The most disturbing flow patterns to a pump are those that result from swirling liquid that has traversed several changes of direction in various planes. Liquid in the inlet pipe should approach the pump in a state of straight, steady flow.
When fittings such as "T" fittings and elbows (especially two elbows at right angles) are located too close to the pump inlet, a spinning action or "swirl" is induced. This swirl could adversely affect pump performance by reducing efficiency, head and net positive suction head (NPSH) available. It also could generate noise, vibration and damage in high-suction-energy pumps.
It is recommended that a straight, uninterrupted section of pipe be installed between the pump and the nearest fitting. This should follow the minimum straight pipe length guidelines listed in the ISO/HI draft standard, which range from:
One to eight pipe diameters (for low suction energy/low specific speeds).
Three to 16 pipe diameters (for high suction energy/high specific speeds).
The specific straight pipe length recommendation depends on the type of fitting(s) and pump. Generally, high-suction-energy pumps have suction nozzles greater than 10 inches (in.) at 1,800 revolutions per minute (rpm), and greater than 6 in. at 3,600 rpm. High specific speed starts above a value of 3,500. If the minimum recommended pipe lengths cannot be provided, flow-straightening devices should be considered.
In addition to the minimum required lengths of straight suction pipe, it is equally important that the NPSH margin ratio (net positive suction head available/net positive suction head required [NPSHA/NPSHR]) guidelines listed in the HI standard on NPSH margin (ANSI/HI 9.6.1) be followed.
These NPSH margin ratio guidelines are dependent on the application and range from:
1.1 to 1.3 for low-suction-energy pumps.
1.3 to 2.5 for high-suction-energy pumps.
It is especially critical for plants to follow these NPSH margin recommendations for high-suction-energy pumps. By doing so, they avoid excessive pump noise, vibration and/or damage, particularly in the region of suction recirculation.
Discharge piping. Pipe fittings mounted close to the outlet (discharge) flange normally will have a minimal effect on the performance or reliability of low-energy rotodynamic pumps. On the other hand, high-energy pumps can be sensitive to flow-disturbing fittings mounted close to the pump outlet flange. These fittings could result in increased noise, vibration and hydraulic loads within the pump.
According to HI, pumps with specific speed values below 1,300 that generate more than 900 feet of head per stage are considered to be high-energy pumps. The differential head value for high energy drops from 900 feet at a specific speed of 1,300 to 200 feet at a specific speed of 2,700.
It is recommended that high-energy pumps meet the minimum straight discharge pipe length requirements in the ISO/HI pump piping draft standard (one to four pipe diameters). The specific straight pipe length recommendation depends on the type of fitting(s) downstream of the pump.
The ISO/HI draft pump piping flowchart for hydraulic considerations is presented as Fig. 2. Although the flowchart is complex, it is critical that plants follow its methodology to ensure reliable pump performance, especially for high-suction-energy and high-energy (discharge) pumps. Again, each of the items in this flowchart represents a section in the ISO/HI draft standard.
Figure 2. Flowchart for use of ISO/HI Pump
Piping Standard (Hydraulic Considerations)
To ensure reliable pump performance, particularly for high-suction-energy and high-energy pumps, chemical plants should follow the methodology shown here.
Although it often is not given the attention it deserves, the design of the system piping attached to a centrifugal pump can greatly influence the pump's performance and reliability. However, most of the published literature currently available on this subject provides only broad general guidelines or is limited in scope. The new pump piping standard being developed by the joint ISO/HI Pump Piping Work Group should go a long way toward filling this critical void. The document is in the balloting stage. CP
Budris is a consulting engineer in Geneva, N.Y., specializing in the selection, application and troubleshooting of pumps and pumping systems. Most recently, he served as director of product development for the Industrial Pump Group of ITT Industries, Seneca Falls, N.Y. He is chair of the ISO/HI Pump Piping Work Group. Contact Budris at firstname.lastname@example.org.