by Thomas Day, Ph.D., Milton Roy Americas
Generally, metering pump technology is associated with process chemicals and compounds usually very fluid and light. Reliability used for years, chemical manufacturers and their staff understand the reliability of process delivery and the longevity of these devices. What is generally not appreciated is that options to metering pumps include the ability to pump abrasive and dense slurries depending on the aggression of the compound and the granular average of the transported solid. High performance diaphragms (HPD) are constructed with a design and materials allowing metering pumps to transport these compounds with the same accuracy and repeatability as they do with smooth or less dense material.
An explanation of their general design and construction will allow the engineer or technician to see that a manufacturer of HPD technology has the ability to convey these compounds successfully. This allows for uniformity in pump purchases, uniformity of spare parts and concentrate technical training to established pump lines without purchasing otherwise exotic or unique technologies.
An assumption is made that the reader is already familiar with the operation and placement of metering pumps. If the reader is unfamiliar, they are directed to prime their understanding by reviewing another of the authors’ papers from this selection.
cps- centapoise, HPD- High Performance Diaphragm, D.E.- Diamateous Earth
Metering pumps have long been known for their accuracy and reliability to provide precise dosing of liquids to a process stream. This understanding was generally limited to homogeneous fluids or liquids under 500 cps. The use of a high performance diaphragm can boost the cps capable of being pumps exceeding 7,000 cps (depending on pump size) and handle large solids content. This has significant application where heavy materials, earth materials or conveyance of other solids bearing fluids is required. This discussion consolidates the selection and application of high performance diaphragms in these specialized applications.
Generally, high performance diaphragms are a specific term to a manufacturers highest quality diaphragm used in its best metering pumps. Both the metallic and plastic versions are covered for their understanding for use in these difficult applications.
Viscosity and Slurries
The best method to begin understanding the High Performance Diaphragm is to understand the unique materials they manage. As stated in the Overview, high viscosity materials and slurries can be well managed by HPD’s1. A short understanding of viscosity and slurries as they apply here is proper to present a base from which to build the diaphragm assembly.
Viscosity is probably the most important characteristic in any pumping operation. It is affected by temperature and pressures, all governed by its specific gravity. Viscosity directly affects the friction coefficients within piping networks and has direct effect on the ultimate NPSH required to make the metering pump system work.
There are two general classes of viscosity2. One is Newtonian and the other is non-Newtonian. When the fluid is a linear or near linear function of temperature, the fluid is considered Newtonian. Shear is directly or almost directly proportional to the pressure imposed. Non-Newtonian fluids are affected either completely or near completely by flow or shear. They can be broken down into three sub-classes that are important to understand as they relate to metering pump applications. The Three sub-classes of non-Newtonian fluids are:
- Pseudoplastic- Viscosity is increased exponentially (or at a factor greater than linear) as the flow rate decreases. Polymers tend to behave in this manner;
- Thixotropic- Viscosity varies as a function of time to a constant pressure. Few chemical compounds tend to fall into this classification. In centrifugal pump definitions, Thixotropic and Psudoplastic materials are grouped together3
- Dilantant- Increase in viscosity with increase in flow. These are generally limited to paints and gels, and other specialty chemicals. These have significant danger to the metering pump if not understood. If the upper range of viscosity is not accounted for, there exists the problem of the viscosity possibly exceeding the rated capacity of the pump and stalling it.
It is important to know not only the viscosity of the fluid to be transported, but its characteristic (Newtonian or non-Newtonian) and viscosity range.
Solids content is the difference between a homogeneous fluid and a non-homogeneous fluid. Slurries generally are classified by one of four methods although the definitions become general at best as they depend on industry and material. In any event, the classifications are:
- Particle size as a factor of screening. This is particularly true for air and water filtration, or specific paper mill operations,
- Physical properties including abrasion, relative hardness and ability to form deposits,
- Overall composition of solid
- Settling factor using weight, specific gravity or settling rate
Diatomaceous earth, lime and carbon slurries that are common in mining operations and reclamation efforts are generally classified by weight. Paint pigment concentrations are generally by solids composition and can have concentrations as high as 50% and still be managed by a metering pump with an HPD. The example here shows the disparity in measurement techniques for the classification of slurry. It also puts the onus on the system designer to ascertain the solids concentration and the method of determining that concentration for the manufacturers’ sizing programs.
In any sizing and specification effort, the viscosity and, where applicable, the solids content in the slurry must be understood.
Temperature and Pressure
The contributing variables to viscosity are the factors of temperature and pressure. As noted above, in both Newtonian and non-Newtonian fluids, temperature is a critical factor in the determination of viscosity. It is therefore important to be acutely aware of the temperature changes or the temperature consistency in the environment around the metering pump and associated piping.
The various pressures that result within a piping network and as a result of pumping action constitute a family of reactions within the system. Any flow through a controlled volume causes this variety of pressure reactions that need to be understood and accommodated. Many of these pressures are a result of shock4 and must be managed in the head and piping design. Most shock forces are a result of reflection of the pressure wave reflection from a fixed boundary, or distinct structures such as piping elbows and injection quills.
Therefore, the major contributing pressures resulting within the HPD head to consider are:
- NPSH- for the overall pumping system
- Friction Forces- generated by viscosity as a result of pressure and temperature
- Shock Forces- from the movement of the fluid within the system reflecting off the fixed boundaries as a result of the pulsating diaphragm
Pressures and temperatures play significant roles in the condition and effect the conveyed fluid have in the system. All must be accounted for in the HPD because of the extreme service they are expected to peform.
Characteristic of High Performance Diaphragms
The moving disk within the wetted end of a metering pump is referred to as the diaphragm. High Performance Diaphragms (HPD) consist of specialty materials with the ability to provide a uniform thrust of motion conveying the material of interest into a process reaction or stream. While general diaphragms tend to be limited to around 500 cps, the HPD can, depending on the pump and wetted end, exceed 7,000 cps at normal operating temperatures.
HPD’s require several conditions for successful operations on a metering pump. First, they will only work on hydraulic metering pumps. The pressures required to move high viscosity liquids and slurries tend to exceed the general capacity of a mechanical pump5. However, always check the manufacturers’ test data for viscosity data to see the limits imposed. Second, there must be a method of balancing the hydraulic fluid in the plunger section to maintain exact uniform pressure on the diaphragm. This has to be balanced by not allowing any of the hydraulic fluid to leak out of the system. Leakage and balance is covered by the use of two technologies. These technologies are material science and fluid hydraulics.
The material HPD’s use in diaphragm construction themselves assist in sealing the hydraulic fluid is a high performance teflon, usually the PTFE variety. The use of this material ensures the proper seal and performance over the long term.
In order to maintain the integrity of the shape of the diaphragm and its associated pumping action, a hydraulic recirculating system for maintaining proper hydraulic pressure is employed6. This recirculating system releases a specific amount of hydraulic oil used in diaphragm movement when the pressure increases over a prescribed limit, and adds oil to increase the internal pressure when the pressure decreases below a prescribed limit. These significant changes are tolerances of less than 1% and are calculated so the net pressure, or the sum of the forces on the diaphragm is zero. Obviously, there will be net pressure changes as the diaphragm moves back and forth.
It is important to realize that with the extreme forces the pump is required to move within the system, the forces are transmitted into the conveyed fluid. This makes the fluid the conveyor of the shock forces as well as the internal pressures of fluid motion.
Another factor is that the HPD system does not require packing, as with standard packed plunger pumps. This allows for the high pressures and changing viscosities that are associated with the characteristic of this pumping option.
There are several characteristic actions that result in the operations of HPD pumps. These characteristic actions are inevitable and sometimes beneficial in the overall pump system.
NPSH is significantly improved through the use of the hydraulic recirculator. Since the adjustments are automatic and pre-set, no further adjustments are required. This mitigates the pressure limitations and contour place loss, allowing for more pressures to be brought into the system.
It can also be assumed that the higher the stroking speed, the lower the viscosity that can be pumped. It is a general characteristic of such pumping systems. Therefore, the range of metering pump viscosity pumping ranges should be verified along with the temperature and pressure range.
These factors are critical in pump sizing, providing a device that will not lock up with changing viscosity and will perform under the pressures and temperature ranges necessary.
It is quite possible and very probably to pump materials of high viscosity or with high solids content using HPD technology. It is important to understand the parameters and chemical characteristics of the materials to be pumped. It is also important to size the pump so the change in viscosity and pressure do not exceed the physical limitations of the HPD and pump body. By understanding the types of viscosity, effects of pressure and temperature and general chemical characteristics of the process fluid being conveyed, both Newtonian and non-Newtonian fluids can be accurately dosed into process applications.
1. MRC Technical Manual 339-0014-000
2. Metering Pump Handbook, McCabe, Lanckton and Dwyer, Industrial Press p 178, 179
3. Pump World, Tutorial, P.O. Box 176, Forest Hills, MD
4. Compressible Fluid Flow, Shapiro, Ronald Press Company, Sec. 16.3 inc.
5. These are generally referred to as ‘Mechanically-Activated Pumps’
6. In Milton Roy Products, this is referred to as the ‘MARS’ valve, standing for Mechanically-Actuated Refill System
About the Author: Thomas Day, Ph.D. is the Product Manager of Systems, Instrumentation & Controls Technology for Milton Roy America. For more information on Milton Roy’s metering, pumping and fluid control products, email firstname.lastname@example.org, call (800) 564-1097, or visit www.miltonroy-americas.com.