What are Reciprocating Pumps?
Reciprocating pumps can be best defined as equipment which pushes/moves liquid by a piston, plunger or a diaphragm (also known as three classes of reciprocating pumps) that results in a reciprocating motion inside a closed fitted cylinder. Let us first understand the components used in a reciprocating pump which shall help understand the working of the same.
The key components used in reciprocating pumps are
- Piston, Plunger or a diaphragm: All these parts have the basic functionality of moving the liquid inside the cylinder. Piston is a lubricated sliding shaft which moves inside the cylinder and pushes the liquid in forward and backward motion, creating a cavity and a high volume pressure at the outlet. In a diaphragm pump, diaphragm is used to avoid leaking of the liquid since it completely seals the liquid to penetrate outside, and hence they are especially useful when the liquids are dangerous or toxic. In a plunger pump there is a high-pressure seal which is stationary and a smooth cylindrical plunger slides through the seal.
- Crank and Connecting rod: Crank is a circular disk attached to the motor and used to transfer the rotation motion of the motor to the piston. Piston, in turn, moves in a reciprocating motion with help of a connecting rod.
- Suction pipe: Liquid flows from this pipe into the cylinder. One side of the pipe is immersed in the liquid and the other end in connected to the cylinder.
- Delivery pipe: This can be understood as an outlet pipe. One end is connected to the cylinder while the other is towards the discharge/Outlet.
- Suction and Delivery valve: It adjusts the rate of the flow of liquid at the suction and discharge points.
Reciprocating pumps are different from Centrifugal pumps on basis of its working, features, applications etc. The main difference is that Impellers are used in Centrifugal pumps where as in reciprocating pumps piston is used to move the liquid. Centrifugal pumps continuously discharge the liquid unlike reciprocating pump. They are used for high viscous fluid and are lighter in weight, less expensive as compared to reciprocating pumps.
The basic Quality standards of reciprocating pumps include ISO13710, API (American Petroleum Institute) standard 674, API standard 675 “Positive Displacement Pumps- Reciprocating” and Reciprocating Pump Standards , Hydraulic Institute.
Types of Reciprocating pumps
The following are commonly known types of reciprocating pumps:
- Single-acting reciprocating pump: This has one suction valve and one discharge valve. When the piston is moved backward, suction happens and when it moves forward, the delivery valve opens up to discharge the liquid.
- Double-acting reciprocating pump: Unlike single acting pump, here there are two suction and delivery valves. When the piston is moved forward or backward, with each stroke, both suction and expulsion happen simultaneously. Thus it requires two inflow pipes and two outflow pipes. Some of the common applications of these kinds of pumps are in Salt Water Disposal, Well Service, Descaling, Hydraulic Fracturing, and Oil & Gas Pipelines.
- Double acting – Air and Steam pumps: These are double acting pumps where steam, air or gas is used to transmit power to the liquid through the piston. The ratio of total steam force (steam pressure per unit area x area of steam piston) to total liquid force (pump head x area of liquid piston) helps determine the efficiency with which the pump produces pressure. They can operate at any point of pressure and flow, within a flexible range. Because of these features, steam driven pumps are mostly used in the refineries for pump-out service, with low NPSH and the fluids used are hydrocarbons mostly with high viscosity and high temperature. National, Gardner Denver, Oilwell, Gaso and Wheatley are known as key manufacturers of such pumps.
- Simplex, Duplex, triplex, Quintuplex Pumps: Many reciprocating type pumps are simplex(one), duplex (two) or triplex (three) cylinder . Duplex pumps are usually used where the two pumps can be used alternatively. Such pumps are commonly used in oil-line pumping, mine de-watering, and chemical and petroleum products transfer, but has many more applications. A triplex pump consists of three plungers, with the aim of reducing the pulsation of a single reciprocating pump. Quintuplex pumps are designed with a gear case that assists in a high pressure task. Common applications of which are in cement slurries, sand-laden fluids, crude oil, acids, mud and other oil well servicing fluids. Well Known manufacturers for these types of pumps are National, Gardner Denver, FMC, SPM, Oilwell, Kerr, Union, Gaso, Emsco, Aplex and Wheatley.
- Metering Pumps: A metering pump is usually used where the rate of flow of the liquid needs to be adjusted in a specific time period. Most of the metering pumps are piston driven and are called Piston pumps. Piston pumps can pump at a constant flow rate against any kind of discharge pressure. Both Piston pumps and Plunger pumps are reciprocating positive displacement pumps that use a plunger or piston to move fluid/substance through a cylindrical chamber. Manufacturers such as Lewa, Watson Marlow and Bredel offer metering pumps at competitive prices.
Reciprocating Pumps can also be classified according to the number of cylinders: Single cylinder and double cylinder pump. They are also sometimes classified according to their operation, known as simple hand-operated reciprocating pump & power-operated deep well reciprocating pump.
Working of a Reciprocating Pump
The working principle of reciprocating pumps is very simple. Since the main components have been explained above, it will be easier to understand how it works. These kinds of pumps are often operated by a power source such as electric motor or engine. When the operation is started through the power source, it gives rotary motion to crank with the help of connecting rod, which results in reciprocating motion to piston in the closed fitted cylinder. The cylinder is equipped with suction (Inlet) and a discharge (Outlet). When piston moves from backward to forward i.e. from the suction pipe towards the discharge pipe a vacuum or cavity gets created in the cylinder that helps in suction of the fluid. When piston moves from forward to backward, the piston forces the water out from the outlet or delivery value.
It is important to note that these are also known as positive displacements pumps, basically because of their functioning, which means that the fluid can only move in one direction and can never reverse back. Therefore, the pump is always started with outlet valve kept open otherwise the pressure will become high and this may lead to rupturing of the pipeline or even the pump itself. However, a relief valve is often fitted to control the pressure.
Factors determining efficiency of a Reciprocating Pump
There are several performance indicators of a reciprocating pump which determine how effectively it works. Following are some of the key performance indicators:
- Brake Horsepower (BHP): How much is the actual power requirement at the input shaft to achieve a desired pressure and flow.
- Capacity: It can be define as total volume of liquid/flow delivered per unit of time.
- Slips: Slip is the capacity loss as a fraction or percentage of the suction capacity.
- Mechanical efficiency: Its working at full load pressure and speed is 90% to 95% depending on the size, speed and construction.
- Pressure: Mainly, the suction and the discharge pressure in a pump.
- Displacement: Also known as GPM, it is the calculated capacity of the pump with no slip losses.
Advantages and Disadvantages of Reciprocating Pumps
Reciprocating pumps are best suited for applications requiring a wide high pressure range. Reciprocating pumps operate at higher efficiencies as compared to other pump designs. However, there may be a lot of maintenance required. They are mostly used to pump low viscous fluid. They do not have any problem of priming. But these are heavier as compared to centrifugal pumps. Also, these pumps are difficult to install and require more space in terms of floor area. If these pumps are well maintained, they can last for years or even decades; however, they can undergo rigorous wear and tear in absence of regular maintenance.
If there are problems like excessive noise and vibrations, no liquid discharge, excessive power consumption, excessive heat, pressure pulsation- induced cavitation, small bore attachment failure etc., then one must thoroughly get the machine checked for replacements of faulty component on time.
Common Application of Reciprocating Pumps
Reciprocating positive displacement pumps are highly effective, where a high degree of accuracy and reliability under different range of conditions are required. Reciprocating pumps with very high efficiency are often available in a wide range of hydraulic, mechanical, and material options. They are widely used across industries such as chemical, petrochemical, refinery, pharmaceutical, cosmetic and water treatment. Typically, these types of pumps are used for applications such as Salt Water Disposal, Well Services, Descaling, Hydraulic Fracturing, and Oil & Gas Pipelines. All types of reciprocating pumps are easily available in the market to meet the diverse demands, as per different processes and applications. Piston pumps are widely used in applications such as Energy Recovery, Steam Recovery and hazardous area pumping and are available with manufacturers such as Union, Gardner Denver, Worthington and Wilson Snyder.
Diaphragm Pumps are commonly used for Sludge Transfer, Acid Pumping and Chemical Fluid Transfer and are easily available with manufacturers such as Wilden, Sandpiper, ARO, Roughneck, and Graco.
All the mentioned manufacturers in this article, offering various kinds of pumps hold good reputation with respect to quality, price, revenue (value) and market share and are preferred by many consumers. However, a thorough check of all its features, specific to your process application, should be ideally done to buy the most suitable reciprocating pump, which can be used for a longer period of time requiring low maintenance, ease of operation and with easy availability of its spare parts.
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