I was surprised: a centrifugal pump actually pumped a 300-cP detergent. A careful examination later showed the pump was near its thermal minimum-flow limit. This means it couldn’t be expected to reliably pump continuously. Nonetheless, another presumption of pump lore bites the dust.
So, let’s look at pumping of viscous liquids — and limit our discussion to pumps for regular processing, not metering or sampling.
Viscosity in pumping is a great bugaboo of our business. Most pump manufacturers will tell you not to use a centrifugal pump for liquids with viscosities beyond 100 cP. And, despite what I witnessed, let’s agree that a centrifugal pump usually isn’t a good choice for viscosities above 100 cP, let alone 300 cP. Such services generally call for some type of positive displacement pump (see: “Consider Positive Displacement Pumps"). These pumps are self-priming.
A good choice often is the lobe pump (see: “Keep Lobes in Mind”). It’s a rotary pump good up to its maximum case pressure; it can handle a maximum of perhaps 2,000 cP (which is about the viscosity of honey at 25°C).
If you need to move a really high viscosity material, you must avoid pumps that require high pressure. Keep in mind the operating limit on 150-psi flanges is only 276 psig up to 100°F; clamps may have a lower rating depending on the manufacturer.
One popular choice is the progressive cavity pump, which is limited to below one million cP. Other potential options include the piston pump or the gear pump (see: “Get Up to Speed on Gear Pumps”). Both the progressive cavity and piston pump require a much greater suction pressure than that needed for lobe, gear or centrifugal pumps. Piston pumps and rotary pumps are very sensitive to abrasives while progressive cavity pumps aren’t.
What if you need a pump for a wide variety of products, say, with viscosities ranging from 4 cP up to 2,000 cP? Believe it or not, a progressive cavity pump is a good choice. It can pump almost anything. If this pump type has a limitation, it relates to handling lower flow rates. In addition, as I recall with TiO2 slurries, seals and bearings sometimes are a problem with progressive cavity pumps.
A lobe pump usually has a lower limit of 100 cP but might work if you go with a smaller model with tighter space between the lobes and case wall, and operate at very high speed to compensate for slippage. Indeed, a lobe pump might be preferable because it is more compact and perhaps more reliable than a progressive cavity pump.
There really isn’t a good way to know whether a lobe pump will work with 4-cP liquid. The best option may be to increase the motor size to raise the flow rate to 10–15% above the design rate; this may compensate for the slippage expected at low viscosity. Hopefully, testing provides a happy surprise.
Now, let’s consider viscosity itself. Too often it’s difficult to find reliable viscosity data: safety data sheets frequently don’t include this valuable information. If you’re not afraid to get your hands dirty, consider measuring viscosity yourself. While spindle-type viscometers run $2,000–$3,000 and require annual calibration, a set of Zahn cups costs less than $600 and easily fits in a suitcase. You only may need a single Zahn cup, which is a bargain at about $135. Measuring with a Zahn cup requires a 500-ml beaker, 400 ml of sample, a stopwatch as well as a scale and a 100-ml beaker for estimating density. Adding a hook above the beaker will allow you to free your hand during the test. Zahn cups measure viscosity in cSt: cP = specific gravity × cSt. Run the test at 25°C. Take measurements at other temperatures to enable you to develop viscosity/temperature correlations. For additional information, go to: http://goo.gl/HUiXZ0 and http://goo.gl/ymFMte.
DIRK WILLARD is a Chemical Processing contributing editor. He recently won recognition for his Field Notes column from the ASBPE. Chemical Processing is proud to have him on board. You can e-mail him at email@example.com