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.
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
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.
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
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.
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:
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:
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:
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.
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.
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.
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):
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.
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
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:
Parameter
Bubble cap tray
Sieve tray
Valve tray
Capacity
Moderate
High
High to very high
Efficiency
Moderate
High
High
Entrainment
High
Moderate
Moderate
Pressure drop
High
Moderate
Moderate
Turn down
Excellent
About 2
4-5
Fouling tendency
High, tends to collect solid
Low
Low to moderate
Cost
High
Low
About 20% more than sieve trays
Application
Rarely used in new columns
Most applications if turndown is not important
Preferred for high turndown is anticipated
Share of market
About 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
What is Piping Slope and Why is it Required? Pipe Slope Calculation
The piping slope is the change in elevation with respect to its flat horizontal position. The piping slope is provided in various piping systems mainly due to free-draining requirements. A slope in the pipe helps the liquid to flow easily in the downward direction. So, it helps in avoiding the accumulation of liquid inside the piping system. This in turn helps the system to eliminate two-phase slug flow problems.
Examples of Piping Slope
There are many lines inside a complex process industry that have piping slopes. Some typical examples of pipes and equipment that are provided with slopes are:
All piping in the main steam and hot and cold reheat line, turbine extraction system, condensate system, and turbine drains of power plants.
Compressor suction lines between the knockdown drum and the compressor should be as short as possible, without pockets, horizontal, and sloped toward the compressor.
Fuel-oil lines in the fuel Oil Burner piping system should be sloped from the burner shutoff valves toward the burners to provide natural drainage.
Steam trap discharge lines should be sloped for drainage where possible.
Drains and vents should be provided and piping sloped to facilitate liquid drainage and gas venting.
In extensive sewer systems (except in hilly areas), most of the pipes will have mild slopes.
Piping Slope Symbol
The piping slope indicates the inclination of the pipe with respect to the horizontal ground or reference level. The application of slope in piping or pipeline systems forces the liquid to go to the next low point of the line. The requirement of the slope is usually mentioned in the process P&ID. The slope given is indicated to the construction team using piping isometrics. A special symbol is used to indicate slope in Process P&ID and Piping isometrics.
Piping Slope in Isometric
Refer to Fig. 1, which is part of a piping isometric drawing.
Fig. 1: Piping Slope Symbol in isometric drawing
In the above piping isometric drawing, the piping slope is clearly mentioned. As can be seen, there is a fall of 1 mm for every 200 mm of pipe run. The value or magnitude of the piping slope is indicated as 1: 200, 1:100, 1:500, etc. All these terms signify that there will be an elevation change of 1 unit for each piping length of 200 units, 100 units, or 500 units respectively.
Note that, the piping slope can also be denoted as 1/100 or 1%. Both of these are the same as 1:100. All of these define the height deviation with respect to a certain length.
The slope in piping is symbolized using a right-angle triangle. The triangle will be placed adjacent to the line having a slope requirement. The elevation will increase towards the direction of the height (side) of the right-angle triangle. In a similar way, the elevation will drop towards the angle created by the base and hypotenuse. So, in the above image, as per the piping slope symbol specified, the elevation will decrease while moving from the south to the north direction. Similarly, for the pipe run traveling in the E-W direction (top ellipse in Fig. 1), the elevation will increase when moving from the West to the East direction.
Piping Slope in P&ID
Now refer to Fig. 2 below, which shows the part of a process P&ID.
Fig. 2: Slope in Piping as symbolized in P&ID
In the above figure, the slopes to be provided in piping are clearly indicated with a triangle symbol. The magnitude of the slope is written directly near the triangle or sometimes covered with notes as can be seen above. Notes 11 and 10 will explicitly mention the slope requirement. Also, the piping slope symbol mentions that there will be a drop in the elevation of the pipe towards the vent stack.
Piping Slope Calculation
As already mentioned, the required piping slope is usually mentioned in the P&ID (Refer to Fig. 2 above). So, once the slope value is known the change in elevation can easily be calculated. Let’s take the example of the E-W piping run in Fig. 1 which is highlighted towards the top end.
The total length of the element is 5357 mm and the slope towards the west direction is 1:200. So, we can use this information to mathematically calculate the change in elevation from one side to the other. So, the elevation difference of the line will be (1/200)*5357=26.785 mm. So calculating the piping slope is quite simple once these values are given. So, in Fig. 1, the Eastside edge will be at 26.785 mm higher elevation than the Westside edge. That we can check from the elevation values given in the isometric. Upon calculating the elevation differences given in the isometric we get 102541-102514=27 mm which is the same as what we calculated. In a similar fashion, we can easily calculate the piping slope for any other leg.
The value of the piping slope required is usually decided by the process engineers following industry experience and thumb rules. Some of the thumb rules that are prevalent in the industry are listed below:
All piping in the main steam and hot and cold reheat line, turbine extraction system, condensate system, and turbine drains, of power plants are sloped down a minimum of ¹⁄₈ in/ft (10 mm/ m), in the direction of flow.
It is common practice to design sanitary sewers with slopes sufficient to provide for velocities of 2 ft/s (0.6 m/s) when flowing full. Experience shows that with such slopes, trouble from deposits is seldom encountered.
Piping Slope Formula or Equation
The formula to calculate piping slope is given by the following equation:
Piping Slope=100*(Elevation or Height Change/Length of Pipe)
The above pipe slope formula calculates the slope in % terms. Both the height or elevation change and pipe length have to be in the same consistent unit. In general, in the FPS unit, the pipe length is provided in feet, and in the metric system, the length is given in meters.
Supporting Pipes having Slope
The usual practice for supporting pipes with slopes in process and power piping is to use a pipe shoe. In this type of piping shoe, the height of the shoe is variable from one side to the other corresponding to the pipe slope. A typical example is flare headers. They are supported with variable-height pipe shoes. Fig. 3 provides a typical shoe support that is used for supporting pipes having slopes.
Fig. 3: Supporting of Sloped Pipes
Advantages of Pipe Slopes
The main requirement of slopes in piping is to avoid the accumulation of liquids inside the pipe. Other advantages are:
When a piping slope is provided, the flow of the medium is easily maintained and gravity flow happens in the downward direction.
The piping slope separates the flow of liquid along with the gaseous phase. For example, in the flare header the liquid is separated and returned back to KO Drum and the gas phase flows through the stack. This separation helps in avoiding slug flow problems.
Types of Piping Slopes
There are two types of piping slopes; positive slope and negative slope. When the pipe elevation drops in the direction of fluid flow it is known as a positive piping slope. On the other hand, if the pipe elevation increases in the direction of fluid flow it is known as negative piping slope.
Drain Pipe Slope
Based on the International Plumbing Code, the drainage piping system should be laid with a uniform slope. The amount of drainage piping slope depends on the pipe diameter. The usual minimum slope for drainage piping is as follows:
For Pipe Size 2.5 inches and smaller: The minimum Slope requirement is 0.25 inches per foot
For Pipe Sizes 3″ to 6″: The minimum Slope requirement is 0.125 inches per foot
For Pipe Size 8 inches and larger: The minimum Slope requirement is 0.0625 inches per foot
Piping Slope Test
The Piping Slope Test or the Pipe Fall Test is the term majorly used in drainage piping systems. This is a process to ensure that the piping/pipeline system is installed with the correct slope or gradient. For systems requiring gravity flow, the piping slope test is crucial.
The Piping Slope Test procedure involves the following steps:
Setting up a level: At one end of the pipe, a level is set up.
Measuring the height: The height is then measured at the other end of the pipe.
Calculating the slope: The pipe gradient or slope is then calculated by dividing the elevation difference by the pipe length and then multiplying the same with 100 to denote in % values.
Frequently Asked Questions
What is a 2% slope in piping?
A 2% slope in piping means that the pipe has a vertical rise or fall of 2 units for every 100 units of horizontal length. To give an example, if a pipe has a length of 10 meters, then a 2% slope means that the pipe will rise or fall by 0.2 meters from one end to the other. A 2% slope is equivalent to a 1.15° angle.
What is the proper slope for a pipe?
The proper slope for a pipe depends on the type, application, and purpose of the pipe. Generally, pipes must slope downhill to drain correctly. The slope should be enough to allow the liquid to flow easily. In general, 1 in 100 is considered as a standard slope in oil and gas piping.
What is a 1% slope in piping?
A 1% slope in piping informs that the pipe will vertically rise or fall by 1 unit for every 100 units of horizontal length. For example, if a pipe has a length of 10 meters, then a 1% piping slope means that the pipe will rise or fall by 0.1 meters from one end to the other. A 1% slope is equivalent to a 0.57° angle.
What does a 1 in 100 slope mean?
A 1 in 100 slope means that the pipe has a vertical rise or fall of 1 unit for every 100 units of horizontal length. It is another way of expressing a 1% slope.
What is zero slope?
A zero slope means that the pipe has no vertical rise or fall, and is perfectly horizontal. A zero-slope pipe does not drain by gravity and may require a pump or other device to move the fluid.
Why slope is given in piping?
Slope is given in piping to facilitate the drainage of liquid or vapor from the pipe. Piping Slope also helps to prevent the formation of pockets or traps in the pipe, where liquid or vapor can accumulate and cause problems such as corrosion, erosion, pressure fluctuation, vibration, or blockage. Slope is especially important for pipes that carry two-phase flow, such as steam, condensate, or flare headers.
Snow and Ice loading is an environmental load and must be considered in pipe stress analysis where the climatic condition specifies the possibility of snow or ice formation. Piping projects in various cold geographical locations like Canada, Russia, the USA, Europe, etc where winter temperature falls below a certain limit that causes snow or ice formation must be designed considering the impact of snow loading. Both snow and ice loads are considered as live loads on piping.
Codes and standards stipulate that the system must be designed against snow and ice loading. They provide a lot of guidance for snow loading calculation and application for system design to safeguard them from failure for building structures. But for non-building structures like piping systems, the calculations for snow loading are too limited. In this article, we will try to explore the philosophy of snow loading in piping and pipeline systems and its application in the pipe stress analysis process.
What is Snow and Ice Loading?
Snow and ice loading is a type of sustained loading. The possibility of snow loading will be mentioned in the project environment conditions of the region. Ice and snow loading for piping systems is basically additional downward-acting forces exerted by the accumulated snow and ice. The increase in support load and system stress due to the weight of the accumulated snow and ice must be considered in the structural design.
Snow and ice loading is only significant for outdoor piping installations and is treated similar to other deadweight loads.
In general, snow and ice loading is considered in pipe stress analysis as a uniform load placed over the exterior part of the pipe and fittings. The entire system can fail if the snow loads exceed the allowable of the piping systems. Snow loads are being applied to all pipe elements above ground, and fittings are in a vertical direction downward. The loads depend on the slope angle of each element. The snow loads are given to the elements by using the snow factor.
Though both snow and ice are made up of water there is a slight difference. Snow falls as precipitation of frozen water whereas ice is simply frozen water. In general snow in the piping system is usual for climatic regions. Whereas ice loading generally refers to ice storm deposits over pipes that are decided based on region-specific weather reports.
Snow Load Calculation Philosophy
Snow load calculation philosophy is based on the consideration that the snow accumulated on top of the piping system will take the shape of an equilateral triangle with its base equalling the pipe outside diameter.
The common equation used for calculating the snow loads is as follows:
Ws=(1/2)*Do*So
Here
Ws=design snow load to be added to other distributed loads acting on pipe, lb/ft
Do=outside diameter of the pipe (for bare pipe) or insulation (for pipe with insulation), in
So=Snow Factor that considers the probable snow loading for the region where the piping system is installed, lb/ft2
Ice Loading Calculation Philosophy
Ice loading is also applied in the system as uniform loading and calculated using the following formula as specified in the piping handbook:
Wice=1.36*t*(Do+t)
Here,
Wice=unit loading on the pipe, lb/ft
Do=outside diameter of the pipe (for bare pipe) or insulation (for pipe with insulation), in
t=ice covering thickness, in
Application of Snow Load in Caesar II
Once the uniform load for snow or ice loading is known or calculated, the same is applied in Caesar II and proper load cases are created to account for it. The steps follow are provided below:
Step 1: Snow load is usually not considered along with wind or seismic load. So, to add snow load in the piping system, simply click on uniform load and add the calculated uniform load value (Note this will vary depending on pipe size) in the input screen as shown below:
Fig. 1: Input screen of Caesar II for inputting Snow load
In the above example, the calculated snow load per unit length is 0.346 in N/mm for the 18-inch pipe size. The negative sign for the force will act downwards.
Step 2: Create load cases to account for the above input value as shown in Fig. 2 below:
Fig. 2: Load cases for Snow loading in Caesar II
Step 3: Now you can run the analysis and check the output results. Support loads considering snow loads must be transferred to the Civil team for structural consideration.
Ice loading on the piping system can be applied in a similar fashion if the ice loading is because of the ice storm. However, if the ice generation is because of the cryogenic fluid inside the pipe and moisture condensation, then the same can be considered similar to pipe insulation. Simply, the thickness of ice generation needs to be calculated and that thickness along with ice density can be added as insulation. This means you are roughly considering the pipe is insulated with ice.
