Pump Handbook

Pump Installation (Cont.)

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Discharge Piping

The size and configuration of discharge piping is primarily an economic issue. There are, however some areas of the discharge piping design that should be considered, among them are:

  • Keep in mind that the material and installation cost of the piping system is a one-time cost. Money saved at installation may be far less than the added operating cost of the pump over its lifetime.
  • Reducing the size of the discharge piping to save cost at installation may increase initial equipment cost by requiring a larger or more expensive pump and monitor. The increased equipment cost is due to the increase in head required by the system, which relates to a higher horsepower demand.

Other than economics, there are some conditions that can cause problems and should be considered in the design of the discharge piping system:

  • Avoid large air traps! If it is not practical to design the discharge piping without potential air traps, include a means to vent the air periodically.

    Liquid containing small amounts of entrained air can cause problems if the fluid velocity is insufficient to carry the air completely through the piping system. This problem occurs when the piping system is routed over large objects, such as building, and then back down. The air in the liquid passes through the pump, into the discharge pipe, where it travels to the high point in the system. If the fluid velocity is not sufficient to carry the air down the other side, it becomes trapped. The trapped air then acts to reduce the effective area of the pipe, creating increased frictional losses.

    Other things can happen also, such as entrained air collecting into a pocket that releases large bubbles of air periodically, causing problems with the flow meters and control valves.

    A similar, but opposite condition can occur when pumping solids with a high specific gravity. If there is a tall vertical run of piping on the discharge side of a pump handling a water-like liquid with a low percentage of high specific gravity solids. It is necessary to design the system with a great enough liquid velocity to carry the solids through the system. To imagine how this happens, think of a rotameter without stops at the top or bottom. The float would remain in the rotameter gauge glass as long as the flow rate was within its designed range. If the flow were to be increased significantly above the design flow rate of the meter, the float would be carried away through the piping system. On the other hand, if the flow was reduced significantly, the float would fall, possibly all the way to the pump discharge. In this analogy, the float represents a high specific gravity solid in a piping system.
  • Avoid "dead heading" the pump. For centrifugal pumps short-term "dead heading" may not be a problem, however, dead heading of centrifugal pumps, in general, will cause increased radial loading of the impeller which can affect the life of the pump bearings and mechanical seals/packing. If the pump is allowed to operate for extended periods with a closed discharge, the pump will generate heat, heating the pump and liquid contained in the casing. This can cause damage to the pump, and more importantly, create a possibly dangerous situation. DON'T OPERATE A CENTRIFUGAL PUMP WITH A CLOSED DISCHARGE. If the system operation requires that no fluid is to be flowing at certain periods, design the system with a bypass line or recirculation line from the pump discharge back to the supply source. Check with the pump manufacturer or your local distributor for information about minimum flow requirements for the pump you have specified.
  • A positive dispacement pump should never have it's discharge restricted. Positive displacement pumps are just that, positive displacement, which means that the pump will deliver a given amount of liquid for each revolution or stroke. If the discharge of a positive displacement pump is restricted or closed, the pump will try to overcome this line resistance, increasing the pressure in the system until the weakest part fails. To protect the system from over-pressurization by a positive displacement pump, many are manufactured with, or offer as an option, an internal relief valve. Typically, however, the internal relief valve is for protection of the pump against pump generated pressures only. This type of relief valve is not intended to be a safety relief valve to protect the system from pressures generated by other means, such as thermal expansion, or pressures developed by other pumps in the system. Systems containing positive displacement pumps should be designed with a system safety relief valve.

    Positive displacement pumps are frequently used in applications requiring varying flow rates. Properly equipped, positive displacement pumps are used as metering devices. When it is necessary to vary the flow rate from a positive displacement pump, there are typically three basic methods used:

    • Varying the pump speed.

    • Diverting the unneeded flow from the discharge back to the supply source.

    • Varying the pump displacement per revolution or stroke.

    As with suction piping, certain things should be considered as typical for discharge piping:

    • Always provide a pressure gauge connection in the discharge line between the pump discharge and the discharge isolation valve. The gauge connection should be located as close as possible to the pump's discharge connection.

    • Support the piping independently of the pump.

    • Use expansion loops or flexible connectors in the systems where thermal expansion of the piping will occur.

    • It may be necessary to install a pulsation dampener if the pump has a pulsing flow.

 

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