Most pump troubles should never happen - Visitors Contribution # 2

MOST PUMP TROUBLES SHOULD NEVER HAPPEN:

“GENERAL RULES OF THUMB”

Mark Brein

General Manager 

Penguin Pumps

• Size a pump to operate on the midpoint, plus or minus ¼, on its performance curve.  This simple rule has been around for many years, and it still holds true.

• Pump suction conditions:  

The piping on the suction side of the pump is much more important than the piping on the pump discharge. If any mistakes are made on the discharge side, they  can usually be compensated for by increasing the performance capability from the pump.  Problems on the suction side, however, can be the source of ongoing and expensive difficulties, which may never be traced back to that area.

READ MORE ...

• To maintain NPSHa > NPSHr ?.

      Apply pumps in:

1. Open systems under flooded conditions, 
2. Near sea level.
3. Operating temperatures lower than the vapor point of the liquid being pumped  

•     Reduce the friction losses.  

    Oversize one pipe diameter larger, or more.  Keep the   length as short as possible, < 10 ft preferably.  Keep suction velocities as low as possible, 5 ft/sec preferably, 8 ft/sec maximum.  Bigger means better for pump suction lines. 

·  No 90 degree elbows.

 The only thing worse than one 90 degree elbow is two 90 degree elbows.  There is always uneven flow in an elbow, and an elbow introduces uneven flow into the eye of the impeller, creates turbulence and air entrapment, inefficiency, and vibration.

   Use two 45 degree elbows instead of a 90 degree elbow. Also A well established and effective method of ensuring a laminar flow to the eye of the impeller is to provide the suction of the pump with a straight run of pipe in length equivalent to 5-10 times the diameter of the pipe.

·  Stop air or vapor from entering the suction line.  

   Primary sources of air: Existing air present, the liquid being pumped, and the mechanical equipment. 

   

   Any high stop in the suction line can become filled with air or vapor, which if transported into the impeller, will create an effect similar to cavitation, and with the same results.  Services which are particularly susceptible to this situation are those where the pumpage contains a significant amount of entrainment air or vapor, as well as those operating on a suction lift, where it can also cause the pump to lose its prime.  A concentric reducer can cause a similar effect.  The suction of the pump should be fitted with an eccentric reducer.

     If a pump is taking its suction from a sump or tank, the  formation of vortices can draw air into the suction line.  As the submergence over the suction pipe decreases, the tendency to develop a vortex increases. To prevent the formation of vortices, it is necessary to provide a certain minimum submergence varying essentially with the square of the velocity at the opening itself. This submergence is completely independent of the NPSHr by the pump. For example, suction velocity of 5 ft/sec would require a 3 ft  minimum submergence. The use of a bell-mouth connection and/or baffles can reduce the suction velocity by a factor of 4 or even more, thus permitting the pump to operate satisfactorily with less submergence. However, it is a good rule of thumb to have a minimum 2 ft submergence based on my experience. 

·  Correct piping alignment.  

  Piping flanges must be  accurately aligned before the bolts are tightened and all piping, valves and associated fittings should be independently supported so as to place no strain on the pump.  This is a common problem with plastic pumps, for they do not have the higher structural strength of most metallic pumps.  Pumps housings will start to lean to one side and soon thereafter the impeller breaks from hitting the pump housing inner wall and the entire pump is ruined and will require replacement. Use thermal expansion joints in high temperature applications.

     Piping design is one area where the basic principles involved are regularly ignored, resulting in hydraulic instabilities in the impeller, which translate into additional  shaft loading, higher vibration levels and premature failure of the seal or bearings. Because there are so many other reasons why pumps could vibrate, and why seals and bearings fail, the trouble is rarely traced to piping. Even when satisfactory pump operation is obtained that does not automatically make a questionable piping practice correct. It merely makes it lucky.

• Reverse rotation.  

The possibility that a 3-phase electric motor driven pump is rotating in the wrong direction should never happen if a simple precaution is taken during the installation of the pump as well as any time there arises the possibility that the electrical leads at the motor or at the switch gear have been disconnected.  Before the pump is ever started, it should be uncoupled from the motor, when is then gently “bumped” for a few resolutions to ascertain its direction of rotation. The odds of wiring a 3-phase motor correctly the first time is 50/50. One must ask themselves if they feel lucky today. One must remember, for example, that a 2-pole electric motor operated on 60 hz will turn 3,450 rpm, revolutions per minute, which is equal to 571/2 revolutions per second.  A plastic impeller spinning into a plastic suction cover inside a plastic pump will heat weld these parts together from heat friction in < 1 minute, and minutes later the motor winding is burnt out.  This is a common, costly mistake that should never be allowed to happen from neglect, laziness,  or ignorance.  In the real world today, reverse rotation issues are blamed on the pump 50% of the time in lieu of on the electrician. The reservoir the pump is to pump liquid from should have a volume three times the pumping flow rate. This simply means that you don’t try to use a 200 gpm pump to recirculate liquid in an open 50 gallon tank.