Positive Displacement Pump: Constant Flow, Variable Head

Positive Displacement Pump Is A Constant Flow Variable Head Device Th

Positive Displacement Pump is a constant flow variable head device. There are two types of positive displacement pump such as reciprocating and rotary pumps (According to Forsthoffer P:10). Reciprocating Pumps increase liquid energy by pulsating action. Types include power and metering pumps (According to Forsthoffer P:11). Power pumps are used for high pressure, low flow applications, such as carbonate, amine service, or high-pressure water or oil services, and can be horizontal or vertical. The power end consists of components like the crankshaft with bearings, connecting rod, and crosshead assembly (According to Forsthoffer P:12).

Metering pumps are used for precise chemical injection control, with types including diaphragm and plunger pumps. Packed plunger pumps involve the process fluid contacting the plunger and are used for higher flow applications, whereas diaphragm pumps isolate the process fluid with a hydraulically actuated diaphragm for lower flow or hazardous liquids (According to Forsthoffer P:12-13). Metering pumps can have single or multiple pumping elements, with double diaphragms alarms for toxic or flammable liquids (According to Forsthoffer P:13).

rotary pumps are positive displacement pumps that do not cause pulsation, unlike reciprocating pumps. Types include screw and gear pumps (According to Forsthoffer P:13). Screw pumps displace fluid axially between rotors, often used in lubrication and petrochemical industries (According to Forsthoffer P:14). Gear pumps move fluid between teeth of external gears and are suited for small volume high-viscosity liquids like asphalt or polyethylene (According to Forsthoffer P:14).

Centrifugal pumps are dynamic machines that use centrifugal force to increase fluid pressure. They come in single-stage and multi-stage types (According to Forsthoffer P:15). Single-stage overhung pumps have an impeller affixed to a shaft outside the bearing system, widely used in industry (According to Forsthoffer P:16). Single-stage inline pumps mount between pipe flanges, suitable for low head and flow applications; they can be mounted vertically without bases, requiring strict shaft alignment (According to Forsthoffer P:16). Integral gear centrifugal pumps serve low flow, high-head needs, with impeller speeds exceeding 30,000 RPM, featuring pump bearings and an integral gear (According to Forsthoffer P:17). Double flow pumps, with flow between bearings, are used for low NPSH requirements, needing balanced piping to prevent cavitation and vibration (According to Forsthoffer P:18).

Magnetic drive pumps (MDP) are seal-less, environmentally friendly options ideal for hazardous, toxic, or flammable fluids, with the motor shaft connected via flexible or rigid couplings, supported by bearings (According to Forsthoffer P:21). They require precise alignment similar to centrifugal pumps with mechanical seals. Multistage barrel pumps are used for service conditions exceeding horizontal split case design limits, often with a thrust balance device and a tight joint assembly (According to Forsthoffer P:19). Multistage horizontal split pumps feature a casing that splits horizontally, allowing vertical rotor removal, working up to approximately 2000 PSI and 600°F, with impeller configurations either inline or opposed (According to Forsthoffer P:18).

Sump pumps, both single and multistage, are designed for handling rainwater runoff and chemical liquids in applications with a setting limit of around 10 feet. These include an enclosed line shaft with external lubrication, with the pump shaft coupled to a motor supported by a bracket (According to Forsthoffer P:20). Submersible pumps also come in single and multistage forms, with the electric motor directly coupled to the impeller, fully submerged, increasingly used in environmental and industrial applications due to stricter regulations (According to Forsthoffer P:20).

Pump failure can result from pulsating discharge in reciprocating designs, causing noise, vibration, and cavitation, which damage pumps. Cavitation occurs when the local pressure drops below vapor pressure, leading to vapor bubble formation (According to Forsthoffer P:22). Failure causes include internal recirculation, operation outside optimal efficiency points, high radial loads, bearing, and seal failures, as well as high internal temperatures.

Operating pumps at excessive flow rates can lead to overloads, cavitation, and increased NPSH requirements, harming the system’s efficiency. Conversely, oversizing can cause internal recirculation and damage to impellers. Proper system design, regular maintenance, and correct sizing are essential to prevent such failures, ensuring longevity and operational efficiency. Ancillary equipment like drives, controls, sealing technologies, hoses, and valves play crucial roles in comprehensive pump system service (According to Forsthoffer P:22-23).

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Pumps are vital components in fluid handling systems, serving various applications across industries such as petrochemical, water treatment, and manufacturing. The selection and operation of pumps—whether positive displacement or centrifugal—depend on the specific requirements of flow rate, head, pressure, and fluid properties. Understanding the different types, their mechanisms, advantages, limitations, and failure modes is critical to optimizing performance, ensuring safety, and reducing downtime.

Positive displacement pumps, characterized by delivering constant flow regardless of pressure changes, are especially suited for high-precision applications such as chemical injection and high-pressure systems. These include reciprocating types, like power and metering pumps, and rotary types, such as gear and screw pumps. Reciprocating pumps operate through a pulsating action that can cause vibrations and noise but excel at high pressures and low flows (Forsthoffer, P:10-13). Power pumps are often used in high-pressure industrial processes, driven by crankshafts and geared mechanisms, with their design optimized for durability and efficiency (Forsthoffer, P:12).

Metering pumps, designed for accurate chemical dosing, utilize diaphragm or plunger mechanisms to isolate the process fluid. Diaphragm designs are preferred for hazardous chemicals, providing safety through double diaphragms and leak detection capabilities (Forsthoffer, P:12-13). Rotary pumps, including screw and gear types, are appreciated for their smooth flow and minimal pulsation, making them ideal for lubrication and high-viscosity liquids (Forsthoffer, P:13-14).

Centrifugal pumps, fundamental to dynamic fluid transfer, operate by converting rotational energy into hydrodynamic energy to increase pressure and flow. They are classified into single-stage and multi-stage variants. The single-stage overhung pump features an impeller attached to a shaft supported outside the bearing system, providing widespread use due to simplicity and efficiency in moderate-duty applications (Forsthoffer, P:16). Inline pumps facilitate easy installation in piping systems, with the benefit of vertical mounting configurations (Forsthoffer, P:16). Multi-stage centrifugal pumps offer higher head capabilities by adding impellers in series, suitable for high-pressure applications in refining and chemical processing (Forsthoffer, P:17-18).

Magnetic drive pumps represent advancement in seal-less technology, leveraging magnetic fields to transmit torque from the motor to the impeller without traditional seals. This design excels in handling hazardous, toxic, or corrosive fluids where leaks are unacceptable, aligning with environmental safety standards (Forsthoffer, P:21). Multistage barrel and horizontal split pumps further extend the operational range, with specialized features such as impeller configurations and casing designs tailored for high-pressure, high-temperature scenarios (Forsthoffer, P:18, 19).

Submersible and sump pumps cater to specific operational environments. Submersible pumps, with their fully submerged motor and impeller assembly, are ideal for wastewater, sump drainage, and chemical processing, especially with increasing environmental regulations (Forsthoffer, P:20). Sump pumps are particularly relied upon in rainwater runoff and non-corrosive liquids, featuring long, enclosed shafts and external lubrication systems (Forsthoffer, P:20).

Despite their efficiencies, pumps are susceptible to various failure modes. Pulsation in reciprocating pumps can induce damaging vibrations and cavitation—vapor bubble formation caused by local pressure drops below vapor pressure, impairing impeller surfaces and reducing overall lifespan (Forsthoffer, P:22). Over-sizing or operating outside optimal points leads to internal recirculation, bearing and seal wear, excessive internal heating, and reduced efficiency. High flow operation can overload the system, cause cavitation, and elevate net positive suction head (NPSH) requirements, risking catastrophic failure if not properly managed (Forsthoffer, P:22-23).

To mitigate failure risks, comprehensive maintenance, proper pump selection, accurate system design, and the use of suitable ancillary equipment are essential. Incorporating modern controls, seal technology, and corrosion protection measures enhances reliability and safety. Proper training of personnel in pump operation and regular inspection routines further extend operational lifespan and efficiency, making the pump system a reliable backbone of industrial fluid management (Forsthoffer, P:22-23).

References

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