Peristaltic hose and tube pumps add cost savings, versatility and maintenance ease for Nashua treatment facility.
Nashua, N.H., is one of many U.S. cities and municipalities selecting peristaltic pumps over other technologies. A wet weather facility in Nashua, N.H., recently purchased two peristaltic dosing hose pumps and four peristaltic metering tube pumps. This wastewater treatment facility will use the dosing pumps for the control of sodium hypochlorite for disinfection.
These pumps were ordered with a special Halar coating for protection against the aggressive sodium hypochlorite in the unlikely occurrence of a hose failure. The pumps were also equipped with hose leak detection to shut the pump down in the event of a hose failure and were packaged with an integral, variable speed motor with 4 to 20 mA input for control. The tube pumps will be used to pump sodium bisulfite. These pumps are highly sophisticated and fully equipped metering tube pumps.
The Difference Between Hose and Tube Pumps
You may wonder what the distinction is regarding tube and hose pumps. Typically hose pumps have a large diameter. They use a reinforced rubber hose, and they also have a glycerine bath for lubrication. In shoe designs, the lubrication is used for heat dissipation.
Tube pumps usually have a smaller diameter, and they use tubes that are extruded from materials such as Norprene, Tygothane or other engineered materials. Tube pumps do not have a glycerine bath.
A large majority of simple tube pumps are limited to 30 or 60 psi pressures. More advanced tube pumps are capable of pressures up to 125 psi and have sophisticated functions based on electronic features. Most hose pumps, on the other hand, can produce up to 150 psi pressures as standard and are capable of different levels of control capabilities.
Only One Compression Per Revolution
Roller tube and hose pumps are unique. They incorporate a distinctive rolling compression of the hose based on an eccentric cammed shaft that uses a roller to compress the hose. This roller compresses the hose only once for every 360-degree revolution. Many hose pumps use two or more shoes to compress the hose every 360-degree revolution. Therefore, the hose is compressed two or three times more frequently in normal operation. Since the number of compressions is the most important determining factor in how long the hose will last, this feature plays an important role.
Since a roller is incorporated in these hose pumps, virtually no friction and heat build up. Friction and heat build-up are typically associated with the sliding shoe style hose pumps. The frictional heat drastically affects the performance of sliding shoe style devices as it limits the pumping volume and the continuous operation of that kind of hose pumps. Any heat creation may be harmful for pumping mediums that are temperature sensitive. For example, sodium hypochlorite, widely used in water treatment industry, easily crystallizes when exposed to heat, creating severe operational problems. Roller hoses typically last three to five times longer than those of other hose pumps, which equates to considerable savings over the life cycle of the pump.
Roller metering peristaltic tube pumps have some impressive features, such as:
- 125 psi pressure capability
- Patented tube leak detection
- IP 66 enclosure
- LCD display with touch pad control
- 4 to 20 mA control and feedback in the standard product
- Multilingual LCD display in English, German, French and Spanish
- Auto priming function
Mechanical Diaphragm Metering Pumps Versus Peristaltic Pumps
In Nashua New Hampshire's equipment selection process, both mechanical diaphragm metering pumps and peristaltic pumps were considered for the application. The original specification called for diaphragm metering pumps for the sodium bisulfite and hose pumps for the sodium hypochlorite.
Both types of technology have advantages and disadvantages. Electrical diaphragm metering pumps are more energy efficient than tube or hose pumps. For instance, in a diaphragm pump the outward stroke requires a significant amount of energy. However, the return stroke has virtually no energy requirement. For tube and hose pumps, the energy requirement is constant. Also, note that electric diaphragm pumps were mentioned above and not pneumatic diaphragm metering pumps. Pneumatic diaphragm pumps can be energy inefficient.
|Peristaltic dosing pump|
Diaphragm pumps in general require a very clean pumped medium. Diaphragm pumps have check valves. These check valves may become clogged or fail in use with dirty or solid ridden liquids. Also if these check valves become clogged, then the pump is likely to lose its prime and also lose its ability to meter accurately. When this occurs, the check valves will need to be removed, and maintenance will need to be performed. If the liquid is always full of solids, then this can be a never ending problem when diaphragm pumps are used.
Diaphragm pumps are easy to prime against low back pressure, but in some cases, they may have difficulty creating a prime against high back pressure. Some advanced electric diaphragm pumps also have adjustable stroke capabilities. If the diaphragm pump does lose its prime and the end user tries to regain the prime while the diaphragm pump has a shortened stroke length, then he/she may have great difficulty priming the pump. A longer stroke will be the most successful in regaining the prime.
Roller peristaltic pumps, tube and hose, offer stable flow and metering capabilities under diverse operating conditions. The output flow of peristaltic pumps is unaffected by the variation of discharge line pressure. Peristaltic pumps can also handle severe variation in suction line conditions in regard to suction lift and viscosity. However for the utmost accuracy in volume-based metering application on site, calibration is recommended.
Hose and tube pumps, on the other hand, are slightly less energy efficient than electric diaphragm pumps. However, they are much more user friendly. Peristaltic pumps have less limitations or potential problems than diaphragm pumps. They are excellent for both clean, high solids slurries and highly viscous materials. Peristaltic pumps will never lose their prime and in fact are very good for high suction lift applications.
Peristaltic pumps also do not rely on check valves (or any valves for that matter) for normal operation. These pumps operate as their own shut off device when the pump is stopped. When the pump is stopped, the hose is under compression from the roller. Therefore, no flow can occur through the pump. When moving high vapor pressure (low NPSH) fluids, peristaltic pumps excel because they do not suffer from vapor blocks. Vapor blocks occur when trapped gases are in liquid or in cases when the sudden pressure loss temporary takes the pressure below the vapor pressure of the liquid. These gases may accumulate in one point in the system creating vapor blocks or loss of prime. Peristaltic pumps can also run dry for any length of time without damage to the pump. If an end user has particulate in the pumped medium, limitation in operator experience and limitations in maintenance resources then the hose and/or tube pumps are an ideal alternative.
Roller peristaltic pumps used as thickener or clarifier underflow pumps are recommended for any thickened slurries, sludge, grids or thickening applications. Peristaltic pumps are widely used in mineral processing applications in which solids content exceeds 30 percent or particle size causes reliability issues, so they are ideal for this type pumpage.
The control properties of peristaltic pumps in underflow applications with high positive pressure in suction side are excellent because the pump's flow rate is directly proportional to the pump speed. As mentioned earlier, no flow can pass through the pump when it is not idle; neither can siphoning phenomenon nor gravity feed effect the flow. These features together mean that the control ability of the tube or hose pump is 100 percent throughout the whole operating range—the flow range being 10:1 with hose pumps and 2,500:1 with tube pumps.
Hose pumps can manage a significantly wider flow range with a variable frequency drive (VFD). Peristaltic pumps can handle solid particles up to a quarter of the diameter of the hose bore's nominal size. On the other hand, suction capabilities—almost full vacuum at any given application—can help to clear any blockages caused by foreign particles or sedimentation.
Technical and Commercial Merits and Long-Term Operational Advantages
In the decision process of the Nashua wet weather facility, all these factors were discussed and considered. The original specifications included both diaphragm pumps and hose pumps. Tube pumps in lieu of the diaphragm pumps and offered its new eccentric rolling design hose pumps in lieu of the specified shoe design hose pumps were selected due to the superior tube pump and control technology as well as the operating cost savings of eccentric design savings.
The engineering consultant, the city of Nashua New Hampshire elected officials and contractor all formed a common consensus to order and approve the use of the roller tube and hose pumps. These were chosen on the technical merits, commercial merits and long-term operational advantages.
Pumps & Systems, June 2011