The sweet zone
Consider the following graph, Figure 9-14. Radial loading on the shaft rises if the pump is operated too far to the left or right of the best efficiency zone. Another interpretation of the same concept is to say that the maintenance and problems rise when the pump is operated away from its BEP. Many pumps have a rather narrow operational window. These pumps can be very efficient if they are correctly specified and operated. This is discussed completely in Chapters 7 and 8 Pump Curves and System...
Wear rings
Wear rings provide for a close running, renewable clearance, which reduces the amount of liquid leaking from the high pressure zones to the low pressure zones in the pump. They are commonly fitted in the pump casing and on the impeller Figure 16-1, next page . These wear rings are lubricated with the fluid being pumped. Eventually they will wear. Tolerances open and more liquid passes from the discharge end back to the suction end of the pump. The rate of wear is a function of the pumped...
Larry Bachus
Larry has almost 30-years experience in maintenance with industrial pumps. His areas of expertise include diagnosing pump problems and seal failures. Larry is highly regarded for his hands-on personalized consulting. He speaks fluent English and Spanish. He has taught pump and seal improvement courses all over the world. His investigations into pump failure have led to inventions, tools and devices used in the chemical process industry. He is an active member of American Society of Mechanical...
The Submergence Laws
Fluid Velocity in Suction Pipe in Feet per Second, g Fluid Velocity in Suction Pipe in Feet per Second, g Use a 'Y' branch and not a 'T' branch. This will reduce turbulence. Don't drain from the bottom of the tank Don't drain from the bottom of the tank Correct suction piping leading to the pump Figure 17-15 . You should respect 10 pipe diameters before the first elbow in the suction piping Figure 17-16 . Example If the pump has a 6 inch suction nozzle, you should have 60 inches of straight...
Alignment techniques
There are a variety of shaft alignment procedures. The configuration and size of the equipment determines the best alignment method. Generally the driver or motor should be aligned to the pump. The motor shaft centerline should be shorter and brought up to the pump shaft centerline with shims or spacers. The pump is generally fixed and attached to the suction and discharge piping, so it is almost impossible to move. The volute casing aids in supporting the piping, so it should be fixed to a...
Coupling alignment
Don't use a flexible coupling to compensate for misalignment between the pump and motor shafts. The purpose of the flexible coupling is to compensate for temperature changes and to permit some axial movement of the shafts without interference, while they transfer energy from the motor to the pump. There should be enough space between the coupling halves so that they don't touch should the motor shaft move forward toward the pump. This space should also consider movement due to wear in the pump...
Advantages of ORings
The majority of mechanical seal manufacturers make models that incorporate o-rings as secondary seals. These o-rings offer advantages over other forms of secondary elastomers. 1. The O-ring can flex and roll - There is no need for the shaft to slide and rub under the o-ring while under an axial load. 2. Availability - It's the only elastomeric seal form easily available in most cities and towns. 3. Material of construction - Almost all elastomeric compounds are available in the o-ring...
The lantern ring
The lantern ring performs three basie functions 1. To supply new lubricant to the packing rings, which is normally lost in operation. 2. To supply 'back pressure', which aids in impeding the entrance of abrasive and corrosive material into the stuffing box. Abrasives and corrosives will damage the shaft or sleeve, and disintegrate the packing. 3. To cool the packing and shaft and dissipate the heat generated between these members, which will increase the normal service life of the packing and...
Definition of NPSHr required
It is the energy in the liquid required to overcome the friction losses from the suction nozzle to the eve of the impeller without causing vaporization. It is a characteristic of the pump and is indicated on the pump's curve. It varies by design, size, and the operating conditions. It is determined by a lift test, producing a negative pressure in inches of mercury and converted into feet of required NPSH. An easy way to understand NPSHr is to call it the minimum suction pressure necessary to...