QUESTION FROM MAY'S CHEMICAL PROCESSING
Toluene is used in manufacturing paint. Fresh solvent is pumped from a tank farm (Figure 1) to the color tanks. Tanks 1, 2, and 5 perform without a problem but tanks 3 and 4 are a problem with both pump 1 and 2. Cavitation is especially bad during the summer months; the tanks and pumps are outside. Pump 2 is the most troublesome. Seals are replaced twice as often with pump 2 as for pump 1. The pump inlet flange is directly connected to the concentric reducer for pump 2 but there is a 10-diameter spacer between the reducer and inlet flange for pump 1. Both pumps were installed with concentric reducers instead of eccentric reducers to save delivery time for fittings. Any suggestions on what can be done to improve the operation?
Probe all the problems
Pump problems are an accumulation of errors. Not a single one is directly responsible but acting together they are causing you trouble. You have long horizontal runs of pipe, concentric reducers, 90° elbows for the splits between the pumps, no straight pipe for one of the pumps, etc. Trouble begins when you cut corners. A popular one seen is tank inlet pipes terminating above the fluid level. The fluid then cascades into the tank picking up air (or in a padded tank, nitrogen). You can get around this if your tank retention time is long enough, but in the theme of cost reducing and cutting corners again, retention time these days is a minimum. The end result is entrained air. The last thing you want to do is give the air somewhere to gather. Once it starts to collect, it will eventually choke off the flow. A concentric reducer wont help here. If the eccentric reducer is installed upside down with the flat side down, it will make matters worse. Choked flow happens occasionally on end-suction pumps where the air collects at the eye of the impeller. In this case, you can burp the pump by shutting it off. The air will pass through the pump. It can then be re-started for a period of time until the air chokes the flow off again.
Now, for this particular scenario, lets assume this is not the case. However, youre pumping a highly volatile liquid. Youre heating it up in the summer. Plus, with the concentric reducer, youre now increasing the speed of the fluid, and thanks to Bernoulli, lowering the pressure. I suspect, and will assume, the toluene is flashing (creating its own vapor) in the reducer. The worst position is at the end of the reducer. No. 2 is taking this directly into the pump. No. 1 has a run of pipe to straighten the flow out (at least 10 pipe diameters). To fix the problem, you have several choices:
- Redo the piping: move the reducer away from the pump and change to an eccentric reducer (install flat side up) and for No. 2, pull its stream off at a 45° angle;
- Increase level in supply tank to increase suction pressure/NPSHA;
- Increase the pad pressure in the tank, thus increasing suction pressure/NPSHA (This could be negligible for atmospheric tanks.);
- Put the piping in the shade. Build an awning or cover.
Richard A. Nardone, product manager
ITT Industries - Goulds Pumps, Seneca Falls, N.Y.
Experiment to find the root cause
Problems with cavitation and lack of seal longevity are often due to how hard the pump has to work to get primed. The less work a pump must do, the longer the seals will last. In high pressure liquid chromatography, we elevate the solvent reservoirs so static head helps achieve prime. Since these large tanks cannot be elevated, perhaps venting will help? (Or, increase the pad pressure on the tank.) Another solution might be to change the diameter of the suction pipe, leaving more energy for the pump. On the discharge, increasing the outlet diameter may help. Another possible cause of seal problems might be seal materials talk to the seal vendor to confirm compatibility. Pump speed may also cause cavitation. A high RPM may cause problems with both pumps but be more apparent with pump 2. Cavitation causes the pumps to run dry, shortening the seal life. Problems with pump 2 may give some insight into ways to improve the performance of both pumps. I can relate this problem to one I had with cooling pumps in a Ford V8 Flat-head hot rod engine. When I put metal discs with holes in the discharge line, the water pumps ran cool without the discs, the pumps cavitated. Another thought is: could toluene viscosity be a factor? Viscosity decreases during the summer. This could cause the pumps to draw in air. With a few experiments, the pump performance could improve. (Look at the impeller style and pump internals as well as piping.)
Richard Ashley, process engineer
Barr Labs, Inc., Pomona, N.Y.
Install a Cheng
A Cheng imparts a spin to the flow which counteracts that imparted by the pump/impeller. It is doing work, so there is probably some pressure drop associated with it check the effect on NPSHA. I think minimal. If the concentric reducer is working well for pump 1, why is replacing the reducer in pump 2 so effective? I assume we were looking for a cost-effective alternative to re-piping. Alternatively, replacing the seal, with another type of seal, might improve seal life but not eliminate the cause of the short life or alleviate the cavitation. So...what goes next, the pump casing or impeller from cavitation erosion? Do the bearings fail because of vibration? I would caution against ignoring the noise. Another thought is concern for toluenes low dielectric constant (2.4 compared to 80.4 for water) it has a low conductivity. Ive investigated three accidents (one with a fatality) involving toluene transfer operations. Pumping toluene generates a streaming current of high voltages even when piping and equipment are properly grounded.