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Details about Spectacle Blind and Spacers

Written by Anup Kumar Dey in Engineering Materials,Mechanical,Piping Design Basics,Piping Interface

What is a Spectacle blind and Spacer?

Spectacle blinds and Spacers are pressure-retaining devices used in the piping/pipeline industry to temporarily or permanently blind (shut) or isolate part of the piping or pipeline system.

  • A spectacle blind is a forged product and a combination of a flange spade and a ring spacer in one single product.
  • One end of the spectacle blind will have an opening that allows fluid flow through the pipe during operation and the other end is made solid to block flow during isolation or maintenance time.
  • One can simply rotate the spectacle blind by 180 degrees to close or open the fluid flow through the piping system.
  • However, when there is space constraint, Flange spades and ring spacers can be used as separate products.
  • As one of the two discs is always outside the pipe, one can easily understand if the blind is in an open or closed position.

As it looks like the numeric number Eight or a pair of eyeglasses, a Spectacle blind (Fig. 1 & 2) is also known as a figure 8 blind or figure 8 spectacle blind.

Design Code and Standards for Spectacle Blind

ASME B16.48 is the governing code for spectacle blind design. This specification provides the dimensions, pressure-temperature ratings, materials, marking, dimensional tolerances, and testing requirements of the Spectacle blind. In general, Blinds are available in sizes NPS 1/2 – NPS 24, and these are installed between ASME B16.5 piping flanges in the 150, 300, 600, 900, 1500, and 2500 Pressure Classes. They are manufactured from steel plates and are available in forged material grades like ASTM A105 (high-temperature carbon), A350 (low-temperature carbon), and ASTM A182 grades (alloy, stainless steel, duplex).

Bigger size spectacle blinds with a diameter of>24 NPS are also available, but these are custom-made. Spectacle blinds of Carbon steel Material are normally manufactured from a single piece of steel, whereas alloy / stainless steel blinds can be produced by welding more pieces together.

Various spectacle blind standards used in industrial applications are:

  • ANSI/ASME Standard: ANSI B16.5, ANSI B16.47, MSS SP44, ANSI B16.36, ANSI B16.48
  • DIN Standards for Spectacle blind: DIN 2527, DIN 2566, DIN 2573, DIN 2576, DIN 2641, DIN 2642, DIN 2655, DIN 2656, DIN 2627, DIN 2628, DIN 2629, DIN 2631, DIN 2632, DIN 2633, DIN 2634, DIN 2635, DIN 2636, DIN 2637, DIN 2638, DIN 2673
  • BS Standards for Spectacle Bild and Spacer: BS 4504, BS 4504, BS1 560, BS 10

Definition of Terms used with Spectacle Blind

Ring Spacer or Spacer Ring or Paddle Blank or Paddle Spacer

Spacer Rings are made to match the pipe ID with the same thickness as the “single-blind” that it replaces.  During the installation of spacers, the flange and associated piping should be pulled together and a “ring spacer” has to be installed to fill the gap. Normally, a handle is attached to the blank by welding.

Single/ Line Blind or Blank or Spacer Blind or Flange Spade or Paddle Blind

Single-blind is a positive shut-off device that is generally installed along with a valve so as to prevent accidental flow. In general, they fit inside the bolt circle of mating flanges. However, Plastic, Fiberglass, and Cast Iron, are bolted. Single blinds use standard gaskets.

To differentiate between a spacer and blind in the installed line, the handle of the blank is made solid whereas a formed eye or drilled hole is made for the spacer.

Spectacle Blind

A combination of a ring spacer and a single-blind is used as a spectacle blind. As required they are rotated to serve the purpose. When spectacle blinds become too heavy, a pair of paddle blank and paddle spacer is used instead of a single spectacle blind.

Spectacle Blind and Spacer
Fig. 1: Spectacle Blind and Spacer

Test (Hydrotest/Pneumatic Test/Service Test) Blank

During leakage testing, test blanks are used. Test blanks are specifically designed blanks and used only for testing purposes for the test duration.

Vapor Blind

Similar to a “single-blind”, Vapor Blinds are positive sealing devices used for preventing vapor leakage into a pipeline or vessel. Generally, they are thinner compared to single-blind and normally 1/8″ to 5/16″ (3-8mm) thick.

Jack Screw Flanges

For larger-size blinds, Jack Screw Flange is required. Jackscrew help in spreading the flanges and replacing the spacer/blind or turning of spectacle blind.

Use of Spectacle Blind, Spades, and Spacers

As Spectacle Blind, Spades and Spacers belong to the family of pipeline isolation devices, and they are used to provide flexibility during operation and maintenance. Hence, it is required to consider this during the design of the Piping system. Blanks are normally installed in a horizontal line. On average, spectacle blinds are installed on the following

  • At inlets and outlets of rotary and static equipment
  • In bypass line
  • At the unit battery limit
  • Where double isolation is required for high-pressure lines.
Spectacle Blind is installed position
Fig. 2: Spectacle Blind is installed position

Spectacle blinds in Piping Material Specification (PMS)

In the PMS following details need to be clearly specified for spectacle blinds:

Common Spectacle blinds are available in the following facing types

  • Raised Face (for high-pressure applications)
  • Flat face (for low-pressure applications)
  • Male and Female Ring-Joint Blanks (used in conjunction with ring-joint gaskets)

Materials of Spectacle Blind and Spacers

Spectacle blinds and spacers are manufactured from various materials as listed below:

  • Carbon Steel Spectacle Blind: ASTM A105.
  • Stainless Steel: ASTM A182 F304, F304L, F304H, F316, F316L, F316Ti, F310, F310S, F321, F321H, F317, F347, F347H, F904L.
  • Alloy Steel: ASTM A182 F1, F11, F22, F5, F9, F91.
  • Duplex Stainless Steel Spectacle blind: UNS S31803, UNS S32750
  • Nickel Alloys: Monel 400 & 500, Inconel 600 & 625, Incoloy 800, 825, Hastelloy C22, C276.
  • Copper Alloys: Copper, Brass & Gunmetal.

Symbols of Spectacle blinds

The following spectacle blind symbol is used in P&ID to denote spectacle blind.

Spectacle Blind Symbol
Fig. 4: Spectacle blind Symbol

Difference between Spectacle Blind and Spade and Spacers

Spade and Spacer combinedly are comparable to Spectacle Blind. So, a spectacle blind is the combined form of a spade and spacer. For larger pipe sizes the weight of the spectacle blind becomes too heavy. So sometimes, as the requirement arises, a spade or spacer is used. The word “SPADE” or “SPACER” are stamped on the handle of the spade or spacer to clearly indicate what is installed in the line.

In the installed position the handles of the spade and spacers are visible from the outside. The common handle configurations are provided below in Fig. 5

Spade handle Configurations
Fig. 5: Spade handle Configurations

Few important Considerations for Spectacle blinds

  • Spectacle blinds must be accessible from grade, platform, or if below 4500 mm by a portable ladder, or temporary scaffolding.
  • The proper weight of the Spectacle blind needs to be considered during pipe stress analysis when near the equipment nozzles.

Few more Resources for you…

“Pipe Coupling”-A short Introduction for the piping professionals
A Literature on Piping Nipples for piping and plumbing industry
Piping Elbows and Bends: A useful detailed literature for piping engineers
Reducers used in Piping Industry: A short literature
Tee Connection: A short literature for piping engineers
Difference between Stub-in and Stub-on Piping Connection

What are Insulating Gaskets? Their Types, Components, Applications, Installation

Written by Anup Kumar Dey in Engineering Materials,Mechanical,Piping Design Basics,Piping Interface

What is Flange Insulation Gasket Kit?

The most common problematic area for a Piping/Pipeline System design is flanges. So we have to be doubly sure to seal it properly to prevent flange leakage. At the same time, It must also be insulated cathodically so that stray currents which are known behind the undue corrosion and eventual metallic breakdown be prevented. Insulating gasket sets or Flange insulation kits play an important role in such cases.

  • They are designed for electrically insulating the flanges.
  • They act as an insulator between dissimilar metallic flanges.
  • Also, they are used to isolate sections of pipework (dissimilar metals) electrically in the cathodic protection systems; thus preventing the electrostatic charge flow along the pipelines as they remove the possibility of galvanic cell system creation.

Major Applications of Insulating Gasket Kit

Insulating gasket kits can be used in all such places where galvanic corrosion protection and electrical insulation are needed. Common Application areas are piping/pipeline systems with seawater environments, offshore installations, chemical installations, oil refinery pipelines, etc.

Components of Flange Insulation Gasket Kit

Each flange insulation Kit (Refer to Fig. 1) constitutes of

  • one central flat or oval section gasket,
  • one insulation sleeve per bolt and
  • two insulating steel washers per bolt and
  • two plated steel washers per bolt.

All these components are produced using special materials possessing chemical stability, dielectric properties, and low water absorption so that the purpose is solved. The full sets are packed individually and labeled clearly mentioning the flange size, rating, type, and material combination. Insulation kits are generally available upto 24 inches, but custom-made higher nominal sizes can be produced on the requirement.

Insulating Gasket Kit
Fig. 1: Components of Flange Insulating gasket Kit

Assembling of the flange insulation gasket kit components

The polyethylene insulating sleeve and the mounting stud are the main components of the insulating gasket kit. Contrary to the conventional nut and bolt system of flanged joints, the insulation kit uses a stud with a washer and nut on each end of the flange joint. The insulating sleeve is then inserted into the bolt holes using one insulator on each side of the flange. So, Two insulators per bolt hole are inserted into the flange. Then the mounting stud is inserted into the insulating sleeves until threads on each end of the stud are visible outside of the flange. A flat steel washer is slid onto the stud at each end, and a nut is threaded onto each end of the stud and tightened.

As a nut is tightened on each end of the stud, the flange insulation kit becomes much stronger than the typical nut-and-bolt type connection. In the traditional manner of nut-bolt tightening, all of the clamping force is applied to the single set of threads on the bolt while in the case of a double-ended stud such as in a flange insulation kit, the clamping force is divided equally between the threads on each end of the stud as well as inside both nuts. The stud is protected and insulated from any electric charge and corrosion. This creates a flanged connection that will not corrode or fail due to a chemical reaction between two dissimilar metals.

Types of Flange Insulating Gaskets

Four major types of gasket isolation kits are available in the market those are designated as F, E, D, & O. Each type of flange kit fits flanges with a specific type of flange face (RF, FF, RTJ).

F Type Gasket

Type F type gaskets are produced to fit the raised face flanges. The inside diameter of the bolt-hole circle is made slightly smaller than the external diameter of the gasket which assures automatic positioning of the gasket. To avoid the accumulation of foreign materials, Special band protectors can be used for the joint assembly to strengthen the cathodic isolation further.  These are manufactured from 1/8″ thick fabric-based phenolic sheets either without coating or a with a nitrile rubber coating on the two faces. Type F gaskets are also available in non-asbestos fibers with high isolation capacity.

E Type gasket

In full-face type, Type “E” Gaskets, the outside diameter of the gasket matches the external diameter of the flanges. So the gasket completely covers the whole flange surface.  There are produced with precision-located bolt holes and are easy to position in the center. This design will prevent foreign material accumulation between the flange faces and thus enhances the isolation properties of the kit. Type E flange insulation kits are available in phenolic, neoprene-faced phenolic, and high-temperature materials.

D Type Gasket

Type “D” Gaskets are manufactured to fit into the ring groove of RTJ flanges (style R, RX, and BX). They are available in medium weave reinforced phenolic and other materials are sized to ANSI specifications available in a basic oval as well as an octagonal shape. This type of gasket is known as an “API ring joint”.

O Type gasket

Type O Gaskets are extra special as they feature an additional sealing element on both sides of the device, generally, PTFE, Nitrile, or Viton Rubber, and are available in both ring and full-face designs (type E and F).

Insulating gasket Material

  • All materials should be selected to assure long-term effective sealing and electrical insulation of the flanged joints.
  • To take advantage of the best characteristics, sometimes, gaskets are made with combinations of materials.

Normal Gasket materials are G3, G7, Nitrile-Face, Plain Phenolic, G10, Neoprene-faced Phenolic, Klinger, Teflon, Durlon, etc. Popular Sleeve materials are Phenolic, Mylar, Polyethylene, Minlon, or Nomex.  Common washer materials are Phenolic, Steel, G7, G3,  or G10 available as a single washer and sleeve kit, a one-piece washer kit, a double washer kit, or a one-piece double washer kit (Minlon)

Sleeves and Washers of Flange Insulation Gasket Kits

One-piece sleeves and washers

One-piece sleeves and washers are available molded from acetal resin. They should generally be used as single washer sets since they are molded to specific lengths and are in many instances longer than the thickness of a single flange. They are available for bolt diameters of ½” to 1-1/2″ (12.7 mm to 38.1 mm), and are limited to applications where the flange temperature does not exceed +180^oF (+80^oC).

A special feature is a washer centering ring that positions the steel washer on the unit properly to avoid uneven pressures on the washers.

Insulating Washers

Standard insulating washers are made of high-strength phenolic and provide tough, positive insulation at temperatures of +3000F ( +1500C). They are available for bolt sizes from ½” (12.7 mm) through 3 ½” (88.9 mm) and are made to fit over the insulating sleeves. Fiberglass reinforced high temperature phenolic insulating washers are available on special order in the same sizes as the standard insulating washers

Insulating Sleeves

Insulating sleeves are available in Mylar, polyethylene, phenolic, and Nomex. They are through 3-1/2″ (88.9 mm) and are designed to fit easily over the flange bolts, and may be used with standard-sized bolts and bolt holes even with some misalignment. Insulating sleeves are used with separate insulating washers and have a standard wall thickness of 1/32″ (0.79 mm). They are available for standard American bolt sizes from ½” (12.7 mm) to 3 ½” (88.9 mm), as well as metric bolt sizes from 12 mm and larger.

Steel Washers

Steel washers are designed to fit over the insulating sleeve or retainer ring on the one-piece sleeves and washers. The outside diameter is sized to fit within the bolt facing on ANSI standard flanges. They are of 1/8″ (3.2 mm) thick plated hot-rolled steel. Pyrox insulating sleeves and washers are also available.

Flange Insulation Gasket Kit Installation procedure

The Gasket ID (inside diameter) is normally made slightly smaller than the flange ID. While installing ensure that the gasket is properly centered over the bore. It will prevent the build-up of foreign material between the flange faces during pigging.  

Single washer insulating sets are recommended for underground flanges. This should be installed on the unprotected side of the flange, which will provide cathodic protection for the studs as well as the nuts. For above-ground flanges, It is suggested to use Double washers for ease in testing.  

The use of alignment pins is always suggested which will ensure proper alignment of flanges and gaskets. The size of each Pin shall be a minimum of 3/32″ (2.38 mm) larger than the bolt.  

A diagram showing a recommended bolt tightening sequence is normally supplied with each insulating gasket set.

Common Vendors for Insulating Gasket Kits

Common vendors who manufacture and supply insulating gaskets are

  • PSI
  • Pikotek
  • Central Plastics
  • Garlock
  • Trojan
  • Lemons
  • Advanced Products and Systems

Information needed for Ordering Flange Insulating Kits

To order an insulating gasket kit, the following information needs to be provided:

  • Flange Specification (ANSI/ASME, DIN, API, MSS SP44, BSI, AWWA Standard)
  • Nominal Pipe Size, Pressure Rating, and Bore Size
  • Operating Pressure, Temperature, and Media
  • Gasket type (D/E/F/O)
  • Required Seal Material
  • Isolating Sleeve Material
  • Isolating Washer Material
  • Metal Washer Material
  • Quantity

Few more Resources for you…
Methods for Checking Flange Leakage
Guidelines on selection of various types of Flanges
Few points on Gaskets for leak Proof Flanged joints
PROCEDURE FOR FLANGE-BOLT TIGHTENING OF VARIOUS SIZES OF FLANGES
Functions of Gaskets for leak-proof Flanged joints

Reference

Guidelines for Modeling and Supporting of Large Diameter Pipes / Pipelines

Written by Anup Kumar Dey in Piping Supports

During the course of pipe stress analysis, we find a few lines in any complex project to have a very large diameter. Caesar II Support Modeling of such pipes always creates confusion on whether to consider radial thermal growth or to be modeled as centreline supporting. There is still confusion among several engineering organizations and the design approach varies. In this article, We will discuss pipe support modeling in Caesar-II, support selection, its detailing & functionality. In this article, Pipes /Pipelines having a diameter of more than 24’’ are considered large-diameter pipes.

Support Modeling Philosophy in Caesar II

Up to 24’’ pipe size, support shall be modeled at the centerline of the pipe i.e. the radial expansion of the pipe shall be neglected & support shall be assumed to be acting at the center of the pipe as shown in Fig. 1

For the Large diameter bare pipes (>26″ NB), the Stress Engineer shall model a rigid element from the center of the pipe to the bottom of the pipe up to the supporting point considering the pipe radial expansion as shown in Fig. 1.

Pipe Support Modeling for bare pipes at Caesar II
Fig. 1: Pipe Support Modeling for bare pipes at Caesar II

For Large diameter insulated pipes, the temperature gradient and the actual point of action of guide and resting can also be fed as shown in Fig. 2.

Pipe Support modeling for large diameter insulated pipes at Caesar II
Fig. 2: Pipe Support modeling for large-diameter insulated pipes at Caesar II

The stress engineer shall decide whether to provide the reinforcing pad or not at the support point (trunnion type) in the following cases:-

  • If the slope of the piping is more than 15˚, the Stress engineer shall decide whether to use a reinforcement pad or not, depending upon pipe size, support load, support function (Line Stop/Guide), unsupported pipe span, etc. Refer to Fig. 3 for an illustration.
Use of R.F. Pad for Sloped Lines > 15˚
Fig. 3: Use of R.F. Pad for Sloped Lines > 15˚

The reinforcement pad at the Directional Anchors or line stops for large pipes shall be used as shown in Fig. 4 below.

Requirement of RF Pad for Supporting large diameter pipe
Fig. 4: Requirement of RF Pad for Supporting large diameter pipe
  • Irrespective of any size, the stress engineer shall provide a reinforcement pad for higher loads on the support based on the trunnion check calculation.
  • RF pad shall be provided at all support for thin pipes. The pipe shall be considered a thin-walled pipe if  D/T> 96, where D is the outer diameter of the pipe and t is its thickness.
  • Wear Pad shall be used at all support locations in case of uninsulated large pipes

For large-diameter pipes, two or more bottom-type springs may be used as shown in Fig. 5, Fig. 6, and Fig. 7.

Illustration of Two Bottom Springs used for Large pipes
Fig. 5: Illustration of Two Bottom Springs used for Large pipes (Note: Provide stiffener plate if the shoe base plate extension is large)
Illustrating Four Bottom Type Springs used at one Supporting Location with guide for Large Pipes
Fig. 6: Illustrating Four Bottom Type Springs used at one Supporting Location with a guide for Large Pipes
Illustration of Four Bottom Type Springs used for Large pipes
Fig. 7: Illustration of Four Bottom Type Springs used for Large pipes

For large diameter pipes, it’s better to avoid single lug support due to the tendency of ovalization of the pipe because of Self-weight. Supporting shall be done using a pipe clamp or two trunnions having two different clamps instead of a single lug as shown in Fig.8.

Use of clamps for supporting Large Pipes
Fig. 8: Use of clamps for supporting Large Pipes

Approach for Caesar Modeling Vs Actual Supporting

Generally, the following points are to be taken care of while converting Caesar II modeling into practical support. Depending upon supporting and practical function of the support we should revisit our Caesar II support modeling.

Supporting for guide modeled with zero gaps

Guide modeling & supporting for Large Pipes
Fig. 9: Guide modeling & supporting for Large Pipes

Supporting a guide with resting

Modeling & supporting of Guide with resting for Large Pipes
Fig. 10: Modeling & supporting of Guide with resting for Large Pipes

These are a few standards practices. These may vary from one consultancy to another. Please provide your input in the comments section.

Few more articles related to piping supports for you..

Supporting of Piping Systems: Few Guidelines
A Brief Description of Sway Brace, Strut and Snubber (Dynamic Restraints) for pipe supporting for process industries
Co-Efficient of Friction for pipe supporting during Stress Analysis using Caesar II
Supporting of Dual Insulated Piping System
Purpose of Pipe Supports
Pipe Support Span for Aboveground Piping

Online Course on Pipe Support Engineering

If you want to learn more details about pipe support engineering then the following online course is a must for you:

Recorded Webinar on Expansion Joints in CAESAR II

Written by Anup Kumar Dey in Caesar II,Piping Stress Analysis,Piping Stress Basics

Expansion Joints are one of the critical devices that piping stress engineers use in piping systems having less flexibility. But it becomes quite a headache to choose the right kind of expansion bellow and then model it exactly considering all real configurations. Whether to choose a tied one? or whether to consider the thrust force? Such questions always arise in our mind and most of the time remain unanswered simply because there are very little literature and description available for modeling and analysis of systems considering expansion joints.

So now we all have a chance to learn from the experts and clarify our doubts. Thanks to the Caesar II owner company, Hexagon PPM who has arranged a webinar on 25th February 2020 at 10:00 AM CST as per the below-mentioned details.

Expansion Joints in Caesar II
Expansion Joints in Caesar II

Webinar Details

Date and time: February 25th, 2020 at 10.00 AM CST (Please convert to your local time and mark it on the calendar else you may miss it)

Main Focus Points of the Webinar

This webinar will briefly throw light on the following points:

  • Various ways of expansion joint inclusion in the piping system.
  • Different types of expansion joint assemblies are available.
  • Selecting the appropriate expansion joint assembly.
  • Using the Caesar II Expansion Joint Modeler.
  • Zero-Length expansion joint.
  • Complex, Detailed Expansion bellow Model
  • Evaluating the joint

About the Presenter

David Diehl, P.E.: With more than 30 years of engineering and technical support experience is the Director of Training for CADWorx & Analysis Solutions – Hexagon PPM. He is the lead instructor for CAESAR II and the principal author of the CAESAR II online training course. He also served as a Director for the Society of Piping Engineers and Designers (SPED) for 16 years and currently, he is Chair of the B31.3 Process Piping Committee. 

How to Register

To register and view this webinar simply click here and submit your details to book your seat.

Few more Resources for you…

Design Considerations for a Piping System with an Expansion Bellow
Piping Stress Analysis using Caesar II
Piping Stress Analysis Basics
Piping Stress Analysis using Start-Prof

Injection Point and Mixing Points for Process Streams

Written by Baher Elsheikh in Mechanical,Piping Design Basics

Injection and mixing points are locations used frequently in refineries, petrochemical, and power plants and carry a potential risk of increased degradation rate compared to the mainline due to the changes in temperature, pH, phase changes, and the concentration of corrosive species. Because of that, dedicated publications and literature addressed the requirements for the design and inspection of the mixing areas

Injection and Mixing Point
Figure 1: Injection points

What is an Injection Point?

Injection Point: Injection Points are locations where chemicals, or process additives, are introduced into a process stream. Corrosion inhibitors, desalter demulsifiers, neutralizers, process antifoulants, oxygen, hydrogen scavengers, caustic, and water washes are most often considered to be requiring special attention during injection point design. [1]

What is a Mixing Point?

Mix Points: Mix points or Mixing Points are points of joining of process streams of differing composition and/or temperature where additional design attention, operating limits, and/or process monitoring are utilized to avoid damage mechanisms (e.g., corrosion).

Design Precautions of Injection Devices

Injection of corrosive chemicals like caustic, and sulfuric acid can cause serious corrosion issues and it is best to be avoided, injection facilities should be designed to allow proper mixing and dilution of the injected chemical in order to avoid the concentration of chemicals on hot metal surfaces. [4]

Static Mixer
Figure 2: Static Mixer
Injection Quills
Figure 3: Injection Quills

Material Selection for Mixing Zone

While selecting materials for the mixing zone, consideration shall be made for local erosion, corrosion-erosion, and corrosion rates influenced by flow regime and turbulence. High corrosion-resistant alloys or internally lined injection facilities can be used based on the injected chemical, mixing facility configuration, and operating temperature.  

Each lined pipe and fitting shall be provided with a venting system that will release any annular pressure between the polymer liner and the host metallic component.

NOTE —Venting is not required with PVDF, PP, ETFE, or PVDC liners.

PTFE Lined spool with vent holes
Figure-4: PTFE Lined spool with vent holes

Injection devices

Injection device designs should include the design of piping tees, spray nozzles, quills, and/or static mixers.

Design Considerations for Quills

  • Quills should discharge co-currently into the center of the receiving stream to enhance dispersion and prevent contact of undiluted injectant with the pipe wall.
  • Quill designs shall include flow calculations to determine the natural frequency and whether fatigue is an issue.
  • Quills with beveled ends should be considered, as they promote better dispersion and mitigate potential corrosion or fouling caused by concentrating chemical injectant at the pressure boundary.

Considerations for Spray Nozzles

  • Spray nozzles should be considered for injecting liquids into gas streams or when the injectant is required to coat/wet the pipe wall. Spray nozzles are used to disperse the injectant stream.
  • The co-current injection is suggested when wetting is the desired result.
  • Cross-flow injection should be avoided because it results in impingement on the pressure boundary wall. Impingement can also occur when the injection is too close to a change in direction.
Injection Nozzle Discharge Orientation
Figure-5: Injection Nozzle Discharge Orientation
Injection Devices
Figure 6: Injection Devices

Injection into Vessels or Tanks

For injection into pressure vessels or tanks, designs that tend to concentrate corrosive media in a small area should be avoided. For example, tank inlets should be designed in such a way that the concentrated solutions are mixed and diluted when they are introduced. Otherwise, excessive corrosion can be caused by localized pockets of concentrated solutions. The poor design causes concentration and uneven mixing of incoming chemicals along the vessel wall (circled areas). Good design allows concentrated solutions to mix away from vessel walls. [3]

Injection to Vessels or Tanks
Figure-7: Injection to Vessels or Tanks

Temperature Difference  

High-temperature differences between the injected stream can the mainstream can cause thermal shock, this shall be considered in the design of the injection nozzle to avoid direct contact of the injected stream with the metallic wall of the mainstream. Another approach can be by changing the temperature of the injected stream (heating or cooling) to approach the temperature of the mainstream.

Operational Considerations

Plant operators should understand the potential risk of the improper use of the injection points. For the operating schemes where the flow of the mainstream is not continuous, the chemical injection shall be stopped while the mainstream is flowing, then the mainstream can be stopped. And for starting chemical injection, the operator shall ensure that the mainstream is flowing before starting the chemical injection.

This can be best achieved by design through the application of interlock. Otherwise, operating procedures with a checklist shall be developed with proper protocol to avoid mistakes.   

Inspection Considerations

Injection points are sometimes subject to accelerated or localized corrosion. Those susceptible should be treated as separate inspection circuits, and need to be inspected thoroughly on a regular schedule. [2]

At the time of scheduled periodic inspections, extensive inspection should be applied to an area beginning 300 mm upstream of the injection nozzle and continuing for at least 10 pipe diameters downstream of the injection point. [2]

Typical CML locations
Figure-8: Typical CML locations

In the case of the use of plastic-lined pipes equipped with vent holes, in the event of a liner failure the steel housing is likely to corrode evenly within the pipe and cause a catastrophic and unannounced failure. Vent holes shall not be Plugged. If the line is insulated, extend the vent out of the cladding and include the check of leaks through the vent holes in the periodical inspection and patrolling checklist.

References

  • [1] NACE SP0114-2014: Refinery Injection and Process Mix Points
  • [2] API 570: Piping Inspection Code: In-service Inspection, Rating, Repair, and Alteration of Piping Systems
  • [3] ASM Metals Handbook, Volume 13 – Corrosion
  • [4] API 571, Damage Mechanisms Affecting Fixed Equipment in the Refining Industry
  • [5] [ASTM F1545 Standard Specification for Plastic-Lined Ferrous Metal Pipe, Fittings, and Flanges]

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