High Temperatures Link to homepage.

 

Overview

High-temperature applications are becoming more prevalent in the fluid handling industry.  Consider an application to be high temperature when the operating temperature is above 110�C / 225�F.  Pumps intended for use at ambient temperatures are not recommended for use at high temperatures without some modifications.  In fact, if an engineer does not take into account some of the problems that can result from using a standard or "stock" pump on a high temperature application, a variety of consequences may result -- from leaky gaskets and stalled drives to broken pumps and shattered casings.

There are a number of reasons why a liquid needs to be handled at a high temperature:

  1. The liquid is a solid or semi-solid at ambient temperature and heating permits it to be moved more easily or permits it to be applied, spread, processed or otherwise utilized. Materials in this category are asphalts, molasses, roofing tars, sulfur, lead, phthalic anhydride, etc.
  2. The liquid is a process by-product or is being used in a product that uses heat as a catalyst to initiate, sustain, accelerate or complete a reaction. Such liquids include those found in soybean processing(solvent extraction), refineries (cracking, blending, mixing, distilling), asphalts and syrups (blending and mixing), etc.
  3. The liquid is being used to transfer heat. This type of application is be coming more popular as industry finds it desirable to have a constant, dependable, easily controlled, economic source of heat with quick temperature response. Plastics manufacturers and molders, snow removal equipment manufacturers, food processors, candy makers, as well as chemical and pharmaceutical manufacturers are finding the convenience of low pressure, high temperature heat transfer liquids much to their liking.  Most of the liquids falling into the heat transfer category are are produced by large chemical and petroleum companies and are marketed under trade names such as Dowtherm (Dow Chemical), Therminol (Monsanto), Mobiltherm (Mobil Oil) and Ucon (Union Carbide).

Rather than discuss the particular problems in selecting pumps for specific high temperature applications, this page will discuss considerations needed for the various pump parts in a high-temperature application.

External Metal Parts

TABLE 1.
Information on Metals Relevant to Proper Selection as
External Metal Parts for High-Temperature Service

Metal

Coefficient of Thermal Expansion

Relative Corrosion Resistance

Maximum* Practical Temperature (�C / �F).

Cast Iron

6.5

Fair

345 / 650

Low Carbon
Cast Steel

7.0

Fair

425 / 800

Ductile Iron

7.0

Fair

As cast (Pearlitic) 345 / 650

Heat treated (Ferritic)
425 / 800

*425�C / 800�F is widely regarded as the maximum pumping temperature.

External metal parts are considered to be those that are in contact with the liquid on the inside and exposed to ambient conditions on the outside.  These parts include the head, casing, bracket or rotor bearing sleeve, relief valve body, bonnet and cap, head jacket plate, and packing gland. Idler pins and shafts could possibly be included in this list but will be covered under separate headings. 

Consider the following factors for selecting a suitable metal for external pump parts for a high temperature application:

  1. Dimensional changes occurring as a result of thermal expansion.
  2. Resistance to corrosion.
  3. Maximum practical temperature for use in PD pumps as determined by thermal shock resistance, loss of strength, or other factors.

Table 1 gives information relative to these factors for the various metals commonly used for external metal parts for high temperature applications.

Dimensional Change

Dimensional change resulting from thermal expansion is an important factor in the selection of a material for the external metal parts for two reasons:

  1. The effect on the running clearances within the pump.
  2. The effect on various interference fits used in the pump (for example the idler pin in the head, and the bushing in the casing, bracket or rotor bearing sleeve).
Figure 1.

Interference fits may increase or decrease at elevated temperatures depending upon the thermal expansion rates of the various parts. For example, an idler pin of low or medium expansion rate installed in a head with a high expansion rate will lose its interference or "fit" at some definite elevated temperature (see Figure 1). In actual practice, this loss between any pin and head taken from a group of subassemblies would occur some place within a temperature range rather than at a predictable temperature because of the variation in initial interference due to manufacturing tolerances on the parts involved.

A loose pin could be forced out of the head causing pump failure and cause hot liquid to be sprayed around a work area. Particular attention should be given to this problem of loss of fit when considering a tungsten carbide idler pin for high temperatures. Its low coefficient of expansion (see Table 2) may require increased interference fit when used on high temperature applications. If the expansion rate of the idler pin is equal to or greater than that of the head, the interference fit remains the same or increases. Considering the metals normally used for head and idler pin, there is not enough increase in interference fit to cause either of the metals to be stressed beyond their elastic limit.

TABLE 2.
Information on Idler Pin Materials Used for High Temperature Service

Idler Pin Material Coefficient of Thermal Expansion
IN/IN�F x 10-6
Average Hardness at Room Temperature Hardness Retention at High Temperature Relative Corrosion Resistance Machinability Maximum* Practical Temperature (�C / �F) Comments
Carbon Steel (Induction Hardened) 7.0 62 Rc Poor Fair

Good (before hardening treatment)

235 / 450 A common idler pin material.
Nitralloy 7.0 92 R15N
(66 Rc Min)
Very Good Fair Good (before hardening treatment) 425 / 800 Frequently used for idler pins for temperatures above 235�C / 450�F.
440C Stainless, Hardened 5.6 55 Rc Good Good Fair (before hardening treatment) 425 / 800 Used when some degree of corrosion resistance needed at high temperature.
Coated Stainless Steel 9.0 56 Rc Very Good Good Poor - finish by grinding only 425 / 800 Good corrosion resistance in addition to good hot hardness.
Tungsten Carbide 3.0 91 RA
(70 + Rc)
Excellent Very Good Very poor - finish by grinding with diamond wheel only 425 / 800 Used when no other material resists wear.   Requires shrink fit.

*425�C / 800�F is widely regarded as the maximum pumping temperature.

Similar "fit" problems should be considered when selecting a bushing for casing, bracket or rotor bearing sleeve. If a bushing comes loose or is stressed beyond its elastic limit rapid wear and pump failure normally result.

Corrosion Resistance

With few exceptions, an increase in temperature makes a liquid more corrosive. In some instances a metal that is quite satisfactory for handling a liquid at ambient or room temperature re will do only a fair job when handling the same liquid at high temperatures. Thus, when working at elevated temperatures, it may be wise to consider a metal with a greater degree of corrosion resistance than normally required. The terms in Table 1 give an indication of corrosion resistance and are relative, but they may explain why it is sometimes necessary to use a different and generally more expensive metal than first thought necessary.

High Temperature Limits

What is Thermal Shock?
Thermal shock occurs whenever sudden changes in the temperature of metal in the pump take place.  Examples include spraying water on a pump handling hot oil during cleanup or letting preheated liquid into a cold pump on start-up.

The high temperature limits given in Table 1 are based on present usage, experience, laboratory testing, and extensive research.  The use of cast iron is limited to applications 345�C / 650�F and below because of the low resistance to failure by thermal shock. Some manufacturers have case studies of cast iron pumps performing satisfactorily in the 290-345�C / 550-650�F range.  As cast, ductile iron has a limit of 345�C / 650�F  because of thermal shock characteristics. Steel and heat-treated ductile iron both have useful properties above 425�C / 800�F.

 

 

Internal Metal Parts

Figure 2.

The rotor and idler (Figure 2) are the only internal metal parts considered for this discussion.

Generally, the selection of the metal for the internal parts is based on the same considerations as those involved in the selection of the external metal parts (Table 1). As with the casing or bracket, the selection of the idler material must take into account the coefficient of expansion to prevent "loss of fit" between idler and bushing and overstressing of bushing material. This consideration is even more critical with the idler and bushing because they are operating at a temperature even higher than the casing or bracket and bushing. In several instances the use of an idler and/or rotor of a metal other than that used for the external parts of the pump is desirable.

Consider using cast iron internals for an application above 345�C / 650�F that requires a pump with steel or ductile iron externals and is subject to a low chance of internal shock.  The advantages of cast iron over other metals are:

  1. cast iron is less expensive
  2. cast iron replacement parts can be manufactured and delivered more quickly
  3. cast iron internals with steel externals requires less extra clearances because of less tendency to gall.  This results in increased capacity.

Using hardened steel internals with cast iron externals is justified when pumping a high temperature abrasive liquid such as filled asphalt.

Be aware of differences in thermal expansion rates when using different metals for the internal and external parts.  Additional extra clearances may be necessary to compensate for difference in expansion rates.

 

Bushings

Figure 3.

The sleeve bushings (sleeve or journal bearings) are typically the idler and bracket bushings. In some pumps, the bracket bushing could be a casing or rotor bearing sleeve bushing (see Figure 3).

The idler bushing is the more critical of the two because it runs faster, is more heavily loaded, is in a hotter area, and has less chance of an outside source lubricant staying on the bearing surface. If a material will serve as an idler bushing, it will be adequate for the bracket bushing. In some instances less expensive materials can be used for the bracket bushing.

In general, sleeve bushings used in internal gear pumps operating at ambient temperatures should be self lubricating, compatible with the liquid pumped, and strong enough to be installed easily and carry the required load.  Moreover, sleeve bushings should be inexpensive, good conductors of heat, and hard enough to resist any abrasives in the liquid being pumped. High temperatures will compound the problem of bushing material selection with loss of strength, loss of components (lead sweat), loss of fit because of differences in thermal expansion coefficients, and thermal shock. Probably the most important and least understood complication is the one brought about by differences between the thermal expansion characteristics of the bushing material and the material of the idler or bracket.

A material such as bronze (high coefficient of thermal expansion), installed as a bushing in an idler such as iron (medium coefficient) has a finite temperature at which the compressive stresses in the bushing and the different rates of expansion exceed the elastic limit of the bushing material. The result is plastic deformation, or yielding, of the bushing which will then be loose in the idler when cooled.  Conversely, when a bushing material such as carbon graphite with a low coefficient is used in an iron idler, there exists a finite temperature at which a loss of press fit interference occurs. Again, the bushing will come loose. Table 3 compares various kinds of bushing materials.

Use shrink fits with bushing materials such as carbon graphite to increase the interference fit beyond that possible by press fitting. Shrink fits can be made by using idlers or brackets with reduced bores or by using bushings oversize on the O.D. The procedure for installing the bushing is to heat the metal part to an elevated temperature to expand the bore.  Then install the cool bushing in the expanded bore by dropping it in or by a light press fit. This operation is expensive and somewhat dangerous.   In addition, there is a practical limit to the amount of shrink fit that can be used without exceeding the compressive strength of the bushing material.

The bushing problems of plastic deformation or loss of fit are compounded by the necessary manufacturing tolerances on the bushing O.D. and the bore I.D., as well as the fact that most materials exhibit reduced yield points at elevated temperatures.

TABLE 3.
Information on Bushing Materials Used for High Temperature Service

Bushing Material Coefficient of Thermal Expansion
IN/IN�F x 10-6
Average Hardness at Room Temperature Self-
Lubricating Properties
Relative Corrosion Resistance Maximum* Practical Temperature (�C / �F) Comments
Bearing Bronze
80-10-10
10.3 65 BHN
(500 Kg)
Good Fair 235 / 450 Common bushing material.
Cast Iron 6.5 160 BHN
(3,000 Kg)
Poor Fair 345 / 650 Requires a break-in coating. Low load carrying ability.
Tungsten Carbide 2.5 91 RA
(70 + Rc)
Fair Very Good 425 / 800 Normally requires shrink fit for high temperature service. Standard bushing material in abrasive liquid pumps.
Ceramic 4.5 9 Moh
Scale
Very Poor Very Good 425 / 800 Low thermal shock resistance.   Normally requires shrink fit for high temperature service.
Carbon Graphite 3.0 70-100
Scleroscope
Excellent Excellent 425 / 800 Standard bushing material in alloy pumps.  Most frequently used material in pumps handling heat transfer liquids.
Carbon Graphite (High Temperature) 2.7 70-100
Scleroscope
Excellent Excellent 425 / 800 Carbon graphite material used when a shrink a fit bushing is required.

*425�C / 800�F is widely regarded as the maximum pumping temperature.

A sleeve bushing will run better and last longer if it is well lubricated. There are several ways to get a lubricant to the idler and bracket bushing. One way is to use the liquid being pumped as the lubricant. This is the easiest way, but if the liquid is very thin, there may not be enough film strength to support the load.  Another option is to use an outside lubricant, such as a high temperature grease. Unfortunately, some applications will be so hot that the grease will not have much value as a lubricant. Other applications may not be able to tolerate the contamination. Another way to get liquid to the bearing surface of the idler bushing is to drill through the bushing between the idler teeth.  The pumped is forced through these holes as the idler and rotor teeth mesh. This method of lubrication is frequently used with liquids such as asphalt where the liquid may have a tendency to build up on the idler pin if there is not positive flow to the surface. This cannot be done with bushing materials such as tungsten carbide that cannot be drilled.

Still another way to insure a flow of liquid through the idler bushing is to bring liquid from the discharge port through the idler pin into the bushing area. This can be done either internally or externally. A desirable but expensive way of lubricating both idler and bracket bushing is the injection of a cool supply of the liquid being pumped. This requires an outside supply source and a means of pressurizing it, possibly with another pump. If none of the suggested ways of lubricating the bushing are feasible, you may want to select carbon graphite because it offers superior self-lubricating performance.

 

Idler Pins

Idler pins are one of the most important functional parts to be found in the entire assembly. Factors to consider when selecting an idler pin are:

  1. Strength and hardness at ambient temperatures.
  2. Retention of strength and hardness at elevated temperatures.
  3. Thermal expansion characteristics of the pin material.
  4. Corrosion resistance of the pin material.
  5. Machinability.
  6. Overall cost.

Retention of hot hardness is an important factor when considering a material for an idler pin because of the relationship between hot hardness and wear resistance. High bearing loads, the relatively high speed of the idler (the idler turns faster than the rotor by the ratio of the number of teeth), and the almost complete lack of lubricating value of many liquids at high temperature all stress the need for a wear-resistant material for the pin.

The use of an idler pin material having a coefficient of thermal expansion different from that of the material in the head will result in an increase or decrease in interference fit between the head and pin. A decrease in this interference can result in a loss of fit with a loose pin or leakage along the pin. Steam can leak along the idler pin on jacketed pumps using pressurized steam as the heating agent.

Proper running clearances between the idler pin and idler bushing are very important. Different expansion rates of the various parts involved (pin, idler, and idler bushing) may require modification of the I.D. of the bushing to provide proper running clearance at high temperature.  The corrosion resistance of the idler pin material should be equal to or better than the other parts of the pump because of the critical part the pin plays in the successful operation of the pump.  Remember, high temperatures usually accelerate the corrosive activity of a liquid.

Idler pins are available in a variety of configurations.  Pump size, pumping principle, and material of construction will determine the configuration to some extent.   Examples of the different types include: straight-plain, straight-externally lubricated, straight-internally lubricated, stepped-plain, stepped-externally lubricated, and stepped-internally lubricated. Because of the difficulty of machining the super-hard pin materials (e.g., tungsten carbide, Stellites), these pins are usually furnished in a straight-plain configuration only. The selection of pins with design features such as lubrication holes, grooves, and steps must be restricted to materials that can be machined, heat treated to the required hardness, and ground to final size.

Table 2 compares the properties of many idler pin materials. There is no one material which is ideal in every respect for an idler pin. For example, the cost effectiveness and desirable thermal expansion rate of a 291 pin must be discounted when hot hardness and corrosion resistance are the determining.g factors. On the other hand, a pin of tungsten carbide may have the necessary hot hardness and corrosion resistance, but may have a low rate of thermal expansion that could cause loss of fit between the head and pin at elevated temperatures.  In this case, 760 Stellite may be a good compromise.

 

Shafts

The same factors are considered in the selection of shaft material as are considered in the selection of idler pin material. Different emphasis is placed on the shaft factors, however, since the shaft functions under somewhat different conditions.

Hot hardness, most critical when considering the idler pin material, is not quite as much concern when considering the shaft material since

  1. the PV factor (bearing load and surface speed) is not as high
  2. the temperature at the bracket bushing which supports the shaft (could also be the casing or rotor bearing sleeve bushing) is somewhat lower than at the idler bushing
  3. the wear occurs over the entire circumference of the shaft instead of one location as on the idler pin
  4. it is frequently easier to get outside lubrication to the bracket bushing area.

Consideration of thermal expansion characteristics is as important in selecting material for a shaft as for an idler pin, possibly even more so. There is a possibility of loss of interference fit between the shaft and the rotor. If the rotor is of a material that expands faster than the shaft material it is sometimes necessary to mechanically fasten the two by cross pins or set screws.  Or, it may be necessary to shrink fit the rotor onto the shaft. There is also the possibility of loss of end clearance ( see Figure 4) between the end of the rotor teeth and the head. The shaft may expand more than the rest of the pump because of a higher coefficient of expansion or because of the pickup of additional heat through the stuffing box area. If this happens the shaft being anchored on one end by a thrust bearing will force the rotor toward the head, decreasing the end clearance. This may progress to a point where the pump will bind and stall or the metals will "pick up."

Corrosion resistance of the shaft material should normally be similar to that of the idler pin. In those Instances where the liquid that leaks through the stuffing box becomes more corrosive as it is diluted by ambient conditions, or where the pump is located in a corrosive atmosphere, it may be desirable to use a shaft material that is more corrosion resistant than the material in the idler pin.

Consider the machinability of the material since shafts may be turned, milled, ground, and sometimes drilled. This eliminates materials such as Stellite and tungsten carbide because they can only be finished by grinding. It is because of this need for machinability of shaft material that it is necessary, if a hard surface is required, to consider a material that can be hardened after machining.

The physical strength of the shaft material is naturally of prime concern. The basic design of the shaft in many PD pumps is based on the physicals of standard shaft materials plus a sizeable safety factor. The physicals of the shaft materials normally used change very little within normal high temperature limitations of 107�C/225�F to 425�C/800�F. Thus, the strength of the shaft material is not of real concern except for unusual applications, and then only when characteristics of the application other than high temperature, (e.g, viscosity, speed or pressure) approach the limits of the pump capabilities.

TABLE 4.
Information on Shaft Materials Used for High Temperature Service

Shaft Material Coefficient of Thermal Expansion
IN/IN�F x 107.0-6
Average Hardness at Room Temperature Relative Corrosion Resistance Machinability Maximum* Practical Temperature (�C / �F) Comments
1045 Steel 7.0 215 BHN
(3,000 Kg)
Fair Good 425 / 800 Frequently used shafting material for high temperature.
Nitralloy 7.0 92 R15N
(66 Rc Min)
Fair Good (before hardening treatment) 425 / 800 Very hard shafting material, readily available.
Nickel, Chrome, Moly Steel 7.0 300 BHN
(300 Kg)
(32 Rc)
Fair Fair 425 / 800 Used when still higher strength is needed.
410 Stainless 5.6 250 BHN
(3,000 Kg)
25 Rc)
Good Fair 425 / 800 Used when some degree of corrosion resistance is needed.
316 Stainless 9.5 290 BHN
(3000 Kg)
(31 Rc)
Excellent Poor 425 / 800 Used when maximum corrosion resistance is needed.

*425�C / 800�F is widely regarded as the maximum pumping temperature.

For many applications basic SAE 1045 steel shafting is adequate for applications involving high temperatures.

 

Extra Clearance

All PD rotary pumps require finite amounts of running clearance between rotating and stationary parts for proper operation. Standard running clearances have been established by many years of experience and have proven to be close enough to result in excellent pump performance characteristics and free enough to assure ready interchangeability of parts, economical manufacture, and lack of interference or binding. For many rotary pump manufacturers, pump parts are manufactured to , standard basic dimensions.

The location, type, and magnitude of these standard running clearances are dependent upon many factors. The basic design of the pump; normal intended service; degree of tolerance control exercised on mass production operations; possible distortions induced by mounting, piping, internal hydraulic forces, and stress relief due to casting aging, are but a few of the factors affecting the establishment of running clearances in a rotary pump.

Deviations, oversize or undersize, from standard basic dimensions are termed "extra clearances" and are applied to parts in a given pump for one or more of the following considerations:

  1. High operating temperature.
  2. High viscosity.
  3. High operating speed.
  4. Galling or seizing characteristics of the materials of construction.
  5. Build-up tendencies of the liquid being pumped.

The common materials of pump construction exhibit the property of expanding in length and volume when raised to a higher temperature. This property is known as thermal expansion. For a specific material, the rate of change in length per unit of length for each degree of temperature rise is called the coefficient of thermal expansion. This coefficient is normally expressed in inches per inch per degree Fahrenheit, or in/in/�F. Average values of this coefficient, over a useful temperature range, vary from .000003 to .000011 in/in/�F for commonly used pump materials.

Figure 4.  Location of extra clearances.

At high temperatures, dimensional changes in the various pump parts occur to the extent that extra clearances are required over and above the standard running clearances. These extra clearances are required at high temperatures for two basic reasons:

  1. A difference of expansion due to the use of materials having different coefficients of thermal expansion.
  2. A difference of expansion due to two parts of the same material being at different temperatures.

In many cases, the following dimensions are varied to change running clearances for high temperature service:

  1. Bushing bores - increased.
  2. Rotor inside diameter - increased.
  3. Idler outside diameter - decreased.
  4. Rotor outside diameter - decreased.
  5. End clearance - increased.
Figure 5.

Figures 4 and 5 show the respective parts and extra clearance locations. Extra clearances are required for the bushing bores in the following cases:

When a bushing with a low rate of thermal expansion (carbon graphite, for example) is installed in an idler or bracket of medium to high rate of thermal expansion (iron, steel, 316 stainless) the bushing is free to expand at its own rate since the housing material expands at a higher rate. An idler pin or rotor shaft of medium to high rate of thermal expansion, running in the bushing is free to expand. It will do so until the normal running clearance is reduced to the point where inadequate lubrication causes an acceleration in heating. The result is binding or seizing of the bushing by the idler pin or on the rotor shaft.

A bushing with a high rate of thermal expansion (bronze, for example) installed in an idler or bracket with a medium rate of thermal expansion (cast iron, steel, ductile iron) will be restrained from freely expanding by the housing.  Thus, the bore of the bushing will not be as large at high temperatures as a free bushing of identical original size and material. Depending upon the material combinations the bushing bore size at high temperature may be the same or nearly the same as at room temperature. The rotor shaft running in the bushing or the idler pin upon which the bushing is running is free to expand and will do so until the normal running clearance available at room temperature is reduced to the point where inadequate lubrication is available, creating additional heat, more expansion, and eventual drastic wear and/or seizure and failure of the pump.

In both cases the answer to these problems is to provide ample extra clearance at assembly to assure proper running clearances at the operating temperature.

Extra clearance is applied to the rotor O.D. for two reasons. The first is the same as for the rotor I.D. and idler O.D. except the rotor O.D. will tend to rub or strike the casing bore instead of the head crescent. The second is that while the rotor is subject to the actual temperature of the fluid being pumped and expands accordingly, the casing is subject to a cooling effect due to its external environment. For this reason the casing is subject to a thermal gradient and expands less than the rotor. In addition the rotor O.D. extra clearance accommodates rapid rotor expansion when hot liquid is introduced into a cold pump.

Extra end clearance is required to provide for expansion of the various parts, preventing seizing or wear at elevated temperatures. Heavy-duty pumps feature rotor shaft restraint outside of the pumping zone. The increase in the length of the shaft from the thrust bearing to the rotor (see "Shafts") requires extra end clearance.   This is especially true of rotor shafts made of materials having a relatively high rate of thermal expansion such as 316 stainless steel. 

The actual amount of extra clearance is determined by the size of pump parts, materials of construction, and operating temperature.

As mentioned, there are factors other than high temperature (e.g., high speed or viscosity) when extra clearances are necessary. If a number of these factors exist in a given application, the factor requiring the largest values of extra clearance determines what extra clearances are used on the pump. The extra clearances required for each factor are not accumulated or added.

Certain alloy pumps present special problems and receive extra consideration when specified for high temperature applications due to the practice of using increased selected running clearances at ambient temperatures to prevent galling or seizing of the various parts.

 

Mechanical Seals

Mechanical (also known as "face") seals prevent leakage along the rotor shaft.  Over the years, their use has grown dramatically, especially over packing, for the following reasons:

  1. They are virtually leak-free; this saves product, reduces mess, reduces corrosion, eliminates possible health hazards.
  2. Do not require frequent attention and adjustment.
  3. Do not require outside lubrication.
  4. May in some instances be less expensive.
  5. Reduce inward leakage of air on vacuum applications.
  6. Eliminate shaft wear.

Mechanical seals do have some disadvantages, however:

  1. When a seal does fail, there may be significant leakage requiring immediate shutdown of the pump.
  2. It may be necessary to completely disassemble the pump to replace or repair a seal.
  3. The selection of one seal type and material for use in a pump or line of pumps intended for a wide variety of services is difficult if not impossible.
  4. For unusual or severe operating conditions a seal can be very expensive, sometimes exceeding the cost of the pump.
  5. Successful operation may depend upon considerable auxiliary equipment, e.g., flush lines, pressurizing systems, and cooling lines.
Figure 6. Basic seal components.

Almost all mechanical seals contain five basic parts (see Figure 6):

  1. Seal seat.
  2. Seal seat gasket.
  3. Rotating sealing ring.
  4. Shaft sealing member.
  5. Spring.

Some mechanical seals also use a positive means of driving the rotating sealing ring. Each of these parts or components takes on many different forms and materials depending on the type of seal, the application, and the supplier.

Seal Location Within the Pump

Figure 7. Seal mounted between the rotor
and cashing bushing.

Mechanical seals are typically found in three locations in a pump. The most common is between the rotor and the bracket or casing bushing as shown in Figure 7. Temperatures at this point in the pump are the same as the fluid pumped.  A seal in this location cannot handle a liquid any hotter than the seal materials will withstand.

A second location is in the pump stuffing box (Figure 8). This location is desirable from the standpoint of placing the seal away from the heat of the liquid pumped. This location also facilitates flushing, cooling, and quenching. Disadvantages resulting from putting a seal in this location are rather minor but should be kept in mind. Circulation around the seal is limited since the liquid is dead-ended in the stuffing box -- shaft modifications are required on some models.  It is also necessary to face off the end of the stuffing box and to drill and tap holes into the casing or bracket to provide access to the seal drive set screws. A flush line can circulate liquid to the seal area for liquids that tend to solidify.

Figure 8.  Seal mounted in stuffing box.

A third location for the seal is basically the same as the second except the seal is mounted with the rotating portion on the outboard side of the stuffing box. This places the seal still further from the heat of the liquid pumped, gives the seal the benefit of being cooled by the air flow around it, permits easy visual inspection of wear and leakage, and allows for simple adjustment. On the other hand, this exposes the seal to dirt and limits the amount of pressure on certain types of seals.

 

 

 

TABLE 5.
Temperature Capabilities of Seal Component Materials

Material Normal Temperature Limit Range, �C/ �F
Seal Component
Seal Seat Seal Seat Gasket Rotating Sealing Ring Shaft Sealing Member Spring
Cast Iron 260 / 500 -- -- -- --
Ni-Resist 260 / 500 -- -- -- --
Stellite 260 / 500 -- -- -- --
Ceramic 260 / 500 -- -- -- --
Buna -- 110 / 225 -- 110 / 225 --
Viton -- 175 / 350 -- 175 / 350 --
Aflas   205 / 400   205 / 400  
PTFE -- 235 / 450* -- 235 / 450* --
Kalrez   260 / 500   260 / 500  
Carbon -- -- 260 / 500 -- --
Steel -- -- -- -- 175 / 350
Stainless -- -- -- -- 260 / 500
235�C / 450�F is generally thought to be the approximate temperature limit of PTFE.  If the liquid being pumped is greater than 235�C / 450�F, move the seal to the stuffing box and cool it.
Frictional heat generated by the seal faces often raises face temperatures 10�C / 50�F to 38�C / 100�F above the liquid temperature.  This must be kept in mind when selecting seal material, location, and auxiliary equipment.

Compatibility of the Material of the Seal Components with the Product Pumped

The commonly used seal seat materials-cast iron, Ni-Resist, Stellite and ceramic-have increasing corrosion resistance in the order listed. When selecting a seat material for high temperatures several factors should be kept in mind: (1) most all liquids are more corrosive at high temperature than at ambient conditions, (2) no corrosion can be tolerated on the sealing face since this would destroy the smooth, flat surface that enables the seal to function and prevent leakage, (3) some seat materials have a tendency to heat check due to localized heating at the faces (this occasionally happens to Stellite) and (4) other materials such as ceramic are susceptible to fracture from thermal shock if there are sudden temperature changes.

The most frequently used materials for the seal seat gasket and the shaft sealing member are Buna, Viton and PTFE listed in order of increasing resistance to solvents or other chemicals. Viton has a useful range of chemical resistance somewhat better than Buna and can be used at temperatures above the range of Buna. PTFE is relatively inert to most liquids to temperatures of 235�C / 450�F but is the most expensive and difficult to work with.

Carbon, the generally used rotating seal ring material, is acceptable for use with most liquids handled at high temperatures.

The rotating elements, e.g., springs, drive collar, retainer, spreader, etc., can generally be furnished of a metal or alloy that will resist the corrosive action of the liquid pumped. Steel and 300 series stainless are normally used, with some of the less common alloys such as Carpenter 20, Hastelloy and Monel being used when necessary.

Figure 9.  Seal end plate showing connections for flushing, cooling, and quenching.

Special Features

Figure 10.  Double seal in stuffing box.

Pump Modifications Required to Accept the Seal

In the standard line of heavy-duty pumps there is little or no modification necessary to accept a seal for high temperature when located behind the rotor. When a seal is mounted in the stuffing box area it is necessary to have the end of the stuffing box faced off to accept a gasket; it is generally necessary to drill a radial hole into the bracket or casing to permit access to set screws in the driving collar of the rotating portion of the seal, and some pump models require reducing the shaft diameter or increasing the bracket bore to give more radical clearance for seal installation. Additional modifications may be necessary, particularly in the case of double seals, flushing lines, etc.

Cost

If the temperature of the liquid being handled goes up, the cost of the seal goes up. Cast iron, Buna N, carbon and steel can be used at 95�C / 200�F. At 205�C / 400�F a possible selection might be Ni-Resist PTFE, carbon and stainless, and at 260�C / 550�F a selection might be Ni-Resist, PTFE, carbon and stainless with auxiliary cooling and mounting in the end of the stuffing box. The higher the temperature, the higher the cost of the seal.

 

Packing

When application temperatures exceed 260�C / 500�F, consider the packing to determine whether it is suitable for the temperature involved.  Figure 3 illustrates a typical arrangement found in many rotary pumps. 

The standard packing is braided expanded PTFE with ultrafine graphite and mineral oil lubricant. It is generally recommended for solvents, acids, chemicals, and similar severe services.

Figure 11.  Cross section of packing area high temperature combination packing.

For higher temperatures, a high temperature packing combination is used as shown in   Figure 11. It consists of one inner and one outer ring (bull rings) of braided graphite filament yarn and the remaining center rings are die formed graphite tape containing no resin binders or inorganic fillers. This combination of packing can be used for pH ranges of 0-14 and temperatures to 425�C / 800�F.

In some instances metallic packing is used for high temperature applications, but normally it is furnished at customer request.

To adjust packing, start the pump and tighten the gland nuts until the leakage is decreased to a tolerable minimum. Make sure the gland bolts are tightened evenly. Check the pump packing area to see if it is overheating. A little leakage during the break-in period is necessary to help lubricate and cool the packing .

The following tips have been used with varying degrees of success to overcome a variety of packing problems:

  1. Use suckback groove in the casing to reduce the pressure in the stuffing box and thus reduce the amount of leakage.
  2. Flush an outside liquid through the bracket bushing or lantern ring to keep abrasives or sticky waste-type materials out of the packing.
  3. Introduce lubricant under pressure by lubricator or accumulator to the stuffing box through the bushing or lantern ring to keep liquid pumped out of stuffing box.
  4. Heat packing area prior to startup to soften a material that has a tendency to "set up" in the packing.
  5. Cool the packing area to increase the viscosity of the liquid pumped and thus make it easier to hold.
  6. Use alternating rings of different packings to get the benefits of the good features of both.

There are many packing manufacturers marketing a wide variety of quality packing materials. Frequently, a particular supplier's packing has proven superior in some respect over another supplier's for a specific application.

 

Gaskets

Gaskets provide sealing between mating parts. Examples include head gaskets, bracket gaskets, relief valve gaskets, and companion flange gaskets.  Common gaskets are made of treated paper, plastic, and compressed organic fiber.

Treated paper and plastic gaskets are considered satisfactory for temperatures up to 150�C / 300�F. For temperatures in excess of 150�C / 300�F, compressed organic fiber gaskets are usually specified. These gaskets are generally considered good for almost all high temperature applications.

In certain instances involving submerged pumps, no gaskets are used. On these pumps sealing is not critical. Normally, special internal pump modifications are required and it is best to consult the manufacturer regarding submerged pumps intended for high temperature applications.

Occasionally gaskets will leak. This can be especially dangerous with high temperature liquids. There are several causes for gasket leakage:

  1. temperature capability of the material is exceeded (e.g., using treated paper at 205�C / 400�F).
  2. dirt between the mating surfaces.
  3. cap screws are loose.

The following general comments may be helpful in solving problems or answering questions regarding gaskets:

  1. Gaskets of the softer materials - treated paper and compressed organic fiber - normally compress 30 to 40% of their thickness when squeezed between mating surfaces. This is important, particularly when gaskets are used for adjusting end clearance.
  2. The port flange gaskets are left with the center in them so they can serve as port covers during shipment. At the time of the installation the center is removed from the gasket and it is then used as a flanged connection gasket.
  3. It is always good practice, and frequently necessary on high temperature applications, to replace a full set of gaskets whenever a pump is torn down.
  4. Gasket cements such as Titeseal are all helpful in getting a gasket to seal but their effectiveness in high temperatures is limited.
  5. Capscrews, studs, and bolts stretch and gasket materials relax during the heating cycle; this may result in the fasteners being loose when the pump is cold. Fasteners should be checked and retightened as required.

O-rings (a form of self energizing gasket) have been used to a limited extent for high temperature applications. A number of commercially available O-ring compounds able to resist temperatures exceeding 110�C / 225�F are limited. Viton, PTFE, Kalrez, and Aflas are frequently used compounds.  For example, Viton is suitable for the 150-205�C / 300-400�F range and PTFE is good to 260�C / 500�F.

 

Antifriction Bearings

Anti-friction bearings, as differentiated from plain bearings or sleeve bushings, are those in which rolling is the primary form of motion rather than sliding. Ball bearings and roller bearings are the two broad classes of anti-friction bearings. Needle bearings, tapered roller bearings, and spherical roller bearings, are all variations of the basic roller bearing.

Most bearings are made from Type 52100 high carbon chrome steel. Bearing manufacturers generally agree that operating temperatures of 110�C / 225�F should not be exceeded when using bearings of 52100 steel. Operation of these standard bearings at higher temperatures results in dimensional changes, reduction in hardness, and early bearing failure.

It is generally a good idea to maintain the temperature at the thrust bearing area to 110�C / 225�F. It is easier to provide an auxiliary means of cooling the bearing on those applications where the bearing temperature may exceed 110�C / 225�F than it is to get involved in special bearings. Cooling of the area can be accomplished by means of cooling glands, cooling jackets, an air flow.

The successful operation of any antifriction bearing is dependent upon a wide variety of factors such as load, speed, duty cycle, mounting arrangement, size, type, lubrication, and environmental atmosphere.  All of these factors are a concern when the bearing is to operate near 110�C / 225�F.

 

Lubrication

The lubrication provided for close-running and load carrying parts is vital to the successful operation of any PD pump in a high-temperature application. 

Lubrication is required for the surfaces of the rotor, casing, idler and head having relative motion, the idler bushing on the idler pin, the rotor shaft in the casing, the rotor bearing sleeve or bracket bushing, the shaft under the packing, and the anti-friction thrust bearing used on heavy duty pumps.

By design, the internal rotating parts of many PD pumps are lubricated by the liquid being pumped. Many liquids handled at high temperatures have very low viscosity and subsequently provide very little lubrication.

Carbon graphite, as a bushing material for high temperature pumps, has proven to be very successful due to its inherent self-lubricating nature. Other bearing materials may be considered providing the lubricating quality of the liquid pumped is sufficient to assure good operation.

Simply apply grease with a hand-operated or pressure-actuated grease gun via the grease fittings (Zerks).

When selecting grease, be sure to use a good quality product which is temperature rated at least for the temperatures encountered at various points of application to  the pump. Generally, this will be the maximum anticipated operating temperature of the pump for application directly into the pump and approximately 110�C / 225�F for application to the anti-friction thrust bearing assembly used on heavy duty pumps.

 

Miscellaneous Concerns

Springs

Coil springs are often used in internal and return-to-tank pressure relief valves.   Stainless steel springs have a practical maximum temperature limit of 290�C / 550�F.  Steel springs have a maximum temperature of 175�C / 350�F.

Spring performance will vary somewhat with temperature changes and some adjustment of the relief valve setting may be necessary after the pump has been put into service.

Lip Seals

Most commercially available lip seals feature elastomeric sealing elements which are satisfactory for temperatures up to 110�C / 225�F. Special elastomers are available from certain manufacturers that are suitable for temperatures in excess of 110�C / 225�F.

Grease Fittings

Upon occasion, customers have specified that grease Zerks be furnished in special construction to withstand high temperatures. The primary difference is the replacement of a small plastic ball used as a check valve with a similar ball of metal. This modification has not proven necessary on the great majority of high temperature pump applications; however, some users have found this to be a solution to problems peculiar to their particular application.

 

Means Of Heating The Pump

As mentioned in the introduction, one of the reasons for handling liquids at high temperatures is that they are easier to pump. Materials such as asphalt, molasses, sulfur, lead, etc., are either solid at ambient temperatures or so viscous that it is impractical to try to pump them. If a pump is used for handling liquids such as these and is then permitted to stand idle while the liquid in the pump cools, it will be impossible to start without first heating the liquid.

Figure 12.  Pump with jacketing features.

Starting the pump with "cold" material can result in breaking a pump part, stripping a pinion, slipping a belt, or "kicking out" a motor. A pump with jacketing is normally recommended for this type of application. Pumps with jacketed parts can be seen in Figure 12. 

Jacketed pumps are designed to have steam or hot oil circulated through the jackets. Steam pressure is usually limited to 8.6 or 10.3 BAR / 125 or 150 PSIG, depending on the kind of pump in question.  This in turn limits the temperature in the jacket to 175-185�C / 350-365�F. Hot oil can be used in the jackets to a temperature range of 235-345�C / 450-650�F, depending on the pump.

Means other than jacketing are sometimes used to supply heat to the pump. Steam tracing the pump and piping is frequently done; also the pump may be wrapped with electrical heating tape or cable. In some instances heavy insulation is applied to the pump over the tracing lines or heat cables to help hold the heat.

If there is any doubt as to whether the material in the pump is liquid, it is wise to turn the pump over by hand or to stand near the motor starting switch or engine clutch so that the power can be quickly turned off if the pump does not rotate.

Be sure that the material in the lines to the pump is also liquid. Solid or viscous material in the suction line will starve the pump - in the discharge line it will cause the liquid to bypass through the pump pressure relief valve, or will cause excessive pressure to be built up at the pump if there is no over pressure protection.

 

Sample Applications

Three typical high temperature applications will be analyzed.

Application #1:  Asphalt, 70M3/Hr / 300 GPM, 175�C / 350�F pumping temperature, 3.1 BAR / 45 PSI discharge pressures, flooded suction, 150�C / 300�F hot oil available for circulating through the jackets of the pump, no safety relief valve required,  8 to 12 hour per day service during road building season.

Selection: A jacketed pump with bronze bushings, compressed organic fiber gasket, hot oil packing, a hardened steel idler pin, and extra clearances should work well. A Nitralloy idler pin might give somewhat longer service, but the economics from the end user's standpoint normally does not justify the added expense of the Nitralloy pin. Normally this is quite rugged service for a pump and frequently the entire pump is considered expendable.  

Application #2:  Heat transfer oil, 5.7 M3/Hr / 25 GPM, 21-235�C / 70-450�F pumping temperature range, viscosity 500 cps to 0.82 cps, 2.8 BAR / 40 PSI discharge pressure, flooded suction, pressure relief valve required, 16 hours per day six days per week service, mechanical seal required.

Selection: A pump with relief valve set at 5.2 BAR / 75 PSI, carbon graphite bushings, Nitralloy idler pin, high temperature non-asbestos gaskets, mechanical seal mounted between rotor and bracket. The seal materials are PTFE, carbon, stainless and Ni-Resist, extra clearances, grease chambers filled with high temperature grease.

Comments: When selecting a pump for handling heat transfer oils, remember that the pump will have to handle the liquid when it is cold. Depending on the particular oil and the temperature, the oil may be quite viscous. Since most heat transfer liquids become very thin and have little lubricating value at their normal working temperatures, anything that can be done to reduce the operating pressure that the pump must develop will extend the service life. A pump for heat transfer oil service is seldom jacketed.

Application #3: Organic Liquid-nonabrasive, 9.1M3/Hr / 40 GPM, 260�C / 500�F pumping temperature, 5.2 BAR / 75 PSI discharge pressure, viscosity 4,400 cSt / 20,000 SSU at pumping temperature, stainless steel construction required, packed stuffing box, jacketing required, batch process approximately two hours per 24 hours service.

Selection: A 316 stainless steel pump with high temperature carbon graphite bushings, extra clearance, and high temperature grease. The pump will have the jacketed head feature.

Comments: On an unknown liquid such as this it is necessary to make sure the user is aware of difficulties that may develop if the viscosity increases when the liquid cools down. Thus, a pump larger than appears necessary may be in order to permit some variation in viscosity without affecting the pump performance.

Half the battle in making a proper selection is getting all of the details.  In addition to required capacity, liquid, temperature, and operating pressure, other useful information is:

  1. viscosity at ambient temperature.
  2. suction conditions.
  3. duty cycle.
  4. maximum operating pressure and normal operating pressure.
  5. type of liquid and temperature of liquid used in jackets.
  6. material of construction if liquid not identified or if there are special requirements because of location or abrasiveness of the liquid.

Viton and Kalrez are registered trademarks of DuPont Dow Elastomers, L.L.C.