An Up-and-Coming Metering Solution
Peristaltic pumping is the fastest growing pump technology in the world, displacing more complex and higher-maintenance positive displacement (PD) pumps for chemical metering.
Acids, caustics and solvents attack diaphragm and progressive cavity pumps, causing downtime and increased life-cycle costs. The peristaltic principle mitigates these costs. Peristaltic pumps are valve-less and seal-less and have no mechanical parts in the product stream. The fluid only comes into contact with the inside of a hose or tube element, which is a low-cost, low-maintenance and easily serviceable component.
A peristaltic pump’s operation is elegantly simple. A hose or tube element is positioned along a stationary pump housing and is compressed from the outside by a roller (tube pump) or a shoe (hose pump). Fluid is pushed toward the discharge as the roller or shoe moves along the outside of the element while the restitution of the hose or tube element behind the shoe draws more fluid into the pump.
The heart of the pump is a hose or tube that is available in different elastomers specifically formulated to balance long mechanical pumping life with resistance against concentrated acids, bases and solvents. The “wetted end” simplicity of a peristaltic pump stands in contrast to a diaphragm pump, where one must properly select materials for the pump housing, diaphragm, ball checks and seals to prevent pump failure from corrosive attack.
PD pumps are normally chosen to accurately meter or dose exact amounts of chemicals. In a peristaltic pump, flow is proportional to pump speed, and the complete closure of the hose or tube element at all times gives the pump its positive displacement action, preventing flow drop or erosion from backflow and eliminating the need for check valves. Without check valves, peristaltic pumps eliminate the primary source of metering inaccuracy and will not vapor-lock.
The “turndown ratio” or “flow range” of a peristaltic pump is unparalleled. A high-quality peristaltic pump is capable of a more than 2,000:1 flow range by simply controlling one parameter—the rotor speed. With the added versatility of putting different tube sizes into one pump, the flow range expands to 1,000,000:1 range with a single pump. Diaphragm pumps are normally limited to 20:1 speed range with typical controllers.
While the initial capital cost of a peristaltic pump can be slightly higher than other PD pumps, an assessment of costs associated with ancillary items, installation, maintenance downtime and spare parts quickly tip the life-cycle cost calculation in favor of the peristaltic pump.
With the tube or hose element being the only wetted part, peristaltic pumps are virtually maintenance-free, with no expensive seals to replace, no check valves to clog and no rotors and stators to wear out. In a peristaltic pump, the hose or tube element needs periodic replacement. Industrial-duty peristaltic pumps will provide thousands of hours of reliable and repeatable hose or tube life before the element fatigues and needs replacement. When it comes time to replace a hose or tube element, the procedure takes less than an hour and in some cases only a couple of minutes. Additionally, the change of the element is done quickly and safely where the pump sits without the need for any special tools.
Process control requirements and chemical compatibility often make peristaltic pumps a lower-cost capital item than other PD pumps, delivering financial benefits even before the reduction in maintenance and ancillary costs are considered. For example, a progressive cavity pump’s price can be exponentially inflated when expensive metallurgy becomes the only corrosion-proof option for its components.
Additionally, many PD pumps require the additional cost of a separate control panel or variable frequency drive (VFD) for achieving variable flow metering. Peristaltic pump manufacturers build high-turndown, closed-loop speed control capability and expansive I/O connections for DCS and SCADA systems into their pumps.
Historically, there was a technological limitation to applying peristaltic pumps to heavy-duty chemical applications; they lacked in-hose or tube design that could withstand highly corrosive fluids while yielding long life at continuously high pressures and flow rates.
For this reason, peristaltic pumps have found their niche in low-pressure, batch pharmaceutical processes, with limited use in industrial processes. This has turned around over the past 50 years, however. Pumps and chemical duty elements have been developed that are capable of more than 350 gal per minute (gpm), 240 psi, thousands of hours of continuous-duty hose and tube life and metering accuracy better than 0.5%.
Peristaltic pumps have proven to be highly resistant to the acid and caustic chemicals that quickly eat away at the metallic components of progressive cavity and diaphragm pumps. For example, an engineered plastics manufacturing plant in the Southeast uses a SPX10 pump with a rubber hose to meter hydrochloric acid in the production of linear polyphenylene sulfide. Because of the hose pump’s metering capability, the plastics manufacturer does not require separate flowmeters and valves to measure and control the flow. Rather, they adjust the speed of the feed rate by simply altering the rpm of the rotor motor with a VFD.
A power generation servicing company in the Mid-Atlantic region uses peristaltic pumps to meter 65 gpm of sulfuric acid when cleaning out the condensers of a power plant, driving the pH below 4 to breakdown scale. Precise control of the flow of the acid is necessary to ensure that after the scale is dissolved, the discharge to the municipal wastewater system will have a neutral pH.
On the other side of the pH scale, peristaltic pumps are used extensively with sodium hydroxide for many different applications such as bleaching pulp at paper mills and disinfecting potable water. Because the hose in a peristaltic pump is self-cleaning, it does not experience the crystallization that builds up in other pumps and eats away at seals.
Until recently, petrochemicals, hydrocarbons and oil-based products were all beyond the reach of peristaltic pumps due to incompatibility with the rubber hose. A new thermoplastic elastomer hose called PetroProof, introduced by Watson-Marlow Bredel, is now available for toluene and other highly corrosive solvents used in the production of a number of materials.
Frequently, corrosive fluids contain not only corrosive chemicals, but abrasive material as well. Peristaltic pumps, originally designed for caustic and abrasive cement applications, stand up well to the challenge. A Southeastern U.S. building products manufacturing plant also uses peristaltic pumps to make its fiber cement siding products. The cement mixture has little effect on the hose despite being highly abrasive and strongly basic.
Despite all of the advantages of peristaltic pumps, they represent only a modest but growing percentage of the market in the U.S. This is primarily because peristaltic technology is relatively new, whereas diaphragm and progressive cavity pumps have a lengthy history.
However, with the pressure on plant managers to reduce the life-cycle costs of their pumps, the functionality and benefits of peristaltic hose pumps are becoming more widely known, and new peristaltic technology is fast becoming the pump of choice for tough, chemically aggressive and abrasive applications.