The considerations involved in choosing the right thermal fluid for a new thermal fluid system and selecting a thermal fluid heater -- fired and electric -- were discussed in prior installments of this series. Previous articles also discussed pumps for higher temperature service and sealed vs. sealless pumps. This article, the fourth of the series, will center on the process decisions that go into selecting and sizing the pump.
As a brief primer, the pump’s obvious job is to move the fluid through the heater to the equipment using the fluid. Pump selection is critical to proper heat transfer in the system and to the successful long-term system reliability. For proper heat transfer, the pump must supply proper flow to the heater and the rest of the system for optimal performance.
The pump also will be the highest maintenance component in the system, unless the system has rotary joints, which I will discuss in a future installment. To prevent seal leakage and provide reliable service, the pump must be constructed of the right materials and have the correct design. And, obviously the pump has to be properly installed with the correct piping arrangement.
Pump DesignFor purposes of this discussion, I will look at centrifugal pumps and positive-displacement pumps.
Centrifugal Pumps. The vast majority of thermal fluid pumps are centrifugal pumps, which move fluid with an impeller that imparts its velocity to the fluid, so the flow of fluid in the pump is variable and varies with the backpressure encountered by the fluid as it moves through the system. Discharge pressure of a centrifugal pump is correctly expressed in feet of head. A typical pump curve for a centrifugal pump is shown in figure 1.
A number of manufacturers make centrifugal pumps for thermal fluid service. These pumps, to varying degrees, handle the higher temperatures of thermal fluids with upgraded materials, enclosed impellers, improved seals (some with novel methods of keeping the seal cool and away from oxygen), higher-rated flanges, centerline casing support and other upgrades. These pumps carry temperature ratings from 600oF (316oC) to as high as 850oF (454oC), and can handle almost any thermal fluid application. Figure 2 shows a typical pump specifically made for thermal fluid service.
One common mistake is using standard process pumps in thermal fluid service. It often occurs because the owner has a large supply of spare parts common to these pumps. These pumps, while adequate for general process service, generally do not give satisfactory performance in thermal fluid applications, nor do they exhibit the same durability as specific-use thermal fluid pumps.
Positive-Displacement Pumps. While positive-displacement pumps are not seen as often as more conventional centrifugal pumps, they are used with some regularity in thermal fluid service, particularly in small electric package heating systems and in certain fired thermal fluid heaters. Though many different designs for positive-displacement pumps exist, the design seen most often in thermal fluid service is a gear-type pump, which transports fluid through the pumps via a system of meshing teeth. The fluid fills the space between the teeth and is forced from the suction to the discharge of the pump. In order to operate safely and properly, positive-displacement pumps need:
- To be protected from deadheading by a pressure-relief
- To be fitted internally and externally with the proper materials for
heat transfer service.
- To be supplied from the manufacturer with the proper
- To have the proper mechanical seal installed for thermal fluid service.
Mechanical Seals. Mechanical seals in thermal fluid service generally fail due to the oxidation of the fluid on the seal faces. Seal life can be improved by cooling the seal, which significantly reduces the oxidation, and by protecting the atmospheric side of the seal from atmospheric oxygen. Most pumps built specifically for thermal fluid applications have some allowance for cooling and for protection from oxygen included in their design. Some do a better job than others and the end user should evaluate, or employ experienced resources to choose and specify pumps that will offer the most value for the dollars spent.
Information on sealless pumps can be found in “Sealed vs. Sealless Pumps for Fluid Service” (November 2006, p. 22; see Related Sites at bottom of page for link), which compares sealed and sealless pumps.
Pump Sizing ConsiderationsOne of the principal problems that I see when I go to plants to diagnose poorly performing systems is low flow. In this situation, the pump does not deliver enough fluid to the heater for the heater to completely load up all the way, causing poor performance in the process. Low flow can also cause higher than desired delta T (ΔT) or higher than desired fluid film temperatures in the heater, both of which can be a severe problem in many processes.
System Pressure Drop. When purchasing a pump, the end user or designer should lay out the pipe and calculate the pressure drop that is expected with the system operating at full-rated flow. The pump then can be sized to produce sufficient head to overcome the friction loss and provide rated flow to the system. Note also that the pump must be designed for both the fluid’s viscosity and density at the operating temperature, as well as its viscosity and density at the cold (startup) fluid temperature.
Static Head. While static lift (the lift required to get the fluid to the highest point in the system) does not apply in closed-loop systems, the owner should always check the pump’s discharge head against the elevation difference in the lowest and highest point of the system to ensure that the pump can get the fluid “over the top” when the system is filled the first time.
Net Positive Suction Head. As with any pump, making sure enough net positive suction head (NPSH) is available is critical, especially at higher fluid temperatures. (Please refer to a pump text for more information on NPSH.)
Horsepower. Pump horsepower is a function of flow, head, pump efficiency and specific gravity. Many systems are plagued with nuisance pump-motor trips when the system is being started from ambient temperature. This is because the pump motor was sized for the specific gravity of the fluid at operating temperature, not ambient temperature. Thermal fluids expand as much as 30 percent from ambient temperature to operating temperature. This means that the specific gravity of the fluid is much lower at high temperature and much higher at ambient temperature. A motor sized for operating conditions may not have enough horsepower to move the fluid at ambient temperature during startup.
Proper Installation is CriticalAs previously stated, the best pump available, even properly sized and specified, will not perform or give reliable service if not correctly installed. Points to consider during installation include pipe stress, alignment and foundation.
Pipe Stress. Pumps are not pipe supports, and to use them as such is abuse. Piping that may not bear heavily upon a pump when the system is installed and cold may impose high loads when the system is operating at high temperatures. Large systems should have pipe stress analysis performed on the piping design as an integral part of the design process. Certain end users may purchase a packaged heat transfer system and then field route pipe to the heat users. These pumps usually become a maintenance problem due to pipe stress. If the piping was not designed and analyzed, no one will know if the piping system is over-stressed until a problem occurs.
Alignment. Maintaining pump alignment in accordance with manufacturer recommendations is critical to good seal life. Alignment with dial indicators should be considered the minimum standard of care in aligning pumps. Laser alignment is faster, more accurate and well worth the money for having long seal and bearing life. The “straight edge” method is, in the author’s opinion, useless. No one can see well enough to tell if a pump is aligned within 0.005"! Do it right and reap the benefits.
Foundation. The foundation that the pump rests upon is extremely important in maintaining the alignment of the pump and carrying any stress loads to the ground. Wherever possible, it is good practice to mount the pump on a concrete pad. The baseplate should be leveled with shims and then grouted to the foundation. The pumps then can be aligned and will maintain the alignment for long periods of time.
For systems where the pump is provided as part of an integral skid system, the owner should make sure that the structure under the pump is adequate for any anticipated pipe loads and that the skid is properly leveled and secured when it is installed. The pump base then can be leveled and the pump aligned in the field. Factory alignment of pumps is fine for equipment fit-up, but the pump needs to be realigned once the skid has been installed at the point of use.
Pumps in thermal fluid systems can be a performance and maintenance headache or a reliable and trouble-free component of the system. The outcome depends on selecting the right pump design for the application, careful sizing calculations, protecting the seal against high temperatures and oxidation, proper piping design and proper installation. It is up to the owner to perform proper due diligence upfront or spend time, effort and money working on a poorly performing pump for the life of the system.
SidebarUse the links at the bottom of the page to
continue reading this six-part series on specifying a thermal fluid heating
system. You've just finished:
Specifying a Thermal Fluid System: A 6-Part Series
Part 4: Thermal Fluid Pumps
Other parts in this series include:
Part 1: Choosing the Thermal Fluid
Part 2: Fired Thermal Fluid Heaters
Part 3: Electric Thermal Fluids Heaters
Part 5: The Expansion Tank
Part 6: Piping Materials, Valves and Insulation