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.
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
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
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
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:
With no Inlet Device ≤ 1000 Pa
With Half Open Pipe as Inlet Device ≤ 1500 Pa
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.
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.
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.
For Reynolds numbers between 2 and 500: Gravity settling-Intermediate law region to be used.
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.
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.
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:
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:
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:
Head V/S Flow (H-Q Curve)
NPSHR curve
Efficiency Curve
Power Consumption Curve
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.
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?
Minimum flow: Minimum flow is the case in which deliverable flow through the pump is in operation.
Rated Flow: Rated flow is the condition for which the pump is delivering a normal flow rate.
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).
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).
Explosive limits give the concentration range of a fuel (gas/vapor) that will cause an explosion or fire in the presence of an igniting source. There are two kinds of explosive limits that are widely used; LEL or Lower Explosive limits and UEL or Upper Explosive limits. Both LEL and UEL are represented by the percentage by volume of the gas in the air. In this article, we will learn the significance of Lower Explosive Limits (LEL) and Upper Explosive Limits (UEL).
What is Lower Explosive Limit or LEL?
The lower explosive limit or LEL of a vapor or gaseous substance is the lowest concentration of the gas in the air required to ignite/burn and explode in the presence of an ignition source. The lower explosive limit is also known as the lower flammable limit or LFL. To give an example, propane can explode once it reaches 2.1% of the air, by volume. So, the LEL of propane is 2.1%.
The LEL or lower explosive limit varies from one gas to another. In general, for most flammable gases LEL is less than 5% by volume. So, these flammable gases can create a high risk even with a very low concentration of the gas/vapor.
What is Upper Explosive Limit or UEL?
The highest concentration of a gas or vapor that will cause an explosion or burn in the air when ignited is defined as the Upper Explosive Limit (UEL). The other term for upper explosive limit is the Upper Flammable Limit or UFL. For example, propane will explode till the concentration does not exceed 9.5% in the air. Hence, the UEL of propane is 9.5%.
For a fire or explosion to take place, all three elements of the fire triangle must be present simultaneously. Those are fuel, an ignition source, and air/ oxygen. The ratio of fuel and oxygen must be above a certain minimum limit and below a maximum certain limit. This limiting minimum value is known as the Lower Explosive Limit and the limiting maximum value is known as the Upper Explosive Limit which varies with each flammable gas/vapor.
Significance of Lower Explosive Limit (LEL) and Upper Explosive Limit (UEL)
Information about the Lower Explosive Limits (LEL) and Upper Explosive Limits (UEL) of a gas/vapor is very important. Below the LEL level, the fuel and oxygen mixture is very lean and it will not cause burning or explosion. Again there will be a maximum concentration of gas (UEL) above which the fuel and air mixture will be very rich to cause an explosion. So, When the fuel and air mixture falls in between LEL and UEL limit, the condition is hazardous and it may cause fire/explosion in presence of an ignition source.
The above explanation can easily be simplified with an example of Methane gas. The Lower explosive limit of Methane is 5% volume in air and the Upper explosive limit is 17% volume in air. Hence, when the volume percentage of methane in an environment falls between 5% to 17%, the environmental condition is highly hazardous. The range of 5% to 17% is the explosive range for methane gas. When the volume percentage of methane is below 5% or above 17%, there will not be an explosion.
Note that even though the concentrations above the UEL are considered non-burning, they are still hazardous because if the concentration is lowered due to the introduction of fresh air, it will easily enter the explosive range.
Lower and Upper Explosive Limits of Various Fuels (Gases/Vapors)
The LEL and UEL values (percentage by volume) for some common gaseous fuels are provided in the following table.
Flammable Gas/Vapor
Lower Explosive Limit (LEL %)
Upper Explosive Limit (UEL %)
Flammable Gas/Vapor
Lower Explosive Limit (LEL %)
Upper Explosive Limit (UEL %)
Acetone
2.6
13
Ethylene
2.7
36
Acetylene
2.5
100
Ethylene Oxide
3.6
100
Acrylonitrile
3
17
Gasoline
1.2
7.1
Allene
1.5
11.5
Heptane
1.1
6.7
Ammonia
15
28
Hexane
1.2
7.4
Benzene
1.3
7.9
Hydrogen
4
75
1,3-Butadiene
2
12
Hydrogen Cyanide
5.6
40
Butane
1.8
8.4
Hydrogen Sulfide
4
44
n-Butanol
1.7
12
Isobutane
1.8
8.4
1-Butene
1.6
10
Isobutylene
1.8
9.6
Cis-2-Butene
1.7
9.7
Methane
5
15
Trans-2-Butene
1.7
9.7
Methanol
6.7
36
Butyl Acetate
1.4
8
Methylacetylene
1.7
11.7
Carbon Monoxide
12.5
74
Methyl Bromide
10
15
Carbonyl Sulfide
12
29
3-Methyl-1-Butene
1.5
9.1
Chlorotrifluoroethylene
8.4
38.7
Methyl Cellosolve
2.5
20
Cumene
0.9
6.5
Methyl Chloride
7
17.4
Cyanogen
6.6
32
Methyl Ethyl Ketone
1.9
10
Cyclohexane
1.3
7.8
Methyl Mercaptan
3.9
21.8
Cyclopropane
2.4
10.4
Methyl Vinyl Ether
2.6
39
Deuterium
4.9
75
Monoethylamine
3.5
14
Diborane
0.8
88
Monomethylamine
4.9
20.7
Dichlorosilane
4.1
98.8
Pentane
1.4
7.8
1,1-Difluoro-1-Chloroethane
9
14.8
Propane
2.1
9.5
1,1-Difluoroethane
5.1
17.1
Propylene
2.4
11
1,1-Difluoroethylene
5.5
21.3
Propylene Oxide
2.8
37
Dimethylamine
2.8
14.4
Tetrafluoroethylene
4
43
Dimethyl Ether
3.4
27
Toluene
1.2
7.1
2,2-Dimethylpropane
1.4
7.5
Trichloroethylene
12
40
Ethane
3
12.4
Trimethylamine
2
12
Ethanol
3.3
19
Vinyl Bromide
9
14
Ethyl Acetate
2.2
11
Vinyl Chloride
4
22
Ethyl Benzene
1
6.7
Vinyl Fluoride
2.6
21.7
Ethyl Chloride
3.8
15.4
Xylene
1.1
6.6
Table 1: LEL and UEL values of gases
LEL Sensors and Meters
The gas concentration must be closely monitored to safely work in hazardous closed spaces with flammable gases. Various LEL sensors or meters are used in industries that can give a warning signal. Infrared sensing elements of these LEL meters measure the lower explosive limits of various gases in an environment.
In general, when the gas concentration exceeds 20% of the gas LEL, the environment is considered unsafe. These LEL gas sensors provide a warning to the operators whenever the combustible gas in the environment exceeds 10%.
Modern LEL meters are highly sophisticated devices with microprocessors-based modular design and digital display. The most widely used LEL meter is the Wheatstone bridge type, which is proven to be effective for most environments. However, these types of LEL sensors have some limitations.
To overcome the drawbacks of LEL sensors, Photoionization Detectors (PIDs) with higher sensitivity sensors are developed which provide more accurate LEL measurements. PID can measure the concentration of inflammable gases and other toxic gases even when present at very low levels.
What are PPM and PPB?
PPM or “parts per million” is a dimensionless measure that provides the ratio of a substance in a mixture to the whole mixture. Sometimes LEL/UEL and toxicity of gases are provided in ppm. Similarly, PPB is parts per billion, which is also used for certain gases.
How to convert %LEL to PPM?
Both %LEL and PPM ratios indicate Volumes. While converting %LEL to PPM, 1% is considered equal to 10 thousand per million. Means,
1% vol = 10,000 ppm.
0.1% vol = 1,000 ppm
0.01% vol = 100 ppm
0.001% vol = 10 ppm
0.0001% vol = 1 ppm
So, using the above values any %LEL can be easily converted into the corresponding PPM. Let’s take the example of hydrogen sulfide (H2S), which is both toxic and flammable gas. The LEL vol% for H2S is 4 (Refer to Table 1). That means,
