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ROBERT E. GILLMOR Eastman Kodak Co., Rochester, N. Y.
Considerations in the Application of Pumps in Chemical Plants Selection of a pump should be based on careful consideration of all the elements: Operating temperatures and pressures Discharge pressure
Flow rate
N.P.S.H.
T H E HISTORY of pumping liquids has been dominated by the need to transport water. The objectives in pumping irrigation water out of the Nile River in ancient Egypt were not materially different from supplying water to a modern city. Even though the method has changed, the vast majority of pumping today is still concerned with moving water. Pumps are generally tested with water and rated by the amount of this liquid that they will move against various pressure heads. '4 pump specified to deliver 1000 g.p.m. against a 200-foot head would provide enough information to procure a satisfactory water pump whereas this would rarely provide a sufficient basis for ordering a chemical pump. While water has widely known and specific physical properties, the liquids encountered in chemical plants include an entire spectrum of physical characteristics. Moreover, chemical processes are generally more complicated than water systems and often create environment problems which are unique and of chemical origin. These points of difference between
chemical fluids and \vater require sufficient looking into to assemble a table of data Xvhich highlights those differences. Overlooking one or more properties or requirements can lead to an unsatisfactory pump installation.
Fluid Properties
-4self-priming centrifugal pump lifting a hydrocarbon mixture out of an underground tank had to be primed by pressurizing the underground tank. The pump was tested to prime with a suction lift of up to 20 feet of water. Still it would not prime this mixture with more than a 4-foot lift because the specific heat, heat of vaporization, and boiling point of the hydrocarbon were sufficiently lower than the same properties of water that boiling took place in the pump before it had evacuated the intake pipe. These properties of many chemicals are recorded in chemical handbooks and "The International Critical Tables" or may be determined by laboratory tests. Another physical property which affects directly the power requirement of centrifugal-type pumps is the specific gravity. Moreover, if the liquid temper-
Fluid properties Environment factors
ature could vary more than 100' F. under any circumstances, a plot of specific gravity us. temperature (Figure 1) will aid in specifying a pump and motor which will not be overloaded under the highest specific gravity condition. Similarly, if the composition of a mixture can change, it is advisable to assume the specific gravity of the most dense of the possible mixtures. Fluid viscosity can also have a direct effect on pump performance. .4lthough centrifugal pumps handle up to 2000S.S.U. viscosity liquids, both the decrease in head developed and the increase in power required change rapidly above 100 S.S.U. (Figure 2 ) . On the other hand slippage in positive displacement pumps decreases with increasing viscosity. A plot of viscosity us. temperature (Figure 3) will indicate this property a t all operating temperatures if the liquid is Newtonian. However, non-Neivtonian liquid such as resin solutions of from 50,000 to over 200,000 S.S.U. viscosity cannot only be difficult to get into a pump but will slip backwards through pump clearances at an unexpectedly high rate. This is due
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> 0,9000
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0.6 00 Oo
100
200
300
400
T E M P E RAT U R E , O F.
Figure 1. A plot of specific gravity vs. temperature aids in specifying a pump and motor which will not be overloaded under the highest specific gravity condition
566
160
60
100 140 CAPACITY, G P M
' 180
200 --lo;
Figure 2. Centrifugal pump capacity and power vs. viscosity shows that fluid viscosity has a direct effect on pump performance Data from R. E. Dolman, Chern. Eng. 59, 155-69 ( 1 9 5 2 )
INDUSTRIAL AND ENGINEERING CHEMISTRY
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SOLVENT
220 '0
50
IO0 I50 200 TEMPERATURE O F
250
Figure 3. Viscosity v5. temperature indicates that slippage in positive displacement pumps decreases with increasing viscosity
PUMP A P P L I C A T I O N
\
I,OOOC
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L 0
too
1000 10,000 PRESSURE DROP ACROSS PUMP, P S I . I
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Figure 4. With non-Newtonian liquids, the apparent viscosity decreases with increasing rate of shear or stress created by the pressurized solution forcing its way back through pump clearances
to the non-Newtonian characteristic of the solution whereby the apparent viscosity decreases with increasing rate of shear or stress created by the pressurized solution forcing its way back through pump clearances (Figure 4). As yet, very little data are available about the rates of shear within positive displacement pumps at varying discharge pressures. Therefore, non-Newtonian liquids should be tested in the pump which is being considered as a final determination that the required pressure level and rate of flow can be attained. Fluid suspensions from colloids to pebbles are successfully pumped where both the system and the pump are designed to overcome the inherent problems. The fluid portion can be described by its physical properties. The solids can be described as t o size and shape, density and hardness, surface texture and felting or packing tendencies. Then the combination of the two in laboratory tests or under simulated operating conditions will reveal such effects as foaming, gas adsorption, and settling rate. These data will give a physical description of the mixture and a basis for further experimentation. Extensive testing may be required to select pump construction materials which will withstand the corrosive and errosive action of the fluid. In some cases, satisfactory materials are either not available or are very expensive. As a result, the most economical solution to the problem may be to choose low cost materials, which give a reasonable pump life, and to replace periodically the worn out parts. However, this is not an ideal procedure. Pump manufacturer's data sheets usually provide a space for recording the results of experience with the
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50
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I50 200 TEMPERATURE,'F.
250
300
Figure 5. During pumping, the effective vapor pressure of the liquid is the most critical physical property (Data from T. Earl Jordan, "Vapor Pressure of Organic Compounds."
various metals. Where there is no reliable or relevant experience to fall back upon, the corrosivity of the liquid can be approximated from pH, electrical conductivity, and corrosion tests. Insofar as possible the corrosion tests should be conducted under simulated operating conditions of temperature, pressure, and agitation or under conditions which will provide data to be compared with other relevant test results. Erosion tendencies of solid suspensions can be related to other materials which have been pumped in similar fluid mixtures by comparing particle hardness, density, and shape. While these problems are immediate, the potential hazard in pumping explosive or physiologically active liquids should also be carefully considered. The relative explosive hazard is indicated by the liquid flash point, ignition temperature, and explosive limits. In addition, the odor, toxicity, radioactivity, or irritation effects will provide the basis for choosing a pump location or ventilation and influence the choice of shaft seals. In choosing a shaft seal, there are a multiplicity of considerations involved and many of them depend on the characteristics of the fluid. Whereas packing glands are usually satisfactory on water service because it is a reasonably good lubricant and the necessary leakage is rarely objectionable, the same thing cannot be said for many chemical solutions. Therefore the choice of a packing gland, rotary seal, or a sealless pump depends on the relative lubricating character of the fluid, the effects of leakage, and the cost of losing it. The lubricating requirement can be avoided or modified if water, oil, grease, or perhaps some high boiling liquid is
compatible with the pumped fluid, so one of these can be introduced into the lantern ring of a packing box or into an externally lubricated rotary seal. In this way, the consideration of the cost or effects of leakage may be vastly reduced. However, there still remains the determination of acceptable packing materials, seal components, or diaphragm materials. Asbestos and Teflon are chemically inactive but have negligible elasric properties while natural and synthetic rubber will swell and decompose in contact with many hydrocarbons and most solvents. The relatively new fluorinated elastomers, such as Viton .4, are resistant to most of these chemicals and offer tempting possibilities as packing and diaphragm materials or for rotary seal O-rings. Therefore, materials testing is a continuing program to make not only the best use of established materials but to take early advantage of new materials.
The System In pump selection, advantage can also be gained from studying the system design and examining its functions. Looking carefully into each functionsuch as liquid movement, heat transfer, metering, mixing, and temperaturepressure or flow control will not only establish the capacity and pressure head required but may also reveal system design changes which will facilitate pumping. It may seem elementary, but no single cause of pumping failure is as prevalent as the failure to get the liquid into the pump, The best solution to this problem is to locate the pump below the source of supply or pressurize the supply tank to push the liquid up into the pump. \Vhere this is not possible, a means of priming the pump is required. Successful operation then depends upon the elimination of any air leaking into the intake pipe or pump and upon the properties of the fluid. During pumping, the effective vapor pressure of the liquid is the most critical physical property (Figure 5). (A conservative method of determining the vapor pressure is to bring the liquid to operating temperature in a round bottom flask. Evacuate the flask and record the absolute pressure when bubbles begin to rise.) The absolute pressure at the pump intake is the absolute pressure on the liquid surface in the supply tank minus the elevation and friction losses in the intake pipe and minus the liquid vapor pressure. If the remaining absolute pressure head is not sufficient to feed the pump, the liquid will vaporize before it reaches the internal parts of the pump. VOL. 52, NO. 7
JULY 1960
567
Once the liquid is in the pump it is u p to the pump to deliver the necessary quantity and pressure. However, it is desirable to minimize the pressure drop through the system, to reduce the power required, and to allow a wide choice of pumps. Unfortunately, in most systems the pressure required goes u p with increasing flow rates while pumps generally deliver constant or decreasing pressures as the flow increases. With centrifugal action pumps, this may on occasion be overcome by increasing the intake pressure with increased flow or by simply allowing the level in the intake pipe to build up (Figure 6 ) . O n the other hand, the minimum level in an open supply pipe should be above the pump intake or the pump will be starved and cavitation will take place within the pump. Cavitation can result in a pulsating flow and excessive vibration, while collapsing bubbles on the surface of a n impeller have been known to be as abrasive as solid particles. While the fluid is in the pump, it can be mixed or metered. Multiple objectives can be achieved by introducing one liquid component into a lantern ring to flush solids out of the packing or keep more objectionable liquids from leaking out of the stuffing box while the pump blends this added flow into the main stream. The rate of flow can be varied or controlled and metered in variable displacement piston pumps. These and direct pressure control or sanitary design are but a few of the extra services a pump can provide in meeting the requirements of the system. The Environment
I n a similar way, a pump’s environment should be examined before a selection is made. A noisy pump over an office can be as objectionable as a jack hammer. I n examining the pump location, a major consideration is the ultimate cost.
Pump Selection-Process Process Summary Name of process Rate of flow Total head Intake head available Operating temperatures Temperature extremes Fluid Properties Specific gravity Viscosity Physiological action Flammability
Data Sheet“
p-Cymene circulating system 80 to 140 g.p.m. 60 to 96 ft. 10 to 30 p.s.i.a. 300 to 400” F. - 10 to 450’ F.
See graph (Figure 1 ) 5.5 cp. at 100’ F., 0.4 cp. at 300” F. Low toxicity-pungent odor Flash point 141.5O F. Auto ignition point 750° F. Solids None Erosive action Similar to degassed water Avoid hot spots and carbonation Corrosive action Neutral, avoid air-liquid interface Materials of Construction Steel Lubricating Properties Surface tension 28 dynes/sq. cm. at 7 7 O F. Other Properties Affecting Seal Choice Sealless pump or stu5ng box Effect on Seal Materials Lead foil packing is satisfactory Avoid contamination System Requirements Heat transfer from boiler to reactor Pump function Circulate cymene Environment Factors Outside operation Mfr. A’s centrifugal pumps in similar service Explosion proof motor 440 volt power available Direct drive Locate under boiler Data from “Newport Para-Cymene as a Heat Transfer Medium,” Newport Industries Co., New York, N. Y.
Under some circumstances a long horizontal pump may be the most economical and convenient to install. O n the other hand, it may take u p valuable floor space when a vertical unit or a close-coupled pump mounted on a processing vessel would serve the same purpose. Acid fumes and moisture can cause corrosion problems originating from outside the pump. More often leakage from the pump eats up floors and sewer lines or corrodes nearby iron structures and instruments. Other pumps in the plant can influence the choice of a pump because a multiplicity of the same make and general type necessitates fewer spare parts and requires the accumulation of less “know how” to maintain them. Another indirect factor is the type, quantity, and cost of the various power sources which are available. Although electric drives are the most popular, they may not provide either the most economical or convenient installation. If there is use for low pressure steam for process or building heat, it may be economical to expand higher pressure steam through turbine or even piston pump drivers. The same methods will harness otherwise wasted water power. While compressed air is inherently expensive, it can be worth the cost for driving small, mobile pumps particularly in potentially explosive atmospheres where electrical systems must be explosion proof. Even the possibility of dust or hydrocarbon vapors forming a n explosive atmosphere is usually reason enough to eliminate all possible sources of ignition. In such circumstances sparks from belt
drives, from open electrical contacts or the ignition system and exhaust of internal combustion engines are hazardous. Assembly of Considerations
Pump ‘selection will be less haphazard if these physical data, system requirements, and environmental considerations are listed. The above tabulation will assist in choosing the right pump. When suggestions are made or quotations are received, these check points will highlight the need for pump characteristics which require special consideration. Physical properties which differ from water stand out as warning signs of problems which would be solved a t the planning stage. Equally productive is the possibility of not only providing the pressure and flow rate required by the system but also of carrying on other essential functions a t the same time. Finally, there is the satisfaction of knowing that pumping costs are based on the long term view which is often revealed by examining the pump environment. A properly selected, installed, and maintained pump is a pleasure to operate. As this is often more difficult to accomplish for pumping chemical solutions than for pumping water, the choice of a “just right” pump can be a source of pride as well.
40v REQUIREMENT
CAPACIT Y
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w
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“0
50
100 150 PUMPING RATE, G. P. M.
200
Figure 6. Pump capacity plus intake head matches system requirement
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INDUSTRIAL AND ENGINEERINGCHEMISTRY
RECEIVED for review March 28, 1960 ACCEPTED April 4, 1960
Division of Industrial and Engineering Chemistry, 137th Meeting, ACS, Cleveland, Ohio, April 1960.
