#reciprocating pump

Vacuum pump is a machine used to pump air from equipment or production system to make equipment or production system work in vacuum. The main working performance parameters of vacuum pump are: pumping rate, limit vacuum degree, shaft power and rotational speed. The pumping rate of vacuum pump refers to the volume of gas that can be pumped out per unit time (m 3/h). Limit vacuum is the highest vacuum that a pump can achieve.

Vacuum pumps commonly used in chemical production include reciprocating vacuum pumps, water ring vacuum pumps and jet pumps. The structure of reciprocating vacuum pump is basically the same as that of piston compressor.

Vacuum pump is characterized by high pressure ratio. For example, to achieve 95% vacuum, the absolute pressure of nominal inspiration is 0.005 MPa, while the exhaust pressure is 0.1 Mpa, with a pressure ratio of 20. Because of the high pressure ratio, the volume occupied by the residual gap volume after expansion is very large, which greatly reduces the suction capacity and the effective utilization rate of the cylinder is very low.

In order to solve the above problems, several grooves are usually opened on the cylinder wall at both ends of the cylinder, which are thicker than the piston. When the piston runs to the stop point, the grooves are used to balance the pressure of the cylinders on both sides of the piston. For example, when the piston runs to the left stop point, the exhaust of the left cylinder ends, the residual pressure in the clearance volume is higher, the suction in the right cylinder ends, and the pressure is lower. Because of the connection of grooves, the gas pressure in the clearance volume of the left cylinder decreases accordingly. In this way, the volume occupied by the expansion of residual gas is greatly reduced and the suction capacity is increased during the piston return operation. The piston runs to the right stop.

Reciprocating vacuum pump is suitable for pumping non-solid particles, non-corrosive gas. This kind of vacuum pump is widely used in chemical industry because of its high pumping rate and high vacuum degree.

Open Cycle Water Ring Vacuum System

A designation of a reciprocating pump that uses two drive rods.

Duplex pumps are common in both single and dual acting designs. Due to some of the benefits over simplex pumps, and simpler design than other reciprocating pumps, they are often a preferred pump for many applications.

Essentially, a single action duplex pump is two simplex pumps running at 180 degrees from each other. As one plunger or piston is pumping fluid into the outlet port, the other is drawing in from the inlet. A dual action is similar in style but is only offset at 90 degrees. Theoretically, a dual action duplex operates using four single-acting plungers or pistons. Keeping the rotation at 180 degrees would create two overlapping sets of plungers or pistons. A 90-degree offset helps to stabilize the flow. See the flow characteristics below for more information.

Duplex Pump Flow Characteristics

Duplex pumps have a steadier flow than simplex pumps. However, their flow still has a large variation. In a single action duplex pump, the maximum flow velocity is 160% of the average. That is, every rotation of the crankshaft, the flow varies from 0% to 160% of the average flow rate. In a double acting duplex pump, the maximum flow velocity only varies between 111% and 79% of the average flow rate.

Note that the dual action charts are made specifically for a plunger pump. In a dual action piston pump, pump chambers one and three would look similar to that shown above, but chambers two and four would have a slightly lower relative velocity. This is due to the fact that one side of the piston requires the drive rod, reducing the usable volume within these pump chambers.

Duplex Pump Example Applications

Duplex pumps are used in low flow applications across many industries. Several example applications may include:

Crude Oil Transfer

Hydrocarbon Condensate Collection

Coke Oven Tar

Bauxite Slurry

Salt Water Disposal

Hot Oil

Cement

Aluminum Ore Slurry

Coal Slurry

Cryogenic Service

Steam

Ore Slurry

A device installed in a pipe system that dampens pressure spikes and pulsations as a result of changes in the velocity of a fluid.

Cause and Effect of Pressure Pulsations

From the law of inertia, an object in motion will stay in motion unless acted upon by an outside force. Sometimes these forces are hard, such as an egg hitting the pavement, and sometimes they are soft, like jumping onto your bed. When the force is hard or sudden, damage is more likely to take place. Fluids have the same properties. When they are in motion, they have inertia. It takes an outside force to change the direction of the fluid.

Imagine a stretch of pipe with a liquid flowing through it. On one end, a valve is suddenly closed. When the valve closes, the moving liquid suddenly needs to come to a complete stop. Since most liquids can be considered incompressible, the force against the valve is a harsh impact. Similar to the egg hitting the pavement. This sudden change in momentum applies a force against the valve. Since there is nowhere for the liquid to flow, it creates a pressure spike.

A discharge dampener is designed and installed in the pipe to help absorb this pressure spike. The dampener consists of a vessel filled with gas or compressible material. When there are sudden changes in flow, the compressible material is able to compress and expand, similar to jumping onto your bed. The video below demonstrates the effects with and without a dampener.

The pressure spikes caused by a change in flow rate can be damaging to pipes and equipment within a system. The changes in pressure cause the walls of the pipes and materials to rapidly expand and contract. Over time, these changes can develop cracks in piping and equipment walls. If the pressure spike is large enough, the resulting spike may have enough pressure to cause the pipe to explode.

By installing a pulsation dampener, the intensity of these spikes are reduced to controlled levels.A dampener should be sized and installed in all piping that may experience a harmful level of pressure spikes.

Types of Pulsation Dampeners

Pulsation dampeners can be purchased in a variety of shapes, sizes, and designs. It is important to size a pulsation dampener for a specific application. Incorrect sizing or incompatible materials may cause a danger to equipment, systems, and personnel.

Membrane Pulsation Dampener: A membrane type dampener provides a solid separation between the pumping fluid and the compressible gas. The membrane resides within the dampener and allows for the pressure to be transferred to the gas, without any mixing of the gas into the fluid.

Bladder Pulsation Dampener: A bladder type dampener fully encloses the compressible gas within a bladder. This setup ensures there is no leakage of the gas into the pumping fluid. The pressure pulsations are transferred to the gas as the bladder expands and contracts.

Bellows Pulsation Dampener: A bellows type dampener works the same way as a bladder type dampener. However, by using a bellows type design, it can be made with other types of materials such as PTFE or Stainless Steel. This type of dampening system is used when pumping corrosive materials that may deteriorate more common materials.

Pressure Vessel Style: A pressure vessel dampener, sometimes referred to as a "zero maintenance" style dampener does not use any moving parts. They are only effective at very high pressures. Fluids, including water, do have some degree of compressibility. Pressure Vessel Style dampeners allow for the pressure of the fluid to be dissipated within the vessel by the small amount of compressibility within the pumping fluid. In very high pressure applications, a pressure vessel style dampener may be the only type of dampener available. It is important to note that this style of dampener does not operate as effectively as other types of dampeners.

Flexible piping: Although it is not recommended, flexible piping or hose can act as a pulsation dampener in emergency situations. As the fluid flow changes, the flexible piping is able to "move" and allow a dampening effect on the fluid. Flexible piping still requires proper sizing to reduce pressure spikes. Improper usage may result in damage to equipment and endangerment of equipment operators.

Causes of Pressure Pulsation in Equipment

Equipment that rapidly changes flow rates are recommended to have a pulsation dampener. Plunger pumps for example, have highly variable flow rate. The average flow rate of a plunger pump can be accurately predicted. However, each rotation of the crankshaft produces several changes in flow velocities.

Let's look at a single plunger for an example.

In step 1, the plunger is moving back and drawing the fluid into cavity. The fluid is flowing through the inlet.

In step 2, the plunger is stopped. It is transitioning from moving backwards, to moving forwards. The fluid has stopped moving through the inlet and has come to a complete stop, resulting in a pressure spike beginning at the inlet which is then travels through the suction piping. The resulting change in the fluid's momentum is a change from kinetic energy in the form of linear velocity, to potential energy in the form of pressure.

In step 3, the plunger has begun to move forward. As fluid begins moving through the outlet. The stationary fluid of the outlet is suddenly required to move as well. A pressure spike is created in the discharge, beginning at the outlet, and then carried through the discharge piping.

In step 4, the plunger has again come to a complete stop. Fluid is no longer flowing through either port. As it begins to move back, fluid will suddenly needs to begin moving through the inlet.

When a plunger pump is running slowly, these pressure spikes can be ignored. In most cases, they will not produce enough of a spike to create damage. When the pump is running at full speed, this full cycle is taking place many times per second. The pressure spikes caused by these sudden changes will likely need a pulsation dampener.

The image above shows the visible pulsations created by a reciprocating quintuplex pump. Using flexible hoses on the inlet and outlet, the pressure fluctuations through the hose can easily be seen. Since steel piping is more rigid, it may be more difficult to visually see pressure vibrations before damage to piping and the surrounding systems takes place. It is important to correctly determine if a dampener is needed, and to correctly install the required size before operating the pump system.

Reciprocating Pump

A positive displacement pump which utilizes a plunger or piston to change a cavity's volume, and produce a pressure differential.

A plunger pump operates using the reciprocating motion of plungers or pistons. Depending on the design of the pump, the use of a single or multiple plungers may be used.

How a Reciprocating Pump Works

To help visualize how a reciprocating pump works, let's look at a single piston and split the process into four parts.

Action 1: The plunger or piston is pulled back. The action increases the volume of the cavity. As the cavity volume expands, fluid is drawn in through the inlet to fill the expanding cavity.

Action 2: The piston has reached it's maximum displacement. Since it is not moving into or out of the cavity, fluid is not flowing through the inlet or the outlet.

Action 3: After reaching it's maximum position, it is then pushed back into the cavity. During this process, the piston applies enough pressure to the fluid to overcome the pressure in the outlet of the pump. This pressure differential pushes the fluid from inside the cavity through the outlet of the pump.

Action 4: The piston reaches its maximum extension into the cavity. Here the volume of the cavity is at a minimum and fluid is not flowing through the inlet or the outlet. The next action repeats the process, starting again with action 1.

The process of how a reciprocating pump works can be seen here in action from start to finish:

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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. The 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, the 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 rotary 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 is 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 whereas in reciprocating pumps piston is used to move the liquid. Centrifugal pumps continuously discharge the liquid, unlike reciprocating pumps. 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.

Advantages of Reciprocating Pumps

High Pressure, Low Flow Applications: Reciprocating pumps are generally designed to pump in low flow, high head applications. One of the most extreme of these applications is water jet cutting, where only a few gallons pass through the pump per minute but exceed pressures of 10,000 PSI.

Proven, Common Technology: Reciprocating pumps are one of the oldest, most proven pump types. Today, a wide variety of reciprocating pumps can be found in many different materials, types, and sizes. Reciprocating pumps range from less than 1 horsepower to over 3,000 horsepower.

Durability: Reciprocating pumps are used in some of the most abrasive and corrosive applications. Fluid ends and fluid end parts can be made of many different materials such as stainless steel, aluminum bronze, tungsten carbide, ceramic, and more. A wide selection of valve types are used in abrasive applications such as pumping cement, sand slurry, mud, etc.

Efficiency: Reciprocating pumps operate at high a higher efficiency compared to other pump designs. In most cases, at any set point, reciprocating pumps operate around 90%.

Factors that determine the 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 defined as the 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.

Disadvantages of Reciprocating Pumps

High Maintenance / Short Life: The main disadvantage of a reciprocating pump is the high maintenance and short life. There are many parts in the pump works, all constantly changing directions. Unless careful maintenance takes place, the lifespan of the pump is greatly reduced. While pumps such as centrifugal pumps can last 15 to 20 years with little maintenance, a reciprocating pump requires higher levels attention and rebuilding several times within the same time frame. The cost of a reciprocating pump rebuild is usually inexpensive which still makes them cost competitive compared to longer lasting, higher priced pump designs.

Pulsations: A characteristic of reciprocating pumps is the production of pressure pulsations through the pump inlet and outlets. The reciprocating motion of the pump produces these pulsations. Increasing the number of pump chambers can greatly reduce the pulsations produced, but it does not remove them completely. To negate damage to piping and surrounding systems or the pump itself, pulsation dampeners must be installed. Further system design can further decrease pulsations to nearly zero. In all cases, overall system design is important when using reciprocating pumps.

Reciprocating Pump Variations

Plunger pumps come in a variety of styles, shapes, and sizes. The specific type of pump chosen for an application takes into account the pressures encountered, the flow rates needed, measurement and control systems, fluid viscosity and corrosivity, pipe material, etc. Careful attention should be given to the application before selecting a pump. Selecting the wrong pump for a job can result in damage to equipment, piping, systems, and possibly endanger personnel.

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 the steam piston) to total liquid force (pump head x area of the 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.

Common Application of Reciprocating Pumps

Reciprocating positive displacement pumps are highly effective, where a high degree of accuracy and reliability under different ranges of conditions that 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 the easy availability of its spare parts.

For further queries or assistance, please feel free to contact us.

Reciprocating Pump Terms

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