Many food products and processes are acidic or alkaline and will attack cheaper grades of stainless steel. Further, the sanitizing solutions used for cleaning food grade pumps are strongly acidic or alkaline and lower grades of stainless steel, e.g. 304 stainless steel will deteriorate rapidly in contact with them. Lower grades of stainless steel have a higher carbon content - carbon is an impurity over a certain level which reduces corrosion resistance. In addition, 304 stainless steel is susceptible to stress cracking when exposed to hot water.
The 316 or “marine grade” range of stainless steel is considered the best grade for sanitary pumps. For all Nuphlo pumps, we use the 316L version of this grade for its corrosion resistance and weldability if repairs are needed at a later date.
Most stainless steel pump manufacturers make the transition housings/motor adapters (between pump and motor) of their pumps in cast iron with only a paint protection. Although this item is external to the product in the pump, it can be exposed to cleaning solutions and sometimes the pumped product if there is leakage. Consequently, it erodes rapidly and looks unsightly after a short time.
With all Nuphlo pumps, the transition housing is also high surface finish investment cast 316L stainless steel (which can be polished if required for pharmaceutical standard pumps).
All parts of Nuphlo pumps which come in contact with the product are manufactured in 316L stainless steel except the stainless steel motor shrouds which protect the motors from water. As they are easy to clean and unlikely to be exposed to the pumped product, we manufacture our motor shrouds in the 304 grade stainless steel.
Investment castings, using the lost wax process, result in a superior finish and enable the use of detailed and non-compromised shapes (created by dies/molds) rather than the conventional casting method. This in turn minimizes the subsequent machining and polishing as the cast surface finish can be of high quality requiring minimal work to bring it to hygienic standards.
At Nuphlo, we consider it very important to have smooth surfaces to eliminate all imperfections from sanitary pumps. There should be absolutely no place for micro organisms to accumulate in sanitary pumps and easy access to all working areas for cleaning is a must. We take particular care with these details.
Pressed stainless steel pumps are pressed into shape from sheet-metal and are likely to have thin pump case walls. Unless handled with care, they are open to damage, i.e. knocks creating bug traps and mechanical problems. An investment cast pump has thicker pump case walls making it more resistant to knocks and easier to repair damage.
This is particularly important when repairing flexible impeller pumps. If the walls of the flexible impeller pump case are not smooth, its flexible impellers will have a tough life and consequent shortened life spans.
It is nearly impossible to create a true progressing cavity scroll case in centrifugal pumps from pressed sheet-metal and pumps using pressed cases generally feature the inefficient concentric type. Cavitation is more likely to occur in this type of pump … more about cavitation below.
All Nuphlo pumps are investment cast.
Where it is important to control the speed of pumps, often flexible impeller pumps, we recommend fitting a variable speed device (VSD) (also called frequency controller) to the motor.
- Energy is conserved by matching the pump speed to pressure flow requirements using a VSD.
- With Nuphlo pumps, the VSD can be pre-wired and programmed at the time of order so all that is needed is to plug the pump into a power outlet and the unit is ready to use (single phase inlet is optional on motors up to 3 kW). There is then no need to purchase a separate motor starter unit as the VSD in effect, takes over this duty and gives better motor protection against current overload.
- As Nuphlo flexible impeller pumps can be run in either direction, a reversing switch, easily fitted to a VSD, is useful, e.g. if a blockage occurs the product can be pumped back out again.
- Limit switches, temperature control, weigh scale cut-out, etc. can also be easily programmed in to control a VSD.
- VSD’s can be easily linked to computer controlled factory systems.
Often spare parts for pumps, such as mechanical seals, are expensive when the pump manufacturer's own brand is used. Leaving the purchaser the choice of mechanical seal brand assures the best price can be obtained and eliminates the need to hold extra stock.
All Nuphlo pumps, except our Waveplate pump, have readily available standard mechanical seals and 'O' rings. (The Nuphlo Waveplate pump is a special pump designed for larger applications and has a customized mechanical seal).
We recommend a spare seal be kept for each pump and also a spare flexible impeller for each flexible impeller pump. We also recommend stocking the appropriate casing ‘O’ rings.
The mechanical seal which comes as standard with most Nuphlo centrifugal pumps has carbon to ceramic faces with viton rubber trim. A more expensive tungsten or silicon carbide hard faced seal is usually necessary when viscous or crystallizing products are to be pumped. These products can crystallize when the pump is left stationary and prize the faces apart causing excessive wear and leakage on carbon faced seals when the pump is restarted. Hard faced seals are usually positively driven to prevent rotation relative to the housing or shaft and are often supplemented with a flushed double seal.
Flushed double seals are used where intermittent flows can occur (e.g. CIP operations) or crystallizing, viscous or abrasive products are present in the product being pumped. Water is usually used as the lubricating fluid between the inner and outer seals.
Our standard pumps have RJT fittings which is the predominant standard in Australia and New Zealand. We are able to incorporate DIN or tri-clamp fittings (often used by pharmaceutical companies for ease of dismantling) at time of order.
When flow rate and head pressure are decided on, the performance charts for the particular Nuphlo pumps can be requested. For the centrifugal series, these charts plot flow rate against head pressure for each impeller size and the best pump-to-motor match for the job can be ascertained. The type of product is the deciding factor. As a general rule, the more viscous the product, the slower the pump should run and this generally favours positive displacement pumps.
If in doubt, Nuphlo's technical staff will be pleased to assist.
The liquid ring pump works on a similar principle to the centrifugal pump where the radial blades throw the liquid outwards converting rotational energy into pressure energy in the process. Unlike the centrifugal pump, the liquid is not directed into a scroll case but into concentric side channels either side of the impeller, in the pump case and the back-plate. The side channels increase in volume as the impeller passes the suction port (drawing liquid in) and decrease in volume at the delivery port (flowing the liquid out). A portion of the liquid re-circulates forming the so called “liquid ring”. The pumping action is generated as this liquid ring moves in and out of the side channels.
Unlike the centrifugal pump, the advantage of the liquid ring pump is that it is self priming once a small amount of fluid is present in the pump.
Centrifugal pumps use centrifugal force to move the product through the pump and generate head pressure. As the head increases, the flow decreases and this is the basic characteristic of a centrifugal pump. Impeller diameter and speed largely determine the maximum pressure a centrifugal will generate. On a multistage centrifugal, the impellers are ganged up in series so each adds further to the head pressure. Depending on the number of impellers and their diameter, high pressures can be generated this way.
It is possible to restrict the flow with a centrifugal pump by throttling the discharge and the flow can even be shut off for short periods. However, mechanical energy being put into the pump by the motor will heat up the pumped fluid. Control and energy consumption are much more efficiently obtained by using a VSD to regulate speed and pressure, especially on larger pumps. Unlike positive displacement pumps, centrifugal pumps are generally not self priming and are unsuitable for more viscous products. They are, however, the most simple and reliable pump type requiring little maintenance.
As the name suggests, positive displacement pumps displace a fixed amount of product per revolution (or stroke in the case of piston pumps). Pressure generated depends on the design and power applied. The pump will be damaged if the flow is shut off while operating unless a pressure bypass is fitted. Positive displacement pumps tend to be more complicated and consequently more expensive than centrifugals.
However, flexible impeller pumps are a cheap option for positive displacement pumping though their maximum pressure head is limited by the blade design and stiffness. Once this pressure is exceeded, the fluid being pumped leaks backwards as it bends the blades over and breaks their seal against the pump casing. For this reason, flexible impeller pumps are sometimes referred to as semi positive displacement pumps. Flexible impeller pumps generally operate at pressures up to 3 Bar (some larger models will go to 4 Bar, i.e. Nuphlo PD30). As the flexible blades rely on the pumped product for lubrication, these pumps cannot be run dry and a small amount of water should be introduced when first starting if the pump is completely dry. With the cyclical bending, the blades fatigue over time so the impellers are consumable items requiring periodic replacement. Flexible impeller pumps and other positive displacement pumps run more slowly than centrifugal pumps and for this reason are more suitable for viscous products or products that need gentle handling.
Nuphlo's centrifugal tanker pump is a large - 4” (100mm) inlet /3” (75mm) outlet - pump running slowly and under these conditions is successfully used to pump whole milk without causing damage. The same scenario is employed for pumping similar products in dairy factories.
A centrifugal pump can also be used to pump whole milk from the milking machine suction can to the farm cooler vat but a large, slow running centrifugal is not an economic proposition for most farmers. Smaller centrifugals running at high speed are often used but can easily damage the fat particles, reducing the fat count and often generating froth. (It is the fat count and quality of milk which determines the price the farmer receives.)
We consider flexible impeller pumps the most suitable pump for this application on the farm as they handle milk more gently than any other pump which meets the farmers’ requirements i.e. the best price for the least damaged milk. Flexible impeller pumps with Nuphlo white /translucent silicone flexible impellers are now extensively used in New Zealand on dairy farms. Flexible impeller pumps have replaced many diaphragm pumps which are now considered too complicated to clean both manually or with CIP (cleaning in place) in larger farm installations.
The progressing cavity scroll which channels the product through, is an important feature on well designed centrifugal pumps, i.e. the pump case should increase in cross-sectional area moving around the perimeter, being minimal at the cut-off or cut-water point and increasing to a maximum at the outlet (in the manner of a snail shell). This is known as a progressing cavity scroll case. Pumps that have concentric cases, rather than progressing cavity scroll cases, will re-circulate the product before moving it through, sometimes by as much as 20%. Apart from the extra handling of product, during the life of the pump (which should be very many years) the extra power consumed re-pumping product could be quite substantial.
Cavitation is caused by the rapid collapse of vapour bubbles as the pumped fluid is compressed when passing through a pump. It can be recognised by a harsh rattling noise coming from the pump casing.
Likely reasons for its generation are:
- Difficult suction conditions caused by a lengthy, restricted or a too small diameter suction pipe to the pump.
- Negative suction head to the pump, i.e. the fluid level is below the pump’s intake.
- The pumped fluid is at or near its boiling point.
- Poor pump design.
Steps should be taken to remove or reduce the cause as cavitation will rapidly eat away the metal at the site (usually at the edges of the impeller blades). It can be recognized by severe pitting or a sand-blasted appearance at the point where it is occurring. It is important that this pitting is polished out as it can lead to a micro organism foothold. This is particularly important with flexible impellers pumps as a pitted surface on the pump casing will cause premature wear of the impeller and consequent early failure.
Nuphlo pumps are designed to minimize the likelihood of cavitation occurring but if external factors described in 1 - 3 above are present, then cavitation can occur.
Under normal operating conditions, life of any flexible impeller depends on operation time, the type of product pumped, the care taken by the operator of the pump and the condition of the pump.
- Flexible impellers can be damaged by dry running, running them too fast, over pressuring the pump or by foreign matter which can inadvertently end up in the product.
- A more aggressive product can also shorten the life of flexible impellers.
- Flexible impellers run at slow speed tend to last longer.
- When the pump speed is controlled by a VSD, life can be extended.
- Suction conditions/temperature (high suction with hot product may lead to cavitation which will damage the blades particularly in the centre) … more in question above. The pump should be run slower if these conditions are encountered. (High suction can occur where the inlet pipe/hose line is too long or when pumping very viscous products.)
On three phase applications, a pump is sometimes accidentally wired in delta instead of star or vice-versa. This will lead to a high current draw or slow running of the motor. On centrifugals if the motor runs in the wrong direction, the pump will give a reduced flow and pressure. Check the wiring connections and direction of rotation. If a VSD is fitted, make sure it is configured to match the number of poles on the pump motor. If not, the same problems will occur.
The N.P.S.H. (net positive suction head) for a centrifugal pump is basically the distance below the centre line of the pump that it can maintain flow at (once primed and started) for varying head and flow conditions i.e. the pump's lifting ability on the suction side. It depends on a number of factors, i.e. liquid temperature in relation to its boiling point (vapour pressure), specific gravity (S.G.), suction pipe head loss due to viscosity, etc. Other products will be different from water shown.
Centrifugal pumps are not self priming so need to start with a positive head above the centre line. 0.5 meter or more would be ideal but they will work with less. For viscous products, the aim should be to minimize the length of the suction line and keep it as large and straight as possible. Sometimes, it pays to turn the pump discharge to 45 degrees or horizontally above the pump so the air can be swept out of the volute case more easily if there is a chance of vortexing in the supply tank.
Cast iron motors stand up to harsher conditions, particularly the acids and alkalis used in cleaning products. Aluminium motors are particularly prone to attack from these products and often corrode rapidly.
The newer style high efficiency, low energy motors are used with all Nuphlo pumps. Unless a specific motor or motor brand is requested, we will quote a cast iron, high efficiency motor. Our stainless steel shrouds are made to fit these motors.
For special applications, stainless steel motors can also be provided.