Pump and system specification practices that cause seal and bearing problems


PUMP AND SYSTEM SPECIFICATION PRACTICES THAT CAUSE SEAL AND BEARING PROBLEMS GT002

Purchasing well-designed hardware does not bring automatic trouble free performance along with it. The very best equipment will cause problems if it was not designed for your particular application. Here are a few of the more common selection problems we find with centrifugal pumps:

  • The pump was designed for packing not a mechanical seal. The packing acts as part of the bearing system and this feature was lost at the conversion. To get the best seal life you should modify the centrifugal pump
  • The shaft  L3/D4  number is too high, causing shaft deflection problems.
  • Many pumps are purchased oversize to compensate for safety factors added during the selection process
  • The pump has to operate in a large window and it does not have a double volute to prevent shaft deflection. The pump has too narrow an operating window. It does not satisfy all of your  operating requirements.
  • The pump suction specific speed number is greater than 8500 and the pump is cavitating.
  • The impeller has the wrong specific speed number and it is not producing the proper shaped curve.
  • The pump was selected for efficiency not reliability. These concepts are often in conflict.
  • A high speed pump was selected as a low cost solution to a slurry problem. The wear will increase by the cube of the speed. In other words if you double the speed of the pump you’ll get about eight times the wear of the components.
  • The pump components are manufactured from unknown materials. You cannot troubleshoot a material if you don’t know what it is. Many manufacturers try to keep their materials a secret.
  • discharge recirculation line is heating the incoming fluid.
  • The pump lacks a centerline design feature and itis running at a temperature over 200°F (100°C).
  • The open impeller has to be adjusted from the rear and it is interfering with the proper mechanical seal setting.
  • There is no visible oil level indication. You need a sight glass or a dip stick to learn if you have the proper level.
  • The radial bearing is being retained by a simple snap ring.
  • There is no facility for adjusting the closed impeller wear rings. They have to be replaced.
  • If you are specifying that the pump must meet a standard, you might be disappointed with your product. The API standard used by refineries has some flaws
  • Be sure to specify, or have your supplier identify the pump and seal materials, There are many types of stainless steels and seal carbon faces. Make sure you know what you hav,e or troubleshooting will be very difficult.
  • Purchasing the same size pump as the one that came out of the application. That’s OK If the old pump was the correct size, but the odds are that it was too large because of the safety factors that were added at the time of purchase. This will cause the pump to run off of its best efficiency point (BEP) and you’ll spend a lot of production money for the additional power that’s needed to run against a throttled discharge valve or orifice installed in the discharge piping.
  • The pump has the wrong impeller for the application.
  • Making a buying decision based on efficiency and believing it somehow relate to quality. Efficiency is always gained at the expense of maintenance. Efficiency means maintaining tight tolerances and smooth passages that will eliminate reliable double volute designs and keep the maintenance department busy adjusting tight tolerances to maintain the efficiency that you paid for.
  • Series and parallel installation problems.
    • We often find pumps installed in parallel, but no one knows it because the second pump was installed at a much later date and no one has bothered to trace the piping. Pumps in parallel require that they have the same diameter impeller and that they run at the same speed or the larger pump will throttle the smaller one causing it to run off the best efficiency point, deflecting the shaft. The capacity should be considered if the higher capacity pump might exceed the net positive suction head available (NPSHA).
    • When pumps are installed in series the impellers must be the same width and they must run at the same speed. If not, the higher capacity pump will either cavitate because the smaller capacity pump can not feed liquid at the proper volume, or it will run throttled if it is feeding the smaller pump. In either case the larger of the two pumps will be adversely affected.
  • Purchasing a larger pump because it will be needed in the future is a common mistake. This will raise the operating cost to unacceptable levels (Power = head x capacity) as the pump is run against a throttled discharge valve. This inefficient use of power will translate to a higher heat environment for the seal, along with all of the problems associated with shaft deflection.
  • Using a variable speed motor to compensate for a pump curve that is not flat enough for the application. Many boiler feed pumps require a flat curve so that the pump can put out varying capacities at a constant boiler pressure (head). We see this same need if we are pumping a varying amount of liquid to a very high constant height.
    • Varying the speed of a pump is similar to changing the diameter of the impeller. If you look at a typical pump curve you’ll observe that the best efficiency point (BEP) comes down with impeller size to form an angle with the base line (capacity line) of the graph. This means that if you vary the speed of the impeller, the pump always runs off the best efficiency point (BEP). The exceptions are:
      • The point where the system curve intersects the pump curve.
      • Any time the pre-dominate head is system or friction head. You find this type of head in un-loading pumps, and circulating systems.
  • Installing double-ended pumps in a vertical position to save floor space makes seal replacement a nightmare, unless you are using split or cartridge designs.
  • Specifying a desired capacity without knowing the true system head. You can’t guess with this one. Some one has to make the calculations and “walk the system”. The present pump is not a reliable guide because we seldom know where it is pumping on its curve. Chart recorders installed on both the suction and discharge side of the pump can give a more accurate reading of the head if they are left on long enough to record the differences in flow. The trouble with this method is that the recorders will also record a false head caused by a throttled valve, an orifice, or any other restriction that might be present in the piping.
  • Requesting too low a net positive suction head required (NPSHR) will cause you to end up with a suction specific speed. cavitation problem.
  • Failure to request a center line design when pumping temperature exceeds 200°F (100°C) will cause pipe strain that will translate to wear ring damage and excessive mechanical seal movement.
  • The use of inline pumps to save floor space. Many of these designs are close coupled with the motor bearings carrying the radial and thrust loads. Because their L3/D4 numbers are usually very high, the wear rings act as “steady bearings” after the pump is converted to a mechanical seal. The pump should have been designed with a separate bearing case and a C or D frame adapter installed to connect a motor to the bearing case.
  • Radial bearings being retained by a simple snap ring is a design problem found in many low cost pumps. Beyond 65% of its rated efficiency most centrifugal pumps thrust towards the pump volute. The thin snap ring has to absorb all of this axial thrust and most of them cannot do it very well. Intermittent service pumps experience trouble with the snap ring wearing the snap ring groove, allowing excessive axial movement of the bearing and impeller.
  • The mechanical seal has been installed in a stuffing box that is too narrow to allow free seal movement. These original equipment stuffing boxes were designed for small cross-section packing. If a mechanical seal was specified, the pump back plate should have been manufactured with a large diameter seal chamber. In most cases the stuffing box recirculation line should be installed from the bottom of this large seal chamber to the suction side of the pump, or a low pressure point in the system. This is called “suction recirculation.” There are some exceptions to this recommendation:
    • If you’re pumping at, or close to the fluid’s vapor point, lowering the pressure could cause the fluid to flash
    • If the entrained solids have a low specific gravity. Centrifugal force will through the clean liquid outward leaving the solids in the stuffing box
    • If you are using a Durco (Flowserve) pump that adjusts the impeller to the back plate. The stuffing box is at or, close to suction pressure.
    • If you are using a double suction pump where the stuffing boxes are at suction pressure.
  • High temperature pump applications have several special needs:
    • jacketed stuffing box that isolates the pumpage from the stuffing box contents by a carbon bushing to retard heat transfer.
    • centerline design to compensate for thermal expansion.
    • cartridge seal design has many advantages including, allowing open impeller adjustment after the pump has come up to operating temperature.
    • A stainless steel shaft to retard heat transfer to the bearings.
    • A method of cooling the bearing oil, but never the bearings.
    • coupling that will compensate for axial expansion.
    • A “C” or “D” frame adapter to compensate for motor to driver misalignment.
  • Check your piping layout against some recommendations
  • If you’re going to be specifying pumps, you might as well use the best technology

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  • On February 18, 2018