Gear pumps - answers to questions

Modified on Saturday, 31 January 2015 02:24 AM by mpieler — Categorized as: Consultants Corner

The answer to your request regarding gear pumps is an easy one. Refer to the September 1982 issue of Plastics Technology Vol. 28. No. 10. Professor McKelvey did an excellent write-up (as always) titled "New Data Show How Gear Pumps Boost Extrusion Productivity".

There are only a few points that I would perhaps elaborate on:

1. In order to run gear pumps, properly trained operators are required and maintenance is higher than on a basic extruder. Planned yearly preventive maintenance shut-down is recommended.

2. I believe that using a gear pump is overall less energy efficient in terms of straight purchased energy but the efficiency comes about because of more uniform product with less rejected material. As he mentioned. increased output with low viscosity watery type polymers where the back flow term is a relatively high term is possible.

- Paul Limbach



The question concerning melt pumps is timely and one which is asked by many users of extrusion equipment. My list of pros and cons is not suggested to be complete, but may highlight some of the major factors.

PROS

1. Reduce rate surging from extrusion process.

2. Reduce mean melt temperature since the extruder may discharge against a lower pressure head.

3. Improve distributive mixing caused by the flow interruptions of a gear pump.

4. Reduce screw wear in situations where the length to radius of gyration ratio of the screw may tend to cause the screw to buckle under a high pressure.

5. Process isolation is possible where several downstream processes are operated in parallel and where the flow rate to each segment must be controlled.

CONS

1. A pump, drive, and control circuit is expensive and the return on capital investment should be evaluated.

2. Gear pumps are precision machinery. Tight clearances are required to prevent excessive leakage flow. Maintenance cost can therefore be expected to be significant.

3. The control parameters required to maintain a positive "suction" pressure to the pump in a changing down stream condition may be complex and require some time to adjust properly.

4. Another line component will increase the skill level requirement of the operator. Most processors attempt to reduce the operator and maintenance skill level.

5. The added volume of the cavities feeding the discharging the melt from a melt pump will incrementally increase the residence time of the melt in the system between the extruder screw and the die.

6. Foreign particles such as sand, metal chips, and pigment agglomerates may cause the gears to gall or wear. Adequate filtration between the extruder and gear pump may require more capital investment than just a pump.

The questioner requests a comment concerning his example of a custom sheet operation using a gear pump. I feel that in a high quality custom operation, as gear pump may be a welcome addition to control gauge. If coextrusion is to be considered, the odds favoring a gear pump improve.

I would suggest that the questioner contact one or more of the gear pump manufacturers to deter mine the cost of the installation and then contact one or more of the listed consultants to obtain a more objective evaluation of the pro/con ratio of a gear pump installation on his sheet line.

- Robert B. Gregory



The performance of the single screw extruder is analogous to that of a centrifugal pump. Accordingly, its pumping capacity depends not only on the screw geometry and screw speed, but also on the pressure against which it has to pump. The higher the pressure, the lower the pumping capacity. This means that the screw characteristic curve has a negative slope. The magnitude of the slope depends on the channel depth of the screw and on the viscosity of the fluid or melt being processed. Since melt viscosity is a function of melt temperature, the slope of the screw characteristic curve is affected by temperature as well.

That pressure against which the extruder must pump cannot be arbitrarily selected. It depends on the die geometry or rather, the relationship between production rate and pressure drop through the die, which is referred to as die characteristic.

Melt pumps are most appropriate for situations where the die characteristic and screw characteristic combine to give relatively poor pumping performance from the total system. This can arise when die pressures are extremely high (6,000 - 8,000) or when material viscosity is extremely low.

However, the extruder is not merely a pump. As a matter of fact, the extruder is primarily a melting device. When melt pumps are used to in crease production rate by reducing head pressure without correspondingly increasing the screw speed, the solids content of the extrudate will often be increased, thereby producing an inferior quality product. This often creates the need for additional filtration which serves only to increase pressure and may counteract many of the benefits expected of the melt pumps, as well as increasing the investment even further.

Depending on the screw design, the extruder often creates pulses causing production rate to fluctuate. Some products. such as fibers, can usually not tolerate even minor fluctuation. A metering pump can often assist in removing these minor non-uniformities in production rate. However, a well designed screw should produce pressure pulses on the order of 20-50 psi and certainly no more than 150 psi with temperature variations on the order of 1-2 degrees F. The majority of products should not suffer substantially from gauge variations on this order, say 0.5-3%.

Extruder screws, however, are often poorly designed, suffering from solids conveying problems or plugging by solids, that is unmelted plastic, causing more pronounced fluctuations, a phenomenon referred to as surging. These surges can seldom be cured by installing even expensive metering pumps because pressure fluctuations on the order of 10-25% will usually be carried through the pump in any event. Even when electronic controls are used to reduce the surging by varying screw speed, the temperature variations which result from the continual shifting of screw speed may produce gauge variations, internal stresses in the product, loss of tensile strength (especially in an oriented sheet or strap) and occasional loss of feed.

In summary. one can say that while some processing limitations can be eliminated by the use of a pump, it certainly cannot be considered a cure for most processing problems and instabilities arising in the extruder. These problems can and should be solved where they occur: in the extruder screw.

- Dr. lmrich Klein



Reference Material

For those who are interested in some of the recent published papers on gear pumps, the following list may be useful:

J.M. McKelvey, S. Lavin, "Gear-Pump-Assisted Plasticating Extrusion (Part II: Applications)". Advances in Polymer Technology vol. 2, No. 3 1982.

J.M. McKelvey, U. Mare & F. Haupt, "How Gear Pumps And Screw Pumps Perform In Polymer Processing Applications", Chemical Engineering, Sept. 2. 1976,

J.M. McKelvey & W, T. Rice. "Gear PumpAssisted Plasticating Extrusion". Advances in Polymer Technology, Vol.2, No. 1, Winter 1982.

J.M. McKelvey & W. T. Rice "Retrofitting Plasticating Extruders With Gear Pumps. Chemical Engineering Jan, 24. 1983. W.T. Rice, "The Case For Gear Pumps: What’s Behind The New Interest", Plastics Technology, February 1980 W.T. Rice, "Conservation Of Energy and Raw Materials By Utilization Of Gear Pumps In Conjunction With Extruder Processes", ASME Publication 80-Pet-2, Presented at the Energy Conference & Exhibition, New Orleans, La. Feb. 1980.

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