How Fluid Properties Affect Hydraulically Balanced Metering Pumps

Introduction
Diaphragm metering pumps that are hydraulically rather than mechanically actuated offer numerous attractions. Their accuracy is high (typically around 1%), and they can operate leak-free even under high pressures (around 5,000 psi). And, they have rightly gained a reputation for being able to handle slurries and corrosive fluids.

In spite of this versatility, optimal specification and use of these pumps required attention to a broad range of properties of the pumped fluid. these include vapor pressure, viscosity, rheology, temperature, corrosiveness, solids content, and specific gravity.

Vapor Pressure
As with other pumps, the vapor pressure of the fluid being pumped is important because it helps establish the available net positive suction head (NPSH) at the pump inlet. To assure proper delivery of the fluid to the inlet, the available NPSH must be equal to or greater than the manufacturer's stated requirement for the pump. Otherwise, the resulting cavitation can cause the pump to lose metering accuracy or even the ability to move the fluid at all.

Determination of the available NPSH for a metering pump is more involved than that for a centrifugal pump. The metering pump equation is:

NPSHa = Pa + Ph - Pv - ((Ls R G Q) / (C1 d2 ))

where NPSH a is available net positive suction head expressed in pressure terms. Pa is the pressure (usually atmospheric) on the surface of the fluid in the supply tank, Ph is the elevation of the fluid supply relative to the center-line of the pump inlet, Pv is the vapor pressure of the fluid at its flow temperature, Ls is the length of the line connecting the supply tank with the pump, R is the pump's stroking rate (in cycles per unit of time), Q is the volumetric flow rate through the supply line, G is the fluid's specific gravity, d is the pipe inner diameter, and C1 is a constant, specified by the pump manufacturer. The C1 term takes into account an average roughness for the pipe, as well as a typical number of pipe fittings.

The term LRGQ/C1d2 represents the supply-line friction loss typical with reciprocating metering pumps, known as the "acceleration loss". This loss is always significantly greater than the frictional loss associated with uniform flow. If the fluid viscosity is over 50 cP, the acceleration-loss term is replaced with:

[(Ls R G Q) / (C1d2)2 + (L µ Q / (C2d4))2]½

where µ is viscosity and C2 is a constant similar in nature to C1.

Several strategies can be considered to achieve an acceptable NPSH when pumping a fluid with high vapor pressure. The fluid temperature might be lowered, the layout of the system might be changed to increase Ph or decrease Ls, or a metering pump with a feasible stroking rate R that is low might be chosen. Selecting a pump with low stroking rate will usually lead the vendor to supply a pump with a larger piton, internal porting and valves.

Viscosity
Metering pumps, like most other pumps, can handle low-viscosity fluids more easily than highly viscous ones. A given metering pump's acceptable viscosity range is part of the vendor's description of that pump.

Because viscosity is defined as the ratio of shear stress (a fluid property) to shear rate (a dynamic condition of the system), the engineer is offered two basic approaches for lessening the pumpability problems caused by viscosity. The shear stress of a fluid can be reduced, ordinarily by raising the stream temperature. Or, the shear rate can be reduced, usually by selecting larger flow diameters for the system.

Other things being equal, metering pumps that offer minimum internal restriction are the best choices for viscous fluids. Pumps with tubular or conical diaphragms are good choices, because the flow path inside these pumps is less restrictive. Also suitable are flat-diaphragm designs, provided that the pump does not require a (perforated) front diaphragm-support plate.

Rheology
Newtonian fluids are shear independent. If their viscosities fall within the capability range for a given pump, they should pose no pumpability problem.

Non-Newtonian fluids offer a different challenge. Whenever a metering pump must handle a fluid whose viscosity rises with shear stress (i.e., a dilatant fluid), the fluid should be actually tested with the pump before the latter is purchased. Conversely, fluids whose viscosity decreases with an increase in shear stress (pseudo plastic and thixotropic fluids) can be handled by metering pumps even though the nominal viscosity seems too high. As a rule of thumb, a pump may be applied to most fluids of this type for reported viscosities up to four times the pump's viscosity limit for Newtonian fluids.

Temperature
Because metering pumps must deliver precise volumetric flow rates, extreme changes in fluid temperature must be considered. Be sure to take into account the possibility of seasonal and even daily cycles in what is nominally a constant temperature.

For fluids so hot or cold that they exceed the temperature limits of components in the pump, the engineer should consider pump configurations that separate the process-fluid handling (the wet end of the pump) from the pump drive (the working end).

In such arrangements, which can accommodate process fluids from -185o to +800o F, an intermediate fluid with suitable thermal properties transmits the force from the ambient-temperature working end of the pump to a remote wet end operating at the process-fluid temperature. The hydraulic fluid in the working end interacts with the intermediate fluid via a flexible diaphragm; the intermediate fluid then interacts with the process fluid via a second diaphragm.

To limit transmission of heat, hot fluids are pumped with the remote head at an elevation above the working end of the pump cold fluids at an elevation below it. Typically, the transition pipe joining the process end with the working end is about 1 to 3 feet long. Its length and diameter are chosen so as to house a volume of intermediate fluid that is around three times the displacement of the pump's piston. As the overall body of intermediate fluid moves back and forth with the diaphragms, any given element of it does not travel any great distance. Thus, distinct temperature zones will be approximated within the transition pipe if it is sized correctly.

Corrosivity
Compatibility between process fluid and all wetted material is important to assure long equipment life, product integrity, safety and pump performance. Selecting wet-end materials of construction must take into account not only corrosion tables but also the local corrosive environment. In particular, keep in mind that the pump diaphragm moves back and forth, and thus is in a dynamic rather than static condition.

Slurries
Diaphragm metering pumps that are hydraulically balanced can handle slurries with solids contents up to about 20 wt%. Even higher concentrations, of around 30% to 40% can be accommodated when the suspension is colloidal.

A tubular diaphragm is usually the best for slurries, since all areas that may entrap solids are eliminated. Flat diaphragms are suitable if solids do not accumulate inside the pumping chamber at the diaphragm face.

Better metering accuracy can be obtained by using elastomeric seats within the valve assembly. Such seats provide an improved sealing surface and better abrasion resistance for some slurries. However, these pumps are limited to discharge pressures of about 150 psig.

For slurry handling, pumps with only moderate stroke rates (116 strokes per minute or less) should be selected. This will limit the effects of erosion and improve the valve's seating action.

An average flow velocity of at least 6 to 7 ft/sec is required to prevent settling of most slurries. This threshold should be met over the entire planned flow range for the pump. Typically, the minimum required flow is set at 20% of maximum. While the velocity should be kept high, a balance must be made to assure that the high velocity does not reduce the NPSHa to below an acceptable limit nor lead to excessive abrasion.

Specific Gravity
For fluids with specific gravities above 1.5, moderate stroke rates should be used. Otherwise, when the fluid is accelerated on either the suction or discharge side, the fluid momentum can be so great as to become irreversible as the pump reciprocates between its discharge and suction action. This effect, called inertial flow through, leads to greater pump throughput than anticipated, and thus a loss in accuracy.

Also, fluids with a high specific gravity may cause the ball in the check valve assembly to float. With fluids of specific gravity (s.g.) greater than 2.0, this often becomes a particular concern for light ceramic or glass balls, whose s.g. is 2.2 to 4.0. In these cases, a heavier metallic ball (s.g. around 8) is recommended. In extreme cases, tungsten carbide balls (s.g. of 15) may be the only satisfactory choice.