Paraphrasing only
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
Reciprocating pumps are that increase liquid energy by pulsating action. There are many types of reciprocating pumps such as (Power and Metering). (According to Forsthoffer P:11)
power pump
Power Pump is used normally for high pressure, low flow applications, typically carbonate, amine service or high pressure water or oil services. They can either be horizontal or vertical. The power end consists of the It is termed. Crankshaft with bearings, connecting rod and crosshead assembly. (According to Forsthoffer P: 12)
Meterin Pump
Metering pump is most commonly used for chemical injection
service when it is required to precisely control the amount of chemical or inhibitor being injected into a flowing process stream. There are two types of Metering pump such as diaphragm and plunger.
1- Packed plunger pump – the process fluid is in direct contact with the plunger and is used for higher flow applications. (According to Forsthoffer P: 12, 13)
2- Diaphragm pump process fluid is isolated from the plunger by means of a hydraulically actuated flat or shaped diaphragm and is used for lower flow applications or where escape of the pumped liquid to atmosphere is not acceptable.
Metering pumps can be furnished with either single or multiple pumping elements. When the pumped liquid is toxic or flammable, in this case the diaphragm is provided with double diaphragms dripping detector to alarm on the problem (According to Forsthoffer P: 13)
Rotary pumps
Rotary Pumps are positive displacement pumps that do not cause pulsation. All reciprocating pumps produce pulsations that can cause damage to the pumps. There are many types of rotary pumps such as (screw and gear) (According to Forsthoffer P: 13).
Screw Pump
Screw pump Fluid flow is carried axially between the threads of two or more close clearance rotors so that a fixed volume of fluid is displaced with each revolution. This design is frequently used for lube and seal service. The refining and petrochemical industry (according to Forsthoffer P: 14)
Gear Pump
Gear pump with this type of pump, fluid is carried between the teeth of two external gears and displaced as they mesh. Gear pumps are used for small volume lube oil services and liquids of very high viscosity (asphalt, polyethylene, etc.). (According to Forsthoffer P: 14)
Centrifugal Pump
Centrifugal pump can be referred as a dynamic machines. Centrifugal pump widely used fluid handling devices in. they use the centrifugal force to pumping the level of pressure to higher level. There are two types of centrifugal such as single stage and multistage and these types are divided to many types (According to Forsthoffer P: 15)
Single stage overhung pump
Single stage overhung pump is probably the most widely used in the industry. Its construction incorporates an impeller affixed to a shaft: which has its centre of gravity located outside the bearing support system (According to Forsthoffer P: 16).
Single stage inline
Single stage inline this type of pump is finding increased usage in applications of low head, flow and horsepower. The advantage of this pump design is that it can be mounted vertically (inline) between pipe flanges and does not require a baseplate. A concrete, grouted support plate however, is many inline designs do not incorporate bearings in the pump and rely on a rigid coupling to maintain pump and motor shaft alignment. Acceptable pump shaft
Assembled run out with these types of pumps should be limited to 0.001″ (According to Forsthoffer P: 16).
Integral gear centrifugal
Integral gear centrifugal of pump is used for low flow applications requiring high head. The pump case design is similar to the inline, but incorporates pump bearings and an integral gear to increase impeller speeds over 30,000 (According to Forsthoffer P: 17).
Single stage Double Flow
Single stage double flow between bearings is commonly used when flow and head requirements make it necessary to yield low values of NPSH required. When designing piping systems for this type of pump, care must be taken to assure equal flow distribution to each end of the impeller to prevent cavitation and vibration. (According to Forsthoffer P: 18)
Magnetic drive pumps
As a result of more stringent environmental constraints and regulations, seal less pump technology has gained prominence. One such design is the magnetic drive pump (MDP). This is a design such that the motor shaft is attached to the power frame of the magnetic drive pump by means of a flexible or rigid coupling. The outer magnet and shaft assembly is supported by its own bearings. Alignment requirements for this type pump are similar to that for horizontal mounted Centrifugal pumps fitted with mechanical seals or packing. The seal less pump is generally applied when there is a need to contain toxic or hazardous fluids. (According to Forsthoffer P: 21)
Multistage barrel
Multistage barrel it is used for service conditions exceeding those normally considered acceptable for a horizontal split case design. A thrust balance device is required since the impeller configuration is almost always ‘inline’. The
circular mounted end flange results in excellent repeatability for a tight joint as compared to a horizontal split case design (According to Forsthoffer P: 19)
Multistage Horizontal split
Multistage horizontal split illustrates a horizontal split casing design which allows the rotor to be removed vertically after the top half casing is unbolted. This type of pump is normally limited to working pressure of approximately 2000 PSI, temperatures to 600°F and S.G. of 0.7 or greater. Impeller configuration for this type of pump can be either ‘inline’ or ‘^opposed’. The ‘opposed’ impeller arrangement has the advantage of not requiring a thrust balancing device which is required for the ‘inline’ configuration (According to Forsthoffer P: 18).
The Sump Pump
The Sump pump can refer to both Single stage and Multistage. Sump pump illustrated is a popular design for handling run off streams of rainwater, non-corrosive or corrosive liquids. The setting limitation for this cantilever design is approximately 10 ft. This particular design incorporates an enclosed line shaft with external lubrication to the bottom bearing. The pump shaft and impeller are coupled to the driver, which is supported by a motor support bracket bolted to a cover plate (According to Forsthoffer P: 20).
Submersible Pumps
Submersible pumps can refer to Single stage and Multistage. This type of pump consists of an electric motor driver is coupled directly to the impeller/bowl assembly. All components are designed to be submerged in the pumped fluid. In the past, this type of pump did not find widespread use in the refining and petrochemical industry. However, with increasing environmental restrictions, this type of pump is being used more frequently in the refining and petrochemical industry (According to Forsthoffer P: 20).
Causes of pump failure
Pulsing discharge is also a characteristic of positive displacement, and especially reciprocating, pump designs. Pulsation can cause noise and vibration in pipe systems and cavitation problems, which can ultimately lead to damage or failure. Cavitation occurs in centrifugal pumps when the pressure of the fluid at a point immediately upstream or within the pump reaches a value, which is below the vapor pressure of the fluid.
Effects of pumps failure
Effects of pump oversizing Operation
Internal recirculation damage to impeller
Operation at less than best efficiency point
High radial loads
Bearing failures
Seal failures
High internal temperature rise and requirement for minimum flow bypass.
Effects of pump operation at high flows
High to overloading horsepower with reduced system resistance
Operation in the “break” of head capacity curve (significant changes in head
with no change in flows)
Higher NPSH required than available
Recirculation cavitation at impeller tips
Solution
Of course, a complete service is not only about the supply, installation, Maintenance and repair of the pumps themselves. Its also about ancillary equipment, such as drives and controls, sealing, hose and valve technology.
