Robert S. Bryant, manufacturing engineer
Valent USA Corp., Walnut Creek, Calif.
Consider a flow conditioner
It seems the only significant difference between the pump with repeated maintenance issues and the stable pump is the lack of upstream piping. Most pumps require between three and 10 pipe diameters of unobstructed straight run upstream from their inlets (More may be needed depending on the number of bends.) This helps present a symmetric, swirl free and equally distributed velocity profile at the pumps impeller to ensure efficient operation and avoid premature wear. Flow through an elbow creates an undesired asymmetrical, uneven velocity profile with swirl entering the pump which results in cavitation, noise and excessive maintenance. A proven industry solution is a flow conditioner, which eliminates these flow disturbances by reshaping the velocity profile into a symmetric, equally distributed, swirl-free velocity profile. Installation is as simple as swapping out the existing upstream elbow and replacing a flow conditioner.
Don Lundberg, principal engineer
Vortab Company, San Marcos, Calif.
Increase pump 2s straight-pipe and reduce pipe losses
These pumps are experiencing cavitation problems. In the summertime, the liquid in the tanks heats up, increasing solvent vapor pressure, which lowers pump net positive suction head available (NPSHA). Pumps have a minimum required NPSH (NPSHR) to prevent damage to pumps by vaporization; this leads to bubbles forming, which collapse in the bowl of the pump under the pressure of the impeller causing damage. Typically, I would recommend 10 pipe diameters of straight suction piping (pump 2 does not have 10 diameters.) and eccentric reducers, but you may be okay with the concentric, since you are not having problems with tanks 1, 2, and 5. (The type of reducer is less critical than the layout of the suction piping.) Tanks 3 and 4 are piped together, before tying into the common header, which causes additional friction loss, decreasing the NPSHA. The additional elbow and tee (through branch) add the equivalent friction loss of (90 x pipe ID) of additional feet of piping. I suggest the following options for evaluation:
- modify the pump suction piping for tanks 3 and 4 to tie directly to the pump suction manifold, as with the other tanks;
- develop an NPSH calculation for the pump suction, with the tank nearly empty, in the summer (highest tank temperature, not ambient, and highest vapor pressure), based on the longest equivalent length of piping from the tank to the pump then, determine how close the NPSHA is to the NPSHR for the pump. Pump suppliers often recommend the available NPSH be at least 2 ft. higher than the required NPSH. (The ratio of NPSHA over NPSHR should be at least 1.3; for some situations, such as high vapor pressure or temperature, the ratio should be more like 2 (ANSI/HI 9.6.1-1998).)
Depending on the results of this calculation, consider additional modifications to increase the NPSHA such as:
- installing larger suction piping to decrease the friction loss;
- modifying suction piping fittings, valves, etc. to decrease the friction loss; piping changes may be difficult, requiring pump relocation;
- raising the tanks to increase static head (This may be difficult, because of connected piping, wiring, etc.);
- operating the tanks at a higher level of toluene in the tanks to increase static head;
- cooling the toluene in the tanks (This may be costly.);
- operating the tanks under pressure (This may be costly and tanks may not be rated for pressure. (An N2-pad must be considered tanks are padded to reduce vapor losses.);
- possibly modifying suction piping to pump 2 to provide 10 pipe diameters of straight piping before the pump suction, but this may require relocating the pump;
- possibly replacing the concentric reducers with eccentric reducers. Generally, eccentric reducers should be installed with the flat side up.
Tonya E. Wilson, PE, project/process engineer
Lauren Engineers & Constructors, Inc., Knoxville, Tenn.
Make corrections in suction pipe
First I would operate from the fullest tank or operate from two, or more tanks simultaneously, assuming that they are identical contents. Next, I would rerun the suction line to pump 2 straight to the header using an eccentric reducer and ensure that there are 10-diameters of straight pipe at the pump 2 suction. The flat side of the eccentric reducer should be on top for this application. Consider changing pump 1 to eccentric reducer. Replace some of the suction piping to reduce friction loss. Tanks 1, 2, and 5 have only two elbows before pump 1. Tanks 3 and 4 have four elbows. Pump 2 adds an additional elbow. Rerun pipe from tanks 3 and 4 straight into the header, or increase the common line size. Verify that any valves are not a reduced port variety.
Rich Reiter, process engineer
Hercules Incorporated. Brunswick Ga.
Reduce suction pressure (losses)
I would suspect a few improvements could enhance the system and minimize the pump seal losses and cavitation. Connect suction pipes for tanks 3 and 4 directly to the main suction header as for tanks 1, 2, and 5. Similarly, it appears the suction piping to pump 2 affects the performance and seal reliability. Reduce pressure drop in the suction piping. I would install a 45° y at the main suction header, where it splits to both pumps, to provide a branch to both pumps. Another idea would be to replace 90° elbows with 45° elbows at the suction to both pumps. I would install the 10-diameter long spacer and an eccentric reducer to each pump head suction connection. Lastly, consider insulating and possibly cooling (during summer periods) the suction line and pump head for added insurance.