Piping Recommendations

PIPING RECOMMENDATIONS P015

There are entire books written on this subject so in the next few paragraphs I will try to give you some of the highlights of what you should know about piping as it relates to pumps and seals.

  • There should be at least 10 diameters of pipe between the suction of the pump and the first elbow. This is especially critical in double-ended pump designs as the turbulent inlet flow can cause shaft thrusting, and subsequent bearing problems. If an elbow must be installed be sure it is in a plane at right angles to the pump shaft to prevent an uneven flow to both sides of a double suction impeller.
  • Pipe from the pump suction flange to the pipe rack, not the other way around.
  • Make sure eccentric reducers are not installed upside down at the pump suction. The top of the reducer should go straight into the suction flange.
  • Piping should be arranged with as few bends as possible. If bends are necessary use a long radius when ever possible
  • Substituting a globe valve for a gate valve in a piping system is similar to adding almost another 100 feet (31 meters) of piping to the system. On the discharge side of the pump this will cause the pump to run off of its best efficiency point (BEP) with a resultant shaft bending. On the suction side of the pump it will probably cause cavitation problems.
  • Use eccentric reducers rather than concentric reducers at the pump suction. Concentric reducers will trap air. Be sure the eccentric reducer is not installed up side down.
  • If an expansion joint is installed in the piping between the pump and the nearest point of anchor in the piping, It should be noted that a force equal to the area of the expansion joint (which could be a lot larger than the normal piping size) times the pressure in the piping will be transmitted to the pump proper. Pipe couplings that do not provide an axially rigid connection have the same affect. If an expansion join or non-rigid coupling must be used it is recommended that a pipe anchor be installed between it and the pump.
  • It is always a good idea to increase the size of the suction and discharge pipes at the pump nozzle in order to decrease the head loss from pipe friction.
  • Suction piping should be at least one size larger than the suction flange at the pump.
  • If increasers are used on the discharge side to increase the size of discharge piping, they should be installed between the check valve and the pump.
  • A check valve and a stop valve should be installed in the discharge line with the check valve placed between the pump and the stop valve to protect the pump from reverse flow and excessive back pressure.
  • Expansion joints should be installed between the check valve and the pump.
  • Suction and discharge piping should be supported to prevent transmitting forces and bending moments o the pump casing.
  • Suction piping must be kept free of air leaks.
  • The installation of check valves should be avoided in the suction piping although they are often used to reduce the number of valves that have to be operated in switching between series and parallel pump operation.
  • A foot valve is often installed in the suction piping to aid priming. Do not install one if the pump is operating against a high static head because failure of the driver would allow liquid to rush back suddenly, causing water hammer. This is especially true for vertical turbine and submersible pumps that are not designed for use with a foot valve.
  • Foot valves should be of the low loss flap type rather than the multiple spring variety and have a clear passage for the liquid at least the same area as the suction piping.
  • A horizontal suction line should have a gradual rise or slope to the pump suction.
  • Cast iron pumps should never provided with raised face flanges. If steel suction or discharge piping is used, the pipe flanges should be of the flat face type and not the raised face type. Full-faced gaskets must be used with cast iron flanges.
  • The optimum control valve location is within five feet (1,5 meters) of the pump discharge to prevent too much surging of fluid in the system when the discharge is throttled. Vortexing can occur if any of the following conditions are present:
    • Low liquid levels
    • Liquid level falling greater than 3 ft./sec. (1 meter/ sec.)
    • There is a large concentration of dissolved gases in the liquid.
    • High outlet velocities in pipes leaving vessels. Generally greater than 10 feet/sec. (3 meters/sec.)
    • Liquids near their vapor point.
    • High circulation caused by asymmetrical inlet or outlet conditions.
    • Inlet piping too close to the wall or bottom of the tank. Consult the Hydraulic Institute Manual or a similar publication for recommended clearances.
    • In a mixer, the liquid level must be at least one and one half diameters of the blade, above the blade.

The optimum pipe size will consider the installed cost of the pipe (the cost increases with size) and the pump power requirements (the power required increases with pipe friction)

  • Try to limit the friction loss at design flow to 2-5 feet for each 100 feet (1-2 meters for each 30 meters) of pipe).
  • To prevent the settling of solids you need a minimum velocity of about 4 to 7 feet per second (1.5 to 2.5 meters per second)
  • Velocities of no more than 10 feet (3 meters) per second are recommended in the suction side piping to prevent abrasive wear.

In multiple pump arrangements we would prefer to have the suction bells in separate bays so that one pump suction will not interfere with another. If this is not practical, a number of units can be installed in a single large sump provided that:

  • The pumps are located in a line perpendicular to the approaching flow.
  • There must be a minimum spacing of at least two suction diameters between pump centerlines.
  • All pumps are running.
    • The upstream conditions should have a minimum straight run of ten pipe diameters to provide uniform flow to the suction bells.
    • Each pump capacity must be less than 15,000 gpm.
    • Back wall clearance distance to the centerline of the pump must be at least 0.75 of the suction diameter.
    • Bottom clearance should be approximately 0.30 of the suction diameter
    • The minimum submergence should be as follows:
      • 20,000 gpm 4 feet
      • 100,000 gpm 8 feet
      • 180,000 gpm 10 feet
      • 200,000 gpm 11 feet
      • 250,000 gpm 12 feet

The metric numbers are:

  • 4,500 1.2 meters
  • 22,500 2.5 meters
  • 40,000 3.0 meters
  • 45000  3.4 meters
  • 55,000 3.7 meters

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Posted

  • On February 16, 2018