Pumps facilitate the movement of liquids by forming a low-pressure area at the pump inlet, which enables fluids to be forced inside by atmospheric or head pressure. In terms of pump performance, it is important to consider the physical limit that external pressure places on how high fluids can be lifted by the pump. This limitation is taken into consideration by Net Positive Suction Head (NPSH), a term that describes the difference between pump suction pressure and vapor pressure. The NPSH is the most important element to consider in a pumping system.
NPSH involves two parts: net positive suction head available (NPSHa) and net positive suction head required (NPSHr).
- NPSHa refers to the NPSH available at the inlet of the pump and is calculated based on several variables related to the specific system.
- NPSHr, which is supplied by the pump manufacturer, is the NPSH necessary for the pump to operate without experiencing cavitation. Cavitation occurs when bubbles rapidly form at the pump’s inlet, then abruptly collapse to create a shockwave. It is crucial to avoid cavitation as it can cause permanent damage to the pump.
Calculating Net Positive Suction Head
Calculating NPSH is essential in order to prevent cavitation, improve efficiency, and ensure optimal pump performance. When selecting the most suitable pump for a particular application, it is important to make sure that the NPSHa is greater than the NPSHr to avoid cavitation. In other words, the system must have a greater amount of suction-side pressure available than the amount required by the pump.
NPSHa is calculated using this formula:
NPSHa = Ha ± Hz – Hf + Hv – Hvp
- Ha: Absolute pressure, typically atmospheric pressure, being exerted on the liquid’s surface
- Hz: Distance between the liquid surface within the tank and the pump centerline
- Hf: Losses due to friction in the suction piping
- Hv: Velocity head at the pump suction port
- Hvp: Absolute vapor pressure of the liquid at pumping temperature
Using Net Positive Suction Head to Select a Pump
As mentioned, the most important factor to consider when selecting a pump is the NPSH. The margin of error between your calculated NPSHa and the manufacturer-provided NPSHr should be 10% or greater to ensure that cavitation is avoided. Other factors to consider when selecting a pump include:
- Temperature. The higher the liquid temperature, the greater its vapor pressure. This has a significant effect on the NPSH equation, causing the NPSHa to decrease. It is important to consider this, especially if the pump is being used in high-temperature applications.
- Suction-specific speed (Nss). This single value is calculated from three factors: the amount of head generated by a pump, the pump operation speed, and the amount of NPSH necessary for pump operation. High Nss values could indicate that a pump is prone to cavitation.
- Distance between pump and suction source. Due to its effect on the NPSHa, the distance between the pump and the suction source should be considered.
- Pump design. Pump design, age, and the rotational speed of its impellers will all influence the NPSH required for the pump to operate properly.
Net Positive Suction Head Solutions by Thermal Fluid Systems, Inc.
Thermal Fluid Systems, Inc. is a thermal fluid heater and hot oil system supplier with 40 years of experience in the field. Our extensive experience allows us to be involved in every step of our customer’s projects, from design and engineering to fabrication and after-sales support. By closely communicating with each customer to understand their specific project challenges and requirements, we can recommend and install the pump that is best suited for a particular application. Our goal is to save our customers from the stress of having to worry about the expensive repairs or unexpected downtime that can come with selecting the wrong pump for a system.
Additionally, we also perform system and control upgrades, provide onsite-technical support, and offer an extensive inventory of replacement parts. To learn more about our many services and capabilities, please contact us today or fill out a request for a quote.
Thermal Fluid Systems, Inc. specializes in the design, supply, and servicing of hot oil systems, also referred to as thermal fluid heating systems for the process industries. Our services include complete system design and supply, as well as parts supply and maintenance throughout the life span of the system. The systems we offer are expertly designed and serviced to produce consistent and high-temperature heating at low pressures
We offer a wide range of replacement parts— including pumps, valves, burners, control panels—not only for our own thermal fluid systems but for other manufacturers’ systems including Geka, GTS, Konus, Eclipse and Fulton among others.
The types of high temperature hot oil pumps we supply include:
- Mag-drive pumps
- Canned motor pumps
- API pumps
- Mechanical seal type pumps
And we carry top brands including KSB, Sihi, Allweiler, Dean, Kontro, Dickow and more.
What Types of High Temperature Pumps Are Available for Hot Oil Systems?
Selecting high-quality pumps and replacement parts from a reliable manufacturer helps keep hot oil systems working properly and smoothly
At Thermal Fluid Systems, we can source pumps and parts from major manufacturers and customize your systems to use one type of pump throughout the facility. Available for fast delivery and installation, our inventory includes the following pumps:
Magnetic drive pumps—also commonly referred to as mag-drive pumps—use magnets to rotate the internal impellers that push fluid through the hot oil system. The use of magnets removes the need for external shafts and their accompanying seals, which eliminates the risk of leakage. This characteristic makes this type of pump ideal for systems that have expensive, corrosive, or toxic fluids that must be prevented from leaking out of the system as either a liquid or gas.
Mag-drive pumps have several advantages, including:
- Low risk of leaks: These pumps don’t have external shaft components, which eliminates many of the potential leak vulnerabilities possessed by other types of pumps. These pumps are also tested to reduce the risk of fluid (either gas or liquid) leaks.
- Low cost of maintenance: Mag-drive pumps require very little maintenance throughout the life span of the system. While the power requirements may cost more than comparable pump systems, that expense is largely negated by the lack of maintenance and repair costs.
CANNED MOTOR PUMPS
Canned motor pumps serve as an alternative to magnetic drive pumps. These pumps similarly do not rely on mechanical seals and offer a low risk of leakage when employed, as the moving pump parts are contained within a hermetically sealed chamber. Typical applications include use in systems that need to guarantee zero leakage, such as systems with expensive fluids, systems with radioactive or toxic coolants, and any system with corrosive fluids.
Some of the other advantages of using canned motor pumps include:
- Reduced noise: These pumps operate more quietly than other hot oil system pumps.
- Explosion-proof and airtight parts: Designed to handle pressures that are greater than is attainable by most systems, the seals on canned motor pumps are airtight. These qualities also contribute to the leak proof guarantee.
- Space efficiency: These pumps are available in a compact design. Because the motor and pump components are contained within a joint unit, canned pumps require half the space or less of comparable sealed pumps
API pumps are designed to meet the American Petroleum Institute (API) standards for hydrocarbon pumps. Specifically, API pumps meet the API 610 standards for centrifugal pumps that are rated for high-pressure conditions, such as are found in the petroleum, petrochemical, and natural gas processing industries. These pumps are built with durable pump casings that are designed to withstand high pressures and temperatures; Qualities which help to prevent explosions and reduce the need for frequent servicing or maintenance of the pump.
ANSI pumps are another type of process pump which is designed to meet the ANSI, or American National Standards Institute, standards. However, these pumps are built for systems that handle thin liquids such as water and alcohols. API pumps, on the other hand, are built to handle viscous hydrocarbons without cracking under the increased pressure. Common applications for API pumps include oil refineries and other processing plants along oil supply chains.
MECHANICAL SEAL TYPE PUMPS
As suggested by the name, mechanical seal type pumps utilize a mechanical seal that acts as a check valve and slider bearing for the pump. This component prevents fluids from leaking out of, and air from leaking into, the pump during operation. However, through regular use, it experiences wear, and eventually fatigue, necessitating replacement of the seal component.
TFS, Inc. also offers mechanical seal pumps that are available as air- or water-cooled.
CONTACT THERMAL FLUID SYSTEMS TODAY
Using the right pump for your facility’s thermal fluid heating system can make it safer, more efficient, and more profitable. To find out how Thermal Fluid Systems, Inc. can help save you time and money, contact us, or request a quote today with the details of your system design or part replacement needs