piping are extremely important. The piping must be selected and installed, so pressure losses are minimized and sufficient liquid can flow into the pump when it is started and operated. Many net positive suction head (NPSH) problems can be directly attributed to improper suction piping systems.
Suction piping should be short in length, as direct as possible, and never smaller in diameter than the pump suction opening. If the suction pipe is short, the pipe diameter can be the same size as the suction opening. If longer suction pipe is required, pipes should be one or two sizes larger than the opening, depending on piping length.
Suction piping for horizontal double-suction pumps should not be installed with an elbow close to the suction flange of the pump. The only exception to this rule is when the elbow is in the vertical plane. A suction pipe of the same size as the suction nozzle approaching at any angle other than straight up or straight down has the elbow located ten pipe diameters from the suction flange of the pump. Vertically mounted pumps and other space limitations require special piping.
There is always an uneven turbulent flow around an elbow, and when it is in a position other than the vertical, it causes more liquid to enter one side of the impeller than the other (fig. 6-34). This results in high unequalled thrust loads that overheat the bearings and cause rapid wear in addition to affecting hydraulic performance.
When operating on a suction lift, you should ensure the suction pipe slopes upward to the pump nozzle. A horizontal suction line must have a gradual rise to the pump. Any high point in the pipe becomes filled with air and thus prevents proper operation of the pump. When reducing the piping to the suction opening diameter, use an eccentric reducer with the eccentric side down to avoid air pockets.
Figure 6-34. - Unbalanced loading of a double-suction impeller.
When operating on a suction lift, you should never use a straight taper reducer in a horizontal suction line, as it tends to form an air pocket in the top of the reducer and the pipe
To clean the pump liquid passage witnout dismantling the pump, bolt an increasing suction nozzle to the suction flange. If this is not done, a short section of pipe (Dutchman or spool piece), so designed that it can be readily dropped out of the line, can be installed adjacent to the suction flange. With this arrangement, any matter clogging the impeller is accessible by removing the nozzle (or pipe section).
DISCHARGE PIPING. - If the discharge piping is short, the pipe diameter can be the same size as the discharge opening. If the piping is long, the pipe diameter should be one or two sizes larger than the discharge opening. On long, horizontal runs, it is desirable to maintain as even a grade as possible. Avoid high spots, such as loops, that collect air and throttle the system or lead to erratic pumping.
VALVES IN PIPING. - When installing valves in the suction piping, observe the following precautions:
If the pump is operating under static suction lift conditions, a foot valve should be installed in the suction line to avoid the necessity of priming each time the pump is started. This valve should be of the flapper type, rather than the multiple spring type, and sized to avoid excessive friction in the suction line. (Under all other conditions, a check valve, if used, should be installed in the discharge line.)
When foot valves are used or where there are other possibilities of "water hammer," close the discharge valve slowly before shutting down the pump.
When two or more pumps are connected to the same suction line, install gate valves so any of the pumps can be isolated from the line. Gate valves should be installed on the suction side of all pumps with a positive pressure for maintenance purposes. Install gate valves with horizontal stems to avoid air pockets. Globe valves should not be used, particularly where NPSH is critical.
The pump must never be throttled by the use of a valve on the suction side of the pump. Suction valves. should be used only to isolate the pump for maintenance purposes and should always be installed in positions to avoid air pockets.Continue Reading