Selecting pumps for chemical pumping can be challenging. The pumped medium's characteristics can lead users to a certain pump style that best fits a given application. This article examines certain issues for selecting pumps to be used in chemical pumping.
With flocculants, polymers or reagents that have long polymer chains, certain types of pumps will not be a good fit. For example, a high-sheer pump will destroy the polymer chains. The net result will be an increased consumption of flocculants or polymers. This does not mean that the high-sheer pump will perform poorly, but it will damage the flocculants and cause the user to use significantly more flocculants or polymers. A low-sheer pump will not destroy the polymer chains and will allow the reagents to reach maximum effectiveness, resulting in less overall consumption.
Every process has variables that end users must consider to determine the most ideal pumping technology for their applications. A highly corrosive medium may require exotic alloys to survive chemical attack; however, certain types of pumps utilize Teflon or rubber components that may eliminate the need for exotic alloy housings and internal components such as titanium, Hastelloy C, Monel or others. The rubber or Teflon components are much less expensive than building complete pumps out of titanium or other exotic alloys. Also, diaphragm pumps and peristaltic pumps may be good alternatives to these other pump types.
Peristaltic pumps utilize rubber hoses and smaller diameter polymeric tubes that have extreme chemical resistance. The advantage is the pumps' hosing does not have to be made of exotic alloys. The rubber or polymeric hoses and tubes isolate the pump casing from the medium that is passing through the pump. These types of pumps are typically equipped with hose and tube leak-detection devices that alert the user when the internal elements have failed. The pump is then shut down and can be cleaned to prevent corrosion from taking place in the pump housing.
Table 1 shows a list of chemicals that may be pumped by pumps incorporating rubber as the wear components within the equipment.
Please note that high temperatures and high concentrations of the chemicals shown in Table 1 may cause some of the listed materials to become incorrect selections.
Abrasion is another example of how certain pumps built with metal internals in direct contact with the medium may fail. There are two ways to combat severe abrasion: Make the internal components harder or make the internal wear components softer.
Certain styles of centrifugal pumps have been successful with rubber coating impellers and all internal surfaces with rubber linings and coatings. This practice has worked well for many years in the mining industry. Also, peristaltic pumps, some rotary lobe pumps and progressive cavity pumps rely on rubber internal components as the wearing part.
Variables such as driving head or NPSHA may lead the consumer into different styles of pumps. Positive suction head is not typically a problem for most pumps. When driving head is weak or even negative, the style of pump may need to be varied. For example, certain pump styles may destroy themselves in applications where insufficient head is available. An example is a progressive cavity pump that runs dry and destroys the rotor and stator in minutes, if not seconds. Other pump styles such as peristaltic pumps have the ability to run dry at any time and can even perform suction lift duties when negative head exists.
When high pressure is required, certain pump styles such as multi-stage progressive cavity pumps, gear pumps and piston pumps may be the best fit. Other styles of pumps such as diaphragm pumps and peristaltic pumps will be limited to relatively low-pressure applications less than 240 pounds per square inch gauge (psig). Temperatures above 300 degrees F may completely eliminate pumps utilizing rubber components.
One example of a challenging medium involves pumping lime slurry or milk of lime. Milk of lime is a suspension of calcium hydroxide in water, and it is an effective alkali. Uses for industrial lime slurry are for municipal water treatment, pH adjustment in chemical production, metal precipitation, flue gas desulfurization and odor control. Lime slurry is typically not pumped at high pressures because of its abrasive nature. In many cases, the lime is pumped at approximately 30 to 40 percent solids and pressures of 150 psig or less. Often it is pumped at less than 75 psig.
Lime is a suspended solid that is jagged and abrasive, and it can scale internal components of pumps and piping systems. Lime is not the most abrasive application for a pump, but it ranks with many formidable opponents. The typical pressure at which it is pumped is manageable by many pumps, but the abrasiveness and the scale aspect can cause problems.
Picking a Peristaltic Pump
If all the variables have been evaluated and the peristaltic pump has been determined as the best alternative for pumping lime slurry, it is time to begin research on the types of peristaltic pumps that are available and decide which type is best for the application.
Peristaltic pumps have been in existence for about 85 years. Rubber manufacturing and durability have made quantum leaps in those 85 years. Even just 30 years ago, automobile tires lasted up to 20,000 miles. In 2015, automobile tires are available with 80,000-mile guarantees. This surge in rubber technology has allowed peristaltic pumps to move from laboratory pumps to main production pumps based on their reliability. But the peristaltic pump operating principle limited the production availability until about 15 years ago.
In the first 70 years, peristaltic pumps were manufactured with a fixed rotation point in the center of the pump hosing; two metal shoes rotate on fixed arms to compress the rubber hose. This design generates a significant amount of heat that, over time, damages the rubber hose and limits its longevity. To help combat the heat, the pump is filled with glycerin used for both lubrication and heat dissipation. Large amounts of this glycerin help transfer the heat to the pump casing. When the hose in this style of peristaltic pump begins to fail, the acid or slurry starts to mix with the large amount of glycerin. This is a hassle because a large amount of contaminated glycerin needs to be disposed, and the glycerin typically costs more than $100 per gallon.
A 2.5-inch or 65-mm shoe design peristaltic pump may require 5 gallons or more of this glycerin. So with every hose change the consumer must dispose of more than $500 of useless glycerin. A chemical plant that has 10 or 15 shoe peristaltic pumps may consume more than $30,000 of glycerin annually.
Approximately 15 years ago, a newly designed peristaltic pump entered the market. This new design incorporates a roller fixed on a cam that rolls over the rubber hose rather than grinding against it. In the shoe design peristaltic pumps, there are two fixed points where the shoes compress the rubber hose twice per revolution. The newly designed peristaltic pumps compress the rubber hose once every revolution.
A common misconception with peristaltic pumps is that the medium determines how long the rubber hose will last. Of course, the medium has an impact, but the primary determining factor in how long a hose lasts in a peristaltic pump is how many times the rubber hose is compressed. So a pump that only compresses the rubber hose once per revolution is going to have hose life that is twice that of a pump that compresses the hose twice per revolution.
The second advantage of rolling rotor design is that it requires only a fraction of glycerin that a shoe design peristaltic pump requires.
The reason the rolling design does not require massive amounts of glycerin is that the rolling design does not generate the friction and heat. For example, your car tires drive over the road for hundreds of miles and may get warm, but they do not get extremely hot and require a bath in glycerin to cool them down. The single rolling design peristaltic pump only requires a half-gallon of glycerin for a 2.5-inch or 65-mm pump. This glycerin is required for light lubrication of the rubber hose only and not for the dissipation of heat.
This is important because the shoe designs have significant limitations on rotation speed, but the rolling designs do not face the same limitations. Shoe designs will literally burn up the rubber hose if run at a high rpm continuously. Also, the continuous friction caused by two rubbing shoes can have a detrimental effect on a hose's lifetime even in a pump that is rotating at a slow rpm.
Rolling designed peristaltic pumps can produce hose lifetimes that are three to five times longer than peristaltic pumps utilizing shoe designs.