There has always been difficulty finding a reliable and cost-effective pump for applications where there is a combination of abrasives and corrosives. The equipment tends to be expensive to purchase and even more costly to maintain. Another issue is the hazardous nature of many of these applications and the extreme safety requirements for the personnel working on the equipment.
Pumping a liquid that is only corrosive is relatively simple: find the correct materials of construction for the pump and piping and the system will generally be reliability. Due to the hazardous nature of these applications, magnetic or seal-less centrifugal pumps are used extensively to eliminate seal leakage and provide zero emissions from the pump. An alternative is a double or tandem mechanical seal with a barrier fluid. All of these designs tend to be subject to failure during system upsets.
When moving an abrasive liquid, the problem is more difficult because the material rarely acts like a simple Newtonian fluid, the specific gravity tends to be high, and it is more difficult to find the correct materials of construction. The velocity of the fluid in the system becomes a large factor in the system reliability; any increase in liquid velocity through the pump greatly accelerates the wear of the parts. If the pump does not operate at the correct point on the curve, recirculation within the pump causes excessive wear of both the casing and impeller. Specially designed slurry pumps made of hard metals such a 28% chrome or NiHard are generally used for these applications.
The combination of both corrosive and abrasive makes these applications far more complex. The corrosive/abrasive liquid requires a high alloy material or a plastic and also requires a very hard or very resilient material to survive the abrasives. If a seal-less pump is used, there are high velocities and close tolerances around the sleeve bearings as well as other issues increasing abrasive wear. Generally, hard abrasion-resistant materials such as 28% chrome are not available in pumps designed for corrosives. If a pump can be found, using a hard metal such as NiHard or high chrome will help with the abrasive but the hard metals do not tend to be highly corrosion resistant. Replaceable rubber-lined centrifugal pumps are used but have the same recirculation and velocity issues as all centrifugal pumps. Ceramic epoxy coatings for the wetted parts of the pump is another alternative—but with limitations due to variations of coating methods and special machining of the pump and its components as well as the high velocity and recirculation within the pump.
Positive displacement pumps require close clearances or interference fits to move liquid efficiently. The capacity and pressure will quickly degrade as the internal clearances become larger. Progressive cavity pumps (See figure 1) have limited life due to internal slip at higher pressure causing both stator and rotor wear. Standard double-diaphragm pumps or AODs tend to operate too fast with undersized valves. Gear pumps have metal parts working together and the corrosive/abrasive mix is in between the parts causing excessive wear on the gears. Hose pumps have limited mechanical and chemical hose life. In all positive displacement pumps there is some level of slip within the pump; it is inherent in the design. There is an internal recirculation liquid through the clearances. As the recirculation increases with wear, the volume of slip increase exponentially, accelerating wear and decreasing flow and pump life exponentially.
Let us examine the velocity and recirculation issue more closely. In all types of pumps velocity is one of the key components. Most standard centrifugal pumps are designed with a specific speed below 8,000 rpm. This represents a radial design impeller and turns the liquid within the impeller basically 90° as it moves through the impeller. The rotation of the impeller accelerates the liquid to a defined velocity based on its diameter and rpm. As the slurry moves from the impeller through the volute, the liquid velocity is reduced and the head increases, which is what we see on a gauge. In general, a specific velocity is required to create a specific head: therefore, the velocity is a function of the required head. In a simpler form; the higher the head required, the higher the velocity required and the higher the wear.
Simply reducing the speed of the pump is the standard solution to increase pump life, and there is some increase in life with speed reduction. However, the primary abrasive/corrosive issue is velocity of the liquid inside the pump. As stated above, the impeller creates a certain liquid velocity to fit the required design head. Whether the impeller is small/turning fast or large/turning slowly makes little difference. A 12-in. (30-cm) impeller turning at 1,750 rpm has a velocity at the outside diameter of 5,500 ft/min (1,746 m/min) and creates the same velocity as a 21-in. (53-cm) impeller turning at 1,000 rpm. The larger impeller will also have a greater minimum flow as well as greater shaft deflection and bearing load problems. The velocity as the liquid leaves the impeller is approximately the same. A larger pump will have greater recirculation issues because the impeller is larger and is designed to pump more liquid.
Positive displacement (PD) pumps have much the same issue. In applications with a significant discharge pressure, the abrasives will open the pump clearances and the corrosion will increase the degradation of the material. The result is high recirculation within the pump and short life. Many PD pump manufacturers will derate their pumps to a maximum pressure of only 60% of design pressure on a clean liquid or reduce the speed of the pump. In both cases, this will increase the size of the pump, the initial cost and maintenance cost. It also decreases pump efficiency.
As demonstrated by the pump curve shown in figure 2, as the pump moves to greater head the flow is reduced, and the corrosive/abrasive liquid begins to recirculate within the pump. The high velocity and recirculation tend to rapidly increase wear. If the liquid is also corrosive, any oxides that form on the surface of the metal are wiped away by the abrasive, exposing new metal which forms a new oxide coating that is in turn wiped away. The combination exponentially increases pump wear.
What are the criteria for a successful corrosive/abrasive pump? Low internal velocity without allowing the slurry to settle, high corrosion resistance, good abrasive characteristics, reasonable initial cost, standard parts with good availability, zero leakage or no seal to fail would create a successful pump application. By using a reciprocating pump with a low stroke rate and a varying acceleration head, a change of the fluid velocity with each stroke of the pump is created. This change will tend to keep solids in suspension at low velocities of less than 2 ft/sec. Rubber and ETFE linings on the wetted parts can be used to offer both corrosion and abrasive resistance (See figure 3). Because the pump is lined, there is no oxide coating for the abrasive particles to wear away. The linings can be a variety of materials—Neoprene, Nordel® (EPDM), Nitrile® (Buna N), Viton® (FKM) or ETFE—to tailor the pump to the application. By using an elastomeric diaphragm as the driver element, the pump becomes seal-less with zero emissions and again the material of the diaphragm can be tailored to the applications. Using standard, bolt-on design check valves allows a variety of valve designs to be offered including a 90° ball check, inline ball check and full port flap. The design is easily varied to accommodate particle size and type. This combination of materials and design characteristics allows for standardized parts, reducing cost and increasing availability.
Air is an excellent driver for the diaphragm and liquid because it equalizes the pressure on the diaphragm, greatly extending its life. Air will easily compensate for the changing solids volume and weight of most slurry. It will offer varying flow to compensate for changes in density and viscosity of the corrosive/abrasive liquid without overloads or catastrophic failures, matching the pump to the system requirements.
What has been described above is a single diaphragm air-driven pump, as shown in figure 4. RamParts has been manufacturing this style of pump for more than 30 years, offering high reliability in the most difficult applications. The pump is constructed with standard parts and various linings. With the introduction several years ago of ETFE linings, successful applications have expanded to include the most corrosive and abrasive slurries—copper and silver slurries with hydrochloric or sulphuric acid and arsenic; hydrofluoric acid; titanium dioxide and others, for example—all of which can be pumped with efficiency, high reliability and low life-cycle cost.
Information for this article was provided by RamParts Pumps (www.rampartspumps.com). For inquiries, contact Daniel Urquhart, applications engineer, [email protected].