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CENTRIFUGAL AND VERTICAL PUMPS, PIPING, VIBRATION, SUCTION, CAVITATION
AND VARIABLE SPEED DRIVES
By Amin Almasi
Centrifugal pumps are an important class of turbomachinery. Attention must be paid to related piping, Net Positive Suction Head (NPSH), vibration and the specifics of their application.
Incorrect pump piping, for example, can result in hydraulic instability, mechanical issues, cavitation, high vibration, pump bearing and seal issues, premature failure of components or even catastrophic failure of a pump. The suction piping is particularly important since the liquid should arrive at the eye of pump impeller in the right conditions (pressure, temperature, and so on.). In addition, this piping should ensure the smooth uniform flow of liquid that is free of air or gas.
Cavitation is known to increase pump vibration and noise, as well as reduce head and cause major damage, such as impeller pitting. The localized boiling of liquid and re-pressurization can lead to a series of implosions and the development of energy levels exceeding impeller metal yield strength. Cavitation, however, can be eliminated by increasing the pressure of the liquid at pump suction.
The effect of piping diameter and the design of the pump’s suction passage on pressure drop are significant. As a rough indication, pressure loss (due to friction) is in inverse proportion to the 5th power of the piping diameter.
For example, a 10% increase in piping diameter could result in a 40% reduction in head loss. In a similar way, a 20% increase in diameter would result in a drop of around 60% in head. In other words, frictional head loss would be less than 40% of the head loss at the original diameter. This could be a significant factor for pump suction piping design.
Suction piping should be as simple, short and straight as possible. A centrifugal pump is usually provided with a straight run of around 5-to-10 times the suction piping diameter to avoid turbulence. A temporary suction strainer is generally required, although a permanent suction strainer is often discouraged.
Suction recirculation can occur due to complex hydrodynamic circumstances (turbulence and swirling, for example). Such issues often occur at a flow lower than 50% of the Best Efficiency Point (BEP).
Some centrifugal pumps can resist suction recirculation instabilities at flow rates as low as 35% of BEP in some cases. For some other pumps, however, suction recirculation can become apparent at 65%. It can result in damage and pitting, usually around halfway along the pump impeller vanes.
Discharge recirculation is another type of hydrodynamic instability. It is seen at the discharge side of pump at low flow. Inappropriate designs of components or incorrect clearances are some of the reasons for this issue. Once again, damage and pitting of impellers can take place.
For a better NPSH margin, there is a tendency among some engineers to reduce NPSH required (NPSHR) and increase NPSH available (NPSHA). However, the focus on the reduction of NPSH required (NPSHR) is not the correct way to deal with this issue.
The reduction of NPSHR is a difficult and costly option. There are a limited number of possibilities. NPSHR is a function of pump design and pump speed. Often, a centrifugal pump with a larger impeller eye can offer a (slightly) smaller NPSHR. However, in the design or selection of a pump impeller to achieve a lower NPSHR, many factors are involved, and sacrifices must be made.
A larger impeller eye usually results in operational and hydrodynamic issues, such as recycling problems. This approach is not usually a solution to pump cavitation. Nevertheless, it is worthy of consideration by pump manufacturers and engineers in some specific pump designs and selections.
A slower pump usually requires less NPSHR. Speed is an important factor in pump selection and for proper operation of a variable speed pump. Larger pumps with bigger dimensions and higher capacities often require more NPSHR. But there are usually challenges for high-power, high-speed, high-pressure pumps with regard to NPSHR.
Sometimes, it is difficult to find a pump that matches NPSHA for large, high-pressure pump service. Physical site constrains, such as the pump location, and suction vessel or tank elevation can impose some limitations on NPSHA. This is the usual story in revamp and renovation projects as site layout often cannot be changed — yet the renovation plan requires a larger, high-pressure set of pumps to be added to existing facilities.
A booster pump is sometimes a way around this. It is a smaller, lower-speed pump with a low NPSHR. This pump should offer the same flow-rate as the main pump. The booster pump is usually installed a close distance upstream of the main pump. Note, that this is only an option where all other possibilities have been evaluated and exhausted.
While many low NPSHR pumps are in use, some lead to operational or reliability problems. A good example is pumps equipped with specially designed large-eye impellers. This can result in high recirculation issues, low efficiency and low reliability.
Another example is low-NPSH pumps with unsuitably low speeds. This can bring about a low efficiency bulky and heavy pumps, and operational challenges. Some special pumps with significantly low NPSHR can also have poor reliability.
It is generally best to increase NPSHA than to lower NPSHR when there is a need to obtain a better NPSH margin.
Vertical centrifugal pumps have the electric motor installed at the top with the pump with vertical shaft at the bottom. These pumps offer significant benefits in specific services. However, for most applications, horizontal pumps are superior.
Vertical pumps have a compact footprint. They have been used in many applications, such as seawater pumping services, large cooling water pumping systems, and various utility water services. There are different types of vertical pump and each is employed for a specific set of services and applications.
Vertical inline pumps are popular options for applications which demand compact units. They are also commonly employed in revamp and renovation projects where there are limits on footprint or budget. In some cases, the pump requires no bearing, since bearings are on electric motor driver. This pump concept, however, has lost popularity. Another disadvantage of vertical inline pumps is that the NPSHR is often greater compared to most end-suction horizontal pumps. They are only recommended in small pumping systems.
Vertical sump pumps are seen in many sumps, tanks, vessels and reservoirs where the sump pump is suspended from the top. The electric motor driver is usually installed at the top of the sump on an assembly or baseplate. This may have safety and reliability benefits. The discharge nozzle on the pump casing is usually horizontal and most often discharges into an elbow that connects to vertical piping.
Another example of vertical pumps is the submersible pump. It is a closely coupled pump and electric motor that can be located and operated while immersed in the pumped liquid. Submersible pumps offer compact packaging and good flexibility for many applications.
They can be used in a sump, well, vessel, tank or reservoir and for permanent, temporary, portable, mobile or moving applications. There is also the possibility of changing the location and elevation of a submersible pump. A set of slide rails with special pump connections or flanges can be used to change elevation, for example.
Vertical sump pumps are usually specified for ordinary, low-depth sumps, or vessel and tank applications. Submersible pumps tend to be employed for special cases, such as temporary services, deep-well applications and other challenging services. There are advantages for vertical sump pumps over submersible pumps. Submersibles offer limited opportunities for monitoring and inspection. Vertical sump pumps are more accessible. Electric motor, systems and accessories of submersible pumps are also more expensive.
Vertical turbine pumps can be used for process and utility applications. They usually incorporate a circular casing with a symmetric passage-type diffuser. Their impellers are usually installed vertically and concentrically within the casing.
They are popular pumps for low NPSH applications or services for temperature-sensitive liquids. In these pumps, the column and bowel assembly are usually incorporated with an ASME Section VIII-designed pressure-containing section. This pump type can reduce NPSHR by lowering the first-stage impeller into a lower suction can, usually with the lowest possible elevation.
One concern about many vertical pumps is high vibration. Their thin-walled shells and long, slender shape add risk of vibration resonance. Vibration characteristics of vertical pumps should be analytically and experimentally verified before operation.
Parametric studies should be done for vertical pumps to investigate the effects of variables on the dynamic characteristics of the pumps and to propose strategies for improvement of dynamics. There are many parameters and variables to be considered. One is the elevation of the liquid in the sump or reservoir.
Mixed-flow impellers or axial impellers are sometimes used in vertical pumps. The performance curves of these pumps could be different than conventional pumps. The curve is relatively steep, and the shutoff head is often much higher than the head at BEP. The shutoff head may be two or three times the BEP head. These pumps usually require a bypass to avoid driver overload when operating points move toward the shutoff point.
Centrifugal pumps are often employed for industrial wastewater treatment. Once water has served its industrial uses, it should be transferred and treated to be reused or treated to return to the environment in a manner consistent with water quality regulations.
Many modern centrifugal pumps in wastewater services have variable speed drives (VSD) that operate over a wide range of flows, head, speeds and conditions. Large pumps are often needed yet space is usually limited. Therefore, vertical pumps are commonly specified. They offer a compact configuration and better economics compared to horizontal pumps in many, though not all, wastewater services.
Priming has been a major issue. As a result, pumps should operate with a positive suction head. Preferably, each pump should have an individual suction line. The suction source and piping layout should avoid turbulence to prevent vortex formation.
The fill time and minimum pump cycle time should be considered in the sizing of storage units, tanks and wet wells. Effective volume (between pump start and stop) should ideally be designed to provide no more than one or two starts per hour.
VSDs are usually needed as pump systems and controls tend to operate at varying delivery rates. Capacity should be based on peak hourly flow and should be adequate to maintain the wastewater velocity within minimum and maximum limits to avoid operational problems, high head loss and so on. A minimum velocity of 0.7 m/s and maximum velocity of 2.5 m/s are recommended.
For industrial wastewater treatment plants and facilities, it is common to have pumps working in parallel. If the maximum rated capacity of the pumping system is supplied by two or more normally operating units, the capacity of standby units should be at least 50% of the system’s maximum rated capacity.
Attention is needed when pumps operate in parallel. One pump should be started at minimum VSD speed. Pump speed is increased as demand rises until the pump is at maximum speed. A further rise in demand initiates the start of a second pump and in turn a third pump once two pumps are running at full speed. The pump stop should follow a similar sequence based on falling of demand.
For wastewater and abrasive or corrosive liquids, material selection is vital. Fluid properties, corrosion, erosion, economy, construction and other factors should be used in pump selection. It is best for any hardware in contact with industrial wastewater to be fabricated from proper corrosion-resistant materials. Type 316 stainless steel material is often suitable. Stainless steel vertical multistage pumps and booster pump sets are commonly used in industrial wastewater handling and pumping systems.
Take the case of a pumping station moving wastewater from a plant discharge to a wastewater treatment unit located offsite. The plant’s operating capacity and head were 2,000 m3/h and 27 m respectively, during the first year. After that, other production came online, the flow increased to 7,500 m3/h and head to 60 m. Because of the difference in head, the only way to use the same pump model for both development stages was to use VSDs.
The first group of pumps operated as 1+1 arrangement (1 operating + 1 standby) during the first year. It ran at low speed. Five pumps were needed in the second year (two existing VSD pumps and three new pumps). These operated in 3+2 arrangement (3 operating + 2 standby). Each pump provides a capacity and head of 2,500 m3/h and 60 m, respectively.
In this way, two VSD-driven pumps can be installed for the first year of operation. Each pump has its own suction line. Proper valves and flanges are provided at the discharge header so three additional pumps of the same kind could be installed later without any interruption in operation.