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What is Nitrogen Purging? Applications, Procedures, and Benefits of N2 Purging

Written by Animesh Chakraborty in Health & Safety,Process

Nitrogen purging plays an important role in the safety and functioning of various plants that are susceptible to fire hazards. In fire and explosion protection engineering, an inert (ie, non-flammable) purge gas (like nitrogen, helium, argon, etc) is introduced into an enclosed system (eg, a container or process vessel) to prevent the formation of a fuming flammable atmosphere. This inert gas helps to mitigate the risk by overriding hazardous fire/explosion-forming agents like oxygen. In this article, we will explore more about Nitrogen purging, Its definition, procedure, benefits, etc.

What is Nitrogen Purging?

Many a time people in safety engineering ask what nitrogen purging actually means. Nitrogen purging is the process of introducing nitrogen into closed vessels, pipelines, containers, etc to displace undesirable hazardous atmospheres and to clean the inner walls. Nitrogen in the nitrogen purging process pushes out oxygen and moisture and creates a stable non-combustible environment, thus reducing the potential hazard.

Why Nitrogen Purging is Required?

Chemical Industries like acetylene production plants, Oil and Gas plants require nitrogen purging on a regular basis. The main benefits that nitrogen purging provides are:

  • Nitrogen purging helps in removing the oxygen % from the internal surfaces of the pipelines and equipment. So, the possibilities of sparks and fires are greatly reduced during operation.
  • Nitrogen purging removes moisture that may be present which helps in lowering the dew point.
  • The nitrogen purging process ensures a safe environment for the workers and nearby residents.
  • It prevents chemical alteration of the product.

Uses of Nitrogen Purging System

A lot of industrial manufacturing processes use nitrogen purging to eliminate moisture or oxygen-rich air. Introgen purging equipment is sometimes integrated with oxygen-sensitive operations to avoid unfavorable conditions. Major industrial applications of the nitrogen purging process are:

  • Transformers and other volatile electrical environments enhance safety by using the N2 purging procedure.
  • Nitrogen purging in the brewery industry extends the shelf life of beer.
  • Ships and tankers use nitrogen blanketing to remove potentially combustible environments.
  • Organic compounds generating chemical and petrochemical industries widely use nitrogen purging processes to eliminate toxic gases from process chambers.
  • Atmosphere Packaging and Food industries apply the nitrogen purging process to remove moisture, oxygen, and other gaseous impurities.
  • Nitrogen purging is widely used in oil and gas pipeline projects for drying, cleaning, and limiting oxygen concentrations.
  • To eliminate compounds affecting weld quality, metal fabrication industries use N2 purging systems.

Why Is Nitrogen Used for Purging?

Nitrogen is dry, non-combustible, and economical as compared to other inert gases. This makes the nitrogen purging process more affordable.

Difference between Nitrogen Purging and Inerting?

The purge gas is inert. By definition, it is non-flammable, or more precisely, non-reactive. The most common purge gases available commercially in large quantities are nitrogen and carbon dioxide. Other inert gases, eg. Argon or helium can be used. Nitrogen and carbon dioxide are not suitable for purge applications in some cases because these gases can chemically react with fine dust from certain light metals.

Since an inert purge gas is used, the purge procedure can (erroneously) be called inerting in everyday language. This confusion can lead to dangerous situations. Carbon dioxide can be considered a safe, inert purge gas. Carbon dioxide is an inert gas that is not safe for inactivation as it can ignite vapors and cause an explosion.

Difference between nitrogen purging procedures from other industrial explosion prevention methods?

Fires and explosions can also be prevented by controlling ignition sources. However, purging with an inert gas (nitrogen) provides a higher level of safety because it is guaranteed that no flammable mixture is formed. Therefore, it can be said that primary prevention is relied on to reduce the possibility of a spreading explosion, and ignition source control relies on secondary prevention to reduce the possibility of an explosion. Primary prevention is also referred to as essential safety.

Types of Nitrogen Purging Procedures

There are four main industrial nitrogen purging procedures that are widely used. They are:

  1. Displacement purging (Plug effect)
  2. Dilution Purging
  3. Pressure swing Purging, and
  4. Vacuum purging

Displacement purging (Plug effect)

In a displacement purge, an inert gas is injected into an open vessel to evacuate hazardous or noxious gases. Slow flow is maintained (velocity < 10 m/s).  

Displacement Purging Process Configuration
Fig. 1: Displacement Purging Process Configuration

The displacement nitrogen purging procedure is mainly used for high H/D (height/diameter) ratios. Ideally, the inert gas should be denser than the displaced gas on Fig. 1 shows how nitrogen is used for the displacement purge of the vessel. Gas is transported in tankers. Liquid nitrogen is vaporized in the vaporizer and gaseous nitrogen is injected into the vessel. Nitrogen pushes the atmosphere out of the vessel through an outgassing valve. The amount of nitrogen required is relatively small, typically 1.2 times the capacity of the vessel.

Dilution Purging:

A dilution purge involves introducing an inert gas to reduce the concentration of hazardous gas.

Dilution Purging Process Configuration
Fig. 2: Dilution Purging Process Configuration

The dilution nitrogen purging procedure is used when the H/D ratio of the machine is low. The amount of nitrogen required is approximately 3.5 times the capacity of the vessel. The configuration in Fig. 2 shows how nitrogen gas is vaporized and injected into the device with the outlet valve open. The diluent gas, which consists of noxious gases and nitrogen, is released into the atmosphere or further processed.

The following  equations can be used for this process:

The number of volume changes:                               i = ln [Ca ⁄ Ce]

The Volume of inert gas required:                              VN = i · VB   

Here,

  • i = Volume change
  • Ca = Initial concentration
  • Ce = Final concentration
  • VN = Volume of inert gas
  • VB = Volume of vessel

Pressure swing Purging:

In pressure swing purging, the closed device is sprayed with an inert gas. When the gas is released, the dangerous or harmful gas disappears. The process (closed-injection-open-exhaust) continues until the desired concentration of harmful gases in the device is reached. Variable pressure purge is used, for example, when the inlet and outlet are close to each other. The device must also be a pressure vessel. In the pumping pressure, there is a difference between vacuum purification and excessive production. One of the main goals of the pressure swing occurs due to hazardous substances or harmful substances, such as oxygen. The residual concentration of harmful substances can be calculated in the following formula:

CSR=(P1/P2)n * (CSG-CSI) +CSI

Here,

  • CSR = hazardous substances residual concentration
  • CSG = Concentration of hazardous substances in mixtures
  • CSI = Concentration of hazardous substances in inert gas
  • n = Number of pressure swings
  • P1 = Pressure 1 (before inerting)
  • P2 = Pressure 2 (after inerting)

Vacuum purging

Vacuum purge involves the use of a vacuum pump to remove harmful gases and then supply inert gas to the evacuated unit. This process is repeated until the desired hazardous gas concentration is reached. Vacuum purge is particularly suitable for machines with multiple dead zones.

The effective inert gas requirement  is calculated as follows:

VN = VB * f * n

Here,

  • VN = Inert gas requirement in m³ 
  • VB = Vessel volume in m³
  • f = Pressure change ratio
  • n = Number of pressure swings

The pressure change ratio equation is as below:

f=1-(P1/P2) (P1/P2)<1

Here,

  • f = Pressure change ratio
  • P1 = Pressure before inerting
  • P2 = Pressure following inerting

Nitrogen Purging in Pipelines

Newly laid pipeline network and sometimes after maintenance and shutdown work nitrogen purging in pipelines is performed. This process is important to remove retained moisture, oxygen, and other impurities that may otherwise change the quality of the fluid being transported.

Nitrogen purging in pipelines is a pretty straightforward procedure. Pressurized nitrogen gas is forced through pipelines that force out all gaseous and particulate impurities present inside. However, Pipeline nitrogen purging may sometimes involve risks. Hence, To safely conduct the pipeline nitrogen purging process, the operators should take the following steps:

  • Ensure proper instruments/apparatus handling
  • The operation must be performed by trained personnel.
  • Emergency protocols for shutdown and personnel evacuation must be well informed to all.
  • Personal protective equipment must be worn by all personnel involved in purging operations

What is Inerting? Gases Used for Inerting and Their Selection Criteria

Written by Animesh Chakraborty in Health & Safety,Process

Inerting refers to the process of introducing an inert gas into an enclosed space to release the gas already present, resulting in a kind of hazard. Although this process can be used to displace toxic gases, it is most commonly used to displace oxygen or to reduce the oxygen concentration in a space when the space is completely enclosed and substitution is not possible.

An inerting system reduces the combustion potential of flammable materials that are stored in a confined space. A common example of such a system is a combustible liquid-filled fuel tank that transports gasoline, diesel fuel, jet fuel, aviation fuel, or rocket propellant. Once the fuel tank is filled and during use, a space vapor barrier above the fuel known as the ullage is created. It contains evaporated fuel mixed with air containing oxygen, the important element necessary for combustion. An inerting system is used to replace the air with inert gas, such as nitrogen, argon, helium, etc. to avoid the combustion hazard.

Reason for Inerting

There are many chemical processes where inerting is required

  • to make the system or process explosion-proof
  • to eliminate unwanted reactions 
  • to keep food away from moisture 
  • to ensure safety during maintenance work. 

Engineers often rely on inert gases and specialized inert equipment, as these goals cannot always be achieved by technology and equipment design alone. There are many situations where deactivation is the only way to meet safety standards during the production process and maintenance. In other cases, inerting is used to improve product quality.

Basic Principle of Inerting

The basic principle of inerting is to completely or partially replace air containing oxygen and flammable and/or toxic gases that often contain moisture or inert gases.

Inerting is based on the principle that a combustible (or combustible) gas can only burn (explode) when mixed with air in the correct proportions. The flammability limit of the gas determines this ratio, i.e. the flammability range. In terms of combustion technology, it can be said that the inert gas inlet dilutes the oxygen below the limiting oxygen concentration. Inerting prevents the formation of a flammable mixture that spreads by definition. Inerting introduces an inert gas to make the flammable mixture safe.

Gases Used for Inerting

The most widely used gases for inerting are Nitrogen and Carbon dioxide. Other gases such as argon and helium are applied in certain instances. Steam and exhaust gases are used in some industrial applications.

Selecting the Inert Gas

There are various criteria that influence inert gas selection. Some of these are:

Dangers of fire and explosion and/or their effects or actions 

Nitrogen and carbon dioxide are not completely inert, but they are the best gases at room temperature. At high temperatures, nitrogen reacts with very few substances such as lithium, which forms lithium nitride. 

6 Li + N2 → 2 Li3N

Nitrogen can react with magnesium, Silicon at temperatures between 400 ° C and 1800 ° C for nitriding, titanium, and other metals:

2 Ti + N2 → 2 TiN

The reaction with oxygen occurs only at very high temperatures. In comparison, carbon dioxide reacts with a longer list of substances containing strong bases. Amine, anhydrous ammonia, lithium, potassium, sodium, magnesium, beryllium, aluminum, chromium, manganese, titanium, uranium, and acrolein. The decomposition temperature of Carbon dioxide is 2000° C, but this is rarely a problem. However, increasing the pressure increases the solubility of this gas.

Effect on product and exhaust gas

When steam is used for inactivation, the condensate produced can sometimes be hazardous. inactivation. When shipping, readily available exhaust gases are used in the oil tank, so the fuel-to-air ratio must be carefully controlled. 

Cost

Inert gases are generally too expensive to use for inertization and are not as effective as inert gases. nitrogen. However, they are often used to inactivate light metal dust. For example,  Aluminium dust explosions can be prevented by using argon and that is why it is used in fire extinguishing systems for such explosions. 

Specific heat capacity of inert gas

 The specific heat capacity of a given inert gas determines its effectiveness. The higher the specific heat capacity, the higher the inert efficiency. Table 1 shows the corresponding values.

Inerting AgentSpecific Heat* (Btu/(lb mole/0F)Specific heat* (kJ/kmol/K)
Nitrogen729.308
Carbon Dioxide8.836.844
Helium520.934
Argon520.934
Steam1771.176
Table 1: Specific Heat of Selected inerting agents at 00C and at constant pressure (* Values at constant pressure, steam at 270C, other values at 00C)

The reason for this is that diluting with a high specific heat inert gas reduces the areas at risk of ignition. According to this logic, carbon dioxide is again favored over nitrogen because less inert gas is required.

Molecular structure of inert gas

The molecular structure of an inert gas directly affects its efficiency. Carbon dioxide is a triatomic molecule, nitrogen is a diatomic molecule, argon, and helium are monoatomic molecules. Polyatomic molecules can absorb more energy because they contain more free molecular vibrations. Therefore, the inert effect decreases in the following order:

CO2 → N2 → He, Ar

Figure 1 shows the effectiveness level of the inactivating agent, using the flammability of methane as an example.

Influence of Various inerting agents on the flammability of methane
Fig. 1: Influence of Various inerting agents on the flammability of methane

The density of inert gas

Density also plays a particularly important role. If the density of the Inactive gas is greater than the density of the replacement gas, the free space above the Inert gas injection point may not be deactivated.

Table 2 shows a comparison of the density of the selected gas to that of air.

Types of GasDensity in Kg/m3 at 1013 m-bar and 00CDensity relative to Air
Air1.2931
Nitrogen1.25050.967
Hydrogen0.08990.0696
Argon1.78371.38
Oxygen1.4291.105
Carbon dioxide1.97671.529
Table 2: Density of selected gases and their densities relative to air

Application of Inerting in Chemical Industries

The chemical industry uses a wide range of materials, technologies, and devices. Inactivation or inerting is especially used for: 

  • Reactor, Mixer 
  • Centrifuge and vacuum filter 
  • Wheat mixing plant 
  • Tank farms, ships 
  • Dryer, silo 
  • Gas station 
  • Oil line and fuel line 
  • Industrial service

Objectives of Inerting Activities

The predominant objectives of those inerting activities are to

  • Prevent explosive atmospheres from forming in equipment along with reactors
  • Ensure secure begin-up and shutdown of flora and equipment 
  • Avoid explosion dangers in the course of garage and delivery of flammable substances
  • Protect merchandise in opposition to atmospheric oxygen whilst oxidation reactions might impair  fine 
  • Protect in opposition to atmospheric moisture, both to keep product fine or to make sure optimum  downstream processing, as an example in grinding
  • Prevent fitness and protection risks in the course of the renovation of flora, equipment, and pipelines.

Different Modes of Inerting Application

Continuous Applications:

  • Protection of the production process to prevent fire, explosion, and oxidation (for quality assurance) 
  • Inert solvent containers and transport equipment to prevent fire and explosion

Intermittent applications:

  • Purge pipeline
  • Tank purging
  • Deactivation of the filter system
  • Inactivation of silos
  • Deactivation of grinding equipment
  • Smoke evolution in the mine

Types of Inerting

Depending on the application, inerting can be classified into two groups as mentioned below:

Partial inerting:

The oxygen concentration is reduced to a level low enough that the mixture will no longer explode.  In the case of partial inertia, the goal is to reduce the oxygen concentration in the mixture to a level where it will no longer explode.

Complete inerting:

Complete inertia involves increasing the ratio of inert gas to a combustible material to a level at which the addition of any amount of air cannot produce an explosive mixture.

Zero Velocity Valves for Water Hammer Prevention

Written by Anup Kumar Dey in Piping Design Basics

Zero Velocity Valve consists of a Power-assisted Spring-loaded closing disc that prevents the reverse flow when pump trips. These valves protect the system from harmful water hammers or surge problems. The design principle of Zero velocity valves is to arrest the quick-moving water column at zero velocity. So it eliminates the establishment of any return velocity which subsequently eliminates pressure peaks.

The zero-velocity valve constitutes an outer shell and an inner fixed dome. In the center, the valve closing disc is mounted. One or more conical compressed springs held this disc in a close position in situations when there is no water flow. A bypass connects the upstream and downstream of the disc. The springs produce such forces that the disc will be in a fully open position when the water velocity is 25% of the maximum design velocity. Again, when the velocity becomes less than 25% of the maximum velocity, the disc starts closing and fully closed at zero velocity. At this stage, the upstream water column is prevented from creating pressure surge waves.

Zero Velocity Valve
Fig. 1: Typical Zero Velocity Valve

Zero velocity valves are made of barrel or flanged ends as per requirements. To keep the valve free from corrosion, a high-quality epoxy coating finish is provided.

Advantages of Zero Velocity Valves

As the zero velocity valves are self-actuating, they can be installed at remote locations. The major advantages of Zero-velocity valves are:

  • Controlled closing characteristics and
  • Low head loss due to the streamlined design

Other benefits of a zero-velocity valve include

  • Long life
  • Low maintenance
  • Trouble-free, smooth operation
  • Robust construction provides heavy-duty operation.
  • High-quality leak-proof sealing
  • Cost-effective solution
  • Easy installation
  • Silent operation.

Working Principle of Zero Velocity Valves

The working principle behind the design of zero velocity valves is to arrest the forward-moving water column at zero momentum. When the velocity is zero, no return velocity is established.

When a pump suddenly trips, the forward velocity of the water column decreases due to gravity and friction. When the forward velocity becomes less than 25%, the flaps of the zero-velocity valve close at the same rate as the water velocity. The flap comes to the fully closed position when the forward velocity approaches zero magnitudes. Thus the water column on the upstream side of the valve is prevented from getting a reverse velocity to create a pressure surge. The bypass valve maintains balanced pressures on the disc. It also avoids the vacuum creation on the downstream side of the valve if that column experiences a certain reversal.

Installation of Zero Velocity Valves

The following steps are usually performed while installing zero-velocity valves in piping/pipeline systems:

  • Clean the pipeline thoroughly by flushing it to remove any material that may damage the valve.
  • Clean the valve from the inside and outside to remove any foreign particles.
  • Zero velocity valves are usually installed in horizontal or inclined pipelines.
  • During installation ensure proper alignment of the valve flange and pipe flange.
  • Adequately support the upstream and downstream piping

Zero velocity valves are usually manufactured of carbon steel, cast iron, S.G. Iron, cast steel, and fabricated steel material. However, depending on the requirement, other materials can be used. The common size range for the zero velocity valves is 80 mm to 3000 mm.

What is a Non-Return Valve (NRV)? Types, Working, and Symbols of NRV

Written by Anup Kumar Dey in Piping Design Basics

A non-return valve is a single-way valve that allows the fluid to flow only in one direction. The main importance of non-return valves is their working of allowing flow in the downstream direction and preventing the flow in the upstream direction. In this article, we will briefly learn about non-return valves, their types, functions, working, uses, and symbols.

Non-return valves are also known as NRVs. They are usually small, simple, and inexpensive. There are various types of non-return valves but the main function is the same. They are also known as one-way valves, check valves, clack valves, reflux valves, or retention valves. In the year 1907 Frank P Cotter developed the first simple model of a non-return valve.

Working Principle of a Non-Return Valve

Non-return valves use the mechanism to allow the medium only in the downstream direction. It has two openings: one inlet and the other outlet. A closing member (ball, clapper, or disc) separates the inlet and outlet, staying in between. When the fluid enters the non-return valve through the valve inlet, the fluid pressure keeps the closing member open. On the other hand, when the fluid attempts to flow in the backward direction from the outlet side to the inlet side, the closing member closes the entrance, which prevents the flow. Non-return valves work automatically without the need for control of any external element.

Applications of Non-Return Valves

Non-return valves find a range of applications in industrial and domestic sectors. Some of the uses of nonreturn valves are:

  • Pump Discharge: A non-return valve known as a check valve is always installed at the pump discharge piping to prevent the backflow of the fluid.
  • Transportation Fluid System: Transportation fluid systems use non-return valves in their pipelines to avoid the backward movement of the fluid.
  • Nuclear Industry: Nitrogen feed systems, dump lines.
  • Industrial Sampling systems
  • Fluid Injection Systems of aircraft, spacecraft
  • Laboratory Purification system
  • Mixing chambers: A non-return valve is installed at each individual line to avoid gas mixing with the original source.
  • Domestic Uses like Sprinkler systems, Home heating systems, inflatable mattresses and boats, drip irrigation systems, Rainwater harvesting systems, Hydraulic jacks, etc.

Types of Non-Return Valves

There are various types of nonreturn valves found for industrial applications. Their exact working mechanism may differ slightly, but the main function is similar. They are two-port valves. Since most non-return valves work automatically, they usually don’t have any handle or stem.

Depending on the exact working mechanism of the movable part that allows or blocks the fluid flow, non-return valves are classified as follows:

Swing check non-return valves:

In this type of non-return valve, the movable part is a disc. It swings on a hinge or trunnion to allow/block the fluid flow. A variety of different types of disc and seat designs are found to be used in different applications. In general, soft-seated swing check valves are more leak-tight.

Stop check non-return valves:

Stop check non-return valves possess an override control that stops the flow. It does not depend on the fluid pressure or flow direction.

Ball non-return valves:

The ball type of NRV features a movable spherical ball to block the flow. Sometimes they are spring-loaded.

Diaphragm NRVs:

This type of non-return valve uses a diaphragm (usually made of flexible rubber) that is controlled by fluid pressure.

Lift check non-return valves:

In a lift check NRV, a disc known as a lift operates to allow/block the flow. When the inlet pressure is high, the disc is lifted and the flow is allowed. When the pressure drops gravity force or outlet pressure lowers the disc and the flow is stopped. Normally used for high-pressure service.

In-line NRVs:

This type of non-return valve uses a spring and the flow is allowed when the upstream pressure exceeds the spring tension. Again when the pressure goes below the pressure to overcome spring tension the flow is blocked.

Folding disc non-return valves:

Mainly used for gaseous or low-pressure liquid service, this type of non-return valves are made in a wafer body pattern. They are also known as double-disc or split-disc check valves.

Tilting Disc NRVs:

Tilting disc types of non-return valves are suitable for turbulent, pulsing, or high-speed flows. The disk of this type of NRV floats within the flow and fluid runs on its top and bottom surfaces.

Foot Valves, Duckbill Valves, etc are also examples of non-return valves

Non-Return Valve Symbol

The non-return valve symbols vary slightly from company to company. The common non-return valve symbols in P&IDs are provided below in Fig. 1

Non-Return valve Symbols
Fig. 1: Non-Return Valve Symbols

Selecting a Non-Return Valve

Selecting the proper type of NRV helps in smooth, trouble-free, low-maintenance, long-term operation. The selection of an NRV is influenced by various parameters like:

  • Type of fluid to be carried.
  • Location of the non-return valve
  • Pressure and temperature of the flowing fluid.
  • Hydraulic characteristics like minimum and maximum flow rates
  • Closing time (Slow, normal, or fast-acting)
  • Installation type; Vertical or Horizontal
  • Cost

The table in Fig. 2 provides a sample guideline for selecting a proper type of NRV.

Non-Return Valve Selection
Fig. 2: Non-Return Valve Selection (Ref: avk.fr)

Functions of Non-Return Valves

The most important functions that a non-return valve performs are:

  • It prevents damage to the upstream equipment due to reverse flow and
  • Preventing reverse flow after shutdown (Water hammer/Surge)

Because of this, non-return valves are also known as safety-critical valves.

Non-Return Valve Maintenance

To ensure suitable maintenance of non-return valves, providing isolation of the check valve/NRV is always considered. In general, a gate or ball valve is used as an isolator. As a general engineering practice, the non-return valve or check valve is installed at a distance of 5D (D=Pipe/Pipeline OD) from the pump or any pipe fittings to avoid turbulence. The maintenance of non-return valves is carried out based on the manufacturer’s guidelines.

Difference between Non-Return Valve and Check Valve

Both the check valve and non-return valve perform the same duty of restricting and allowing flow only in one direction. Both are one-way valves. All valve types that act as unidirectional valves are non-return valves and the check valve is one of them. Most of the time check valves and non-return valves are used synonymously. There are various types of nonreturn valves such as foot valves, duckbill valves, etc. So, in layman’s language we can say:

All Check Valves are Non-Return Valves but All Non-Return Valves may not be Check Valves.

Can non-return valves (NRVs) be installed vertically?

Yes, some non-return valves can be installed in a vertical orientation. However, not all non-return valves can work in a vertical line. While installing an NRV, the manufacturer’s direction of the flow arrow must be studied and installed accordingly.

Difference between a reflux valve and a non-return valve

Both reflux valves and non-return valves are used to control the direction of fluid flow and prevent backflow in piping systems. They usually denote the same type of valve, and thus there is no difference between them.

Frequently Asked Questions

Let’s find the answers to some of the frequently asked questions that professionals wish to learn about non-return valves.

What is the meaning of a non-return valve?

A non-return valve is a type of unidirectional valve that allows the flow of fluid in the downstream direction but prevents the backflow.

Where is the Non-Return Valve or NRV placed?

The non return valves are positioned where there is a requirement of preventing backflow. The placement of NRVs will be clearly marked in the P&ID drawings. A typical example is the pump discharge line. Additionally, the NRV is installed in a location such that it can be easily accessed and maintained.

How do you identify a non-return valve?

The identification of a non-return valve is quite easy as the manufacturers place the flow direction on the valve body and mark an X mark at the end of the valve.

What is the main purpose of a nonreturn valve?

The main purpose of a non-return valve is to allow flow only in one direction and prevent the flow from the reverse direction.

Further Studies

For more details about the check valve types, components, working, and applications, follow the following article:

What is Flange Facing? Working and Types of Flange Facing Machines

Written by Anup Kumar Dey in Construction,Maintenance,Pipeline

To ensure that old flanges continue to work smoothly without any issues with their joint integrity flange facing is required to be performed. Flange facing is a machining service on flange surfaces performed during the maintenance and repair period. Timely maintenance and repair of flanges must be performed to avoid leakages and corrosion, which in turn increases the service life of the flanges.

What is Flange Facing?

Flange facing is the process by which the flanges are resurfaced by machining work to create new mating surfaces that ensure a perfect seal when assembled. Flange facing is done using a tool known as a flange facing tool, flange facing machine, or flange facers.

Flange facing is a very important activity in oil and gas, petrochemicals, refinery, pharmaceutical production, pipelines, food processing, chemical, and power generation industries. Additionally, the following industries use the flange facing operation while maintenance and shot down of their plant:

  • Nuclear industry
  • High-Purity industry
  • Diesel Engined
  • Defense
  • Tube Processing
  • Shipyards
  • Fluid control industris

Why is Flange Facing Required?

Flanges are always pressurized during operation and hence continuously experience damage due to turbulent flow. Additionally, they experience impacts with other components while construction and installation, or cuts from gasket leaks.

Flange facing cuts the flanges and provides a spiral grooved finish that helps flanges to be less susceptible to leakages as fluids are forced to travel in a spiral path rather than across the flange face. During plant shutdowns or maintenance activities, Flange facing is one of the most important repair jobs.

Applications of Flange Facing

The flange-facing work is required for the following activities:

  • Re-facing of pipe or pipeline flanges.
  • Repairing of a heat exchanger and other equipment nozzle flanges.
  • Re-surfacing large pump base housings.
  • Re-machining the gasket seal on tube sheets.
  • For sealing, weld preparation, facing, and beveling of the pipe are required.
  • Repairing flat face raised face and phonographic finish flanges.
  • Ship thruster mount facing, drilling, and milling.
  • Repairing piston rod mating flanges.
  • Boiler feed pump flanges.
  • Cutting new grooves or repairing ring grooves.
  • Vessel and plate weld prep.
  • Re-facing ship hatch sealing surfaces.
  • Re-machining bearing surface of rotary cranes.
  • Re-facing valve flanges and repairing heat exchangers.
  • Flange milling wind tower section

Flange Facing Machine

Flange-facing machines or Flange facers are very useful tools to mechanically polish or cut disks, collars, rings, or flanges. Flanges in the piping industry usually get deformed (scratches, dents, etc) or corroded during operation or handling. To ensure the flange joint’s integrity, these damages are removed using a split frame/clamshell cutter known as a flange-facing machine.

A flange-facing machine is also known as a portable lathe machine as they allow to repair of flanges of any diameter without replacing them.

Working of Flange Facing Machine

The cutting tool of the flange-facing machine travels in a spiral path across the flange face and removes the damage from the flange face. This machining or cutting/polishing operation is done by successive strips to ensure face flatness and regularity. Once the flange facing is done, the quality of the flange and the proper sealing capability are achieved.

Finishes as per ASME B16.5 specifications are created and achieved using geared fixed feeds. Flange-facing machines are flexible in operation. They can be mounted at any angle, including the inverted orientation. The facing arm of the flange facer is balanced using adjustable counterweights when not mounted horizontally.

The flange-facing process usually involves the following steps:

  • Cleaning of the flange face surface
  • Machining with a cutting tool
  • Low rotation, low feeding.
  • Fine machining with insert bits.

Types of Flange-Facing Machines

Depending on how the flange-facing machines are powered, various types of flange-facing machines are available in the market. The common types of flange facers are:

  • Pneumatic flange facing machine
  • Hydraulic flange facing machine.

Depending on the mounting mechanism, flange-facing machines are of two types:

  • Internal Diameter (ID) mounted flange facer and
  • Outside Diameter (OD) mounted flange facing machine.
ID and OD mounted Flange Facing Machine
Fig. 1: ID and OD mounted Flange Facing Machine

Selection of a Flange-Facing Machine

The following parameters dictate the selection of an appropriate flange-facing machine:

  • Flange Size (Smaller machines are used for 2″ to 12″; Larger ones for 45″ to 120″ sizes)
  • Pneumatic or Hydraulic type
  • ID or OD mounted
  • Price
  • Toolkit required

Operation Features of a Distillation Column: Flooding, Weeping, and Entrainment

Written by Partha Pratim Panja in Process

The flow patterns on a tray of a distillation column with gas rising continuously through the downflowing liquid are pretty complex. The tray internals is selected & designed keeping in view the complexity of the flow and problems that arise out of it. The factors that arise due to the mal-operation of distillation are termed as different terminologies that are weeping, flooding, Entrainment, etc that are not desirable at all and immediate measures should be taken to control these complexities.

What is the Weeping of Distillation Column?

If a very small fraction of the liquid flows from a tray to the lower one through perforation or openings of the tray deck, the phenomenon is called ‘weeping’. Weeping causes some reduction of the tray efficiency because the liquid dripping down to the tray below through the perforation has not been in full contact with the gas or vapor. On the other hand, dumping is an extreme case of leakage through the tray deck if the vapor velocity is low and the vapor pressure drop across the tray is not sufficient to hold the liquid. In a practical scenario, a slight weeping may occur intermittent basis while sieve trays are used due to an instantaneous pressure difference.

There are two things to be considered i) weep point and ii)weep rate.

The weep point is defined as the velocity of vapor becoming significantly low which reduces the tray efficiency. The weeping phenomenon increases with

  • larger hole area
  • Higher liquid rate
  • Higher weir height
  • The lower surface tension of the liquid
  • Closer spacing between holes

What is the Entrainment of Distillation Column?

Entrainment is the phenomenon when gas bubbles through the liquid pool continuously and the droplets of liquid are continuously formed in the vapor space by quite a little mechanism including the shearing action of the gas jet or breakage of the film of the liquid because the gas bubble collapse. The droplet may descend back into the liquid on the tray or may be carried into the tray above based on the size of a droplet, its projected velocity, and the drag force acting on it due to the gas velocity. This carryover of the suspended liquid droplet into the upper tray is termed ‘entrainment’. The chances of entrainment are more if the droplet is small, if the gas velocity is large, or if the tray spacing is small.

Entrainment causes three major problems:

  1. It causes the mixing of the entrained liquid from the lower tray with the liquid on the upper tray. This adversely affects the mass transfer which reduces the tray efficiency.
  2. The carryover of a substantial mass of liquid as droplets into the upper tray increases the liquid flow rate and downcomer load of that tray.
  3. The next problem may lead to the ‘flooding’ of the tower.

Entrainment is expressed as kg (droplet entrained)/s, kg/kg vapor, kg/kg liquid flow, or kmol/ kmol liquid flowing.

What is Flooding of Distillation Column?

Flooding of a distillation column is a phenomenon when liquid flows across a tray and goes toward the outlet weir. The liquid starts overflowing the outlet weir and drains through the downcomer to the tray below. Vapor bubbles through the holes of the sieve trays, or caps of the valve trays, on the tray deck, where the vapor comes into intimate contact with the liquid. The function of a tray is to mix the vapor and liquid together to form foam. This foam should separate back into a vapor and a liquid in the downcomer & if it is not drained fast from a downcomer onto the below tray, then the foamy liquid or froth will back up onto the tray above. This condition is called flooding of the distillation column.

Cause of Flooding in Distillation Column

Loss of downcomer seal

As per the figure depicted in Fig.1A, it has been clearly shown that downcomer B is flooding. The reason is the loss of the downcomer seal. The height of the outlet weir is less than the bottom edge of the downcomer from the upper tray. This allows the vapor to flow upwards to downcomer B. The upgoing vapor drives away the downflowing liquid. The vapor pushes the liquid up onto the tray above which is the main reason for flooding.

Inadequate downcomer clearance

The next reason for flooding is inadequate clearance of the downcomer and tray deck which is shown in the figure below (Fig-2). If the bottom edge of the downcomer is too adjacent to the below tray then a higher pressure drop is needed for the liquid to escape from downcomer B onto tray-1 & which causes the liquid level in downcomer B to back up onto tray-2. As a result of that Tray-2 gets flooded. Once tray-2 floods, downcomer C (shown in Fig. 1B) will also back up and flood. This condition will be continued till all the trays and downcomers above downcomer B are flooded. At the same time, all trays below downcomer B will get dry on the loose liquid levels.

Thus, the following rules apply:

  • When flooding commences on a tray, all the trays above the flooding point will also be flooded, but trays below that point will get dried up.
  • Loss of liquid level in the bottom of the column is an early indication of flooding in a distillation column
  • If the downcomer clearance (the distance between the bottom edge of the downcomer and the tray below) is too large, the downcomer becomes unsealed. Then vapor flows up the downcomer, and flooding occurs.
  • Liquid starts backing up in the downcomer if the downcomer clearance is too small, and the trays above become flooded.

To calculate the height of liquid in the downcomer, due to liquid flowing through the downcomer clearance:

ΔH = 0.6 × V2 

where ΔH = inches of clear liquid backup in the downcomer, due to the head loss under the downcomer V = horizontal component of liquid velocity, in ft/s, as the liquid exits from the downcomer.

Flooding in Distillation Column
Fig. 1: Flooding in Distillation Column

To guarantee a proper downcomer seal, the bottom edge of a downcomer should be about 0.5 inches below the top edge of the outlet weir. This dimension should be carefully checked by process personnel when a tower is opened for inspection. It is quite easy for sloppy tray installation to distort this critical factor.

Concept of Incipient Flood

The control of the distillation tower is such that both the pressure and bottom temperature are kept constant. This indicates that the percentage of propane in the bottoms product (butane) is held constant. If the operator increases the reflux flow to the distillation column then the following condition will occur (refer to Fig-2):

A simple C3-C4 Splitter
Fig. 2: A simple C3-C4 Splitter
  • The top temperature of the distillation tower decreases.
  • The weight percent of butane in the overhead product (propane) decreases.
  • The bottom temperature of the tower starts to decrease.
  • The duty of the reboiler increases to regain the bottom temperature of the distillation tower to its set point.
  • The weight flow of vapor and the velocity of the vapor through the tray increase.
  • The height of the spray section, or entrainment, between the trays of the distillation tower increases.

As per figure (Fig-3A) Point A is called the incipient flood point, that point in the tower’s operation at which either an increase or a decrease in the reflux rate results in a loss of separation efficiency. One can call this as optimum reflux ratio which would be an alternative description of the incipient flood point.

Flooding Concept of Distillation Column
Fig. 3: Flooding Concept of Distillation Column

How to Calculate the Height of Liquid on Tray Deck?

As the liquid height on a tray increases, the height of liquid present in the downcomer that is fed to this tray will increase by the same amount. In addition to that, the excessive liquid present in the downcomer or froth levels causes flooding and loss of efficiency of the tray of the tower. The liquid level on a tray is governed by both of the following factors:

  • Weir height
  • Crest height

The height of the weir of the trays can be adjusted. It usually adjusts the weir height to between 2’’ and 3’’. This produces a significant depth of liquid on the tray deck to develop effective mass transfer.

The height of the crest is similar to the height of water overflowing a dam or a river.

The formula for the calculation of crest height is,

Crest height = 0.4 (GPM ÷ inch (outlet) weir length)0.67

where crest height = inches of the level of clear liquid overflowing the outlet weir; GPM = gallons (U.S.) per minute of liquid that leaves from the tray.

Total Height of Liquid in the Downcomer

The total height of clear liquid in the downcomer weir is the summation of four factors stated below:

  • Liquid exit velocity from the downcomer onto the below tray.
  • Height of the Weir.
  • Height of the Crest of liquid overflowing the outlet weir.
  • The pressure drop of the vapor flowing through the tray above the downcomer.

But in the actual scenario, there is no clear liquid exists either in the downcomer, on the tray itself, or outlet weir. The liquid actually is froth or foam in nature which is called aerated liquid. The factor that compensates aeration effect is 0.5. So 50 percent is often used for many hydrocarbon services.

This signifies that if we calculate a level of clear liquid of 12 inches in the downcomer, then the actual level of foam in the downcomer is  12 inches/(0.50) = 24 inches of foam. If the total height of the downcomer along with the height of the weir is 24 inches, then the height of the foam in the downcomer is 24 inches resulting in downcomer flooding. This is frequently called a liquid flood.

What is Jet Flooding?

Jet flooding occurs when the downcomers and trays consist of froth or foam, there is a quantity of entrained liquid that is lifted above the froth level on the trays of the tower. The driving force that causes this entrainment is the vapor flow through the distillation tower. The height of the spray section of this entrained liquid is governed by two factors:

  • The height of the foam resides on the tray
  • The velocity of the vapor through the tray
Jet Flood due to Entrainment
Fig. 4: Jet Flood due to Entrainment

High vapor velocities in conjunction with a high level of foam will cause the height of the spray section to hit the underneath of the upper tray. This results in the mixing of the liquid from a lower tray with the liquid on the upper tray. This back mixing of liquid causes the reduction of separation, mass transfer, or efficiency of the tray of a distillation tower.

If the vapor flows through a tray increases, the froth height in the downcomer draining the tray will increase as well. This will not have any impact on the foam height on the tray deck until the downcomer fills with liquid foam. Then a further increment of vapor flow causes a significant increase in the foam height of the tray of the distillation tower, which increases the height of the spray section. When the height of the spray section from the below tray hits the upper tray, then it is called the incipient flood point or termed the initiation of jet flooding.

Relation between Tower Pressure Drop and Flooding

The relation between the pressure drop of a distillation tower and flooding is important to understand for the prediction and prevention of flooding. The common parameter of process equipment is that smooth operation is reached at neither a very high nor a very low loading. The intermediate equipment load that results in the most efficient operation is called the best efficiency point. For trays of the distillation tower, the incipient flood point corresponds to the best efficiency point. We have correlated this best efficiency point for valve and sieve trays as compared to the measured pressure drops in many distillation towers. We have derived the following formula:

where

  • DP = pressure drop across a tray section, psi
  • NT = the number of trays
  • TS = tray spacing, inches
  • s.g. = specific gravity of the clear liquid, at flowing temperatures

On the basis of hundreds of field measurements, we have observed

  • K = 0.18 to 0.25: Tray operation is close to its best efficiency point.
  • K = 0.35 to 0.40: Entrainment occurs—an increase in reflux ratio significantly reduces tray efficiency.
  • K = ≥0.5: Tray is fully engulfed with flood—opening a vent on the overhead vapor line will blow out liquid with the vapor.
  • K = 0.10 to 0.12: Low tray efficiency, due to tray deck leaking.
  • K = 0.00: There is no liquid level on the tray, and quite likely the trays are lying on the bottom of the column.

What is the turndown ratio?

Turndown is a term that is frequently used with respect to the capacity of the plant. A plant is designed for a particular capacity range that may have to operate at an enhanced or reduced throughput depending upon the changes in the production rate or demands or various factors. It is therefore desirable that the trays should have some degree of flexibility to accommodate variable throughput. Such flexibility is called the turndown ratio is defined as the ratio of the design vapor throughput to the minimum operable throughput.

Sieve trays have a low turndown ratio of about 2. It means sieve try can normally be operated up to 50% of the design vapor throughput. This turndown ratio can be increased by reducing the fractional hole area. Valve trays normally have a turndown ratio of 4 while bubble cap trays have a still larger turndown ratio.

What are Hydraulic Gradient and Multipass Trays?

The difference between the clear liquid heights at the points of the inlet and outlet on a tray is called the ‘hydraulic gradient’ or ‘liquid gradient’ where ‘Gradients’ means the rate of change of a quantity with the position. But the hydraulic gradients are really the ‘difference’ of liquid heights. Basically, this is the requirement of the liquid head to overcome the resistance to liquid flow on the tray.

The value of the hydraulic gradient on a tray should not be more than a fraction of an inch. Preferably, it should be kept within ½ inch. An excessive liquid gradient causes severe malfunctioning of the tray as most of the gas flows through the holes near the middle of the tray and at the outlet weir section (where the ‘effective liquid depth’ on the tray is low) and only a small part of flows through the holes at the liquid inlet side of the tray. Such maldistribution of the gas or the vapor called vapor channeling severely reduces tray efficiency.

So, the hydraulic gradient is a very important operational feature that needs to be checked during tray design. It remains pretty small for the sieve tray. But for the bubble cap tray, it may be significant because the bubble caps offer a larger resistance to liquid flow.

Comparison between several trays

A quantitative comparison of the three frequent trays used in respect of capacity, efficiency, flexibility, cost & other criteria is given in the table below:

ParameterBubble cap traySieve trayValve tray
CapacityModerateHighHigh to very high
EfficiencyModerateHighHigh
EntrainmentHighModerateModerate
Pressure dropHighModerateModerate
Turn downExcellentAbout 24-5
Fouling tendencyHigh, tends to collect solidLowLow to moderate
CostHighLowAbout 20% more than sieve trays
ApplicationRarely used in new columnsMost applications if turndown is not importantPreferred for high turndown is anticipated
Share of marketAbout 5%25%70%
Table 1: Quantitative comparison of the three frequently used trays in the Distillation Column

Reference and Further Studies

The following book you can use as a reference and for further studies:

  • Troubleshooting Process Operations by Norman Lieberman