Question { National Petrochemical Company, 4345 }

We do have a problem in the company that is one PT connected to PlC which is mean 1 out of 1 in PLC system we do have the PAHH is 55 mbar and PALL 8 mbar with a timer of 15 sec for the PAHH the problem started when PAHH goes more than the set point which is 55 it reached up to 71 but due to timer doesn't trip . We force it and we checked everything nothing there . We replaced the transmitter problem still there the reading goes more than the 55 bar the range for transmitte is 0- 60 mbar . So we did loop check , aculaizing valve , wiring all are okay . What the possible causes for this ???

Answer

Although the post has lots of information, it isn't clear
exactly what the problem is.

I think the problem is that the PLC does not alarm when the
pressure exceeds (goes above) 55mbar. But I'm not sure.

Given this description: when PAHH (pressure alarm High High)
goes more than the set point which is 55 it reached up to 71
but due to timer doesn't trip . We force it and we checked
everything nothing there, let's suppose that the PLC is not
indicating an alarm state.

a) what should the PLC do at the alarm condition?
b) How is an alarm indicated? A digital output (DO) goes
high? A lamp is driven by a DO lights up?
c) What exactly does not work as it should?

If the PLC does not indicate an alarm, yet the indicated
pressure goes higher than a high alarm, the problem is NOT
the field transmitter, the problem is in the alarm setup or
alarm annunciation.

Troubleshooting depends on
- an explanation of exactly what should be happening
- an explanation of how you know it is not working; that is,
what the evidence is that it is not working.

Yes, the pressure went above 55mbar.
How do you know that?
The transmitter's local display?
A milliampmeter in the analog input circuit?

Did the alarm ever work? Or is this a new commissioning and
you're trying to get the alarm to work?

Carl Ellis
Measure First

Question { Essar, 7192 }

transmitter in 3 connection +,-,And test mode so how Use To Test Mode

Answer

The 'test' connections on a 2 wire, loop powered pressure transmitter have a diode behind them.

If a milliampmeter is connected across the test terminals (in parallel with the diode), the impedance of the meter is less than the diode's, so current will flow through the milliampmeter. There is not enough voltage drop to forward bias the diode and the diode will not conduct with a low impedance meter circuit connected across it. The diode turns off and the current flows through the meter.

Test terminals are provided for loop troubleshooting so that loop current can be measured without breaking or opening the loop circuit so that the milliampmeter can be put in series in the loop circuit.

Carl Ellis
Measure First

Question { Shell, 10137 }

why we are giving negative supply for vibration sensors?

Answer

Because negative voltage is what the Bentley Nevada circuits use.

The regional guy from Bentley Nevada was in the other day so I asked him. His answer, "Historical legacy. It's always been that."

The point is, if the circuits need a negative voltage, that's what's used.

Carl Ellis
Measure First

Question { OMV, 8865 }

WE Know that different supply voltage of solenoid valve like 24,110,220 etc..how we check the solenoid supply is 24v or 220 NOTE: No specification at device.

Answer

1) Put a voltmeter on the solenoid terminals and see what the control signal supply voltage is. It could be AC or DC.

2) Go back to the control signal source and trace the wires until it becomes apparent what the control signal voltage is.

3) Dig out the electrical drawings and see what control signal the circuit was designed for.

4) Measuring winding resistance is dicey (not terribly reliable) because the wattage ratings on solenoid coils covers a range of about 3:1, so the difference between a DC coil and an AC coil, or between a 6 or 9 watt coil is overlapped by different wattage coils for different services.

Carl Ellis
Measure First

Question { Shell, 10343 }

What is capacity of impulse line tube ...answer in psi

Answer

Theoretical burst and working pressure in psi at 100 degrees F

Pressure Formula:
      Internal Burst = 2 St/D 
Where 
P = Internal pressure psi
S = Tensile stress psi
t = Minimum wall thickness in inches
D = Nominal outside diameter in inches

Working pressure = 1/4 internal burst pressure

Carl Ellis
Measure First

Question { OMV, 14895 }

why use to 250 ohm for hart protocol and why hart communicator not communicate without 250 ohm 

Answer

My friend Dan has an article on why HART needs a minimum 
resistance in the loop to work.

http://blog.lesman.com/2011/10/06/add250-ohm-resistors-to-make-hart-work/#more-68

In summary, the loop power supply acts as a very low pass filter that tends to filter out the HART signal. 

According to the HART Foundation spec, the 1200-2400 Hz HART signal needs a minimum (230 ohm) resistance in the loop to create a HART signal with enough amplitude to meet the sensitivity requirements of a HART modem receiver. 

Carl Ellis
Measure First

Question { 13259 }

how to calibrate TT transmitter

Answer

Disconnect the field sensor.

Connect a calibrated/traceable temperature signal calibrator.

Apply LRV value.

Trim the 4.0mA output.

Apply the URV value.

Trim the 20.0mA output.

Check as many interim points as the calibration protocol requires and note the output and the deviation from the expected value.

Determine whether the transmitter is within spec from the deviation values seen.

If transmitter passes, reconnect the field sensor to the input.

Fill in the paperwork.

Carl Ellis
Measure First

Question { 6536 }

Can 2 transmitters be connected to Integral orifice meter taps? Does an integral orifice comes with 2 set of taps?

Answer

Integral orifice plate assemblies are either designed to install as part of the process piping with flange mounts for a manifold valve, or for direct mounting to the process heads of a DP transmitter so that no transmitter piping is required.  They're designed primarily for low flows.   

Although intended for direct mounting to a manifold valve or process heads, optional fittings are available for high temperature applications that move the transmitter away from the integral orifice assembly.   Presumably, fittings for dual transmitters could be fabricated as well.

Commercial orifice flange unions have a single tap on each flange.  It would be possible to tap a 2nd DP transmitter into the piping for those taps.   

Dual or even triple transmitters on a single flow element and set of taps are sometimes referred to as 'stacked' transmitters. There are commercial adapters for close coupling dual DPs with a special manifold valve from vendors like Century Valve.   This has a long history of useage to cover a wide flow range with multiple DP transmitters, each of has a relatively low or limited turndown.

An SIS safety calculation has to account for a single-point-of-failure with a single set of taps.

There's always the possibility of a corporate engineering policy discouraging or forbidding dual DP transmitters on a single flow element and set of taps.

Carl Ellis
Measure First

Question { Samsung, 13094 }

What is the compensation of thermocouple

Answer

Cold junction (CJ) or ice point compensation is the temperature measurement of the terminal connection temperature.  The difference between the CJ measurement and the freezing point of water is added to the thermocouple's gradient temperature measurement.

Cold junction can be measured by thermistor, RTD, or solid state junction.  

Question { JAL International, 13339 }

why pressure 3-15psi is used as industrial standard and what is the reason

Answer

do a Google search for
allinterview 181560

This question was answered in that thread

Carl Ellis
Measure First

Question { Process Control Technologies, 3977 }

If a flow transmitter (magnetic type FT) that can measure flow and temperature, then why in some industries people use flow transmitter and temperature transmitter in the same line 1 after another? Even after 1 single flow FT is capable of measuring flow as well as temperature.

Answer

I'm aware of few high end flow meters (coriolis) that measure process temperature as well a flow rate, but I'm not aware of a magnetic flow meter that does so. There are magmeters that measure the electronic temperature of the electronics for diagnostics, but the temperature value is really to used for warranty replacement evaluation (to make sure that the failure wasn't due to overheating).

The multimeasurement concept is certainly valid - fewer insertions into the process. As to why a process temperature output is available but is not used but a separate temperature sensor is used, I can only speculate that either the instrumentation designer was just unaware of the availability of the temperature from the flow meter or was subject to the "we've always done it as a separate sensor" philosophy.

In general though, the vast majority of flow meters are just that, flow meters; they are not multivariable meters with a flow output and a temperature output. With the increased use of fieldbus, that's likely to change over the next decade.

Carl Ellis
Measure First

Question { Process Control Technologies, 5441 }

Why is the length of cable layyed from RTD to temp. Transmitter should be minimum? Why does heating effect takes place in their?

Answer

Why is the length of cable layyed from RTD to temp. Transmitter should be minimum? Why does

heating effect takes place in their?

An RTD develops a very low voltage signal, on the order of tenths of a volt. Low level signals on

that magnitude can be overwhelmed by industrial electrical noise. And voltage signals, unlike a

current loop signal, diminishes (decreases) over distance.

There is also a limitation on the maximum wire resistance (which is proportional to wire length

and gauge) that an RTD input on a temperature transmitter or controller analog input can handle.
These reasons are why RTD temperature transmitters are so popular; the transmitter connects

close to the RTD minimizing noise and producing a current loop signal that is relatively immune to

noise and that does not decay over distance. The loop power is delivered over the same to two

wires as the output signal uses.

The self-heating effect is at the RTD element itself, it is not in the extension wire/cable

Unless it's a cryogenic (low temperature) application, the self heating effect is usually ignored.

Look at how little heat is developed:
A 1ma (0.001A) excitation current through 120 ohms (approx 52 Deg C) drops 0.120V (E = IR)

0.120V * 0.001A = 0.00012W or 120uW or 0.12mW

That's not a lot of heat to dissipate.

Carl Ellis
Measure First

Question { Process Control Technologies, 7164 }

Why 1-5 vdc a standard?

Answer

1-5Vdc is not a standard, it is a widely used convention for 4-20mA current loops.

A 250 ohm resistor drops a 1-5 Vdc voltage proportional to the 4-20mA current.

DCS analog inputs historically used 250 ohms for 4-20mA analog input resistance and over time that value has become quite common.

But I have seen 1 ohm, 4.7 ohms, 10 ohms, 12 ohms, 24.9 ohms, 50 ohms, 62.5 ohms, and 100 ohm resistances for 4-20mA current input.

Question { Qatar Petroleums, 4709 }

How to calculate flow of air bubbler type level transmiter 
According hight and density 

Answer

Question: How to calculate flow of air bubbler type level transmiter? According hight and density?

I love this question because it deals with the understanding of how a bubbler works - flow is not the issue, pressure is.

The calculated value for a bubbler is NOT flow. The critical, calculated value for a bubbler is pressure, because the air/gas pressure in the bubbler tube is a direct representation of the head pressure of the liquid column; which in turn, can be interpreted as the level of the liquid column.

Measurements of head pressure are always done in units of water column (wc), inches of water column or mm of water column, because water column is universal everywhere in the world.

The column of liquid might not be water, it might be some chemical or solution whose specific gravity (density) is not the same as water. So to determine the level, the head pressure measurement of the air/gas pressure in the bubbler dip tube needs to be corrected by the specific gravity. Level = measurement (wc)/specific gravity

The flow rate of air/gas is typically just enough to create a constant stream of bubbles, proving that the air/gas column in the tube has sufficient pressure to overcome the liquid column head pressure. Flow rates regulated by a constant flow, differential relay range from 0.5-2.0 CFH (15 L/hr to 55 L/hr).

Carl Ellis
Measure First

Question { icl, 5218 }

Guy's need your support,I want to measure closed tank level which is under vacuum -630mmHg using DP Transmitter, tank fluid density 1.38, Tank height is 7ft

Answer

A differential pressure transmitter is required to correct for vacuum pressure. The DP's low side is installed high up, tapped into the vacuum and the DP's high side tapped in near the bottom.

The DP transmitter's minimum range (LRV) is zero

The DP transmitter's maximum range (URV) is:
maximum level height (inches or mm) * specific gravity

Liquid level = measured pressure in water column units (inches or mm) / specific gravity

Many digital indicators just scale the 4-20mA to the desired level range; so a calculation involving specific gravity isn't needed.

For example:
4.0 mA = 0.0 inches level
20.0 mA = 84.0 inches level

Carl Ellis
Measure First