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. All three
options appear in Figure 1.
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.
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 NPSH a
to below an acceptable limit nor lead to excessive abrasion.
