Hydraulic Institute Standards, 13th Edition, 1975—out of print)

NPSH The net positive suction head so essential to trouble-free pump operation is always reduced by losses in suction piping. An economic balance must be obtained between pump size and speed, required NPSH, pipe size, and suction velocity. If the suction source is a tank, such as a deaerator in a power plant, it is quite expensive to elevate the tank. Therefore, the available NPSH is low. The pipe size from the tank to the pump should be large for low velocity and the length should be short for minimum losses. A cooling tower on a hill supplying water to a pump station below could have a much higher pipe velocity and longer length without forcing an extremely low NPSH requirement on the pump.

A dry-pit pump should be as close to the suction source as possible. When the NPSH required indicates a suction lift is possible, the most advantageous solution is to reduce the suction losses by increasing the pipe size, flaring the inlet bell, and keeping the pump suction eye of the first-stage impeller close to the minimum water level.

High-Pressure Inlets Pumps in series build up pressure, so the second and following pumps will have a high-pressure suction piping connection. This emphasizes the need for tight joints and flanges and careful welding. Expansion joints should not be used because the hydraulic forces on the pump would be large, difficult to restrain, and perhaps impos sible for the pump to handle without distortion. As NPSH will not be a problem for pumps downstream of the first pump, the pipe size between pumps can be kept small to minimize design problems and valve costs.

Effect on Pump Efficiency Other than mechanical operation, the greatest effect of flow disturbance at the pump suction is on pump efficiency. The higher the pumping head, the lesser this effect becomes. For very high-head pumps, a high velocity may be ignored completely unless there is an extremely large power evaluation factor. Greater attention should be given to the suction pipe design for pumps producing 100 ft (30 m) of head or less than those with high head, as efficiency may be worth many dollars in power costs over the life of the plant and equipment.

Pump efficiency relates to the efficiency of the whole system, and so it may be well worthwhile to invest more money in a large suction pipe.

Surge and Vibration One of the possibilities arising from a power failure at a pump station is the reversal of a centrifugal pump if a valve fails to close, and its subsequent operation as a turbine. Under rated head, a pump will run from 20 to 60% above rated speed in the reverse direction. In its transition to that phase, the forward motion of the water is interrupted and gradually reversed. At the time of the power failure, the flow velocity in the suction pipe may not decelerate slowly enough to prevent a surge in the direction of the pump. The suction piping should be designed to withstand the resulting pressure rise because absolute integrity of valving and power supply will be too costly as a design parameter (see Chapter 8 for additional information on this subject).

Rotating machinery must have a design vibration frequency that is sufficiently removed from a system frequency to avoid sympathetic activity. Any combination of vibration frequencies near enough to each other to react will do so when a prime source, such as a pump, excites them. When a piping system has been designed to allow only very low stresses to be transmitted to a pump nozzle, it is quite vulnerable to vibration. It is usually good practice to analyze a suction system made up of pipe, valves, hangers, restraints, pump nozzle loads, pump speed and impeller configuration, foundation, and anchors to be sure the system is not "in tune." At the design stage, it is relatively easy to change a valve or elbow location or to add a surge suppresser. It is usually much more costly to change a pump.

Reciprocating pumps are surge producing machines. In particular, they require sufficient energy at suction to overcome pump required NPSH and pipe friction and a form of energy called acceleration head. The pump energy must overcome the acceleration-deceleration pulsation flow in the suction end, which could lead to liquid flashing with pump noise and vibration. Surges large enough to rupture the pump cylinders may also be produced.

The total NPSH required by a reciprocating pump must include the NPSH required by the pump plus frictional loss from the suction pipe plus acceleration head. Acceleration head in feet (meters) is

Renewable Energy 101

Renewable Energy 101

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. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

Get My Free Ebook


Post a comment