Subject: Changes in the discharge head of a centrifugal pump 1-1

We need to know the discharge head of a centrifugal pump for several reasons:

  • To select the correct size pump for a new application.
  • To estimate stuffing box pressure in the event the discharge valve is shut with the pump running.
  • To determine if an installed pump is running at it's Best Efficiency Point (B.E.P.). Any time we go off the B.E.P. the shaft is subject to radial side loading and potential problems with mechanical seals, packing, bearings and couplings.
  • To be sure the pump will have enough capacity.
  • To prevent cavitation problems, if the head is too low.

There are two methods of learning the head :

  • Look at the discharge pressure gauge that's been installed in the system, or a chart recorder that's installed on the discharge side of the pump. Subtract this reading from the reading obtained from a gage installed on the suction side of the pump.
  • Calculate the discharge and suction heads from losses in the piping and associated hardware in the lines.

As easy as this sounds, we can still have many problems determining accurate head information because of variations in the discharge system. In the following paragraphs I'll describe how design, operation and maintenance practices can affect this discharge head.

DESIGN PROBLEMS

  • An automatic or manual control valve is controlling:
    • Level - Boiler feed systems are typical.
    • Mixture - Many processes do this routinely.
    • Pressure - Common with wash down pumps.
    • Temperature - Typical for heat exchanger applications.
    • Flow - Probably the most common application.
  • Two or more pumps are running in parallel:
  • A larger pump is closing the discharge check valve of a smaller pump.
  • Only one pump is running and there's not enough head to stop it from running on either side of it's B.E.P.
  • The wrong size pump was originally installed. This is a common problem in new applications because each individual involved has added a safety margin to the original operating specifications. Every one involved in the pump selection has a real fear of buying a pump that's too small for the application. It's also a common practice to purchase a pump that's known to be too large for the application, because of anticipated future needs.
  • A storage tank is being filled from the bottom and the discharge head varies as the tank fills.
  • The pump motor speed is changing:
  • The packing was removed from a pump driven by an induction motor. These motors have a slippage from 2% to 5% depending upon their quality and load.
  • A mechanical seal was installed and the motor has speeded up changing the B.E.P..
  • A variable speed motor is being used in a system with a high static head. These motors work best where the head is mainly caused by the piping and valves, such as in a hot or chill water circulating system.
  • A higher rpm. pump has been substituted because of system demands, and the piping is offering too much resistance at this increased flow. Keep in mind that the pipe resistance varies by the square of the speed. In other words if you double the speed of the pump you get four times the resistance in the piping.
  • The system has been altered to accommodate a change in the process. Another storage tank may have been added, more valves, fitting or piping etc..

OPERATION PROBLEMS

  • A discharge valve is being closed to save amperage. This is a common practice for starting centrifugal pumps with lower "specific speed" impellers.
  • The product viscosity has changed
  • A different product is now going through the piping. This is a common problem in pipe line applications.
  • You are pumping a dilatant (Its viscosity increases with agitation).
  • If the pump was sized for a higher viscosity product and the temperature has increased, or the product viscosity lowers with agitation (thixotropic products do this) you can also move off of the best efficiency point towards the higher capacity side.
  • The specific gravity of the product has changed, causing a change in the rpm of the motor:
  • A different product is now in the system
  • The temperature of the product has changed. (Specific Gravity changes with temperature)

MAINTENANCE PROBLEMS

  • A discharge valve is being throttled.
    • A common practice in an attempt to stop cavitation. (The lower the capacity, the less N.P.S.H. Required)
    • To compensate for impeller wear or worn wear rings
    • To stop water hammer
  • The impeller has been trimmed.
  • The discharge of the pump is being restricted
  • A foreign object was left in the pipe the last time the system was opened, or maybe the swing check valve seat has come loose etc.
  • There is a buildup of material on the wall of the piping and fittings. Hard water, depositing a layer of calcium on everything is a good example. Many other fluids can also coat out on the pipe walls.
  • The pipe has collapsed (often not visible)
  • A truck ran over a piece of the pump piping.
  • A liner has come loose and is restricting the passage.
  • A frozen water jacket has collapsed the pipe.
  • A restricting fitting has been substituted or added to the system.
  • A reducer has been installed
  • A globe valve has been substituted for a gate valve. A globe valve can add another 50 to 100 foot of head depending on the flow and pipe size.
  • A gasket is protruding into the piping.

To size a pump properly, or to do effective trouble shooting it's important to know where the pump is running on its' curve. There are low cost chart recorders available to help you anticipate some of these problems and receive a clearer picture of what the system is actually doing. I strongly recommend the use of one of these recorders prior to specifying a replacement pump in an existing application.

This discussion was an attempt to show you how we sometimes make an error in our calculations because of lack of knowledge about varying discharge conditions. It should also demonstrate to you the importance of the L3/D4 relationship to compensate for some of these unforeseen variables.

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