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Choose the Rotor for Your Application
The choice of rotor for your progressing cavity pump can be fine-tuned to your application. The benefits are: longer pump service life, and optimum pump performance, even under arduous duty and installation conditions.
What choice as a customer do you have?
Rotor Geometry -
- Standard 1:2 geometry. Suitable for low to medium duty flow conditions.
- Ultra-Pro 2:3 geometry. Double pitch - Offers opportunity to increase flow within existing pump configuration, run the pump slower for same flow rate, hence increase service life.
Rotor Size -
- Standard size - Suitable for fluid pumping temperatures up to 60 deg c.
- Undersized - Suitable for pumping fluids above 60 deg c. Expansion of chiefly the electrometric stator means the rotor is reduced in size to ensure optimum transitional fit, performance and minimum wear at elevated temperatures.
Rotor Coatings -
- Chromed - Chrome provides a hard wear resistance surface to abrasives.
- Double chromed - Simply a double layer of chrome that further extends the rotor’s life against abrasive wear.
- Non-chromed - Provides material choice for pumped fluids that are not compatible with Chrome. Brine, for example.
If you believe your progressing cavity pump can be further customised to your application please do not hesitate to call and we will happily discuss the options available to you.
Whole Life Cost. Pin Joints
As soon as the universal joint in your progressing cavity pump starts to wear the effect will be immediate:
- Reduced operating efficiencies.
- Seal leakage.
- Excessive rotor movement causing high stator wear, this will reduce service life dramatically.
Therefore, it is worth looking in some detail at the universal joint design of the pump you are about to purchase.
The majority of progressing cavity pumps specified today still utilise the original ball and pin universal joint design to compensate for the eccentric motion of the rotor. This simple design has stood the test of time, particularly for light duty applications. It is functional, low cost to manufacture and quick and easy to repair.
Variations of the ball and pin joint do exist. The Robbins & Myers Moyno pump utilises a connecting rod and ball, precision machined and hardened for best fit and joint performance.
You will also see bushed designs, primarily to save replacing a worn drive rod. However, in practice, bushing degrades the contact points because:
- Tolerances accumulate.
- The hole in the connecting rod bush is hourglass shaped, rather than the preferred angularly elongated slot. This is to eliminate the need to index the bushing to the rod axis.
- The load-bearing outer surface of the ball is no longer a hardened surface.
- Bushing also increases the cost of the universal joint assembly and maintenance time.
The Moyno pump rod is supplied as a precision and hardened component, for longer life. Subtly, but crucially, it is also pre-drilled at 90 deg, offering users repeated ball and pin connection. Maintenance of the Moyno component is literally at zero cost and down time is reduced.
Another design you will see is the flexible shaft, whose primary purpose is to eliminate moving parts from the universal joint and reduce cost. Sounds good?
It is, however, a light duty solution, the thickness of the shaft being limited due to its requirement for flexibility. Hence it offers limited torque capability and therefore pumping loads. The shaft strength, in practice, is also weakened by surface damage due to foreign bodies in the fluid and pitting or corrosion. Additionally, the flexible shaft requires the length of the pump’s footprint and servicing space to be dramatically increased.
If your application is light, the fluid consistently “clean”, and pumping pressures stable, then all designs will perform satisfactorily for you. However, factors such as variations in solids content, viscosity, pressure, possible corrosion, all put extra loads and torque requirements on your pump’s drive mechanism.
It is clearly vital to understand your application to determine how robust you need your ball and pin joint to be for the lowest whole life operating cost of your plant!
To go one stage further, if your application is arduous: for example, pumping 30% solids sewage cake sludge, or requiring pressures 20, 30 even 100 bar, then a gear joint universal coupling design should be considered.
For further information on the unique Ultra-Drive gear joint please do not hesitate to request further information.
Pump Failure. Why? A Quick Check List.
Your supplier’s initial pump selection should eliminate problems. In practice, repositioning pumps and changes to original duty conditions can cause the designed service life / pump performance to change.
Here is a general troubleshooting guide to aid corrective action and save you time.
Pump does not rotate
Check motor / inverter, selection and installation. Check for any foreign matter in pump. Check for excessive static friction in new rotor and stator. Check for stator swell due to chemical attack or high temperature. Check for solidification of settled liquid within the pump. Check gland packing is not too tight.
Pump does not discharge
Check suction/ flooding/ excessive suction lift. Check for air leakage into suction pipe. Check pump is primed. Check stator or rotor for excessive wear. Check direction of rotation.
Discharge output low or fluctuating
Check motor / inverter, selection and installation. Check for foreign matter in pump. Check for excessive suction lift. Check air leakage into suction pipe. Check for stator or rotor for excessive wear. Check seal or packing leakage. Check pump speed is not too high. Check liquid viscosity or SG is not too high.
Excessive stator or rotor wear
Check for stator swell due to chemical attack or high temperature. Check settled liquid has not solidified within the pump. Check suction lift. Check dry running. Check discharge pressure. Check pump speed.
If you should have site specific questions please do not hesitate to ask. We are here to help.
Your Guide to Pump Suction Feed Systems
When pumping high viscosity fluids or those with high solids content, as is common for progressing cavity pumps, the pump’s performance is often dependent on the mechanical feed option selected for overcoming poor suction conditions that could easily cause a breakdown to your process and plant.
Use these guides to help evaluate your own plant’s pumping applications and the suitability of installed or quoted pumping equipment.
Viscosity range and typical pump feed aid guide
Viscosity CPS |
Suction Feed Option |
200,000 to 10,000,000 + |
Twin screw feeder |
150,000 to 10,000,000 |
Large top feed auger |
75,000 to 275,000 |
Breaker with auger conrod |
20,000 to 100,000 |
Open throat with auger conrod |
10,000 to 35,000 |
Enhanced feed rotor |
1 to 10,000 |
Standard pump |
High Solids range, for municipal waste and typical pump feed aid guide
Solids Content % |
Suction Feed Option |
18% to 50% + |
Twin screw feeder |
15% to 22% |
Bridge breaker with auger conrod |
10% to 17% |
Open throat with auger conrod |
5% to 12% |
Enhanced feed rotor |
1 to 7% |
Standard pump |
If you should have any questions regarding your own applications, or require further information on the mechanical suction devices described, please feel free to call.
Choosing a Sanitary Pump
Would an industrial stainless steel pump suffice?
A common discussion we have when visiting food manufacturers is the specification of pump required. Do you require a full sanitary pump specification or would an industrial stainless steel pump be suitable?
Dairies are easy: 3A and BISSC sanitary specifications have to be strictly complied with; and the pump must be suitable for CIP (Clean in place) procedures to eliminate bacterial growth and possible salmonella poisoning. There is no choice.
However, sometimes there are not clear guidelines for your pumping application: typically, the addition of food additives or flavours. How do you know which is the correct specification?
To help, the main reason to use a sanitary pump is to significantly reduce bug and bacteria traps within the pump and increase your ability to clean the pump to a sterile condition confidently and easily.
If these requirements are essential to your process then you’re going to need a sanitary pump; if not, a standard industrial stainless steel pump could suffice and offer significant cost savings. But, please, every application needs to be assessed independently so this is a rough guide only.
Here are some of the main features that sanitary pumps give you over an industrial pump:
- Self-draining discharge port to allow drainage of product and flushing liquid.
- DIN and ACME specification flanges, clamp style release, for quick and easy clean; hygienic design.
- Clean in place port on pump suction chamber to allow for automated CIP and fluid bypass.
- Material specification 304 or 316 to class No. 4 surface finish.
Need help deciding on the right standards and specification for your industry and application, we will be more than pleased to help.
PS
Did you know that progressing cavity pumps are used extensively within the food industry for pumping products like margarine?
This is because this type of product can change viscosity dramatically within the process, the result is often slip and backflow within say a Lobe pump causing shear damage to the product and loss of metering accuracy.
The PC pump has a transitional fit between rotor and stator. There is no change to pumping performance or product quality due to wide changes in a product’s viscosity.
Number of Pump Stages = Service Life
Noticeably, many pump performance related problems arise from being tempted by price: the initial lower cost option, say, of a single stage, compared to a two stage pump can be considerable.
To help you see why the additional payment for the extra pumping stages can be of value for the money it is worth revisiting some of the PC pumps’ design characteristics.
The progressing cavity pump is a single helical rotor, rolling eccentrically in a double threaded helix of twice the pitch length. In doing so it forms a series of cavities, 180 deg apart, that progress from suction to discharge. As one cavity diminishes, the opposing cavity is increasing at exactly the same rate; so the sum of the two discharges is constant.
This rotation represents one total pumping chamber or stage, the pumps seal line running from start to finish, measured along the total length of the helix pitch of the single rotor stage. (This is one reason why Robbins & Myers Moyno rotor geometry has a steeper pitch angle than many of our competitors.)
This seal line is responsible for preventing slip or backflow within the pump; however, each stage has a maximum pressure capability: if exceeded, or neared, another pump stage should be added.
Many of your wear or failure descriptions are due to backflow, combined with high pressure and abrasives. This combination will mean wear to your pump will be severe and quick.
The answer is to increase the number of pump stages; but how do you know?
Check out your next pump quotation
The number of stages you might require for your PC pump for longest life and lowest maintenance costs:
- Clean fluids 75psi/stage
- Abrasive applications 20psi/stage
As in all pump selections judgment is required, if you are near the pressure limit of a single stage, but the duty cycle is low, say only a few hours per week, then you could use a single stage pump as service life would still be acceptable.
If your application results in an unclear decision reference number of pumping stages, selection on the side of caution will ultimately increase the pumps service life and save you money. Need assistance? Please call.
If You’re Using a Conveyor, or Suffering Pumping Rat Holing and Bridging Then Please Read On...
Getting product into any pump is probably your chief pumping consideration. If you can do this then most positive displacement pumps will handle an extensive range of viscous products, literarily a product you would initially view as a solid!
The pressure differential between atmosphere and the pump’s vacuum (e.g. 14.75 PSI max) is very limited in helping us to push high viscous product into pump’s suction.
Typically non-Newtonian fluids with high solid content, i.e. sewage cake, tend to bridge and rat-hole; the result: starvation and cavitations within the pump.
Pump manufacturers have developed pumps with open throat suctions and bridge breakers. Additionally these pumps often utilise a single auger feed on the pumps connecting rod for enhanced fill efficiency to the pumps’ pumping chamber. These methods have proved successful across thousands of applications and many makes of pumps; however, as a product’s viscosity and solids content increases the behaviour of such a fluid generally becomes increasingly difficult to predict.
This is why, occasionally, plant engineers operating near the limit of singular auger design pump systems on high solids or super viscous products experience intermittent pump performance drop-off with possible wear or breakdown consequences.
Handling the very worst of unpredictable and unforgiving products, therefore, requires a positive pump feed system that is independent of the pump’s speed and able to adjust to the fluids’ sensed condition at the pump’s suction. The Twin-Screw Feeder (TSF) is designed for this demanding type of application.
Twin-Screw Feeder (TSF) from Moyno Inc.
How it works:
- The twin screw feeder has a separate drive; therefore, feed rate is not locked into the pump’s own RPM. By using a pressure sensor and a PID controller, stuffing pressure control can be automated to the process for consistent feed pressure to the pump
- Your product is physically pulled down into the augers due to the twin feed augers counter rotation. This pull-down effect is unique to the twin augers and cannot be duplicated with a single auger design.
- A positive stuffing pressure is also achieved: the concentric motion of the augers allows for extremely tight tolerances with the sidewalls creating a high tolerance pressure tube area for maximum fill efficiency.
- Product has no opportunity to bridge - side walls to hopper and chute are vertical - the product falls directly onto the augers. The twin augers also create a greater width than a single auger, plus their intermesh action generates a self-cleaning effect, further eliminating bridging opportunity.
Could this approach answer you’re operational and business requirements, please feel free to call for more information.
Five Considerations for Pumping - Abrasive, Shear Sensitive and/or High Viscosity Products
Your main process and plant considerations are probably:
- Quality of pumped product: Unchanged
- Pump reliability: Lowest running costs
Whilst progressing cavity pumps dominate this difficult product category, you will find many different pump designs operating in the field. The most important factor for selecting a pump for these arduous duties is correct selection - not brand, design or price - but matching your pump to the duty conditions exactly. If not, wear and tear, poor performance and pump failure will come round quickly, regularly and expensively.
Let’s have a look at the considerations you should be aware of for most pump types in order to achieve your goals. Examples are PC biased!
Remember it’s free to call, for further selection guidelines.
Design Factors
- You want the straightest and simplest flow path of fluid through the pump to achieve low velocity within the pump and low shear.
The flow-line in a PC pump is a straight line!
- You want to eliminate product backflow or slip within the pump. Particularly at high pressures, backflow of the product can create rapid localised wear and shear damage to the product.
PC pumps utilise, in effect, an elastomer as the outer gear: this is a compression fit between the rotor and stator, creating a continuous seal line per stage. This has the added advantage of making a PC pump suitable for pumping low viscosity and gaseous fluids, as well as high viscosity.
- You want to select the best materials for abrasive duties. Pump manufacturers offer rubber lined, hard metal and ceramic to resist abrasion, chemical and temperature. You can probably find material match from most renowned pump manufacturers.
The PC pump, however, wins due to a combination of stator and rotor material selection, plus low product velocity within the pump. Particles tend to imbed, and deform the stator rather than abrading it, the rotor being protected with, typically, a hard wearing coating. Depending on chemical, temperature and pressure compatibility, stator elastomers with a 50 to 70 durometer (Shore A), typically a high grade of natural rubber, provides the optimum balance of softness and resistance to deformation under pressure. For the rotor, typically air-hardened tool steel, 55-58 Rockwell “C”, with either single layer or double layer of hard chrome plate, offers greatest abrasion resistance.
- You want to think seriously about pump speed. Let’s be honest, you can get everything right, but if you increase the pump speed to purchase a smaller and lower cost pump, then you probably pay for that choice many times over. This generally applies to all designs of pumps.
As a guide, the ideal speed for a PC pump is probably below 350 rpm for abrasive applications; where pump longevity is required, short batch operation is possibly around 600 rpm. Get it wrong and you find PC pump wear for abrasive applications more closely proportional to the speed squared than it is to a direct relationship.
- You want accurate metering or dosing of abrasive slurries, often falling into the non-Newtonian category; meaning pump speed selection is critical to ensure each cavity of the pump has been filled before the next pump cavity is offered to the suction. Getting this right can be a mixture of sum, experience and trial!
Your PC pump offers the advantage of accurate metering and dosing even when the pumped product changes viscosity dramatically: size and selected pump speed for the maximum viscosity and pumping accuracy will be maintained where other pump designs will lose pumping efficiency when viscosity drops to an extent that slippage occurs within the pump.
If you would like more information please do not hesitate to contact our team at T023 8076000 or email scarter@moyno.com
To give you an idea of the pumping performance possible from a progressing cavity pump here are a few examples of what can be done:
- Automobile repair compound
Dealing with viscosity in the range of 300,000 to 600,000 centipoises and highly abrasive. This is typical of many automobile body shop repair compounds which also contain chopped, extremely abrasive, glass fibre. To picture this fluid, appreciate that it’s designed not to run or sag!
- Oil and water mixtures
For many pumps, pumping oils, particularly with any possibility of water present, would result in emulsification of the product. However, the average shear rate of a progressing cavity pump can be less than 100 sec-1 enabling pumping of oil and water mixtures without emulsification.
- Other pumped products in the difficult category that we are considering could include: sandy crude, coal slurries, waste sludge’s, polymers, drilling mud, tallow and grease, sealant, asphalt emulsions, paper stock, refractory mortar, gypsum, caulking compounds, margarine, sausage meat...
Please feel free to call to discuss your pumping application