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Types of Heat Exchangers and Their Selection

Written by Anup Kumar Dey in Process

A Heat Exchanger is a mechanical device that is used to exchange heat between fluids. The fluid used for heat transfer operation can be in the gas or liquid phase as per the availability. The number of fluids used for heat transfer operation maybe two or more. A properly designed heat exchanger can be used for both heating & cooling operations.

Heat exchangers are widely used in chemical plants, refrigeration and air conditioning systems, petrochemical facilities, food and beverage industries, and nuclear and power industries. As the heat exchanger does not have any rotating component, they are known as static equipment. Most of the heat exchangers are designed based on ASME BPVC Sec VIII codes or TEMA guidelines.

Heat exchangers are usually manufactured from materials that are heat-resistant. Some of the most common heat exchanger materials are Carbon Steel, Stainless Steel, Alloy Steel, Aluminum, Copper, Titanium, DSS, Hastelloy, Ceramics, Composite Plastics, etc. In this article, we’ll learn about the Classification of Heat Exchangers.

Classification of Heat Exchangers

Classification of Heat exchangers is dependent on several factors such as

  • Flow configuration,
  • Pass configuration
  • Heat transfer mechanism,
  • Construction method,
  • Function and application,
  • Fluid phases, etc.

A. Heat Exchanger Types based on the Flow Configuration

Flow configuration simply refers to the arrangement (path) provided within the heat exchanger. This is an indication of the fluid traveling direction of the heat exchanger. Flow configuration in heat exchangers consists of the following four types:

  1. Co-Current Flow
  2. Counter Current Flow
  3. Cross Flow, and
  4. Hybrid Flow

Co-Current Flow Heat Exchangers:

Co-Current flow Heat exchangers are also known as parallel flow heat exchangers. In this type of heat exchanger, the flow of fluid streams is parallel to each other and the operating fluids are moving in the same direction. Co-Current flow heat exchangers provide thermal uniformity across heat exchanger walls and usually have lower efficiency.

Counter Current Flow Heat Exchangers:

The heat exchangers where the direction of the fluid is parallel but opposite (divergent) to the other are known as counter-current type heat exchangers. They usually have the highest heat exchange and greatest temperature change.

Types of Heat Exchangers based on Flow Configurations
Types of Heat Exchangers based on Flow Configurations

Cross Flow Heat Exchangers:

When the flow of the fluids inside the heat exchanger is perpendicular (formation of right angle ~ 90° while flowing) to each other, they are called cross-flow heat exchangers. In terms of efficiency, a cross-flow heat exchanger gives better efficiency than the parallel flow configuration type heat exchanger.

Hybrid Flow Type Heat Exchanger:

Hybrid flow configuration uses a combination of the above-mentioned flow configurations, i.e. Co-Current, Countercurrent, or Cross Flow any of the mentioned combinations may be used within the single process. To accommodate the limitations of a single type of heat exchanger, this type of heat exchanger is produced.

B. Types of Heat Exchangers based on the heat transfer mechanism

According to the heat transfer process, heat exchangers are classified into direct contact type heat exchangers & indirect contact type exchangers.

Direct contact type heat exchanger:

This type of heat exchanger does not contain a wall tube for different fluids for the heat transfer process. The fluids come into direct contact to exchange heat between each other & exit via their respective outlet nozzles or line. Due to direct heat exchange, very close temperatures can be attained. Examples: Direct contact type process utilizes by the cooling tower, desuperheater, and Scrubber.

Indirect contact type heat exchangers:

This type involves the heat exchange between hot & cold fluid across a construction wall (The separating wall used is generally a tube/ pipe/vessel). As the wall separates the fluid, they do not mix during the whole heat transfer process.

Indirect contact type heat exchangers are further classified into

  • Direct transfer type,
  • Storage Type, and
  • Fluidized Bed type.

Direct Transfer Type heat exchangers: This type of heat exchanger consist of a separate path for each fluid including their respective passes. The heat transfer is based on the temperature gradient (difference). Direct transfer type heat exchangers are also known as recuperators. Continuous heat exchange occurs from the hot fluid to the cold fluid.

Examples of direct transfer type heat exchangers are Shell & Tube heat Exchangers (STHE), Plate type heat exchangers, tubular heat exchangers, and economizers used in power engineering.

Storage Type heat exchangers: This type of heat exchanger is provided with a fixed bed area across which fluid is about to pass through each bed. Hot & cold fluid passed through their beds respectively & valves at the inlet, the outlet is adjusted so that hot fluid is passed through the cooled fluid bed, same is applicable for the cold fluid which is passed through the hot fluid bed, and hence heat transfer takes place through this mechanism.

Example of Storage Type: An air preheater is a suitable example for this type of heat exchanger, ambient air taken into APH & hot fluid heated from the heater emit flue gases which need to be taken out through a stack, flue gas at high temperatures emissions give lower efficiency to the heater so the temperature of flue gas to be lowered by the ambient air whose temperature is increased at the APH Outlet, hence flue gas temperature decreases as both flue gas & ambient air simultaneously passed through each other.

Indirect Contact Type Heat Exchangers
Indirect Contact Type Heat Exchangers

Fluidized Bed type: Fluidized bed type heat exchangers consist of two-compartment; one for hot fluid & cold air for the inlet distributor plate is placed in each section (i.e. hot gas, cold air). The bed is completely filled with alumina pallets. The hot gas causes the alumina to fall into the lower chamber which is primarily used to heat the cold air. It gets heated up from hot gas which is flowing in the upper chamber. A recirculation path or arrangement is incorporated for recycling purposes.

In the Fluidized bed heat exchanger, solid particles or pallets are placed at the bottom, at low operating fluid velocity fluid is passed through the pallet bed, high velocity causes the solid particles to float in the entire section volume as drag force of fluid is greater than the weight of fluid.

C. Heat Exchanger Types based on the pass configuration:

Depending on the pass configuration used in the heat exchangers they can be grouped into any one of the following types:

Single-pass heat exchanger: In this type of heat exchanger, the fluid can pass once through the length of the heat exchanger.

Multi-pass heat exchanger: This type of passing arrangement utilizes the U bends and the fluid moves through the heat exchanger length more than once. The design of a multipass heat exchanger involves a series of tubes.

D. Types of Heat Exchangers based on the available Fluid phases:

Depending on the fluid media phases involved in the heat transfer process, three types of heat exchangers are found. They are

  1. Gas – Liquid: This type involves the application in the compressor oil cooling arrangement is required.
  2. Liquid – Liquid: This type of Heat Exchanger is used very commonly in the process industry. Shell & tube heat exchanger is a common example.
  3. Gas – Gas: This type of heat exchanger is used in a process where air pre-heating is required which is through APH (Air pre-heating).

E. Heat Exchanger Classification based on Functions and Usage

Depending on the process function and application, there are various types of heat exchangers as listed below:

  • Exchangers
  • Coolers
  • Condensers
  • Chillers
  • Reboilers
  • Non-fired Heaters
  • Evaporators
  • Steam generators

F. Classification of Heat Exchangers based on Construction

Depending on the construction features, heat exchangers can be classified into different types as follows:

  • Shell and tube heat exchangers
  • Plate-type heat exchangers
  • Air-cooled heat exchangers
  • Spiral heat exchangers
  • Finned tube heat exchangers
  • Double-pipe heat exchangers

Selection of Heat Exchangers

Selecting the proper type of heat exchanger from such different options available is a difficult task. The selection of a specific type of heat exchanger is usually done by the process engineer during the design phase. Many parameters must be considered while selecting the right heat exchangers. Some of these factors that impact the heat exchanger selection process are:

  • The type of fluids, the fluid stream, and their properties (Fouling characteristics of the fluids)
  • The desired thermal outputs (Temperature driving force)
  • Size limitations (Plot plan & layout constraints)
  • Application
  • Design and Operating pressures & temperatures
  • Available Utilities
  • Costs
    • purchase cost
    • installation cost
    • operating cost
    • maintenance cost

In general, more than 50% of all heat exchangers installed are Shell-and-tube heat exchangers. The following image provides a rough idea of how the heat exchanger selection varies with design temperature and pressure.

Selection of Heat Exchangers
Selection of Heat Exchangers with respect to Design temperature and Pressure

What is an Access Fitting? Applications, Types, and Components of Access Fittings

Written by Anup Kumar Dey in Piping Design Basics

Access fittings are used in operating piping and pipeline systems. To insert or remove corrosion probes, bio-probes, erosion probes, chemical injection fittings, sacrificial and impressed current anodes, electrical resistance probes, corrosion coupons, hydrogen probes, thermowell, linear polarization probes, etc. a special type of fittings known as access fitting is used. In this article, we will learn about various types of access fittings used in pipe and pipeline systems.

What are Access Fittings?

Access fittings are special pipe fittings that can be installed under full process conditions providing internal access to production plant vessels and pipework. So, the use of access fittings avoids any kind of costly system shutdowns for obtaining various vital information like corrosion data. They are high-pressure fittings and are available for a rating of up to 10,000 psi.

Applications of Access Fittings

Access fittings find applications in various industries. Some of the sectors where access fittings are used are:

  • Oil & Gas
  • Chemical and Petrochemical
  • Heavy Manufacturing
  • Cooling Systems
  • Process Sampling Systems
  • Sand monitoring Systems
  • Pipeline Systems, etc.

Chemical injection equipment uses access fittings to inject a wide range of chemicals like Corrosion Inhibitors, Dewaxers, Biocides, Neutralizing Agents, Boiler Water, Oxidants, Demulsifiers, Oxygen Scavengers, Flocculants, etc.

Typical Access Fittings
Fig. 1: Typical Access Fittings

Access Fittings Styles

Access fittings are available in various styles as listed below:

  • Welded
    • Buttweld
    • Socketweld
    • Flareweld
  • Flanged
    • API Flange
    • ANSI Flange
  • Threaded (NPT)

Special access fittings using Graylok or Techlok connectors are also available commercially.

Components of Access Fitting

The assembly of access fitting has three main parts. They are:

  • Protective cover to protect the external threads of the access fitting body.
  • Plug-It is the carrier of the installed device. May be solid or hollow depending on the application. The plug assembly screws into the body of the access fitting and seals the fitting bore to contain line pressure.
  • Access Fitting Body-the specialized pipe fitting which is permanently attached to the process plant vessel or pipework.
Fig. 2: Components of Access Fittings
Fig. 2: Components of Access Fittings

To perform the online installation and retrieval of the plug assembly, components like retrieval tools and service valves are required. The service valve contains the line pressure when the plug assembly is removed. Maintaining a number of access fitting assemblies are possible using one service valve and one retrieval tool.

Solid vs Hollow Plug Assembly for Access Fittings

Access fittings can be installed with a solid or hollow plug assembly. For example, Access fittings for corrosion coupon and chemical injection fittings use a solid plug assembly, whereas hydrogen probes, bio-probes, and corrosion probes use hollow plug assemblies. The main difference between them is that the hollow plug assembly allows the probe to directly pass through the plug.

Access Fittings for Chemical Injection

For chemical injection, the access fittings are installed on top of piping at locations of high turbulence. They usually have a tee connection with an isolation valve where the chemical injection line is tied in. The tee is generally an NPT thread, flanged connection, or a Nipolet. To avoid the risk of accidental damage, the tee pipe of the chemical injection fitting is aligned with the pipe axis. The injection fitting is supported by the main pipe where it is installed.

Corrosion Coupons

Corrosion coupons are an effective way to detect and track corrosion over time. this is one of the simplest corrosion monitoring techniques. In this method, the corrosion coupon is exposed to a process environment for a predecided duration and then remove for analysis. The basic analysis is weight measurement i.e, the weight loss of the corrosion coupon due to corrosion. Corrosion coupons are also known as Weight loss coupons. They are generally available in the following types:

  • Strip type coupons
  • Multi-disc coupons
  • Ladder strip coupons
  • Flush disk coupons

What is Glass Piping? Considerations for Glass Piping System

Written by Anup Kumar Dey in Piping Design Basics

Glass piping systems are a special type of piping system mainly used for food processing, laboratory service, and some other industrial applications. Glass piping is specifically preferred because of its cleanliness, transparency, durability, and good chemical resistance. However, glass pipes being of special nature, require special consideration of supports and attachments. In this article, we will briefly discuss some salient points of glass piping in general.

Advantages of Glass Piping

Glass pipes find application for some specific piping services in the chemical and pharmaceutical industries. The primary application of glass piping systems is for gravity drainage of various corrosive
liquids (Acid-waste drainage).

The main benefits that glass piping provides are:

  • Smooth, pore-free surface
  • Very high corrosion resistance for a range of aggressive chemicals
  • Their transparent nature is useful in various applications.
  • No possibility of product contamination.
  • Glass piping does not affect the taste, odor, and color
  • Glass pipes are Inert
  • Long service life
  • Low maintenance cost

Glass-lined pipes are increasingly used in various industries as glass-lined piping components offer similar advantages as those offered by solid glass piping. Additionally, the secondary containment of glass-lined pipes increases resistance to shock loads.

However, glass-lined pipes usually suffer from a crazing effect when subjected to repeated thermal stresses. This same problem does not occur with glass piping systems.

Pipes, fittings, and other hardware are available for glass pipes. In general, low-expansion borosilicate glass having a low alkali content is widely used for producing glass piping systems. They are joined by compression couplings.

Supporting Glass Pipes

The below-mentioned guidelines are considered for supporting glass pipes.

  • Glass piping systems can be supported using standard pipe support components. However, padding or cushion must be used to avoid scratching the pipe surface.
  • Point loading over the glass piping surface must be avoided. The support should be installed in such a way that it fit loosely around the pipe, but distribute the load over the largest possible area.
  • Rigid anchor points on glass piping systems are usually reduced to the minimum possible.
  • There is no standard pipe support span, so manufacturer-provided guidelines must be followed for supporting glass pipes. As a general rule, glass pipes usually require two supports per 10-ft (3.05-m) section. One extra support is added when there are three or more couplings in a 10-ft (3.05-m) section.
  • For maximum protection and accessibility, supports are placed about 1 ft (0.3 m) from each joint or coupling.

Glass pipe supports and layouts are usually designed based on the general fundamentals used for other piping materials. However, extreme care needs to be exercised to reduce strain and scratching of the glass. The design of support systems for glass pipes is normally performed in consultation with the pipe manufacturer and reputable support manufacturers.

For glass-lined pipes, the glass lining is extended from the pipe bore to cover the gasket seating areas of flanges. Nonmetallic sheet gaskets suitable for the service are used to minimize compression seating forces exerted on the glass-lined flange face.

Features of Glass Piping

The usually available sizes for glass piping systems are 1, 1 1/2, 3, 4, and 6 inches. The maximum operating temperature is 400 F with a pressure ranging from 0-75 PSIA (1 in. thru 3 in.), 0-50 PSIA (4 in.), and 0-35 PSIA (6 in.). Flange assemblies are used to connect with pipe fittings and equipment.

The borosilicate glass to manufacture the glass pipe components conforms to the ASTM E438 standard. The approximate chemical compositions consist of the following elements:

  • SiO2 81% (Wt %)
  • B2O3 13% (Wt %)
  • Na2O 4% (Wt %)
  • Al2O3 2% (Wt %)

Borosilicate glass pipes are suitable for almost all substances except hydrofluoric acid, phosphoric acid, and hot strong caustic solutions. Phosphoric acid and caustic solutions can be used at lower temperatures, however, hydrofluoric acid service is suitable for Borosilicate glass piping.

Some of the physical properties of borosilicate glass pipes are:

  • Coefficient of mean linear expansion Between 20°C and 300° : (3.3 ±0.1) x 10-6 K-1
  • Mean thermal conductivity Between 20°C and 200°C: 1.3 W/mK
  • Mean specific heat capacity Between 20°C and 200°: 0.98 kJ/kg K
  • Density at 20°C: 2.23 g/cm3

The thermal expansion of glass pipe is approximately 0.022 inches per 100 feet of pipe and 100°F temperature change.

Separation in The Oil & Gas Industry, Their Classification and Working

Written by Anup Kumar Dey in Process

A separator is simply a pressure vessel that is used for the separation of feed mixture which is from the upcoming well containing crude components. This well crude contains a gaseous, liquid mixture.

Classification of Separators

Separators can be classified through various modes; which are listed below:

1. Based on the installation:

Installation of the separator can be offshore or onshore.

  • Offshore: Offshore term in which the separator is placed under the sea region.
  • Onshore: Onshore term relates to the installation of a separator on the land.

2. Based on the installation location In the Plant:

Based on the location of the separator Installation, they are of the following types:

A. Production Separator: Production is situated near the well for first-stage Separation to stabilize the fluid because both of them are operating at High pressure. The stabilizing process involves Multi-stage Operation.

B. Test Separator: The test separator is also placed nearby the well simply next to or corresponding to the production separator. The test Separator function is to measure the small quantity of flow rate (for each available stream or phase) which is coming from the production separator stream using the diverting route of the production separator to the Test Separator

C. IP or MP Separator (Intermediate or Medium Pressure): This type of Separator is placed downstream to Production Separator which typically operated at low Pressure than the production separator.

D. LP Separator (Low Separator): These are used where separation is to be achieved at low operating pressure within the Separation Process.

3. Based on the Orientation of the Vessel:

The orientation of the separator vessel may be vertical, horizontal & Spherical. Selection of orientation will be based on the several factors discussed below:

Factors that affect the separator Orientation Selection:

Vertical: Vertical Orientation may be used where the plot plan provided area is small, it can be used in both application offshore & onshore applications.  Vertical orientation also helps in the Pumping operation where it gives a higher NPSH Option than Horizontal Orientation.

Horizontal: Horizontal orientation is to be used where adequate space is provided & large flow rate separation is to be facilitated (achieve).

Spherical Orientation: Spherical orientation is in cases where the processing area distance equipment from each other is minimal, it is also used for ease of transportation. 

4. Based on the Feed (Incoming fluid) with No of Phases available:

Generally, separators are distinguished based on phase availability. It can be 2-Phase or 3-Phase, In the case of a 2-Phase separator, the Feed fluid may contain Gas & Oil, or Gas &Water. On the other hand, a three-phase separator handles all 3-phase mixtures formed from Gas, Oil, and Water.

Important Terms for Separation Mechanism

Before starting the understanding principle of 2 Phase & 3 Phase Separator, we need to go through some important terms which are used in the Separation Mechanism:

Momentum & Its criteria for the selection of Type of Internals:

Momentum is defined as the force contained within itself. Momentum also plays an important role in sizing the Feed inlet, Gas outlet Nozzle. Momentum is measured in the Pascal (Pa).

Momentum = (Density of Fluid * Velocity2)                                (Pa)

Criteria for Feed Inlet nozzle:

  1. With no Inlet Device ≤ 1000 Pa
  2. With Half Open Pipe as Inlet Device ≤ 1500 Pa
  3. With Schoepentoeter as Inlet Device ≤ 6000 Pa

Criteria for Gas Outlet Nozzle:

Gas outlet nozzle size selection shall fulfill the Momentum criteria limit ≤ 3750 Pa

Criteria for Liquid Outlet Nozzle:

The liquid outlet nozzle shall be based on a velocity that does not exceed the mark of 2 m/sec. Generally, the liquid outlet nozzle shall be taken as 2” (50 NB).

Note! – Indicated criteria for momentum are based on the D.E.P Standards. It may change from project to project, design code standards guidelines.

Mist Eliminator:

The mist eliminator is used for the removal of liquid-entrained particles in gas phases. Generally, liquid droplets size greater than 0.3 – 10 microns are captured through the mist eliminator Element.  Gas-Liquid separation uses the wire mesh type of Mist Eliminator.

Internals of a Separator
Fig. 1: Internals of a Separator

Weir Plate:

Weir Plate is usually used in 3 Phase Separation which is placed in a liquid separation section where oil & Liquid separate. Weir plates act as a barrier for Liquid and allow it to pass the Oil Phase through itself.  Weir Plate work with the difference in density Mechanism.

Retention Time:

It is the average time period for which flowing fluid (Liquid) remains in the liquid section of the separator. It can be determined by the Liquid volume inside the vessel divided by the liquid flow rate.

Working Principle of 2-Phase Separator

2-Phase separators work on the “Principle of Momentum, Gravity Settling Theory” described below:

Inlet Fluid from the well or other sources comes at the inlet Nozzle of the separator where it takes entry into the separator.

Principle of Momentum:

Based on the momentum criteria inlet device is selected, purpose of the inlet device installation is to absorb the momentum, Minimize turbulence, re-entrapment of liquid particles in gas particles & change in direction of the fluid flowing towards the Liquid separation Section. This stated process is performed in the “primary separation section”.

Gravity Settling Theory:

This theory stated that “Settling of dispersed droplets from the continuous phase takes place if the gravitational force is greater than the sum of the buoyant force and drag force acting over the same droplet”.

Terminal velocity is the maximum velocity that can be attained by the fluid which is free-falling in the Separation Process in Liquid Separation Section treated as a secondary separation section where droplets velocity gets reduced as droplets settle within this section.

Liquid accumulation section:

It is the section that is used to provide the retention time for the liquid to efficiently separation of gas & Liquid particles from Each other; where separated gas contains drag force due to which moves upwards & passes through the Mist Eliminator.

Separated Liquid is taken outlet from the separator via the liquid outlet nozzle consisting of a Vortex breaker that keeps the fluid prevents from swirling formation.

Notes: Gravity settling theory* includes various laws within itself which are applicable based on the Reynolds number.

  1. For Reynolds numbers, less than 2: Gravity settling-Stokes’ law region is used which consists of the drag coefficient, and terminal Velocity, Reynolds number works as a function of the shape of the Particles, Settling of the Particles, property of behavior (Nature) of flowing fluid.
  2. For Reynolds numbers between 2 and 500: Gravity settling-Intermediate law region to be used.
  3. For Reynolds numbers between 500 and 200000: Gravity settling-Newton law region to be used.

* Gravity settling theory consists of three laws having the same functional parameter (i.e. Drag coefficient, Reynolds number, terminal Velocity) but the method to determine these parameters or equations changes according to the Reynolds number which is applicable to the law selection.

To know about the design basics of a two-phase separator visit here.

Working Principle of 3-Phase Separator:

3-Phase separator work on the “Principle of Momentum, Gravity Settling Theory & coalescence” described below:

Principle of Momentum:

The principle of Momentum remains the same as the 2-Phase separator.

Gravity Settling Theory:

Gravity settling theory remains the same as the 2-Phase Separator, but the weir plate mechanism is to be defined play an important role in oil & Liquid efficient separation.

The Weir plate works on the density difference in this lighter fluid (Less density) floating upper in the liquid separation section and the level of Liquid as the HHLL (High High Liquid Level) reaches the separator which is usually less than the weir plate height.

This enables the oil flow to be diverted (travel across the weir) towards the oil outlet in the liquid separator section.

Liquid accumulation section:

The liquid accumulation mechanism also remains the same as 2 Phase separator; in addition to that, an oil outlet nozzle is to be provided for the Oil Phase Exit from the Separator.

Click here To learn about the design basics of a Three-phase separator.

Three-Phase Separator
Fig. 2: Three-Phase Separator

Image Credit:

  • https://www.forain.net/products/filtration-separations/three-phase-separators
  • https://www.costacurta.com/products/separation-technologies/separator-internals/inlet-distributors/

What is a Throttling Valve? Definition, Applications, Working, Examples, Selection

Written by Anup Kumar Dey in Piping Design Basics

A Throttling valve is a type of valve that can start, stop, and regulate the flow of fluid from one point to another. In general, there will be a high-pressure difference between the upstream and downstream sides of the throttle valve. The pressure drop increases with the increase in flow restriction inside the throttling valve. A range of control methods is used for controlling these types of valves. Different valves can work as throttling valves in different working conditions. In this article, we will learn about the basics of throttling valves.

Definition of a Throttling Valve

Throttling valves are valves used for opening, closing, or regulating a fluid flow. They are basically regulating valves as the discs of the throttling valves can regulate the flow, temperature, or pressure of the flow medium passing through it.

Examples of a Throttling Valve

Different valves can work as throttling valves. Some common examples of valves working as a throttling valves are:

  • Diaphragm valve
  • Butterfly valve
  • Ball valve
  • Globe valve
  • Pinch valve
  • Needle valve, etc

Control valves are specialized throttle valves. However, not all throttling valves are control valves.

Applications of Throttling Valves

Throttling valves are found to be used in a broad range of industries. Some of the common applications of throttling valves include:

  • Refrigeration systems
  • Air conditioning systems
  • Steam applications
  • Chemical applications
  • Pharmaceutical applications
  • Fuel oil systems
  • High-temperature applications
  • Food processing applications
  • Automobile systems
  • Power generation systems
  • Metering systems

Working of a Throttling Valve

In a throttling valve, an obstruction is created inside the valve so that the parameters like flow velocity, temperature, pressure, etc are obtained as required. The flow is impacted by the designed restriction and friction generated while flowing. In general, the valve stem is raised or lowered to adjust the size of the flow path through the valve. They can even close the valve completely to stop the flow.

Common Valves used as Throttling Valves

All industrial valves are not throttle valves. The design of the valve must be proper to be used as a throttle valve. In general, the following valves are used as throttle valves:

Globe Valves:

Globe valve is one of the most popular throttle valves for industrial applications. The disk/plug of the globe valve provides the required restriction to work as a throttle valve so that only the required amount of media can pass through. However, globe valves require power or an automatic actuator for high-pressure applications.

Butterfly Valves:

The butterfly valve is the most suitable for throttling applications as it can easily create throttling by just opening a bit to allow the media to pass.

Gate Valves: Gate valves are generally not suitable for the throttling process.

Needle valves:

The needle valve is designed with a needle-like disk that moves to regulate the fluid flow. Needle valves are used as throttle valves for high-precision applications. Note that thicker and viscous media are not suitable for use as a throttle valve.

Pinch Valves:

Pinch valves are lightweight and easy to maintain and are used widely as throttle valves for sterility and sanitary applications. Due to the soft liner and smooth walls the pinch valves use, their best efficiency is 50%.

Diaphragm Valves

For moderate temperature and pressure applications, diaphragm valves can be a good choice for throttle valves.

Expansion Valves

Various expansion valves like manual expansion valves, thermal expansion valves, capillary expansion valves, and floating ball-type expansion valves are widely used as throttle valves for refrigeration systems.

Selecting the Throttle Valves for Specific Applications

The selection of throttle valves for a specific application is quite complex. There are various parameters that must be checked thoroughly to zero down on a specific valve to be used as a throttle valve. Some of these parameters on which the throttle valve selection depends are:

What is a Pump Performance Curve? Types of Pump Performance Curves

Written by Anup Kumar Dey in Mechanical,Process

The pump is a device that converts mechanical energy into hydraulic with the help of an external source, it may be centrifugal force through an electric motor. This centrifugal force is generated due to the rotation of the impeller above the suction port of the Pump, which helps in delivering fluid from one location to another via the discharge nozzle outlet. The pump type may be Centrifugal or Positive displacement, the mechanism will change accordingly.

Pump performance curves are very important graphs produced by pump manufacturers. They give information regarding how different parameters like NPSH required, efficiency, and power requirement will behave when the flow is changed. To select the right pump for a specific service, the engineer must know to read the pump performance curves. A pump performance curve is also known as a pump efficiency curve, pump selection curve, pump characteristic curve, or simply a pump curve.

What is a Pump Performance Curve?

The pump performance curve is the design analysis (or indication) of the pump and how it will operate with respect to the changes in operating parameters such as Pressure (Pressure Head is derived from pressure) & Flow rate.

The factors which are dependent upon the operating parameter are Pressure head, shut-off head, impeller diameter, efficiency, NPHSR, and Power Consumption. We will discuss these terms later in this article.

Generally, the pump performance curve is generated by the pump’s original equipment manufacturer such as Flowserve, Netzsch, etc.

Types of Pump Performance Curves

Generally, there are 5 types of curve representation available which are listed below:

  1. Head V/S Flow (H-Q Curve)
  2. NPSHR curve
  3. Efficiency Curve
  4. Power Consumption Curve
  5. Family Curve: It is the curve that includes different functional parameters (Shutoff head, efficiency, impeller diameter, NPSHR within a single curve)

Please refer to the attached pump performance curves in Fig. 1 (first curve) and Fig. 2 (second curve) below. Fig. 2 is a typical example of a family curve.

Typical Pump Performance Curve
Fig. 1: Typical Pump Performance Curve

Cases for which the Pump Performance Curve is drawn or plotted

In general, there are three conditions for which pump performance curves are usually drawn. These are?

  1. Minimum flow: Minimum flow is the case in which deliverable flow through the pump is in operation.
  2. Rated Flow: Rated flow is the condition for which the pump is delivering a normal flow rate.
  3. Maximum Flow: ­Maximum flow rate is the expected rise in flow rate for future provision.

For Example, the Pump vendor requires the flow rate as Flow (Minimum/Rated/Maximum): 10 / 25 / 40* (m3/h) (Values indicated are typical only).

Example of a Family Curve
Fig. 2: Example of a Family Curve

Important Terms Involved with Pump Performance Curve

A basic understanding of the following terms is beneficial to understand the performance curve for any pump.

Flow: Flow is the quantity of liquid available for pumping application (i.e. amount of flow to be delivered through discharge of pump). It is represented on the X-axis of the Curve. Measurement of Unit is m3/h or GPM.

Differential Pressure Head: The head is the equivalent height of fluid energy due to pressure, velocity, or height above the datum (reference Point). The pressure head is generated because of pressure energy. This parameter is plotted over Y-Axis.

Differential pressure head can be calculated from the equation below:

∆P= rho* acceleration due to Gravity* ∆Height

Here

  • ∆P is the difference in pressure available at Suction & discharge.
  • ∆H is the difference in height of liquid at suction & discharge (also known as differential pressure head), and
  • rho is the density of the Fluid.

Curve Interpretation: The head is indicated on curve 1 with green color. The differential head is inversely proportional to the flow rate. Decreasing the value of the head from 60 to 40 meters over the Y-axis increases the flow on X-Axis from 9 to 12.5 thousand Gallons. Differential pressure head is measured in meters (m) units. Refers to the curve indicated above, as the deliverable or discharge flow required is increased pressure head gets reduced.

Shut off Head: Shut off head is the condition in which the pump gives the highest head. It is a condition in which the pump is designed as a discharge outlet of the Pump and is assumed as closed or blocked condition while in this condition flow corresponding to shut off head is Zero as indicated in the above curve. The shut-off head is also measured in meters (m).

Curve Interpretation: Shut-off head is attained in the case where the curve when the volumetric flow rate is zero. Shut off head is marked on Y-Axis over the curve.

NPSHR: NPSHR stands for Net positive suction head required means that a positive suction head requires to keep the pump safe from cavitation or boils off operation. NPSHR should be always greater than NPSHA (Net positive suction head available). The measuring unit for NPSHR is a meter (m).

Illustration: The basic purpose of NPSHR is to keep the pumping fluid pressure above the vapor pressure; vapor pressure is the state of matter in which phase is to be found in the gas phase (Vapour fraction is equal to 1). A positive suction head needs to be sufficiently more than required to maintain the fluid phase as real & causing the pump to any type of wear. 

Curve Interpretation: Refer to curve second, NPSHR is directly proportional to the flow rate, NPSHR increases as the volumetric flow rate is increased; the same is indicated in the curve.  

Efficiency: Efficiency is simply defined as the ratio of hydraulic power output to the shaft power input. In the description, hydraulic power is generated by the pump for developing flow rate & discharge pressure. In the shaft power case, this is Power delivered to the pump through an electric motor or external engine. The efficiency of the pump is usually indicated in (%) percentage.

Curve Interpretation: Refer to the indicated first curve as efficiency increases as the flow rate increases, thus reaching up to a specific Efficiency mark where efficiency is highest; this point is called the Best Efficiency Point. Moving on the curve either the left or right efficiency of the pump gets reduced and so on.

For Example: let’s assume a Car gives generally a 20 Km average but at a certain range of speed where the car produces the maximum average. Exactly is the same in the case of the Pump.

Power Consumption: The power consumption term ultimately relates to the efficiency in which shaft and hydraulic power are included.

Curve Interpretation: Refer to the first curve indicated with the orange line, Power consumption is in direct proportion with the flow rate. Power consumption rises as the pump flow increases. This power input’s main purpose is for the pump to produce a discharge flow rate that will be achieved by the external electric motor.

For Example, A 100 Watts bulb consumes high electricity rates than 10 Watts. Consumption is directly dependent on the Output.

Impeller Diameter: As indicated in the second curve, Affinity law defines flow as directly proportional to the Impeller diameter & Speed; the total head developed is directly proportional to the square of the Impeller diameter & Speed; power consumed through the pump is directly proportional to the cube of impeller diameter & speed. 

Curve Interpretation: Refer second curve Impeller diameter is indicated with three cases as in the curve, the Maximum case head achieved by the pump is maximum and thus higher the head lowers the volumetric flow rate. The same scenario is applicable in the Rated & minimum case of the impeller. The impeller diameter curve starts from the Y-Axis indicated in black color for all three cases (Max. / Rated / Min).

Image Credit: https://commons.m.wikimedia.org/wiki/File:Pump_Characteristic_curve.png