December 2023SPECIAL FOCUS

Tagline Header BoxCase study: Forced-circulation evaporator piping design V. Nelluri, S. S. DARGAN and M. G. CHOUDHURY, KBR, Gurgaon, India Evaporation and crystallization techniques have many applications in the chemical, fertilizer, metallurgical, chlor-alkali, pigment and dye, food, ash treatment and zero-liquid discharge process industries, among others.In general, evaporation is a thermal separation process for the separation of volatile components from a solution. It is necessary and widely utilized in the process industries to concentrate solutions and/or recover solvent fractions. Evaporation processes are common in the chemical process industry to concentrate inorganic and organic solutions, acids and bases, and for systems that concentrate and recycle dilute sulfuric acid streams. The most suitable evaporator type is carefully chosen considering numerous factors, such as physical properties of the product (e.g., boiling point elevation, viscosity, sensitivity to temperature), scaling or foaming tendencies, impurity content, corrosiveness, etc.A_HLFH_SilcoTekPiping and equipment configurations play a vital role in achieving the best efficiency and effectiveness for a forced-circulation evaporator circuit. This article recommends the most reliable forced-circulation evaporator piping system in terms of stress analysis. Using proprietary pipe stress analysis software^a, different piping and equipment configurations were considered for the case study, as well as various aspects like equipment nozzle loads and piping stresses due to weight, temperature and pressure effects. The advantages and disadvantages of each option are discussed in more detail using the results from the study to help readers choose the most appropriate configuration according to their sets of constraints.Forced-circulation evaporator operation. As the name implies, force is used to drive the slurry or acid through the heater tubes, thus producing high tube velocities. The evaporation of slurry or acid is accomplished by a forced-circulation shell-and-tube exchanger (heater) set-up with steam as a heating utility. The slurry or acid is circulated in the heater with a circulation pump, taking suction from the vapor head that is heated in a heater—the hot process fluids then circulate back to the vapor head. The vapor head can be operated under vacuum or atmospheric conditions, depending on the process requirements (mainly, the solubility of the fluid). In the heater, the process fluid is heated with steam from utility and boiling takes place in the vapor head for liquid separation. The heater orientation selection (e.g., vertical, horizontal) is dependent on the expected temperature rise across the heat exchanger: for higher temperature increase, horizontal heaters are preferred (FIG. 1). Nelluri Fig 01CASE STUDYA typical forced-circulation evaporator piping stress system comprised of piping and equipment (pump, vapor head and heater) was considered for the case study using pipe stress analysis software^a and ASME’s B31.3 code. Three different options were included in the case study:

In Option 1, the vapor head, pump and heater were fixed on the structure/foundation and thermal movements were managed by the addition of expansion joints, as shown in FIGS. 2 and 3.

In Option 2, a hanging-type pump was selected rather than a fixed pump, as shown in FIGS. 4 and 5.

In Option 3, the heater was mounted on springs, while the pump and vapor head were secured to the structural steel/foundation, as shown in FIG. 6.

In the stress analysis, the weight, pressure and temperature were used to generate loading effects. For the case study, the material of construction, design parameters (pressure temperature, specific gravity) and component diameters, thicknesses, etc., were considered the same for all three options, as per TABLE 1.Typically, positioning the heater and vapor head fix point at the same level is recommended to avoid differential thermal expansion along the recirculation piping, specifically from the heater outlet to the vapor head inlet nozzle. This alignment ultimately reduces thermal loads and stresses, eliminating the need for expansion joints or bellows, as shown in FIGS. 3 and 4. The flexibility of the vapor head inlet nozzle can be advantageous if the loads are excessive. Sometimes, graphite heaters are used, in which case the expansion joints/bellows should be included at the inlet and outlet side of the heater due to the brittle nature of the graphite material.Nelluri Table 01Option 1: The pump, heater and vapor head are fixed on the structure/foundation. This is the conventional configuration used in a forced-circulation evaporator: the vapor head is fixed on structural steel, the pump is fixed on a foundation through the baseplate and the heater is fixed on structural steel, as shown in FIGS. 2 and 3. The only way to compensate for the thermal growth in the suction line from the vapor head to the pump and pump discharge line is the addition of untied expansion joints/bellows. Considering the service conditions and severity of the service, polytetrafluoroethylene (PTFE) expansion joints/bellows with an internal sleeve arrangement are usually selected. A stainless-steel internal sleeve is preferred to avoid damage to expansion joint/bellow elements and sleeve-wetted surfaces due to the impact of crystals in the flow. All-round guides with proper gaps are required near the bellows for proper balancing/stability of the system—as per Expansion Joint Manufacturers Association (EJMA) requirements—and proper anchors will be required in the piping to transfer the thrust forces and spring forces from the introduction of untied expansion joints/bellows.Nelluri Fig 02Nelluri Fig 03Advantages of Option 1 include:

Since the equipment is properly fixed on steel/foundation, the removal of internals using mechanical handling equipment is easy and safe.

Equipment and piping are isolated properly by the inclusion of expansion joints/bellows, and the equipment is anchored to the structures at appropriate locations.

Since the thermal growth of the piping is compensated with the inclusion of expansion joints/bellows at appropriate locations with proper supports, the thermal loads on the equipment nozzles and the expansion stresses in the piping are the lowest among all the options provided in the case study.

Disadvantages of Option 1 include:

Reliability and maintenance are a concern due to the use of expansion joints in the system.

Pressure thrust develops due to the inclusion of untied expansion joints/bellows in the system—this must be considered in the design of piping supports and equipment anchors, which leads to heavy support structures and foundations.

Pressure thrust is present in all pressurized piping systems (gauge pressure x the inside area of the pipe), and acts at changes in direction (e.g., elbows) and at changes in the pipe cross-section (e.g., reducers). Pressure thrust is normally carried as an axial load by the pipe. However, the inclusion of an untied bellows/expansion joint—which is not intended to carry such axial loads—removes the normal means of resisting the pressure thrust. Therefore, other means (e.g., pump anchors, hardware such as casing or baseplate) are required to carry the pressure thrust load. The primary challenge lies in securing the vendor's agreement regarding the unbalanced thrust on the pump casing and baseplate, which often faces resistance as it deviates from the original pump design not typically embraced by pump vendors.

Option 2: The pump is a hanging type (suspended), and the heater and vapor head are fixed on structural steel. In this configuration, the vapor head and heater are fixed on structural steel and the pump (hanging type) is suspended from the connected piping, as shown in FIGS. 4 and 5. Thermal growth in the suction line from the vapor head and pump discharge lines is allowed freely in a downward direction as the pump (hanging type) is not locked on the foundation and is free to move. The differential downward thermal growth between the vapor head downcomer line from the fixed/anchor point and discharge piping from the heater fixed/anchor point is managed by the horizontal piping section near the pump suction nozzle, and the expansion joints/bellows in the pump suction and discharge lines are eliminated. Hanging pumps are available in two categories: one is a pump and motor coupled with a V-belt drive, and the other is a pump hanging freely from piping with a motor that is fixed on a foundation with a Cardan shaft. In this study, a pump with a Cardan shaft-driven option was used, as shown in FIG. 5. Spring supports are usually added in the vertical section of the piping from the vapor head to the pump suction nozzle to reduce the loads on the vapor head nozzle arising from the piping and hanging pump.Nelluri Fig 04Nelluri Fig 05The advantages of Option 2 include:

It eliminates pressure thrust forces on pipe supports, the pump and other equipment nozzles, and leads to a reduction in the size of support structures and foundations.

Normally, pump vendors are reluctant to make design changes in the pump based on the pressure thrust values determined in a project’s advanced stages. Hence, the same challenge can be eliminated using a hanging pump rather than a conventional pump with a fixed foundation in the forced-circulation evaporator piping.

The disadvantages of Option 2 include:

The heater anchor support lugs must be designed to include the pump weight, as the pump is suspended from the heater through discharge piping.

Although the hanging pump is a proven design for some well-known pump vendors, vibration issues can still occur since the pump is not properly isolated with an elastic foundation. Therefore, proper guiding on the suction piping near the pump is required to eliminate any vibration issues—rigid piping supports are preferred over flexible supports to eliminate vibration.

The hanging pump arrangement with a Cardan shaft will require more space in the layout compared to a conventional pump.

A limited number of vendors offer a hanging-type pump with a head limitation of approximately 10 m. The cost of a hanging pump is high compared to a conventional foot-mounted pump.

Option 3: The heater is mounted on springs, and the pump and vapor head are fixed on structural steel/foundation. This configuration is rarely used in a forced-circulation evaporator piping system. In this configuration, the vapor head and pump are fixed, and the heater is mounted on springs, as shown in FIGS. 6 and 7. Untied expansion joints/bellows in the pump discharge line are eliminated to avoid unbalanced pressure thrust on the pump casing/baseplate by mounting the heater on springs.Nelluri Fig 06Nelluri Fig 07The advantages of Option 3 include:

Pressure thrust on the pump can be avoided by eliminating the expansion joint in the pump discharge line.

This option allows users to make late-stage design adjustments, such as installing springs on the heater rather than a fixed support. This modification is particularly beneficial if the pump vendor faces challenges in qualifying the integrity check of the pump casing and baseplate due to pressure thrust from untied bellows/expansion joints, as well as if the pump’s foundation has been designed and cast without considering the untied bellow/expansion joint pressure thrust loads.

The disadvantages of Option 3 include:

The stiffness of the system is reduced due to the additional use of flexible springs supports, leading to a reduction in the natural frequency of the piping configuration.

Attention must be paid to spring locks to safeguard the pump nozzles, heater nozzles and vapor head nozzle from the pull-and-push effects of the heater springs in an empty condition.

Modifications are sometimes required in the civil structural arrangements and heater support lug drawing to accommodate the additional height due to the inclusion of spring hanger/cans.

RESULTS AND COMPARISONPiping stress per ASME B31.3 code. Expansion stress per ASME B31.3 is plotted in FIG. 8. The maximum expansion stress is seen at the bend in the heater outlet for all three options. FIG. 8 indicates that the expansion stresses in Option 3 are high due to a cumulative growth from the discharge piping and heater in an upward direction from the pump fixed point. Conversely, the expansion stress is less for Options 1 and 2 as the thermal growth is coming from the heater anchor and not from the pump fixed point.Nelluri Fig 08Vapor head nozzle loads. The resultant horizontal force and bending moment presented in FIG. 9 for the vapor head inlet nozzle are the highest for Option 3 since the reactions are from the cumulative vertical growth from the discharge piping and heater in an upward direction from the pump fixed point. Conversely, the loads in Options 1 and 2 are less as the vertical thermal growth originates from the heater anchor and not from the pump fixed point.The nozzle loads presented in FIG. 10 for the vapor head outlet nozzle are the same for all three options since the piping configuration is almost the same from the vapor head to the pump suction nozzle for all three options. Nelluri Fig 09Nelluri Fig 10Pump nozzle loads. The axial force and bending moment on the pump suction nozzle are high for Options 1 and 3 (as shown in FIG. 11) because the load at the support on the elbow in the suction line increases from the untied bellow pressure thrust exerted by the bellow installed in the vapor head downcomer line. Other loads are almost the same for all three configurations.The axial load on the pump discharge nozzle is high for Option 1 (FIG. 12) due to the pressure thrust acting from the untied expansion joint/bellow in addition to the thermal loads. Other loads are almost the same in all three configurations. The pressure thrust is normally carried as an axial load by the pipe. However, the inclusion of an untied bellows/expansion joint, which is not intended to carry such axial loads, removes the normal means of resisting the pressure thrust. Therefore, other means—such as pump anchors and hardware like a casing or a baseplate—are required to carry the pressure thrust load. Nelluri Fig 11Nelluri Fig 12Heater nozzle loads. The nozzle loads presented in FIG. 13 for the heater inlet nozzle are the least for Option 1 due to the use of an expansion joint in the pump discharge piping. The pressure thrust due to an untied expansion joint/bellow is transferred to the heater anchor point but is not included in the nozzle loading. The axial force is more for the Option 2 configuration since the heater is fixed at a structural steel location and there is no provision to add additional spring supports on the pump discharge piping. The resultant bending moment presented in FIG. 14 for the heater outlet nozzle is minimal for Option 3 since the heater is on springs and rotational degrees of freedom are free. Other loads are almost the same in all three configurations. Nelluri Fig 13Nelluri Fig 14Takeaway. This article has explained three different configurations in a forced-circulation evaporator piping system according to a set of conditions and stages of a case study project. The study showed that Option 1 is preferred when the pressure thrust exerted from the expansion joint on the pump is within the permissible limits of the pump allowable loads, or the pump vendor agrees to reinforce the pump baseplate. However, reliability and maintenance are a concern due to the use of expansion joints in the system. Option 2 is preferred if Option 1 is not viable because of higher magnitudes of thrust forces due to high operating pressures in the forced-circulation circuit. However, a very limited number of vendors offer a hanging-type pump. Also, the cost of a hanging pump is high compared to a conventional foot-mounted pump. Option 3 should be used when the pump has already been procured, as the vendor will not manage the additional loads from an untied bellow/expansion joint pressure thrust to the pump. However, modifications are required in civil structural arrangements and heater support lug drawing to accommodate the additional height due to the inclusion of spring hangers/cans. HPNOTES^a Hexagon’s CAESAR-IIFirst Author Rule LineAuthor pic NelluriVenkat Rao Nelluri, based in Gurgaon, India, serves as a piping engineering professional at KBR. With a robust 15-yr background, he specializes in designing and engineering piping systems and pipe supports across a spectrum of industries, including the refining, fertilizer, metals and mining, power and petrochemical sectors. Before joining KBR, his previous affiliations include roles at M.N. Dastur and Fluor Corp.Author pic DarganSukhjinder Singh Dargan is a seasoned engineering professional with a rich 17-yr background in the piping stress analysis domain, focusing on process plants, refineries and fertilizer plants. Since 2018, he has been an integral part of KBR in Gurgaon, India. Prior to his tenure at KBR, Dargan worked at L&T Chiyoda, India, and Fluor Daniel, India, where he honed his expertise in this field.Author pic ChoudhuryMrinmoy Ghosh Choudhury is an accomplished engineering professional with extensive experience in process plant engineering, specializing in piping and plant engineering from initial concept to final commissioning. His expertise encompasses various crucial aspects, including concept layout, stress and support, and materials. Choudhury has a proven track record in resolving complex issues during plant startups, addressing problems such as vibration, rotary equipment alignment and piping/equipment system failures. He has also significantly contributed to leading engineering journals with numerous published articles. His career has seen him contributing his expertise to prominent companies like Reliance Engineering, Toyo Engineering India, Chemtex Engineering India, Engineers India Ltd. and DCPL.CoverAd—Merichem THIOLEXContentsAd—APIMastheadAd—Total EnergiesEditorial CommentAd—Honeywell UOPInnovationsAd—Koch Engineered SolutionsSF Opening pageSPECIAL FOCUS—Nelluri (KBR)Ad—FIPISPECIAL FOCUS—Arbulu (Project Production Institute)Ad—Atlas CopcoSPECIAL FOCUS—Abid (Antonoil Service)SPECIAL FOCUS—Goyal (Black & Veatch)Ad—AIChEHeat Transfer—Malhotra (S&B Engineers and Constructors)Heat Transfer—Russell (Blasch Precision Ceramics)Ad—GEI MappingProcess Optimization—Da Silva (Petrobras)Process Optimization—Akhtar (TAI Engineers)Ad—HPI Market Data 2024Digital Technologies—Rice (Control Station)BAR—Prusha (Emerson)Ad—HV H2Tech CircCarbon Capture—Hopkins (Burns & McDonnell)Ad—GEISales OfficeAd—HP Circ (Back Cover)