Tools For Tradesman

Know your tools is a meant to familiarize you with tools used in the industry. This is not the entire list though it has the core tools listed in it. there are a lot of tools in my tool box that I use in my day to day work, yet still the tools that i use most are my hands, eyes, and ears (if you can call them tools.) If you train yourself to use your eyes, ears, and hands effectively, you will be a much better troubleshooter.

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GLOSSARY

A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z

Temperature Transmitter

To establish a temperature measurement system, electronic temperature instrument needs to be wired to control system . RTD and thermocouple can be directly connected to control system I/O card or through Temperature Transmitter for signal conversion. In most application, temperature transmitter is used to convert RTD/Thermocouple signal and transmit 4-20mA signal, rather than directly wired the RTD or themocouple to I/O card

The choice of using temperature transmitter over direct wiring of RTD or thermocouple to control system provides many advantages:

• The signals generated by Resistance Temperature Detector or Thermocouple are categorized as low-level signal hence it is more susceptible to noise compared to conventional 4-20mA signal generated by transmitter.
• Reduce instrument cable type. It also means less type for cable spare.
• The junction box for temperature transmitter cables can be combined with those from other 4-20 mA transmitters.
• No need to provide special I/O card for thermocouple/RTD. Just connect the Transmitter to the Analog Input I/O card.
• Provided maintenance facilities such as diagnostic feature in smart transmitter.

Instrument Cable Specification

During detailed design project, the cable shall be selected with suitable specification for the  intended application. In the beginning of the project, the specification for each cable part shall be carefully specified to satisfy the requirement of application and environmental condition. Normally the instrument cable consists of conductors, insulation, screen/shield, armor and outer jacket therefore the specification should covers and details each of them. The following information only describes the structure and its example, however the specification for each project may differ and shall be evaluated case by case hence details specified herein is not recommended to be copied and used directly.

Conductor (to transfer the electrical current from higher voltage to lower voltage point)

• Conductor material, commonly used material is copper. Tinned coated copper may also be selected to prevent copper oxides which provides more durable and corrosion resistance
• Design of the conductor, whether solid, stranded, flexible. Stranded conductor means the conductor is not single, instead it consists of several smaller size conductors which allow more flexibility.
• Size of the conductor cross section. Determining the size of the conductor requires information of the device operating voltage and current, length and resistance of the cable as they will contribute to the voltage drop across the cable. Cable shall be sized and ensured that the device at the end of the cable will function properly at a given voltage.

Insulation (to physically and electrically separate each conductors)

• Insulation material

The choice of insulation material is driven by several requirements such as electrical transmission properties, minimum and maximum temperature rating, burning behavior, abrasion and corrosion resistant

• Insulation thickness

The selection of insulation thickness, together with conductor specification, will be determined from the requirement of the voltage rating and cable strength. Note that the insulation thickness also contributes to the the flexibility of the cable.

• Screen (to prevent interference)

Screen is constructed from aluminized polyester film/tape which provide protection from external interference. The aluminum tape is spirally wrapped with 25% overlap to guarantee 100% coverage of cable element including in bending area. Screen also consists of copper drain wire of which shall be electrically in contact with the screen along the cable.

Armor 

(to protect the cable against mechanical stress/load during construction and operation)

The selection of cable armor mainly depends on the cable installation and required mechanical properties such as maximum tensile loads, pressure loads, protection against rodent, minimum bending radius, direct burial installation. There are several types of cable such as galvanized steel wire braid, galvanized steel wire tape and galvanize steel wire round armor which each has advantages over each others.

Outer Sheath / Jacket (to physically protects the internal components of a cable)

Material of outer cable sheath/jacket shall be selected with regards to the following consideration:

• Environmental condition (humidity, temperature, solar radiation)
• Method of installation (indoor, outdoor, direct buried, on trays, etc.
• Possibility of oil, chemical spills or abrasion
• Behavior in fire (low smoke, zero halogen to avoid toxicant)
• Flame retardant or fire resistant
• Sparing philosophy

Determining the number of wire in multipair or multicore cable shall consider future expansion and maintenance.

Other requirement

Twisted pair shall be specified in 4-20mA analog signal cable to reduce the effect of interference. As recommended by API 552 Transmission System, twisted wire shall have minimum of six crossover per foot. Eight crossover per foot is a typical specification

Choosing Valve Actuator Power Source

Valve Actuator, based on the power source, falls into the three categories. Each power source of valve actuator has unique characteristic which provide different advantage to make it suitable for specific application.

Pneumatic Actuator

Pneumatic actuator is the most commonly used type due to the cheapest price and availability of  its power source  i.e. compressed air. The use of natural gas is also applicable if it is not feasible to install air compressor system. It is used both for control valve which operates in throttling mode or actuated valve which intermittently operates.

Hydraulic Actuator

The superiority of hydraulic actuators is its capability of supplying very high torques. Since hydraulic is considered as non-compressible liquid it can provide fast stroking speed. A typical application of hydraulic actuators is for wellhead valves since very high torques and stroking speed become a concern.

Electric Actuator

Electric actuators has smooth operation, provide very high torque values, and retain thrust. In process platform, motor operated valve is used in sea water intake line where large valve which requires high torque is operating in on-off mode.

The selection of valve actuator type by its power source shall consider several issues as follow:

Service in process system

Mode of operation such as on-off or throttling operation. The requirement of valve closing or opening time.

Remote valve location

For pipeline in the middle of nowhere, pneumatic actuated valve with supplementary air or nitrogen bottle which periodically inspected is the only possibility.

Fail-safe requirement

Process requirement of valve to fail open, fail close or fail last. Standard motor operated valve will be fail last when loosing power supply.

Valve operating torque

The actuator torque shall be larger than the required torque by valve in specified operating process and condition. Safety factor shall be taken into account in selecting the size of the actuator.

Tubing and Fitting Design Guide

The following points provide brief guideline in tubing and fitting design for instrument tubing and fittings. The selection preferences may vary between projects.

• Selection of hook up materials shall consider process fluid pressure and pressure rating to ensure good sealing and integrity.  
• Tubing thickness shall be selected based on application pressure rating. Consult with the manufacturer for pressure rating of each tubing thickness.
• Double compression-type tubing fittings are preferred.
• It is absolutely necessary to use a single manufacturer’s type components of tubing and fitting for the whole project.
• Mixing different threaded fittings from different manufacturers is not allowed as it could leads to poor fitting sealing.

Instrument Air Consumption Calculation

Instrument air consumption shall be calculated during detail design to determine the plant utility air requirement by pneumatic operated instrument as well as package which requires air for its utility such as purging or other purpose. Instrument air consumption calculation will be the basis to size instrument air utility system which consists of air compressor, air drier, and air receiver.

Actuated valve such as shutdown valve, blowdown valve and control valve are instruments which require instrument air for their operation. Instrument air requirement of each instrument valves depends on its actuator size and its operation. Shutdown valve and blowdown valve may be considered working intermittently, while control valve  working continuously. It should be noted that the instrument air required by control valve during steady condition is much lesser than during its transient condition.

Assumption

To determine the instrument air consumption, some assumptions should be made (please note that these assumptions are for example only, not as standard reference, and may differ on each project):

Normal air consumption demand

• Control Valve

From total number of control valve, 90% of control valve operates in stable condition hence requires steady state air consumption only, while 10% could be in unstable condition hence requires transient air consumption.

• Shutdown valve / Blowdown valve

Shutdown valve and Blowdown valve only require instrument air when they are operating which is predominantly during start up after shutdown, so it is considered intermittent consumption and one can assume that only some number of valve are working simultaneously. It could be assumed that 10% of the valve will operate simultaneously for normal air demand calculation.

Peak air demand

From total number of control valve, 70% of control valve operates in stable condition hence requires steady state air consumption only, while 30% is in unstable condition hence requires transient air consumption.

• Shutdown valve / Blowdown Valve

Let’s say 100% of the valve will operate simultaneously for normal air demand calculation.

*Note that the number of percentage state here is assumption only and may differs among each projects

• Vendor data

The next step is to gather data from vendor / manufacturer catalog which provides information of instrument air consumption.

The following is data taken from catalog:

• Control valve in stable condition 0.3 scfm
• Control valve in unstable condition 7 scfm

For actuated valve, the instrument air consumption depends on size of actuator (swept volume) which could be obtained from vendor catalog. The required opening or closing time of the main valve will also influence the air consumption rate.

• Calculation

For instrument air consumption calculation, simply multiply the number of valve in the plant with the air consumption data, then sum for all instruments. Do the calculation tabulation for both normal and peak condition. In addition, other package which requires instrument air shall also be included.

Instrument Range Selection

Instrument range shall be selected based on operating range of process to be measured. In common practice, it is recommended to select instrument with full scale range that can give operating process reading between 30% and 70% especially for pressure gauge. The following is an example of selecting pressure gauge range

Given Process data:

Minimum operating pressure:          5 bar

Maximum operating pressure:      9.5 bar

Maximum Design pressure:           25 bar

Available standard Pressure Gauge range: 0 – 10 bar, 0 – 16 bar, 0 – 25 bar

[X] Selecting 0-10 bar will give 95% reading during maximum operating condition. Instrument has only narrow margin above maximum operating pressure.

[X] Selecting 0-25 bar can withstand to 32.5 bar (25 x 1.3) without using overrange protector, however it would give poor measurement indication during normal operating condition.

[√] Selecting 0-16 bar range will give best reading during operating condition. However, the maximum overpressure of pressure gauge is commonly 1.3x of the maximum scale which is only 16×1.3 = 20.8 bar. In such case, pressure gauge shall be equipped with overrange protector to withstand design pressure.

Basic Equation for DP Level Transmitter Range Calculation

Liquid level height could be known by measuring its differential pressure using DP transmitter. Prior installation, DP transmitter shall be pre-set to calibrated range from calculation result. The formula to determine DP transmitter range is simply a Bernoulli principle equation. Having the transmitter set correctly, we can obtain the liquid level height by measuring the differential pressure between two differential points.

Bernoulli equation states:

Since the velocity within a tank is neglected, the equation becomes:

or

P1 + rho. g. h1 = P2 + rho.g.h2 = P3 + rho.g.h3

For any points within a tank, the above total elements shall be constant.

So, the deeper a point within a liquid, the more pressure exerted at that point. In other words, liquid pressure increases with elevation decreases and vice versa. For more clear understanding, see below picture showing their relationship.

Note that above formula shows that liquid density take into account on the calculation hence affects level measurement. The use of DP level transmitter is recommended only for application where liquid density does not vary considerably

Diaphragm Seal

This post tries to describe the use of diaphragm seal since the standard installation of pressure instrument is by using pressure transmitter which is remotely mounted from process taps and connected by impulse line which is commonly a tubing. In some installation, standard installation using tubing is not suitable for the system thus require diaphragm seal in lieu of tubing impulse line.

Compare to tubing impulse line, diaphragm seal costs more thus the use of diaphragm seal must be justified by its functionality against the process requirement where impulse line becomes limitation.

Diaphragm seal provide the following benefits:

• Prevent the process fluid getting clogged in the impulse line due to temperature change that may affect the properties of the process fluid.
• Prevent corrosive fluid contacting with the pressure sensor.
• Prevent slurry or viscous fluid entering the impulse line and the sensor which may be plugging inside.
• In level measurement, the use of diaphragm seal may be placed in lieu of wet leg arrangement which require more maintenance on service where the process fluid inside the wet leg is not stable hence eliminate the needs of refilling.
• For high temperature service, diaphragm seal may reduce more process temperature compare to the use impulse line, hence the transmitter is not exposed to the temperature which above its operating limit.

Hence diaphragm seal should be the choice when it is expected to encounter as such condition.

Zero Elevation

Zero elevation is a condition in which zero value of the measured variable is more than the lower range value. The example of zero elevation is in DP transmitter c/w diaphragm seal and capillary tube installed above the lower process tap in measuring atmospheric tank level, one port is connected to process while the other port is exposed to the atmosphere. In this condition, the fluid in capillary gives negative pressure on the transmitter which cause the DP transmitter provides negative pressure reading at lower range value.

Zero Suppression

Zero suppression is a condition of which zero value of the measured variable is less than the lower range value. Zero suppression occurs in level measurement using DP transmitter installed below the lower process taps.
In this condition, the fluid in the impulse line or capillary gives positive pressure on the transmitter which cause the DP transmitter provide positive pressure reading.

Temperature Element Instrument

There are several temperature element instrument types widely used to measure temperature of a process substance . These types are categorized into mechanical temperature element and electronic temperature element. Mechanical temperature element uses the principle of thermal expansion i.e. the element expands if the temperature rise and vice versa. Electronic temperature element use the principle of electrical properties change following of temperature variation.

Mechanical Based Temperature Element

Solid element

Bimetallic is temperature element instrument formed from two types of metal which have different thermal expansion properties. These two metal are bonded together become one strips. The metal strips will bend if temperature changes. At free end of bimetal, the pointer is attached to indicate the result of temperature measurement.

Gas element

Gas filled system is temperature element instrument based on principle of the ideal gas law. It consist of bulb/stem, capillary and bourdon tube. The sensing element is a rigid bulb or stem containing a gas. If the temperature rises, the volume remain constant since the bulb is rigid, while the gas pressure increase proportionally. The change in gas pressure is measured by a pressure element such as bourdon tube.
The bulb and the bourdon tube is connected by a capillary tube allowing the temperature indacator to be located not directly attached to the piping/vessel being measured. This advantages allows the temperature gauge indicator being mounted on the convenient location whereas the tapping point of process fluid to be measured is not permanently inaccessible.
However the capillary might loss heat therefore the addition of compensation is required to eliminate the error. www.instrumentationportal.com

Liquid element

Liquid temperature element utilizes mercury constrained within a bulb/stem. Mercury  is no longer preferred in most process application although it provides fast response and good accuracy. Mercury mostly used in glass stem thermometer for non-process industry such as body temperature measurement.

Electronic based Temperature Instrument

The most two commonly used electronic temperature element are Resistance Temperature Detector (RTD) and Thermocouple. To indicate the temperature measurement, these elements need to be connected to control system by wired directly or using temperature transmitter.

Resistance Temperature Detector

Resistance Temperature Detector (RTD) works based on principle that resistance of a metal varies with its temperature. RTD is the most commonly used type in process measurement due to its good stability, accuracy, repeatability.

Thermocouple

Thermocouple consist of two dissimilar metals of which the junction generates a voltage proportional to the junction temperature. The thermocouple is selected for service which requires wide range (very high – low temperature). The other advantages of thermocouple over RTD are it has more rugged design  and also provide faster response.

Side Mounted vs Top Mounted Level Instrument

What is the consideration on deciding whether level instrument (transmitter or switch) to be top mounted type or side mounted with external cage type?

Top mounted means the sensor or measuring element of level instrument is inserted into the vessel or tank from the top. Side mounted with external cage means sensor or measuring element is installed integrally in manufacturer prefabricated external cage.

The price of level instrument with external cage is higher than that of top mounted type. This is due to the cost for the cage, moreover for special material such as Duplex Stainless Steel. However, external cage level instrument can have isolation valve installed in between vessel and cage. This arrangement allows instrument maintenance without the need of process shutdown. Therefore external cage level instrument is preferred in most application.

Top mounted level instrument is utilized in some cases such as:

§  Buried vessel, since it has no side access.

§  Semi-submersible tank such as caisson.

§  Liquid which may change properties/form due to temperature change and the use of steam  or electrical heat tracing is not feasible.

The picture shows the arrangement of top mounted level transmitter and side mounted level transmitter. The isolation valve(s) can only be installed on the arrangement of side mounted with .

Instrument Cable Screen Grounding

Why the cable screen should not be grounded at instrument field device?

Cable screen protects the signal carried on the conductor from external interference. Cable screen blocks the external interference and noise, then directs it to the ground. Screen shall only be grounded at one source. 

The grounding is usually made in equipment panel or cabinet while field device point is left ungrounded and insulated. This is to eliminate the ground-loop current that may arise if screen is grounded at both end. In this case, the ground loop current becomes a noise for the signal conductor itself. The following wiring diagram shows ungrounded instrument cable screen at field device.