Instrument Engineer Test 1.pdf

8/14/2019 Instrument Engineer Test 1.pdf

http://slidepdf.com/reader/full/instrument-engineer-test-1pdf 1/22

Q1 : State the meaning of “Fail-Safe Design” and “Fool-Proof Design”.Fail-safe – when it fails, the failure will not cause any damage (or minimum

damage) to others.

Fool-proof – it will not have any failure upon human mistake.

Reference for answer:~ The term fail-safe is used to describe:

• A device which, if (or when) it fails, fails in a way that will cause no harm or at

least a minimum of harm to other devices or danger to personnel. Examples

include:o Toughened glass (tempered safety glass) used in modern automobile side

and rear windows, designed to shatter into small pieces rather than in the

long jagged shards created when common window glass breaks.

o Luggage carts in airports in which the hand-brake must be held down at all

times. If it is released, the cart will stop. See dead man's switch.

o

Air brakes on railway trains and trucks. The brakes are held in the 'off'position by air pressure created in the brake system. Should a brake line

split, or a carriage become de-coupled, the air pressure will be lost and thebrakes applied. It is impossible for the train to be driven with a leak in the

brake system.

o Avionics using redundant systems to perform the same computation with

voting logic to determine the "safe" result.

o Motorized gates - In case of power outage the gate can be pushed open byhand with no crank or key required. See also fail-secure.

• An operation which ensures that a failure of equipment, process, or system does

not propagate beyond the immediate environs of the failing entity.

• The automatic protection of programs and/or processing systems when a hardware or software failure is detected in a computer system. See fail-safe (computer).

• A control operation or function that prevents improper system functioning or

catastrophic degradation in the event of circuit malfunction or operator error.

• A system which has been structured such that it cannot fail (or that the probability

of such failure is extremely low) to accomplish its assigned mission regardless of

environmental factors. Examples include:

o The hardening of a nuclear missile bunker or the dispersion of nuclear

bombers to multiple secret locations

~ fail-safe (f l'sf')

adj.

1. Capable of compensating automatically and safely for a failure, as of a

mechanism or power source.

fail safe

Same as fault tolerant.



fault tolerance (fölt täl··rns)

(systems engineering) The capability of a system to perform in accordance with designspecifications even when undesired changes in the internal structure or external

environment occur.

fool·proof (f l'prf')

adj.

1. Designed so as to be impervious to human incompetence, error, or misuse: a

foolproof detonator; a foolproof safety lock.

2. Effective; infallible: a foolproof scheme.

The adjective foolproof has one meaning:

Meaning #1: not liable to failure

The verb foolproof has one meaning:

Meaning #1: proof against human misuse, error, etc.



Q2 : State the meaning of “MTBF” and “MTTR”.MTBF – Mean time between failure

MTTR – Mean time to recovery

Reference for answer:

~ Mean-time-between-failure (MTBF) is the "average" time between failures, thereciprocal of the failure rate in the special case when failure rate is constant. Calculations

of MTBF assume that a system is "renewed", i.e. fixed, after each failure, and thenreturned to service immediately after failure. The average time between failure and being

returned to service is termed Mean Down Time (MDT).

A common misconception about the MTBF is that it specifies the time (on average) when

the likelihood of failure equals the likelihood of not having a failure. This is only true for

certain symmetric distributions. In many cases, such as the (non-symmetric) exponentialdistribution, this is not true. In particular, for an exponential failure distribution, the

probability that an item will fail by the MTBF is approximately 0.63. For typical

distributions with some variance, MTBF only represents a top-level statistic, thus is notsuitable for predicting detailed time of failure, as the uncertainty in actual failure

distribution manifests itself in variability in the time to failure distribution.

~ Mean time to recovery (MTTR) is the average time that a device will take to recover

from a non-terminal failure. Examples of such devices range from self-resetting fuses

(where the MTTR would be very short, probably seconds), up to whole systems whichhave to be replaced.

The MTTR would usually be part of a maintenance contract, where the user would paymore for a system whose MTTR was 24 hours, than for one of, say, 7 days. This does not

mean the supplier is guaranteeing to have the system up and running again within 24hours (or 7 days) of being notified of the failure. It does mean the average repair time willtend towards 24 hours (or 7 days). A more useful maintenance contract measure is the

maximum time to recovery which can be easily measured and the supplier held

accountable.

Note that some suppliers will interpret MTTR to mean 'mean time to respond', and others

will take it to mean 'mean time to replace/repair/recover/resolve'. The former indicates

that the supplier will acknowledge a problem and initiate mitigation within a certaintimeframe. Some systems may have a MTTR of zero, which means that they have

redundant components which can take over the instant the primary one fails, see RAID

for example. That said however, the failed device involved in this redundantconfiguration still needs to be returned to service and hence the device itself has a non-

zero MTTR even if the system as a whole (through redundancy) has a MTTR of zero.



Q3 : List more than 5 types of flow instruments.Thermal Mass

Coriolis Mass

RotameterPositive Displacement

Vortex sheddingMultiphaseTurbine


~ Flow measurement can be described by

Q = A · v, which means that the volume of fluid passing through a flowmeter is equal to

the cross-sectional area of the pipe (A) times the average velocity of the fluid (v); and

W = r · Q, which means that the mass flow of fluid passing through a flowmeter (A) isequal to the fluid density (r) times the volume of the fluid (Q).

Volumetric flowmeters directly measure the volume of fluid (Q) passing through the

flowmeter. The only flowmeter technology that measures volume directly is the positivedisplacement flowmeter.

Velocity flowmeters utilize techniques that measure the velocity (v) of the flowing stream

to determine the volumetric flow. Examples of flowmeter technologies that measurevelocity include magnetic, turbine, ultrasonic, and vortex shedding and fluidic

flowmeters.

Mass flowmeters utilize techniques that measure the mass flow (W) of the flowing

stream. Examples of flowmeter technologies that measure mass flow include Coriolis

mass and thermal flowmeters.

Inferential flowmeters do not measure volume, velocity or mass, but rather measure flow

by inferring its value from other measured parameters. Examples of flowmeter

technologies that measure inferentially include differential pressure, target and variablearea flowmeters.

Flow computers are often used to compensate flow measurements for actual processconditions, such as pressure, temperature, viscosity, and composition.

Additional flowmeter technologies include flowmeters that measure liquid flowing in anopen channel, and insertion flowmeters that measure flow at one location in a pipe and

use this measurement to infer the flow in the entire pipe. Insertion flow measurement

systems often use a flow computer to compensate for hydraulic effects.



Q4 : List 3 types of flow instruments that no straight run is required.Coriolis Mass Flowmeter

Rotameter

Positive Displacement FlowmetersDifferential Pressure Flowmeter (V-cone style only)


~ Coriolis Mass Flowmeter - This meter uses the Coriolis effect to measure the amountof mass moving through the element. The substance to be measured runs through a U-

shaped tube that is caused to vibrate in a perpendicular direction to the flow. Fluid forces

running through the tube interact with the vibration, causing it to twist. The greater theangle of the twist, the greater the flow

~ ROTAMETERs™ (Variable Area Meters) are named after ROTA, one of the European

inventors of this flow principle in the beginning of the century. ROTA invented therotating float, which is self-guiding and has less friction in the pipe so that a more precise

measurement is possible.

~ Positive Displacement Flowmeters

These meters are used for water applications when no straight pipe is available and

turbine meters and paddlewheel sensor would see too much turbulence. The positivedisplacement are also used for viscous liquids.

~ With its unique ability to self-condition flow, the V-Cone Flow Meter is a real space-

saver, eliminating the need for up/down stream straight pipe runs required by other DPtechnologies, such as orifice plates and venturi tubes.



Q5 : List process data required for sizing the D/P type flow meter for gas service.

Gas/Vapor Pressure, Flowrate, Gas density, Temperature


~ 3.2 General Calculations

~ 3.3 Calculations for Liquids



~ 3.4 Calculations for Compressible Fluids (gases and vapors)



Q6 : Choose correct instrument type that are able to measure the interface liquidlevel

Radar Type (yes), D/p Type (yes), Float Type (yes), Ultrasonic Type (yes), Displacer

Type (yes)


~ When detecting the interface between two liquids, such as the hydrocarbon/brineinterface in a salt dome storage well, the transducer is lowered down to the bottom of the

well. The ultrasonic pulse is sent up through the heavy brine layer to the interface. The

time it takes for the echo to return is an indication of the location of the interface (Figure9-4C).

~ Contact radar gages send a pulse down a wire to the vapor-liquid interface. There, a

sudden change in the dielectric constant causes the signal to be partially reflected. The

time-of-flight is then measured (Figure 9-2B).





Capacitance probes comprise a long metallic probe, which normally enters the top of the

separator vessel and extends to its lowest point.

Liquid level and interface are detected by measuring the capacitance value between thewall of the vessel holding the liquid and the probe itself.

Again, the aggressive nature of most chemical and petrochemical applications willrequire the use of exotic materials, adding to the cost of the installation.

Another complication associated with this technology is the measurement of stickysubstances, which can coat the metal, resulting in measurement uncertainties and poor

readings.

Other factors such as foam on the liquid surface or vibration of the tank can also conspire

to reduce measurement certainty or even render the probe inoperable.

Remote seal differential pressure transmitters probably offer the best solution for themeasurement of liquid interface levels.

With this technique, when the distance between the taps on the separation stack is filled

only with the lighter liquid, the differential pressure is minimum value or the lowestrange value (LRV) of the transmitter.

When it is filled with the heavier liquid, the differential pressure is at its maximum value,

or the upper range value (URV) of the transmitter.

Although this technique overcomes many of the problems associated with the previously

described methods, particularly with respect to corrosion, it does have one maindrawback.

The small difference in both the specific gravity of the two liquids and the distance

between the taps on the separation stack results in a very small differential pressure span.

In many cases, the size of this span is often lower than the recommended minimum span

for most remote seal transmitters.

One way of overcoming this problem is to use remote seals and transmitters which are

sensitive enough to detect very low span changes.



Q7 : Pressure Gauge installed in steam service shall have aWater column


~ Article 66. Pressure GaugesA pressure gauge shall be attached to the steam space or water column or to a steam-side

connection leading to such water column of any steam boiler in accordance with thefollowings:

(1) The direct influx of steam into the pressure gauge shall be precluded.

(2) Any valve or cock shall be so designed as to clearly show whether it is open orclosed.

(3) The steam connection for the pressure gauge shall have such construction that is not

easily clogged.

(4) The highest degree of the scale on any pressure gauge shall be in a range not lowerthan 1.5 times but not higher than 3 times the maximum allowable working pressure of

the boiler(5) The diameter of the face of a pressure gauge shall be such that ensures easy reading.



Q8 : Impulse piping in liquid service for pressure and flow instruments shall beoriented and sloped (downward) so that it remain full of liquid.


~ Application Cautions for Differential Pressure Flowmeters Because of the nonlinearrelationship between flow and differential pressure, the accuracy of flow measurement in

the lower portion of the flow range can be degraded. Plugging of the impulse piping canbe a concern for many services. For slurry service, purges should be used to keep the

impulse piping from plugging. For liquid service, impulse piping should be oriented and

sloped so that it remains full of liquid and does not collect gas. For gas service, impulsepiping should be oriented and sloped so that it remains full of gas and does not collect

liquids. In vapor service, vapor may be allowed to condense in some of the impulse

piping to form a liquid seal between the hot vapor and transmitter in order to protect the

transmitter from heat.

~ The following restrictions and recommendations apply to impulse piping location:1. Impulse piping that runs horizontally must slope at least one inch per foot (83 mm/m).• Slope downward (toward the electronics) for liquid and steam applications• Slope upward (toward the electronics) for gas applications.2. For applications with temperature below 250 °F (121 °C), impulse piping should be asshort as possible to minimize temperature changes. Insulation may be required.3. For applications above 250 °F (121 °C), impulse piping should have a minimum length ofone foot (0.3048 m) for every 100 °F (38°C) temperature increase over 250 °F (121 °C).Impulse piping must be non-insulated to reduce fluid temperature. Any threadedconnections should be checked after the system reaches the intended temperaturebecause connections may come loose with contraction and expansion caused bytemperature change.4. Outdoor installations for liquid, saturated gas, or steam may require insulation and heattracing to prevent freezing.

5. When impulse piping is longer than six feet (1.8 m) the high and low impulse lines mustbe positioned together to maintain equal temperature. They must be supported toprevent sagging and vibration.6. Impulse lines should be positioned in protected areas or against walls or ceilings. Useappropriate pipe sealing compound rated for the service temperature on all threadedconnections. Do not place the impulse piping near high temperature piping or equipment.



Q9 : Impulse piping in gas service for pressure and flow instruments shall beoriented and sloped (upward) so that it remain full of gas.


~ Application Cautions for Differential Pressure Flowmeters Because of the nonlinearrelationship between flow and differential pressure, the accuracy of flow measurement in

the lower portion of the flow range can be degraded. Plugging of the impulse piping canbe a concern for many services. For slurry service, purges should be used to keep the

impulse piping from plugging. For liquid service, impulse piping should be oriented and

sloped so that it remains full of liquid and does not collect gas. For gas service, impulsepiping should be oriented and sloped so that it remains full of gas and does not collect

liquids. In vapor service, vapor may be allowed to condense in some of the impulse

piping to form a liquid seal between the hot vapor and transmitter in order to protect the

transmitter from heat.

~ The following restrictions and recommendations apply to impulse piping location:1. Impulse piping that runs horizontally must slope at least one inch per foot (83 mm/m).• Slope downward (toward the electronics) for liquid and steam applications• Slope upward (toward the electronics) for gas applications.2. For applications with temperature below 250 °F (121 °C), impulse piping should be asshort as possible to minimize temperature changes. Insulation may be required.3. For applications above 250 °F (121 °C), impulse piping should have a minimum length ofone foot (0.3048 m) for every 100 °F (38°C) temperature increase over 250 °F (121 °C).Impulse piping must be non-insulated to reduce fluid temperature. Any threadedconnections should be checked after the system reaches the intended temperaturebecause connections may come loose with contraction and expansion caused bytemperature change.4. Outdoor installations for liquid, saturated gas, or steam may require insulation and heattracing to prevent freezing.

5. When impulse piping is longer than six feet (1.8 m) the high and low impulse lines mustbe positioned together to maintain equal temperature. They must be supported toprevent sagging and vibration.6. Impulse lines should be positioned in protected areas or against walls or ceilings. Useappropriate pipe sealing compound rated for the service temperature on all threadedconnections. Do not place the impulse piping near high temperature piping or equipment.



Q10 : The reason why 3-wire system is used for RTD temperature measurement isto allow lead wire resistance to be compensated.


~ Lead Configuration & Colour Code

Used where lead length is short. There is no compensation

for resistance of lead wires.

Most common type of RTD assembly. Typically connected

to standard bridge circuit, which allows lead wire resistance

to be compensated.

Where higher accuracy is demanded. Lead wire resistance

errors are eliminated in this configuration by measuring the

voltage across the RTD element supplied with a constant

current.



Q11 : Cable between thermocouple element and temperature transmitter is copper


~ 2.1 Terminating the Thermocouple

A practical industrial or laboratory thermocouple consists of only a single (measuring) junction;the reference is always the terminal temperature. If the terminal temperature is other thancontrolled and stable, procedures are necessary to deal with the situation. Possible measures are:-

a) Measure the terminal temperature accurately and compensate accordingly in calculating themeasured value.

b) Locate the terminals in a thermally controlled enclosure

c) Terminate not in copper cable but use compensating or actual thermocouple wire to extend thesensor termination to the associated instrumentation (compensating cable uses low cost alloys

which have similar thermoelectric properties to the actual thermoelement). On this basis, there isno thermal voltage at the thermocouples termination. The transition to copper then occurs only atthe instrument terminals where the ambient temperature can be measured by the instrument; thereference junction can then be compensated for electronically.

Fig. 3

Note : It is essential to use only compensating or specific extension cables (these have thecorrect thermoelectric properties) appropriate to the thermocouple otherwise an additionalthermocouple is formed at the connection point. The reference junction is formed where thecompensating or extension cable is connected to a different material. The cable used must not beextended with copper or with compensating cable of a different type.

d) Use a temperature transmitter at the termination point. This is effectively bringing

instrumentation close to the sensor where electronic reference junction techniques can beutilised. However, this technique is convenient and often used on plant; a transmitter proceduresan amplified "corrected" signal which can be sent to remote instruments via copper cable of anylength.



~ Thermocouple Wiring

!

! "



Q12 : Major parameters for sizing the valve actuator are forces (torque) required tooperate the valves, hydraulic system pressure and fail-safe trip time.


~ Actuator size is a product of both the high forces required to operate the valves andcode requirements which require a mechanical (spring) close to ensure fail-safe closure of

the fuel valves when there is a failure in the control signal or loss of hydraulic pressure .

~ Valve Actuator Sizing

A torque matching actuator was designed for, and tested using a 406 mm line size

butterfly valve with the following specifications: unseating and seating torques of 3028Nm; hydraulic system pressure of between 83-117 bars; and a fail-safe trip time of < 0.5

seconds.

The manufacturer's listed valve actuator requirements were a torque of 3028 Nm during

both seating and unseating operations (0° angle) and a minimum torque of 1170 Nm atthe 70 angle of the valve's rotation. Estimates were made from valve technical literature

that for the 30° and 90° angles of rotation the valve torques are 40% and 15%,

respectively, of the maximum valve seating torque. The dimensions for the new linkages,

geometry and spring size were then derived to meet these design requirements.

Test results showed that the designed actuator satisfied the valve torque requirements

with a torque margin in excess of 35% during the 0° - 90° valve rotation. The test resultsalso showed that the torque increased exponentially at closing, as predicted by computer

simulations

~ Actuators

Actuators also need to be properly sized if the valve they serve is to perform well under alloperating conditions. Consideration needs to be given to the forces associated with the designflows and pressure drops as well as the forces required to completely close the valve against thefull available differential pressure. The shutoff differential pressure for a two-way valve is oftensignificantly higher than is the operating differential pressure because the pressure drop due toflow in the lines to and from the valve is no longer present. A good rule of thumb is to sizeactuators to close the valve at the peak pressure on the pump curve for the system they serve, atthe very least. A little allowance should be added for seating the disc into a resilient seat (if thevalve is so equipped).



Q13 : Control valve characteristics installed in the condition, where most of thesystem pressure drop is not across the valve is INSTALLED CHARACTERISTIC.


~ FLOW CHARACTERISTIC: The relationship between valve capacity and valvetravel. It is usually expressed graphically in the form of a curve. CONTROL VALVEShave two types of characteristics INHERENT and INSTALLED. The INHERENT

characteristic is derived from testing the valve with water as the fluid and a constant

pressure drop across the valve. When valves are installed into a system with pumps,

pipes, and fittings, the pressure dropped across the valve will vary with the travel. Whenthe actual flow in a system is plotted against valve opening, the curve is known as the

INSTALLED flow characteristic. Valves can be characterized by shaping the plugs,

orifices, or cages to produce a particular curve. Valves are characterized in order to try toalter the valve gain.

Valve gain is the flow change divided by the control signal change. This is done in aneffort to compensate for nonlinearities in the control loop.

FIGURE 1. Typical flow characteristics.



Q14 : What process data are required for sizing the control valve ?Total pressure drop, pipe diameter, specific gravity, operating flow


~ STEP #1: Define the systemThe system is pumping water from one tank to another through a piping system with atotal pressure drop of 150 psi. The fluid is water at 70 0F. Design (maximum) flowrate

of 150 gpm, operating flowrate of 110 gpm, and a minimum flowrate of 25 gpm. The

pipe diameter is 3 inches. At 700F, water has a specific gravity of 1.0.

Key Variables: Total pressure drop, design flow, operating flow, minimum flow, pipe

diameter, specific gravity

~ The Most Important Variables to ConsiderWhen Sizing a Valve:1. What medium will the valve control? Water? Air? Steam?What effects will specific gravity and viscosity have on the

valve size?2. What will the inlet pressure be under maximum loaddemand? What is the inlet temperature?3. What pressure drop (differential) will exist across thevalve under maximum load demand?4. What maximum capacity should the valve handle?5. What is the maximum pressure differential the valve topmust close against?



Q15 : Balance bellows type safety valve are used, where the variation in backpressure exceeding 10% of set pressure.


~ When must I specify the use of a Balanced Bellows pressure relief valve?

Balanced bellows as used in our 2600 Series BalanSeal design are generallyspecified for a variety of reasons. The most prominent is to nullify the effects ofback pressure in the discharge system on the valves set pressure. They are alsoused to protect the principal guiding surfaces, spring, and valve top works fromcoming in contact with a corrosive fluid. This may allow for the use of lessexpensive metallurgy as the bellows isolates the trim from the fluid. A balancedbellows valve should always be used when the variation in back pressureexceeds 10 % of set pressure.

~ Balanced safety relief valve - A balanced valve incorporates a means of minimisingthe effect of backpressure on the operational characteristics of the valve.

~ Bellows safety valve - A direct loaded safety valve wherein sliding and (partially orfully) rotating elements and springs are protected against the effects of the fluids by a

bellows. The bellows may be of such a design that it compensates for influences of

backpressure.



Q16 : In the safety valve design based on the ASME Section VIII, over pressure is10% or 3psi whichever is greater.


~ ASME VIII valve - A safety relief valve conforming to the requirements of Section

VIII of the ASME pressure vessel code for pressure vessel applications which will openwithin 10% overpressure and close within 7%. Identified by a National Board ‘UV’

stamp.

~ paragraph UG-125 of ASME Section VIII, Division 1. It states in part,

"All pressure vessels other than unfired steam boilers shall be protected by a pressure

relief device that shall prevent the pressure from rising more than 10% or 3 psi,whichever is greater, above the maximum allowable working pressure except as

permitted in (1) and (2) below."

Sub-paragraphs (1) and (2) mention cases where the pressure rise may be higher.

~ ASME Section VIII, Division 1 clearly states in Paragraph UG-131 (c)(1) that

"Capacity certification tests shall be conducted at a pressure which does not exceed thepressure for which the pressure relief valve is set to operate by more than 10% or 3 psi,

whichever is greater, except as provided in (c)(2)..."

Sub-paragraph (c)(2) covers a fire case.



Q17 : Last major construction drawings for instrumentation to be prepared.

Piping and Instrumentation Drawings (PNID)

Loop Drawings

Documents

Instrument Engineer Test 1.pdf