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Level Gauges: Types, Sizing, and Ordering Information | Level Instrument | Level Indicator

Written by Anup Kumar Dey in Instrumentation,Piping Design Basics,Piping Interface

What is a Level Gauge or Level Indicator?

Level gauges or level indicators are instrument devices that are installed in plants for determining the liquid level in process equipment like drums, vessels, tanks, etc. The level gauge instrument or level instrument has several parts like a head, float, measuring tape, bottom anchored bracket, guide wires, elbows, anchors, coupling, pipe support brackets, and pipework. To suit the requirement of the industry, various types of level gauges (Fig. 1) are available.

Level gauges are widely used for a wide variety of applications requiring level monitoring. Some of the applications include:

  • Industrial processes involve storing or filling liquid in containers.
  • Process control systems for managing flow rates of various equipment.

Major industries that use level gauges are

  • Chemical and Petrochemical
  • Pharmaceutical
  • Medical
  • Manufacturing
  • Food and Beverages
  • Marine
  • Fuel and Energy Management
Level Gauges
Fig. 1: Various Types of Level Gauges

How many Types of Level Gauges are there?

Depending upon application and level measuring techniques, the following types of Level gauges are found:

  • Transparent level gauge
  • Reflex level gauges
  • Magnetic level gauges
  • Tubular level gauge
  • Bi-color level gauge

Transparent Level Gauges

They are made of two transparent glass plates which are fitted with liquid cavities on both sides. By detecting the dissimilarity of transparent characteristics of the two media, Transparent Level gauges determine the fluid level. In some applications, an illuminator from which rays emit is used to make the liquid level easier to see. Such level indicators can be used for almost all installations.

Transparent Level Gauges are sometimes fitted with Mica shields for the purpose of protecting the glass surfaces from the corrosive action of the process fluid.

What is Reflex Type Level Gauge?

The reflex-type level gauge operates by detecting the dissimilarity in the refraction index between fluids and vapors. This type of level gauge works based on the refraction and reflection laws of light.

They are frequently used in vessels or containers. They are durable, can sustain high temperatures and pressure, and are normally made of carbon steel or stainless steel.  In the gas or steam phase, the light is reflected by the prismatic grooves of the glass, which therefore gives a clear appearance. In the liquid phase, the light is absorbed, thus providing a dark indication of the level on the sight glass which is a transparent tube, clamped to the gauge body. Reflex Level gauges are frequently used as they have low initial cost, low operating cost, and easy level reading.

Magnetic Level Gauges

Magnetic level indicators are used to control the fluid level. Operating based on Archimedes’ Buoyancy Principle, such level gauges are highly durable and find their application in high-pressure temperatures and highly toxic, explosive, poisonous, or corrosive environments. These types of level instruments are able to detect the inter-phase level.

Magnetic level indicators consist of a float in a pipe chamber that moves up and down with the level. A flag follower is clamped with a chamber. The individual flags comprise an alignment magnet that couples with the float magnets as the float moves up or down within the piping column.

Float movement rotates the flags and changes their color, the position of the follower, or the point at which the flags change color, represents the true level.

Tubular Level Indicators

The tubular Level indicator is the simplest form of Level indication. Used in low-pressure, non-toxic services, Tubular level gauges detect and report the process fluid level through the direct reading of level in tanks or vessels. It consists of a Glass tube, Packing Material, end blocks, Vent plug, Drain valve, isolation valve, etc.

Bi-Color Level Gauges

Bi-color level gauges are basically transparent gauges having a liquid chamber in a wedge-shaped section. The illuminator on the backside of the device is fitted with two color filters; red and green. When red color light radiations hit the water, they get alerted to one side and absorbed. When the same light hits the steam, the light passes through and shows up as a red color. The reverse happens for light passing through green filters. This allows users to tell how much of each media is in the system.

These are specifically used to measure the media level in a boiler and detect levels by distinguishing the index of refraction between the steam and water. The gauge uses high-quality mica sheets to guard against the wet steam generated in a boiler drum.

Depending on the specific application liquid level gauges are usually termed drum level gauges, tank level gauges, oil level gauges, propane level gauges, fuel level instruments, water level gauges, etc.

Measuring Range of Level Indicators

The measuring range is the level range that the devices can measure and the range is determined by the maximum and minimum media levels. While choosing a level gauge, this parameter must be examined accounting for anticipated media-level fluctuations.

Parameters for Level Gauge Sizing and Selection

The main parameters that affect the level gauge selection and sizing are:

Properties of liquid: Predicting the Properties such as density (specific gravity), viscosity, conductivity, dielectric constant, etc are absolutely critical for the selection of instruments. These properties are responsible for the proper functioning and accuracy of the equipment.

Condition of the liquid: The liquid condition like foaming, frothing, Emulsion, sand, Wax, scaling, etc must be accurately specified to select the required accessories to reduce the effect on the Instrument.

Nature of the Liquid: The nature of liquids like corrosive, non-corrosive, and hazardous depends on their properties and uses to study the compatibility of the material of construction of the level measurement system.

Accurate prediction of vapor space: This is required since vapors can disrupt the functioning of sensors, build pressure, and in turn result in incorrect readings.

Operating conditions: Various operating parameters like temperature and pressure conditions to which the liquids are subjected must be specified. Level-measuring instruments have minimum and maximum limits for these parameters. Considering these limits help to select the Instrument which will withstand them.

Accurate Level Sketch: The accurate level sketch depicting various levels inside the equipment for determining the nozzle positions on the equipment.

Features of Level Gauges

Level indicators have the following characteristics:

  • They are mechanical devices.
  • They do not require a power source to operate.
  • They provide a visual level indication.

Level Gauge Sizes

Level gauges are sized by the body length and the sight length. The body length of a level gauge is measured from the extreme top to the bottom of the gauge. Whereas the sight length represents the available viewable window length. The minimum and maximum level readings that can be obtained from a given size gauge are dependent on the sight length of the level instruments.

How to Order a Level Indicator?

While selecting and placing an order the following points need to be mentioned:

  • Process conditions with Temperature range, Maximum operating pressure, Fluid Density
  • Center-to-center distance
  • End connections (flanged or screwed)
  • Device Medium
  • Mounting Options and
  • Application of the instrument

Few more Resources for you..

What is FMS or Flow Metering Skid?
Types of Flowmeters and their Applications
Overview of Piping – Instrument Interface: An article
An article on Temperature Measurement by Filled Thermal Systems
An article on Conductivity Analyzers
The safe way to install restriction orifices
Other Oil and Gas Instrumentation related Articles

References

Collar Bolts To Maintain Removable Bundles in Heat Exchangers

Written by Baher Elsheikh in Maintenance,Mechanical

What is a Collar Bolt?

Collar bolts are a type of fastener that is extensively used to hold the bundle in the exact place for removable bundle heat exchangers. This ensures the channel removal without interrupting or breaking the seal between the shell and tube sheet. Figure 0 shows a typical schematic diagram of a collar bolt assembly.

Collar Bolts Assembly
Fig. 0: Collar Bolts Assembly

Collar Bolts in Standards

TEMA Standard

The new part (RCB-11.8) was added in the tenth edition of the TEMA (Tubular Exchanger Manufacturer Association) standard, covering the recommendations of the collar bolts in the removable bundles with B-Type bonnet as shown in Figure-1.

Collar stud shall be used on units with removable tube bundles only when specified by the purchaser. Normally, It is recommended for B-Type of bonnets. The Outer Diameter (OD) of the static tube sheet should match the mating flange OD, and shall be through-bolted. It is preferable to have every fourth stud in the bolt circle (with a minimum of 4) as collar type I for type II as shown in Figure 2 below.

TEMA 10th Edition
Fig. 1: TEMA Tenth Edition 2019

Collar bolts are only used to maintain the gasket integrity and position when the channel is removed and torqued prior to pressurizing.

As an alternative to collar studs, every fourth bolt hole in the tube sheet may be drilled and tapped to the size of the stud bolt. The studs in the threaded holes shall be double nutted on the shell side or provided with machined flats to allow the tube side nut without rotating the stud.

API 660 Standard

In API 660, Para 7.5.2.4: A full-diameter stationary tube sheet shall be provided for removable tube bundle exchangers with bonnets (Figure 2). The tube sheet shall be provided with collar studs or tapped tube sheet holes for a minimum of 25 % of the bolts (4 minimum). Hydrostatic testing of the shell side shall be allowed with the bonnet removed and all bolting installed in place.

When collar bolts/drilled-and-tapped holes are used, at least four shall be provided and the location of them shall be identified on the drawings and by stamped markings on the external diameter of the tube sheet.

Type B Stationary Head as per TEMA
Figure 2: Type B Stationary Head as per TEMA

PIP VEV1100M

As per PIP VEFV1100M Vessel/Shell &Tube Heat Exchanger Standard Details, the standard arrangement and configuration for collar bolt dimensions are produced in Figure 3 below.

Collar Bolt and Locking nut for Heat Exchanger
Fig. 3: Collar Bolt/ Locking nut for Heat Exchangers

HEI

Surface condensers are designed to the requirements of HEI (Heat Exchange Institute) per the typical configuration shown in Figure 4. Here, both tube sheets are fixed. Also, without the removal of one of the tube sheets, the gasket between the tube sheet and shell flanges cannot be attended to.

Surface Condenser
Figure 4 Surface Condenser

Surface Condenser without Collar Bolts

A manual of one of the most famous and reputable surface condensers manufacturers in the world alerts the following:

“It is important not to break this seal between the tube sheet and the shell flange. The tubes are expanded into each tube sheet holding them firmly in place, and the shell seal cannot be replaced without retubing the entire condenser. To prevent breaking the joint, it is important that all nuts be removed from the water box flange side and not from the shell flange side. Do not loosen or remove the stake studs and double nuts on the shell side.”

Finding a leak in the shell side causes a huge impact on the plant as it breaks the vacuum. In such a configuration, the collar bolts must be used. 

Despite the illustrated advantages of the use of the collar bolts, there is a debate about the disadvantages as it might be a cause for trouble instead of enhancing the exchanger’s maintainability. In the following section, the main advantages and disadvantages of the use of collar bolts are summarized.

Advantages of the use of collar bolts 

The main objective and advantage of the collar bolts is better maintainability considering that each time the channel is removed, the bundle shall be removed for replacing the gasket between the tube sheet and shell to avoid leakage after pressurizing the exchanger. Bundle gasket replacement is time-consuming and increases MTTR (Mean Time To Repair/Restore). 

Disadvantages of the use of the collar bolts

  1. Some field experience showed that it is nonmandatory to remove the bundle if the channel is removed. This opinion is built on some special experience in using cam profile gaskets and the application of initial proper stress to reach the desired gasket stress.
  2. In case of using tapped holes and the bolts get stuck and the attendance for the holes machining and replacement of the bolts would be time-consuming and might be beyond the readiness of the maintenance crew for the task.
  3. Marking or stamping of the collar bolts has to be adequate to avoid misleading the maintenance crew otherwise, they may remove collar bolts by mistake.
  4. Relative higher cost due to the bigger tube sheet size and the aching required for the bolt holes in the tube sheet. 

Few more resources for you..

Shell & Tube Heat Exchanger Piping: A brief Presentation
An article on Plate Heat Exchanger with Steam
A typical Check List for Reviewing of Shell & Tube Heat Exchanger Drawings
Basics of Shell and Tube Heat Exchangers: A brief presentation
A brief presentation on Air Cooled Heat Exchangers
Basic Considerations for Equipment and Piping Layout of Air Cooled Heat Exchanger Piping

References

  • [1] API Std 660 – Shell-and-Tube Heat Exchangers
  • [2] TEMA Tenth Edition, 2019 (Standards Of The Tubular Exchanger Manufacturers Association)
  • [3] PIP VEFV1100M Vessel/S&T Heat Exchanger Standard Details
  • [4] Explore The World Of Piping – EWP https://www.wermac.org/equipment/collarbolt.html)

What is FMS or Flow Metering Skid?

Written by Anup Kumar Dey in Instrumentation,Mechanical,Pipeline,Piping Interface

What is FMS or Flow Metering Skid?

A Flow Metering Skid is a framed or moduled device on which various other assemblies are fitted for the measurement (flow rates) of gas or liquid products. The major purpose of using a metering skid is for custody transfer. They are designed and manufactured to fulfill the lowest uncertainty. At the same time, they optimize operation and maintenance costs.

The metering skid includes equipment for flow conditioning, filtration, automated or manual operational sequences, draining, venting, safety, maintenance, lifting, proving, sampling, etc. Mass or Volumetric flow rates as per the client’s specification are measured by such skids or packages. Depending on the requirements for measurement, this package can also be used for other treatments like flow control and cleanliness of fluid.

Flow Metering Skid Components

The main components of a metering skid (Fig. 1) are listed below:

  • The structural framework of the skid along with supporting members
  • The piping network
  • Applicable process equipment
  • The electrical power feed including the earthing system, the MCC, and all cabling and trays
  • The local instrumentation and control system includes the flow computer, personal computers, printers, and PLC.
Flow Metering Skid
Fig. 1: Typical Flow Metering Skid

Purpose of the metering skids

Metering skids can be used for various purposes as mentioned below:

  • Pressure regulation and metering stations
  • Fuel gas conditioning systems
  • Border metering stations
  • Offshore gas and liquid metering
  • CNG filling stations
  • Biomethane grid injection systems
  • Underground gas storage metering and control skids
  • LNG metering skids
  • Calibration facilities

Application of metering skid

Offshore metering systems are used in FPSO, FSO, Platform / MOPU / TLP, and FLNG / FSRU. Onshore metering systems are used in Oil Production plants, Oil refineries, Gas Processing Plants, Petrochemical plants, Terminal, Tank farms, Pipelines, etc.

Few important considerations for FMS Design

The type and size of the FMS are based on fluid properties, required system uncertainty, flow rate, pressure drop, maintenance, proving requirements, and more. Different types of flowmeters such as the Positive Displacement meter (PD meter), Turbine meter, Ultrasonic Flowmeter (UFM), Coriolis meter, Orifice meter, V-Cone meter, and Venturimeter, etc. are used for flow metering.

A flow meter must be calibrated and validated at a certain interval of operating life to ensure the reliability and uncertainty of the system. The validation methodology and calibration intervals must be carefully decided as it plays a crucial role in the design of the metering system design.

For the optimum performance and operability of an FMS, the most critical items are the Metering Control System whose design depends on various functional applications and requirements with preferences for Flow Computer, PLC, and HMI manufacturers in line with the preferred operator infrastructure.

In addition to its metering capability-related design considerations, the system has to be designed and developed in the context of various external influences like availability, maintenance possibilities, safety procedures, ATEX and Ingress Protection, etc. 

The design must strictly comply with project specifications and applicable international codes, standards, and regulations, etc.

FMS Packages, in general, contain two numbers of metering runs (2 x 100 % configuration) one-meter run as a duty or operating and another meter run as standby. Under normal operation mode, only one stream (Stream-A) shall be in operation as Duty Stream. This is calculated to be able to provide 100% flow rates required for the FMS skid. In this mode, the other stream (Stream-B) and Proving run should be fully isolated.

Caesar II image of typical FMS Skid
Fig. 2: Caesar II image of a typical FMS Skid

Operating & Control Philosophy

  • Prior to the operation of the FMS Skid, a line walk is required to ensure that all equipment is in good installed condition. 
  • Valve opening-close position shall be ensured to be as per PEFS (P&ID Drawing) requirements.
  • The stream intended to be placed in operation must be pressure-equalized with the inlet pressure.
  • All operation control of the metering stream shall be made using the operation of USV Valves in the inlet, outlet, and proving run of each metering stream. 

Few more Resources for You…

Types of Flowmeters and their Applications
What is Fluid Flow?
Piping Interface Related articles
Piping Design and Layout

Comparison between Piping and Pipeline Engineering : Piping vs Pipeline

Written by Anup Kumar Dey in Pipeline,Piping Design Basics,Piping Interface

Both Piping and Pipeline originated from Mechanical Engineering and many a time, share common activities. For example, both have piping materials, piping expansion, stress, and support problems. So, both piping and pipelines need Stress and Material engineers. Both piping and pipelines are used to transport fluids. On a broad scale, ASME B31.3/ASME B31.1 deals with piping engineering, and ASME B31.4/ ASME B31.8 deals with pipeline engineering. Refer to Figure 1, which shows the piping and pipeline demarcation for a typical plant. Through this article we will try to find out a few other differences that piping and pipeline systems have in a general sense:

Geographical Differences

Piping and Pipelines are normally demarcated by a boundary or fence. Outside the fence comes under the pipeline scope and the inside boundary falls under the piping scope. Generally, a pipeline travels a long distance (across villages or countries) whereas the length of single piping is short (equipment to equipment or pipeline to equipment).

Physical Personality or Action Performed

Piping is normally connected with various equipment and carries fluids inside a complex network that will be processed in that equipment. Whereas Pipelines supply the feed for further processing or deliver the processed fluid or end product and they normally run straight. The number of equipment connections in the pipeline is very less as compared to piping. 

Construction

Pipelines travel aboveground, underground, or sub-sea with the maximum part being buried. Whereas, piping systems are mostly aboveground.

Piping vs Pipeline
Fig. 1: Piping vs Pipeline Scope Demarcation

Pipe Diameter and Fitting Types

In piping systems, pipe size is normally less (the majority of lines in the Process or the power piping systems are less than 36 inches) but the number of pipe fittings used is very high. On the contrary, pipeline diameters are large and the number of fittings is comparatively much less.

Type of Pipe and valves:

In most cases Line Pipes i.e, API 5L code is used for pipeline material and API 6D is used for pipeline valves whereas piping material uses ASTM, BS, API 5L, or various other codes and standards and piping valves are from BS or API standard.

Design temperature

In most of cases, fluid design temperature for pipelines is normally less than 230 deg C, whereas piping systems carry fluids with different design temperatures.

Hydrotesting Pressure

For the piping system, the hydro test pressure is calculated by multiplying design pressure by 1.5 and a temperature factor whereas pipeline design pressure is 1.25 times the design pressure for liquid pipelines and (1.25 to 1.5) times the design pressure for gas pipelines. Also, pressure holding time for pipelines is normally 24 hours whereas for piping the same is generally, 2 to 6 hrs.

Typical Piping System
Fig. 2: Typical Piping System

Pipe Routing

While routing pipelines large-diameter elbows (Normally, Hot bends up to 6D and Cold Bend up to 60D) are used whereas piping systems, in general, do not find such large diameter bends.

Construction Drawing

The construction drawings in the case of pipeline systems are termed as alignment sheets, but the same for piping systems are termed as piping isometric drawings.

Surveys

Various technical surveys like Topographical surveys, Soil-resistivity surveys, Cadastral surveys, Hydrological surveys, Geo-technical investigations, etc are performed to collect various data during pipeline design. On the contrary, Only wind and seismic profile studies are performed for piping systems.

Pigging

Long pipelines are cleaned or inspected by used pigs whereas piping systems are cleaned with steam or nitrogen.

Typical Pipeline System
Fig. 3: Typical Pipeline System

Miscellaneous

  • Pipelines are normally preserved using inert gas or corrosion-inhibited water.
  • Cathodic protection systems are involved with pipelines.
  • Corrosion protection coating is normally applied for pipelines whereas piping systems are painted.
  • The pipeline runs across rivers, below railroads, highways, etc. Hence, special design and constructional considerations are required.

Few more Resources for you..
Piping Design and Layout
Pipeline articles
Piping Materials
Piping Stress Analysis Basics
Piping Stress Analysis

References and Further Studies

Difference between Tee and Barred Tee

Written by Nur Hidayah Haron in Engineering Materials,Pipeline,Piping Design Basics,Piping Interface

Tee or Tee connection in piping engineering is a very important pipe fitting and is frequently used to combine or divide a flow. Two types of Tee are available, Equal Tee and Reducing Tee. However, in pigged pipelines, one special type of tee connection is widely used which is known as Barred Tee or Pigged Tee. In this article, we will try to study a few points about Barred Tee and Tee Connections.

Tee vs Barred Tee
Typical Tee and Barred Tee Connection

Tee vs Barred Tee

Tee is a type of pipe fitting that allows fluid to flow on its main pipe and branch out. The branch can be designed equally the same size as the main pipe (known as Equal Tee), or smaller size than the main pipe (known as Reducing Tee).

Barred Tee is a special type of Tee that is based on a normal tee (can be either an equal tee or a reducing tee) that at a later stage, will be added with bar plates inside the branch outlet (From inside it looks like a steel cage) to restrict the PIG (pipeline PIGGING) from flowing from the header pipe into the branch pipes.

Design Codes and Standards for Tee and Barred Tee

  1. The international standard dimension of the tee will be covered under ASME B16.9 or MSS-SP 75 (for DN16 and above). Click here to know more about Tee Connections.
  2. There is no international standard dimension for the barred tee. It is custom-made using the ASME B16.9/ MSS-SP 75 tee as a base. However, many develop their barred tee based on Shell DEP 31.40.10.13-Gen or ISO 15590-2 standard.
  3. This design can be a guideline to assess Vendor’s design.
Reduced Barred Tee (Reference: Shell DEP 31.40.10.13-Gen Figure 4)
Reduced Barred Tee (Reference: Shell DEP 31.40.10.13-Gen Figure 4)

Design Considerations for Barred Tee

The barred tee will be used when there is a requirement for pigging. Thus, many of its applications can be found in the pipeline or in the subsea field.

The bar plates that are welded internally at the branch are to avoid the pig from changing direction or getting stuck at the branch outlet.

The design of the bar plates must be in sufficient quantity, thickness, and adequately spaced to ensure the smoothness for the pig to run through the main pipe, and at the same time not affect the flow that was meant to flow through the branch. Normal practice is to ensure that the opening in the branch pipeline after guided bars is not more than 40% of the main pipeline area.

The size of the bars in the branch connection has to be small enough not to restrict the flow but large enough to sustain the pressure of the flow.

To ensure smoothness, the bar plates have to be a grind to suit the branch curvature. Any sharp edges spatter, and burs are required to be removed. This smoothness of the pigging process is important to protect the sensor of pig from damage.

Refer to Shell DEP 31.40.10.13-Gen Figure 4 above; the quantity of the bar plates start with two (2) pieces and increase as the ID of the branch increase.

The bar plates ideally will be equally spaced.

For a larger tee (size 14 inches and above), there will be a bridge plate in the middle to support the bar stiffness when getting hit by the pig.

The material of the bar plates is commonly used the same as the tee material for weldability.

Standard practice is to avoid welding guide bars directly on the high-stress concentrated areas of the extrusion neck. Bar ends must be machined to fit the branch.

Weld Repairs on Parent metal are prohibited.

Difference Between Tee and Barred Tee

So from the above discussion, we can summarize the following differences:

ParameterTeeBarred Tee
DefinitionStandard Pipe FittingA special type of piping component
ManufacturingGenerally by Extrusion or forgingMostly Fabricated
UseUsed in both piping and pipeline engineering used in pipeline engineering near the pig launcher/receiver
Design Code / Standard ASME B16.9/MSS SP 75 Shell DEP 31.40.10.13-Gen or ISO 15590-2
Production QuantityLarge scale in bulkSelect small quantities (custom-made)
CostCheaperCostlier than normal Tee
Tee vs Barred Tee

Few more references for you

Piping Design and Layout
Piping Materials
Piping Stress Analysis
Piping Interface

Reference

Stub-in vs Stub-on | Differences between Stub-in and Stub-on Piping Connection

Written by Anup Kumar Dey in Piping Design Basics

Stub-in and Stub-on are methods for making a fabricated branch connection from the pipe. Both types are permitted by many international codes and standards, including ASME B31. However, both of these are weak connections on piping systems and are normally limited only to low-pressure and temperature applications. In this article, let’s explore the differences between Stub-in and Stub-on branch connections.

What is Stub-In?

In the case of a stub-in, a larger hole is drilled in the header or run pipe, and a branch pipe whose end is contoured similar to the inside diameter (ID) of the header is fitted inside the hole. Then both the stub-in branch pipe and the run pipe are welded together to form a connection similar to reducing tee. Stub-in is normally used when the branch is more than one size smaller than the main pipe. For the “stub in” connection, the branch pipe extends to the inside of the main pipe. Stub-in connection is also known as the set-in connection.

What is Stub-On?

On the contrary, In the case of a Stub-On branch connection, the hole that is cut in the run pipe is the same as the inside diameter (ID) of the branch Pipe (Not the Header). The end of the branch pipe connection is contoured the same as the outside diameter (OD) of the header pipe and is then fitted outside the hole on the header pipe. It looks like the branch is seated “onto” the header pipe. Stub-on is generally used when the branch is equal to or one size smaller than the main pipe. For “stub on,” the stub extends only to the outside of the main pipe. A stub-on connection is also known as a set-on connection.

Stub-in and Stub-on Connection
Fabricated Piping Branch Connections

Additional Features for Stub-In and Stub-On

Both stub-in and stub-on branch connections can be made with or without a reinforcing pad as per requirement. This requirement is normally governed by pressure and stress criteria.  The reinforcement pad is basically a ring that is cut from the run pipe or from a plate with the same material as the run pipe. At the center of the pad, a hole is made (the size the same as the branch pipe). When it is cut from a flat plate, it is contoured to fit around the run pipe. The width of the reinforcement pad is normally one-half the diameter of the branch pipe.

The aim of this reinforcement is to substitute the material that was removed for making the branch connection from the header. A small-diameter hole, known as a weep hole (1/4″ NPT) is normally drilled in the pad, which acts as a vent during the welding process for the weld-generated gases to escape. Using full penetration welds, The ring or pad is then welded to the branch and the run pipe. Once, the work is completed, the small hole is fitted using a plug.

Both the stub-in and Stub-on connections, in a sense, reduce the cost of pipe fittings. It saves installation time as well because only one weld is required around the stub hole instead of three welds that are needed for joining welding tee connections.

The welding strength of the Stub-in connection is as good as butt welding but welding steps are difficult in actual conditions. So Stub-in is comparatively stronger than stub-on connections.

Stress Considerations for Stub-in and Stub-on Connections

From the piping stress analysis considerations, the calculated SIF of stub-in and stub-on connections is much higher than weldolet fittings and ASME tees, which is significant when a detailed stress analysis is done on the system. While analyzing, extra caution needs to be considered as the stress generated will be higher. The use of this type of branch connection is not preferred for severe cyclic applications, high-pressure temperature applications, or category M fluid service applications.

Stub-in vs Stub-on Piping Connection | Differences between Stub-in and Stub-on

The main differences between a stub-in and stub-on branch piping connection are tabulated below:

Stub-in pipe ConnectionStub-on branch connection
As explained above, Stub-in is used when the run pipe and branch pipe have a difference of more than one size.On the contrary, a stub-on pipe branch connection is applicable when the branch pipe is equal to the run pipe or only one size lower than the header pipe. 
In the case of the stub-in branch connection, the pipe welding between the branch and header is of butt-weld type.
This welding is quite difficult.		Stub-on branch connections are made using fillet welds. 


Welding is easier as compared to the stub-in welding method.
A stub-in piping connection is able to withstand more pressure.Stub-on branches are comparatively weaker than the stub-in branch connection and hence handle less pressure.
The branch edge of a stub-in connection matches the internal pipe diameter of the header.For stub-on branch connection, the branch edge lies on the outside diameter of the header.
Stub-in pipe connections usually have more weld strength value than sub-on branch connections.Stub-on pipe branches possess comparatively less weld strength value.
Table 1: Stub-in vs Stub-on

Few more Resources for you..

Briefing about Reinforcement Pad
Piping Design and Layout
Piping Materials
Piping Stress Analysis
Piping Interface