Less than 10 percent of the thermal fluid systems that I work with operate at 650°F (343°C) and above; however, most of my time spent assisting clients with pumps for thermal fluid systems is spent specifying pumps for higher temperature systems or troubleshooting pump problems encountered with higher temperature pumps.
Pumps are often the most visible component of a thermal fluid system because they are often the system component requiring most maintenance. Pumps installed in higher temperature service can be very troublesome if they are not specified, installed, operated and maintained properly.
When considering pumps for a higher temperature system, whether the consideration is for a new system or to improve an existing system, the owner will benefit from paying particular attention to four specific areas:
- Pump specification and sizing.
- Pump (and associated piping) installation.
- Pump operation.
- Pump maintenance.
SpecificationMany end users think (incorrectly) that choosing a pump that delivers the correct flow and head constitutes “specifying” a pump. At 650 to 750°F (343 to 399°C), there are many forces that can affect pump performance and even pump safety. There is a large difference between “will work in this service” and “is well suited for this service.”
Items to consider when specifying a pump for higher temperatures include:
- Hydraulic performance, including flow, head and NPSHR.
- Casing design.
- Impeller design.
- Materials of construction.
- Baseplate design to maintain proper alignment.
- Mechanical seal selection and mechanical seal environmental control.
- Use of sealed vs. sealless pumps.
Hydraulic Performance. While it is only part of the task of specification, specifying proper hydraulic performance is the single most critical item affecting pump performance. The specific gravity of thermal fluids used at higher temperatures can change 30 percent or more between ambient temperature and operating temperature. Fluid viscosity also can be very different at ambient vs. operating temperature. Therefore, care should be taken in specifying the motor for the pump. Also, at the higher temperatures discussed in this article, the potentially high vapor pressure of the hot thermal fluid makes it doubly important to include an NPSH (net positive suction head) calculation to make sure that the NPSH available is greater that the NPSH required.
Casing Design. Casing design is affected by three considerations: thermal fluids' tendency to leak, the need to maintain mechanical alignment within the pump, and the large pipe loads that can be encountered in thermal fluid systems.
Because of their low viscosity and low specific gravity, thermal fluids have a tendency to leak through gaskets and valve packing. The common “fix” is to specify ANSI class 300 lb flanges for pumps and piping. If one consults the pressure-temperature tables for carbon steel piping, it will be found that, even at very high temperatures, it is seldom necessary to upgrade the flange rating simply due to temperature and pressure. In addition, 300 lb flanges are thicker and have more bolts than a similar size ANSI class 150 lb flange. The advantage of using 300 lb flanges is the ability to gain extra gasket compression, which makes it more difficult for the fluid to leak.
The casing of common process pumps and lower temperature specialty thermal fluid pumps mount to the pump base at the bottom of the casing. These “foot-mounted” pumps can present difficulties as the pump reaches elevated temperatures. As a foot-mounted case grows, it can push the front of the pump out of mechanical alignment with the rear of the pump and the driver. Most pump manufacturers specify that alignment be maintained within 0.002" in all directions. This degree of alignment is not possible with foot-mounted pumps when they are operated at high temperature. Mounting casings on the centerline, with pedestals or a yoke carrying the weight of the pump down to the base, allows the casing to expand in all directions and not lose alignment.
Pumps are not pipe supports. (Some people don't know that.) Piping design should make every effort to minimize pipe stresses being transmitted to pumps, but this can be difficult in the design of high temperature systems because of the amount of piping expansion from ambient temperature to operating temperature. Figure 1 shows a heavy-duty thermal fluid pump with the features needed for high temperature service.
Impeller Design. Common process pumps have open or semi-open impeller designs. These impellers depend on precise clearances between the impeller and the casing to maintain hydraulic performance. When a thermal fluid pump heats up, the shaft inside the pump heats up and grows, causing the impeller to move forward in the casing. It is therefore impossible to correctly set impeller clearances if open impeller pumps are used in high temperature service. Because of this, the user should make sure that pumps purchased for high temperature service have fully enclosed impellers. It also is good practice for the casing to have replaceable wear rings.
Materials. Many pumps meant to operate at low to moderate temperatures can have a cast-iron casing. The user should consider that a thermal fluid operating at 700°F (371°C) can be operating 400°F (~220°C) above its flashpoint! In the unlikely event of a fire, spraying cold water (or foam) on a hot cast-iron pump casing can cause the casing to crack, allowing fluid to leak from the pump and providing more unwanted fuel for the fire. Specifying a pump with a cast-steel or stainless-steel casing will give the end-user a pump that will not crack if sprayed with water.
Baseplates. While a channel steel baseplate would be considered the minimum base acceptable for this service, a fabricated steel baseplate is more robust and offers a more stable platform to maintain alignment between the pump and driver. It also is extremely important that the baseplate be properly leveled and anchored to the pump foundation. (How to achieve this via proper baseplate installation will be covered in detail later in this article.)
Mechanical Seals. Entire books have been written about mechanical seals, and one paragraph is not going to provide an in-depth understanding of mechanical design and materials.
Mechanical seals for thermal fluid service are generally metal bellows seals with carbon-graphite gaskets and sealing rings. It is important to remember that materials for your mechanical seals should all have maximum temperature ratings that are higher than the temperature of the thermal fluid. This means that O-ring type seals, particularly those with dynamic O-rings, should never be used in sealing high temperature pumps.
A variety of seal environment controls can be employed to improve seal performance and extend seal life. Examples of these controls include jacketed stuffing boxes and tempered seal flush streams that cool the fluid in the vicinity of the seal faces to improve lubricity, and inert quench streams on the atmospheric side of the seal faces to prevent oxidation of the fluid while it is between the faces. Double or tandem seals also have been used where the barrier fluid cools and inerts the seals.
Sealed or Sealless Pumps? So far, this article has not mentioned the use of sealless pumps for high temperature service. Both magnetically driven (mag-drive) and canned motor pumps are available that perform very well in high temperature service. An in-depth article comparing sealed and sealless pumps will appear in a future issue of Process Heating, but a brief synopsis is offered herein.
The advantage of sealless pumps is that they do not leak. They use magnetic couplings (mag-drive) or they combine the pump and motor into one unit (canned motor pumps). The internal shaft, completely wetted by the pumped fluid, is supported by ceramic bearings, which are lubricated and cooled by the fluid. While the ceramic bearings can be reliable and long lasting, they are shock sensitive. Sealless pumps are generally intolerant of cavitation or dry running.
Mag-drive pumps are similar in appearance to conventional frame-mounted pumps. The difference between mag-drive and sealed pumps is the completely sealed liquid end with its magnetic coupling. The very strong magnets that make up the coupling are temperature sensitive and have to be kept cool. A variety of air- and water-cooled systems are offered by manufacturers to effectively control magnet temperatures (figure 2).
Canned motor pumps are very compact and are less sensitive to mechanical alignment issues. Unless they are equipped with available ceramic motor windings, they also need to be kept cool (figure 3).
InstallationThe best-specified pump in the world will not run well if it is not properly installed.
Foundations. An excellent pump base that is not adequately supported or anchored will not keep the pump aligned. Read the manufacturer's recommendations. Level the base and anchor with grout.
Alignment. After the baseplate is level and stable, the pump can be leveled and then aligned with the motor per the manufacturer's recommendations. Laser alignment is more accurate that manual alignment. It also is a good practice to recheck the pump's alignment after the pump has reached full temperature the first time.
Piping. As stated earlier, piping can affect pump performance in a variety of ways. Poor piping design (or no piping design) can result in poor fluid presentation to the pump suction and develop forces inside the pump that can reduce pump performance or shorten pump life.
It is always considered good practice to evaluate pipe stresses in a high temperature system. Pipe stresses applied to pumps can affect alignment and reduce pump life.
Cooling. Most pumps in 650 to 750°F service have some kind of cooling; very often, it is water cooling. It is good practice to have an adequate supply of clean cooling water. It is also a good idea to have a flow indicator or flow switch to reduce the chance of having the pump run without adequate cooling.
OperationThe principal reason for properly specifying and installing pumps is to have a piece of equipment that performs well and operates with minimal unplanned maintenance. It is important to realize that a properly specified pump that is properly installed can be compromised by improper operation.
The owner will be well served to make sure that procedures are in place, and that operators are properly trained in:
- Starting the pump.
- Guarding against cavitation, particularly for sealless pumps.
- Preventing dry-running, particularly for sealless pumps.
- Maintaining the mechanical seal environment.
- Maintaining proper cooling.
MaintenanceNow the owner has a properly specified and properly installed pump that is operated by well-trained personnel. If adequate maintenance is employed to support the pump, the pump can be expected to remain in service for extended periods.
Items that should be included in the pump preventive maintenance program include:
- The pump's cooling mechanism should be monitored for proper operation. If the cooling mechanism involves cooling water, having clean water is very important.
- Proper lubricants are important, but monitoring (and periodic changing) of lubricants is just as important.
- Re-check alignment and re-align the pump at least annually.
- The capability to monitor vibration will allow the owner to predict the maximum life of the pump and rebuild it before is fails catastrophically.
It is possible to specify and install pumps for higher temperature service that will operate safely and reliably. In addition, processors who operate or are contemplating systems operating at lower temperatures also can benefit from more robust pumps. The extra money spent on more robust pumps can pay benefits for the life of the system in reduced system downtime and maintenance costs.
Sidebar: 10 Tips: Pumps for High Temperature Thermal Fluids
- Specifying proper hydraulic performance is the single most critical item affecting pump performance. Remember that the specific gravity of thermal fluids can change 30 percent or more between ambient temperature and operating temperature.
- Casing design is affected by three considerations: thermal fluids' tendency to leak, the need to maintain mechanical alignment between pump and motor, and the large pipe loads that can be encountered in thermal fluid systems.
- Excessive pipe stresses applied to pumps can affect alignment and reduce pump life.
- Thermal fluid pumps should have enclosed impellers, because it is impossible to correctly set impeller clearances if open impeller pumps are used in high temperature service.
- Cast steel or stainless steel casings are preferable because, in event of a fire, spraying cold water (or foam) on a hot cast-iron pump casing can cause the casing to crack, allowing fluid to leak from the pump and providing more unwanted fuel for the fire.
- The materials for mechanical seals should have maximum temperature ratings that are higher than the temperature of the thermal fluid. Seals with O-rings should not be used.
- Make sure that the base plate is level and properly grouted. Then the pump can be leveled and aligned with the motor per the manufacturer's recommendations. Laser alignment is more accurate that manual alignment.
- A properly specified pump that is properly installed can be compromised by improper operation.
- If adequate maintenance is employed to support the pump, the pump can be expected to remain in service for extended periods.
- Sealless pumps do not leak, but they are intolerant of cavitation and dry-running.