All pumps have hydraulic limitations and mechanical boundaries. From allowable speeds to casing pressure to flange loading, there are always limits and no pump can cheat the laws of physics. In the pump selection process, you must consider all of the physical boundaries. In the case of hot liquid applications, most pump manufacturers will have a standard pump selection that will provide satisfactory service up to a range of 250 F (121 C) and perhaps even 300 F (149 C) without modifications or extra options. Operation above 300 F will normally require some modifications on all but the pumps specially designed for high-temperature applications. The primary concern is to provide a safe unit; the strength of the materials must provide both reliable pressure containment and resistance to thermal shock.
Pumps compliant with American National Standards Institute (ANSI) B73.1 are designed for 500 F (260 C) but may require modifications/revisions to operate at that temperature. The same pump can often be made suitable for services up to 600 F (316 C) or higher with added modifications. There are several pump manufacturers that produce pumps specifically engineered for hot liquid applications at temperatures in the 600 F (316 C) and higher range. These specialized high-temperature pumps will use centerline mounting (casing) in lieu of base-mounted support feet, mechanical cooling fans, cooling fins, special materials and extended bearing housings to both distance and ostensibly mitigate the undesired effects of operating at high temperature.
The purpose of this column is to provide a general checklist to consider for hot applications. This is not a comprehensive list, and I suggest you check with each pump manufacturer for their specific guidelines. This list is focused on type OH1 overhung single-stage centrifugal pumps as they are the most ubiquitous industrial style, but many of these checks will also apply to other pump geometries and types.
My recommendations are unabashed and admittedly conservative—and in no particular order.
Pump flange rating
Review and confirm the flange limits for the temperature and pressure relative to the material selection for the pump. Even with Class 300# flanges in stainless steel (SS) or CD4MCu, the maximum pressure allowed will be around 375 pounds per square inch gauge (psig) at 300 F (149 C). I prudently recommend that if the application is above 300 F that Class 300# flanges be used regardless, even if not required by other criteria. For pressures and temperatures that exceed the Class 300# flange rating, you will require a specialty pump or one that is manufactured and rated for (American Petroleum Institute) API service with corresponding higher rated flanges and casing pressures.
Depending on the expected temperature range, your operational/maintenance finesse and the duty cycle, several parameters will factor into the pump selection. For example, if you will operate the pumps continuously (few starts and stops) and initially set the pump up with the proper options, piping design and alignments, a less expensive foot-mounted pump can perform well up to 600 F (316 C). Frequent heat-up and cooldown cycles and other institutionally required operational and maintenance steps combined with experience and staff skill levels may lead you to choose a specialty high-temperature pump. The specialty pump will, at a minimum, utilize a centerline supported casing and other high temperature compensating features. There is overlap in the allowable temperature ranges for both types and the user can decide where to switch from the lower cost alternative after a thorough total cost analysis.
Almost all grease lubrication choices will start to falter around 225 F to 250 F. If your application requires grease lubrication at or above this temperature range, please consult with a lubrication specialist. Some special high-temperature greases may allow operation at a higher temperature.
Oil lubrication should be used above 225 F (107 C) and the higher the temperature of the system, the more I recommend using a synthetic oil specifically selected for high-temperature applications.
Above 250 F (121 C), I recommend high-temperature bolting/hardware for the pump fasteners. As a minimum starting point, consider American Society for Testing and Materials (ASTM) A193 B7/B8/B8M for the high-temperature hardware.
Above 300 F (149 C), I recommend high-temperature casing gaskets and O-rings. Regardless of the brand you use, be aware of the negative effects due to high temperatures and choose materials that will be reliable for the application.
Regardless of the manufacturer, I recommend that you seek consultation to determine the allowable temperature range. Normally above 250 F (121 C), it is recommended that the standard materials be switched for high- temperature alternates.
Stuffing box cooling
My experience with stuffing box cooling is that it typically does not work as intended and makes little difference in the critical temperature at the mechanical seal faces. It does work to remove some heat from the pump in general, which is a good thing. My opinion is that you are better off investing in a high-temperature mechanical seal and supporting piping plan than in a cooled stuffing box. You should consider both the initial added cost of the cooler and the operating/maintenance cost of supplying the cooling water over time.
Alignment to driver
A foot-mounted pump will grow a significant amount (vertically) when operating at higher temperatures. A motor driver that is vertically aligned for ambient operating temperatures will be out of tolerance at operating temperatures. You need to calculate the thermal rise and also perform a hot check alignment to verify.
Speaking of alignments, many ANSI pump manufacturers will have optional offerings for C-face adaptors to reduce the need for tedious but necessary alignments. The adaptors are particularly useful if there will be frequent thermal swings associated with startup and shutdown. Be aware that C-face adaptors are not always perfect solutions. Your experience with your specific application will be the overall guideline. The actual life of the coupling, mechanical seals and bearings will be key decisive factors in the decision process.
There will always be some pipe stress even in the best system designs. Be aware that the higher temperatures create even more pipe expansion and consequential stress/strain. Consider working with a piping design specialist for incorporation of thermal loops and/or expansion joints. Example: A 100-foot run of 6-inch schedule-40 steel pipe will expand over 1.5 inches when heated from ambient to approximately 200 F. The resultant force exerted on the pump flange, if left unrestrained, will be close to 190,000 pounds (lbs).
Even with a design specialist, the pump installer (mechanic/millwright) must also make sure the alignments are correctly compensated for thermal contraction/expansion and the piping that was properly designed is also properly installed for mitigation of the harmful effects caused by the thermal stresses.
Rate of temperature change
How fast you heat up and cool down the system will greatly affect system reliability and equipment life. Almost no one pays attention to the recommended rates for the average industrial application because the cost of process and operating time is weighted over equipment costs and reliability. Refineries and power plants (especially nuclear) will be the exception to this scenario.
The pump operator must ensure the unit is methodically brought up to operating temperature and again when cooling down. I recommend a rate of no greater than 60 degrees an hour from ambient, which is 1 degree per minute.
Thermal expansion and contraction rates
Thermal expansion rates come into play when the pump is starting up and shutting down and sometimes with process upsets (thermal swings with high rates of temperature change). If all of the pump components are of the same material, there is less need for concern with the issues that result from rotating pieces growing into the stationary ones. If the pump is constructed from a combination of cast steel (iron) and 316 SS parts, then you have dissimilar materials with different expansion/contraction rates, but if the pump is 100% constructed from all 316 SS, there is no issue.
Corrosion rates become even more important on high-temperature applications. Corrosion rates increase exponentially with temperatures above 120 F (49 C). Almost all liquids will become more corrosive with an increase in temperature. Cast steel or iron may be satisfactory for applications at 100 F (38 C), but at temperatures above 180 F (82 C) the rate of corrosion can be both accelerated and pronounced. Most manufacturers have a corrosion allowance on the casing material of at least 0.125 inches.
Series 300 austenitic and 400 martensitic stainless steels retain their hardness at higher temperatures much better than carbon steels and cast irons. This helps mitigate and/or preclude the deleterious results inherent with thermal shock issues.
Cast steel is stronger than 316 SS from approximately 50 F to 800 F, but as the process and the material becomes hotter, the 316 SS will be stronger than the carbon steel. Another way to look at this is that carbon steel gets weaker from 150 F to 800 F, but the 316 SS strength remains the same and, after 800 F, 316 SS is stronger than carbon steel, which continues to lose strength.
Specific heat (heat capacity) and thermal conductivity
316 SS has poor heat conductivity properties, which on the surface may seem like a bad thing, but it is not. The 316 SS material will get just as hot (temperature) as the carbon steel for a given application, but due to the specific heat capacity and the thermal conductivity of 316 SS, the key point is that less heat will be transferred. This property works well when you do not want to transfer heat to the bearings, lubricating oil, labyrinths, bearing isolators and mechanical seals. Do not confuse heat with temperature. Heat is energy and temperature is just a relative measure of the magnitude of the heat (see sidebar). If temperature is measured in Kelvin degrees, then its value is directly proportional to the average kinetic energy of the molecules of a substance (heat). Again, temperature is not energy—it is a number proportional to a type of energy.
In summary, I have covered some of the main concerns for operating a pump in a hot application. Of no less importance, but not addressed here, is selection advice for the mechanical seal, the geometry/type of stuffing box, the seal piping plan, calculating thermal rise, high-temperature paints, lubrication cooling versus bearing, bearing housing cooling, water cooled pedestals, the coupling and the net positive suction head (NPSH) margin. Perhaps we will address those in a later column.
For now, stay cool.
Temperature vs. Heat—Or Intensive Property vs. Extensive Property
Temperature is an intensive property.
An intensive property means that the amount of material present will not change the specific traits of the material or substance. As an example, the boiling point of water in an open container at sea level is 212 F (100 C). One gallon of water boils at 212 F, the same as 10 gallons of water boils at 212 F.
Heat is an extensive property.
An extensive property is one that depends on the amount of material present. As an example, consider the amount of heat produced by 1 gallon of boiling water and how that will differ from the amount of heat produced by 10 gallons of boiling water. (Do not confuse extensive properties with the specific heat properties of a material. Specific heat is the heat capacity per unit mass. Consequently, it is an intensive property.)
A comparison of heat and temperature can be demonstrated by the sparks generated and emitted by arc welding, but as a more practical example I like to use fireworks sparklers, which all people can relate to. The sparks that come off the burning sparkler are expelled metal particles at temperatures approaching 5,430 F (3,000 C). These sparks are extremely hot and yet, if touched by them, they will not burn you or your clothes under normal circumstances, even though the temperature is extremely high. The hot sparks (extensive property) have little mass and, consequently, a small amount of heat.