Published: 08 July, 2015
When it comes to pumping solids in industries such as sewage treatment, mining or food there are numerous pump choices available to the process engineer because solids are relatively common in these sectors. However, when it comes to the chemical processing sector the options are more limited due to a range of additional factors, including the need for chemical resistance, ATEX, higher temperatures and the financial pressures. Malcolm Walker, marketing manager for AxFlow Ltd, reports.
The question that the chemical processing engineer faces is: what is the best pump type and technology for the application? Getting the correct answer can, and will involve a degree of research and detailed discussions with manufacturers and distributors. Pumps designed to handle solids are often not the cheapest option, but when the costs of maintenance, production downtime or even cleaning up an escape of corrosive liquids are considered then for many applications they almost always represent the most cost effective option.
The range of pumps that can be considered for handling solids laden liquids, slurries and pastes can be narrowed down to rotary lobe, rotary piston, reciprocating positive displacement, progressing cavity, hose, centrifugal and air-operated double diaphragm (AODD) designs. Making a decision as to which type to select is dependent on several factors including the corrosiveness of the liquid, the nature of the solid content, the flow and differential head required by the process.
Many engineers resort to AODD pumps because they can be constructed from chemically resistant materials, while at the same time being extremely good at handling solids; these can be up to 76mm in size while the inherent action of these pumps make them resistant to abrasives, components can be easily replaced and they are self-priming. The absence of close fitting parts allows solids to pass through the pump without harm, whilst abrasive and viscous products can be pumped without any damage to the fluid characteristics. In addition, this pump can accommodate dead-heading during filling applications, a task that motor driven pumps are not able to handle effectively.
Unfortunately, AODD pumps have some intrinsic disadvantages. Compressed air is required, this being a factor that can contribute to their notoriously high running costs, flows are limited to 50 m3/hr and the flows do pulse even when the pumps are fitted with dampeners. Furthermore, AODD pumps cannot withstand particularly high inlet pressures and there are issues relating to the lack of product containment, should the diaphragms rupture. This particular aspect is being addressed through the development of new robust designs that are manufactured from solid blocks of chemically resistant plastic, and with robust bolting arrangements to improve containment and pressure resistance.
Hose pumps are an extremely popular option for pumping slurries with a solids content of up to 80%, and can deliver flows up to 90 m3/hr. The extensive range of chemically resistant hoses now available has resulted in the pumps being suitable for handling many of the most demanding chemicals.
The hose pump is a positive displacement pump where the fluid is contained within a hose or tube inside the casing. The fluid is moved into and out of the tube/hose by the action of rotating rollers or shoes which alternatively compress and relax the tube/hose. The ‘peristaltic’ pumping action ensures that the fluid is transported accurately and without any variation in the volume, making it an ideal proposition dosing requirements.
What’s more, where chemicals of a high value are involved, the pump is economic because at the end of the processing cycle or operation, the pumping motion can be reversed and any fluid remaining in the hose/tube can be returned to its source for future use. Indeed, when faced with a corrosive slurry hose pumps are often the only option capable of handling the combination of corrosion, solids and abrasion. But, like AODD pumps should a problem of leakage does exist as the rupture of the hose can be disastrous. Other factors that come into play when the pumps is being considered relate to the flow, which is far from smooth, the limitations of the inlet pressure and restriction on the maximum temperature.
Progressing cavity pumps
Another common choice for transporting solids and slurries in chemical industry applications is the progressing cavity pump. They represent a simple and robust design that can deliver flows in excess of 200 m3/hr added to which they have been around for many years, so giving them an air of credibility.
The key elements of this pump are the rotor and stator, the rotor being a helical worm that moves in an eccentric manner within the non-rotating stator. In this way the fluid is transported through the pump in a continuous low pulse motion. Where the accurate dosing of high and low viscosity fluids containing solids is the goal, then it is a popular choice.
Drawbacks that can be encountered include the rather limited range of materials used for construction and component wear which can be quite fast, resulting in the need for regular replacement. Their susceptibility to many types of failure places an onus on the operator to maintain regular inspection and planned programmes of maintenance.
Rotary piston pumps
The rotary piston pump is a proven alternative to the rotary lobe pump within the chemical processing industry sector. Although classed as a rotary lobe pump, it employs an operating principle known as the external circumferential piston (ECP) where the arc-shaped rotary pistons, or rotor wings, travel in annular-shaped cylinders machined in the pump body. The resulting long sealing path reduces slippage and produces a smooth product flow without destructive pulses or pressure peaks, and without the need for valves or complex parts. However, unlike progressing cavity and rotary lobe pumps, they are not adversely affected by slurries which have a propensity to settle in the pump.
Where solids are involved, the large fluid cavities of the rotors, together with the large easy-entry anti-cavitation ports enable efficient pumping. Maximum service life even under severe operating conditions where fluids are non-lubricating or abrasive can be achieved, because there is no contact between the bearings and the pumped fluid.
The rotary lobe pump provides an alternative to the rotary piston pump, employing timing gears which eliminate contact between the rotors and enables them to handle non-lubricating fluids. There are various designs of rotor, including bi-wing and multi-lobe options. Providing low shear and gentle handling of the liquid, thereby minimising product degradation, rotary lobe pumps are also easy to clean in place (CIP) or strip clean between operations for batching applications.
The final category to be considered is the centrifugal pump, which is often ignored within the chemical industry when it comes to selecting pumps for handling solids. There is a misconception that there aren’t any chemically-resistant centrifugal pumps capable of handling solids. It is a situation that does need correcting.
Centrifugal pumps that feature a vortex impeller in stainless steel provide an excellent and well-proven solution. In operation, a whirlpool effect is formed in the volute by the rotating recessed cup-shaped impeller extending into the suction line, drawing the liquid/solids into the pump and then quickly through the discharge. This motion minimises the fluid/solids contact with the impeller and volute. It is a simple design that allows solids to pass through the pump without choking the impeller, so considerably reducing pump wear and the potential for blockages.
The fully recessed cup-shaped impeller and free passageways within the pump casing offer almost total freedom from blockages where solids and fibrous materials can be encountered in process waste water treatment plants. Suitable for ATEX certification, 316 stainless steel stainless steel impellers coupled with mechanical seals incorporating Viton or FEP are the recommended option for the chemicals industry.
It is important not overlook the small number of manufacturers which manufacture ATEX certified lined ISO 2858 pumps that are designed to pass solids. Taking Wernert as an example, their pumps can be supplied with open impellers, larger internal tolerances, flushed seals and a hard wearing polyethylene lining if required. Significantly, they can handle liquids with up to 15% solids that can be as great as 7mm in diameter.
For further information please visit: www.axflow.com