I
EDWARD G. HOFFMAN and D.
M. CALKINS
Merck & Co., Inc., Rahway, N. J.
Design Factors Affecting Selection of Pumps for Chemical Service The design of a pump, like any other piece of equipment, must consider certain engineering factors and should end up as a work of art
A while it is a simple piece of equipment, is claimed to rank second PUMP,
only to the electric motor as the most widely used industrial machine. Its importance is often discounted because of its common usage. More time is often required in the design of a process pump than other items of equipment costing much more. A pump is one of the highest cost items to maintain in a chemical plant because of its high speed, its gland which is difficult to maintain, and the steel bearings which fail a t times to operate smoothly in corrosive environments. The lowest competitively priced pump may satisfy the purchasing department but it may be a constant source of expense to the maintenance department. Therefore, a balance must be reached between the first cost and maintenance cost. Only knowledge and experience in pump design will reach a satisfactory solution. Design factors affecting selection of chemical pumps can be listed in three groups : materials of construction, specific factors, and general factors. Materials of Construction
This is one of the most important design factors to consider because this may decide the type and class of pump which can be used or is available in the required size, and in the time permitted for procurement. A titanium pump may be desired for a n operation, but certainly would not be available in two weeks!
Standard Iron Carbon steel Bronze High silicon iron A I S 1 300 Series S.S. (primarily 316) ACI type CN7M -e.g. Worthite, Ircamet, Durimet 20,and other ‘“20” designated alloys Impervious graphite Rubber-lined
Advantages and Disadvantagesof the Three Most Widely Used Pumps for Chemical Service Centrifugal
Diaphragm
Rotary ADVANTAGES
Wide range of operating conditions Excellent for continuous heavy duty Flexible operating characteristics Good ease of control Optional features for special situations-such as, steam jacketing, selfpriming, and water cooling of stu5ng box-are readily available
Constant volume for set speed Directional flow interchangeability Small space requirements
No packing problems Handles toxic or expensive materials Handles certain slurries with ease
Wide range of head and viscosity Self-priming Handles vapors and gases Low cost
DISADVANTAQEB
Pressure, viscosity, low capacity limitations
Close clearance, limiting the type of fluids
Volume control di5culties
Must be protected by relief valves Limited material construction
Selection of a material of construction for a pump is a complex problem involving one or more of the following: temperature effect, concentration effect, time effect, velocity and impingement effects, effects of impurities, and process changes. Chemical pumps are available in a wide variety of materials of construction. These can be divided into several classifications as follows :
Special Ductile iron Special bronzes Special A l S l 300 Series S.S. (AIS1347 and 329) Hastelloys Monel Nickel Ceramic Lead Aluminum Plastics Glass-lined
Extra Special Penton-lined Titanium Zirconium
Limited in capacity and materials of construction Higher cost Possesses inflexible operating characteristics Produces pulsating flow Requires a relief valve
The choice of materials is broad, the suppliers are numerous, and the prices are commensurate with the material of construction. I t is, therefore, important for the pump designer to have a well rounded knowledge of many factors concerning pumps, particularly the material of construction, before he commits his dollars. T h e source for this data is chemical engineering or equipment manufacturers corrosion data tables, your own research files, or actual laboratory or field tests. Due consideration should be given to selection of a material of construction when .handling a combination of liquids or two separate liquids, alternately. This is particularly true when the passive oxide film formed by the first solution handled might possibly be destroyed by the second solution. Usually the chemical engineer is best qualified to choose the pump material when requesting a design from a manufacturer. VOL. 52, NO. 7
JULY 1960
561
When designing pumps, if piping i s complex, such a system layout or sketch i s necessary to evaluate and calculate head requirement Suction pipe A-B 35’pipe f 55’(globevalve)+ Discharge pipe C-D 70’ pipe 5 5 ’ (globe valve) Static discharge head Static suction head
+
6’(ell)
+ (entranceloss) = 99‘.
+ 6 ’ (ell) = 131 ’
50g.p.m.in2”pipe= l O ’ l o s s / ~ o o ’ A-6 pipe friction= ( - 1 10’
C-D = 1 0 X 1 3 1 / 1 0 0 = =
Total head =
Either the desired material should be specified, or the chemical to be handled should be specified with all of its characteristics.
Specific Factors This second most important design consideration decides the size of the pump and motor. Capacity. First let us consider capacity requirements. I t must be determined whether a constant capacity or a range of capacities with a maximum or minimum can be permitted. Job conditions usually fix capacity requirements. This factor may determine whether to use a centrifugal or a positive displacement type of pump, and what horsepower will be required. The minimum amount of capacity should be selected to permit the smallest pump without affecting process timing. A 500gallon batch pumped in 10 minutes with a 50-g.p.m. pump is tolerable, but a 5000-gallon batch pumped in 1 hour and 40 minutes, with the same pump, could be unsuitable for many reasons. Consideration should also be given a t this point to future pumping requirements in the event increased production is a possibility. I n such a case, a few additional dollars expended could result in the savings of a n addi-
562
(-1 I-)
13’ 60’
(+)30‘ 53’
tional future pump plus the cost of removal of the original pump. Head. At the same time that capacity is selected, it is necessary to determine the head requirrments. The maximum, minimum, and possible future total head against which the pump will be required
INDUSTRIAL AND ENGINEERING CHEMISTRY
to operate, must be satisfied. Many of the troubles experienced in the operation of pumps are attributable to mistakes in estimating the head against which they must work and the size of suction and discharge pipes. Unless one is designing pumps frequently, the only method to evaluate properly the head requirement is by the use of a system layout or sketch, preferably in isometric, if the piping is complex. T h e layout should be based on the actual job. If it is necessary to design from a preliminary flowsheet, which is usually the case in competitive industries such as the pharmaceutical, the designer should rely on his past knowledge of pipe lengths, fittings, numbers of valves, line filters, and building heights for typical installations. Such a system layout together with the calculated head is shown a t left. Note that all valves are converted to equivalent length of pipe. Cameron Tables, T h e Hydraulic Institute, pump manufacturers, and others supply tables which are useful for determining friction in pipe, valves, and fittings. Unknown friction values for heat exchangers, filters, and the like must be estimated as accurately as possible. The total head required is the sum total of all frictional components and the total elevational lift including the suction lift or minus the positive suction head, whichever is the case. Positive suction head can vary but usually this is insignificant in most pumping situations. Heads should always be expressed in feet as most pump curves are expressed in this unit by manufacturers. The chart below illustrates a headcapacity or pump-performance curve for a centrifugal pump. It shows the variation of head and capacity at a constant speed. They usually also show
1750 F ” i 4
~
8 $40
N.FiS.H 15
2
5
IO MARCH2 1954
PUMP D E S I G N FACTORS 100 feet for water, gasoline, or brine. The pressure, however, depends on the specific gravity and will be 43, 32'/2, and 52 pounds, respectively. When applying a pump for a given pressure, convert pounds pressure into equivalent feet head and select pump accordingly. Do not expect a pump to handle a solid-liquid mixture as it would for a pure liquid. Power introduced to a pump is converted by the impeller into pressure energy and kinetic energy of the liquid. Solids cannot possess or transmit any pressure energy; therefore, when solid-liquid mixtures are pumped, solids can acquire only kinetic energy. Solids cannot convert their kinetic energy into pressure. Solids are moved almost entirely a t the expense of the energy imparted to the liquid by the pump. Therefore, as solids in a liquid tend to act as an obstruction to the flow, the resulting loss of energy results in lower pump head. Increases in solids content causes rapid decreases in efficiency. Many performance curves such as this one will show head and capacity characteristics for various-sized impellers for the same speed.
efficiency, horsepower, and net positive suction head, as does this one. T o design pumps properly it is important that performance curves for all sizes of pumps be a t your disposal. Rating charts are sometimes used for selection, but curves give a much clearer picture of the characteristics. Specific Gravity. Capacity is expressed in gallons per minute in the small and medium sizes, and head always in feet. Head is the height to which a liquid can be raised by a centrifugal pump, and is independent of the nature of the liquid-its specific gravity-as long as the viscosity is not greater than that of water. This is important to remember as most mistakes are made by adjusting the head characteristics of a pump by the specific gravity of the solution. T h e head produced by a centrifugal pump depends upon the tip speed of the impeller. A pump should produce the same head whether it is pumping water or mercury. The pressure at the discharge of the pump, however, will vary directly with the specific gravity. This is best illustrated by the diagram shown below. T h e pump shown will deliver a head of
n t
100' (43 LE.)
WATER SP. GR.-I.O
I
T
43 X 2.31
43 LB.
11 WATER SP. GR.-1.0 np, = 200
x
100
3960
x
7o
1.0
=
7.2
n t
P W GASOLINE
43 X 2.31 43 LB. =0.75
SP.G R . 4).75
I I
II
BRINE
SP. GR.-1.2
:134' 43 LE. =
43 X 2.31 I .2
~
=
84 I
This is a convenience to the designer who requires many pumps for a project, and desires to maintain as many of the same sized pumps as possible, thereby reducing spare parts inventory. Curves are also available for various speeds such as 3600, 1800, and 1200 r.p.m. for 60-cycle current; and 3000,1500, and 900 r.p.m. for 50-cycle, where these are required. Designing pumps for use in areas where 50-cycle current is used necessitates curves for speeds in the 3000-, 1500-, and 900-r.p.m. range. For a specified head and capacity, a size or type of pump is selected from manufacturer's curves. A point to bear in mind when reviewing centrifugal pump curves is not to select a pump to operate at less than 20 or 30% of its rated capacity. .Also pumps developing high heads should not be operated at less than 30 to 40% of rated capacity. This guide applies where pumps will operate for long periods. The horsepower required to drive the pump is determined from the curve. Referring to the chart again, a capacity of 50 gallons per minute will require 4 horsepower \vith a 9-inch impeller. This is water horsepower as all curves are based on water. M'hile pumping 66' Be. sulfuric acid (specific gravity of 1.84), it will he necessary to adjust the horsepower requirements accordingly. Here the designer must be careful. Kote that the curve is somewhat flat. A4ssume his head calculation was in error and instead of 85 feet he has only 75 feet. .4s a pump must follow its curve, it means the flow is 90 g.p.m. instead of 50 g.p.m. The pump will then call for 5 horsepower instead of 4 horsepower. Most open motors can be overloaded 1570 for short periods without serious harm, hut closed motors cannot. T o overcome the damage of a burned-out motor it would be advisable to increase the motor horsepower to be safe. In fact, some designers follow the practice of specifying maximum horsepower for full pump capacity based on water for the smaller sized pumps. This practice affords greater flexibility and permits future relocation without the need of changing motors. Before leaving the subject of horsepower, one point should be remembered and that is, never use more horsepower than the shaft can transmit. A few centrifugal pump manufacturers indicate on their performance curve the maximum horsepower permitted without overloading the shaft. Self throttling could also prevent overloading of a motor. This is possible if actual head is less than design head, in which case the capacity will increase to a point where resistance to pipe flow reaches an equilibrium.
6-fdL GASOLINE SP.GR.-0.75
HP. =
200 X 134 X 0.75 = 7.2 3960 X .70
BRINE SP. GR -1.2
Hp, = 200 X 84 X lIZ =
3960 X .70
7,2
I (Top) For a given head (in feet), horsepower will vary according to the specific gravity of the liquid Assume 200 9.p.m. i s required at a total h e a d o f 100 f e e t and a pump efficiency of 70%
(Bottom) For a given pressure in pounds per square inch, the head will vary according to the specific gravity of the liquid, and horsepower will remain uniform Assume 200 g.p.m. is required at a pressure o f 43 pounds and a pump efficiency ai 70%
VOL. 52, NO. 7
e
JULY 1960
563
T o prevent overpumping when total head is not definite, a control valve on the discharge will prevent overloading a motor. Where it is necessary to calculate hydraulic horsepower, the formula is : Horsepower = g.p.m. X feet head X specific gravity 3960 X efficiency Efficiency is often shown on performance curves but for the most part is not a n important factor in process pumps of the smaller sizes. I t is always desirable to operate a t the maximum efficiency and is an important factor on the larger size pumps. Process pumps, unless on continuous service, will not result in undue cost if operated inefficiently. An important point to remember always about pump performance curves is that variations of head, capacity, and brake horsepower with speed follow definite rules known as affinity laws. Within reasonable limits when applied to every point on the performance curve these laws are as follows : When speed is changed, capacity varies directly as the speed, head varies directly as the square of the speed, and the brake horsepower varies directly as the cube of the speed. Expressed as formulas letting Q equal volume, H equal head, and S equal speed,
Also, for constant speed, the capacity of a centrifugal pump varies directly with impeller diameter, and head varies as the square of impeller diameter, and horsepower varies as the cube of the diameters. These rules are most useful when performance curves are in short supply for various speeds or impeller diameters.
Q = AV; H =
v2 -
2g
and V = RDN
ATM OS PHER I C PRESSURE 14.7 R S , k
Suction Conditions. Net positive suction head (N.P.S.H.) has been mentioned before. On a pump performance curve, N.P.S.H. is most always shown by pump manufacturer. Each pump has its individual required N.P.S.H. characteristics which the manufacturer can plot on a performance cume. Values shown are based on tests and are corrected to the centerline of the pump suction. Net positive suction head a t any point on the curve is head in feet of liquid pumped equivalent to pressure in pounds per square inch or push required to force liquid into the pump. A good pump designer must therefore locate the pump and design the suction piping so available N.P.S.H. or push is equal to or greater than required N.P.S.H. needed by the pump. Shown below is a typical pump handling water where the suction head is negative. T h e available N.P.S.H. is calculated by considering the atmospheric pressure, vapor pressure, lift, and pipe friction. T h e required N.P.S.H. is obtained from the performance curve. The available head is greater than the required head which must be the case if the pump is to perform satisfactorily. Why is N.P.S.H. important? If the N.P.S.H. a t the pump intake is not sufficient, cavitation will take place and will impair the performance of the pump. I t may cause considerable vibration and noise. The head-capacity curve will no longer apply and serious pitting and corrosion will result, particularly in alloy metal impellers. If any design results in a questionable available N.P.S.H.-equal to or less than the required N.P.S.H.-consult the pump manufacturer to prevent serious operational trouble after the pump is purchased and installed. Viscosity. This factor in a liquid is often neglected in pump design. Often it is not serious enough to affect a pump’s
performance-but occasionally it can be the cause for considerable additional expense and delay. Viscosity of a liquid is the measure of its resistance to internal flow. I t has a marked effect on the characteristics of a centrifugal pump when compared with the performance of water. I n general, in pumping Newtonian liquids-those that resist shear in direct proportion to the rate of shear-of high viscosity using centrifugal pumps, the head and capacity a t the best efficiency point is reduced by additional friction losses and the brake horsepower increases. This is noticeable with liquids of viscosity over 200 S.S.U. T h e performance of a pump handling viscous liquids is estimated by means of corrections applied to the water performaxe a t maximum efficiency. Performance correction charts can be found in the Stanciards of Hydraulic Institute or are made available from pump manufacturers and should be considered in pump design on viscous materials over 200 S.S.U. Viscosity can be a factor in pump selection. Centrifugal pumps can be used on viscosities u p to 2000 to 3000 S.S.U. Rotary pumps can range from water u p to millions of S.S.U. The practical range of rotary pumps runs from about 70 S.S.U. upward. Low viscosity materials have poor lubricating properties, hence the centrifugal pump has advantages over the rotary. Pipe friction is also greatly affected by viscosity. Total head calculations can be in considerable error by insufficient knowledge concerning friction losses with viscous liquids. Tables should be consulted before assuming everything has the same viscosity as water. Vapor Pressure. This is important to consider when a pump is operating under a negative head or handling a liquid of high temperature. I n applications where pumps are used on the
PUMP
30”HG. Lift, 20
EQUALS
34‘
WATER T
PIPE FRI CTI ON
+
6 FT.
I
I
200
C A PA C I T Y Available net positive suction head at the pump intake must always be greater than the required to prevent poor pump performance left. Available N.P.S.H. The energy from the suction source availabe to 34’-20’-6’-1= 7‘ drive the liquid into the pump-Le.,
’
564
INDUSTRIAL AND ENGINEERING CHEMISTRY
Right. N.P.S.H. (of Pump). The external energy needed in the liquid to overcome the entrance losses of the pump-expressed in feet of heodi.e., with 200 g.p.m. and 60’ head, the required is 5’
PUMP DESIGN FACTORS bottom of vacuum stills or evaporators, vacuum filters or on lift operations, the vapor pressure must be known to determine the N.P.S.H. as mentioned previously.
General Factors
There are other important design considerations which must be analyzed. Cost, availability, spare parts, space availability, and standardization are factors which may decide or influence the selection of a pump. Initial Cost. This must be balanced against its maintenance cost. Your acquaintance with operating department personnel will afford you this knowledge. Maintenance cost records, if available, will dictate what type, alloy, or make of pump should be avoided for certain applications. Failure in service can make the lowest price pump the most expensive in the long run. Availability. Many times this factor will determine what can be used. Today, common materials of construction, such as all iron, bronze, stainless steels, rubber-lined, ceramic and silicon irons, are off-the-shelf items in common sizes. T h e more expensive alloy pumps are longer delivery items and this factor must be considered. Spare Parts. Their cost and availability are usually never a problem to the pump design man unless he happens also to be the maintenance engineer. Thorough acquaintance with a pump manufacturers’ ability to handle your spare parts problems may influence your selection of a pump. On critical service it may be advisable to order spare parts when the initial order for pumps is placed. For foreign projects, the designer should estimate carefully his spare parts requirements and order spares as if they were a part of the original unit. Space Limitations. This may decide that vertical pumps will be the prime consideration in the design selection. Close-coupled pumps will require less room; however, their use in chemical service is not desirable as leakage at the gland will attack the motor housing. “Canned” pumps may be considered as an excellent space saver but their use is limited to clear solutions as the fluid pumped acts as a bearing lubricant. Serious consideration should be given by chemical pump manufacturers to the design of in-line pumps similar to the one recently reported by a West Coast oil company. If one considers that the building space occupied by the average pump with base and motor costs from $50 to $200 to construct, it can be readily seen there are advantages in in-line pumps.
Standardization. This factor should not be overlooked by the designer as it is becoming more important every day. For the past 2 years, a committee of the American Standards Association has been undertaking the problem of standardizing pumps for the chemical industry. This is a giant task considering the number of manufacturers and types of pumps and models involved. Any fairly large organization which is cost conscious must eventually standardize on pumps. Ignoring this problem will result in large expenditures for spare parts and space to store them. Some plants-particularly those which cannot afford shutdown time-must stock spare units for quick replacement. I t is here that standardization is a must. If you have not standardized yet, start tomorrow-your company cannot afford to do otherwise.
Pumps Used in Chemical Service Most of the design considerations mentioned apply to the three most widely used pumps for chemical service: centrifugal, rotary, and diaphragm. T h e centrifugal is the most widely used and versatile. I t is the most competitive, has the largest range of sizes, and greatest diversity of materials of construction, A centrifugal pump is simple in construction, has only one moving part, is lowest in cost, and has small space requirements. T h e rotary pumps most common to chemical service are the gear, vane, screw, and cam. Their use is generally confined to clear, nonabrasive solutions. They find use in handling a wide range of viscous materials, high pressures, nonlubricating materials, volatile fluids including gases or vapors, combinations of heavy fluids and solids. They work well for applications handling low capacities a t high pressures. Diaphragm pumps should be mentioned since they have a definite place in chemical operations.
Stuffing Box This has to do with a little problem at the opposite end of the volute where the pump shaft protrudes. I t is known as a stuffing box-commonly referred to as the end which leaks. The two most common ways to control stuffing box leakage are by means of packing and mechanical seals. A pump should always be designed so that a packed stuffing box or a mechanical seal may be used interchangeably without having to modify either the pump or seal. This flexibility enables the user to decide his perference and permits more latitude.
Conventional packing is held in the stuffing box by means of a gland. It acts as a washer throttling the liquid that leaks between the rotating shaft and packing. Some leakage is necessary for lubrication, cooling, and sealing, thus preventing scoring the shaft and burning the packing. To maintain lubrication and prevent overheating, water, if compatible with the fluid pumped, is introduced into a seal cage or lantern gland located in the center of the packing rings. T h e packing gland should be adjusted to allow the proper amount of leakage which is necessary. There is no magic material yet developed which will completely eliminate excessive leakage at the stuffing box without proper attention. From a design standpoint, the important thing to bear in mind is that the material used to prevent leakage must be resistant to the liquid being pumped. I t is amazing how many chemical pumps are initially installed with a lubricated packing to handle some chemical-containing solvents with the result that after several hours service the remaining asbestos is a dry, hard, abrasive mass. One of the better packing materials for chemical service is molded Teflon in combination with mica or Teflon suspensoid and blue African asbestos. While Teflon packing costs more, its use is recommended where any doubt may exist regarding conventional packing. Mechanical seals have definite advantages over conventional packing, such as controlled leakage, freedom from product contamination, reduced maintenance, and ability to handle highsuction pressures. Mechanical seals are available in practically any material of construction and are not limited to a few basic materials, such as is the case with packing. Seals must be designed for the particular application and is a j o b for the specialist. Special features such as cooling, flushing, quenching, required in a pump are possible with mechanical seals. Sufficient experience has been developed over the past 10 years to ensure today’s mechanical seal from failing suddenly, if properly designed for the particular application. If and when seals fail, make certain spare parts are available for immediate replacement. A brief exposure to the factors discussed will not make you an expert, but it shoud serve as a general guide in achieving the artist’s definition of a designer-the arrangement of details which will make u p a work of art. RECEIVED for review January 29, 1960 ACCEPTED February 23, 1960 Division of Industrial and Engineering Chemistry, 137th Meeting, ACS, Cleveland, Ohio, April 1960. VOL. 52, NO. 7
JULY 1960
565
