To begin learning how to be a tech, you must first understand what the plant is trying to do. Refinery’s refine crude oil and separate it out into usable fuels such as kerosene, gasoline, diesel, and such. Chemical plants will combine chemicals and with the assistance of heat and pressure and a catalyst they will become something else altogether. Plastic plants will make different plastics with ethylene or propylene and at the end of the day each one of these was done by getting everything in the right amounts and at the right situation.
There are four main (P.V.)process variables that the unit will monitor. Flow, Level, Temperature, & Pressure, these transmitters will send a signal to the (D.C.S.) distributive control system. The DCS will control the process and feed the right amount and maintain the levels and control the temperatures and basically control the process. Each transmitter communicates with the DCS and most do this through an analog signal. An analog signal is where the transmitter puts out 4 – 20 ma to equal 0 to 100%. Let’s do a simple one. We have a pressure transmitter PT-101 in the field and the range is 0 to 300 PSI
300 PSI = 20 ma = 100%
225 PSI = 16 ma = 75%
150 PSI = 12 ma = 50%
75 PSI = 8 ma = 25%
0 PSI = 4 ma = 0%
The transmitter is powered up with 24 volts DC. The transmitter will then apply the right amount of resistance to that voltage to achieve the right ma output for what pressure it’s seeing. Understanding this is half the understanding to do basic instrumentation. This signal to the DCS from the transmitter is an analog input to the DCS. To control the process the DCS has to be able to control the valves in the unit. For this it will do an analog output to the valve. The DCS will send 4 – 20 ma out to the valve and this will stroke the valve. 4 ma and the valve will be closed and 20 ma and the valve will be fully open. So what would 12 ma out give you? Right, 50%. This is for control valves. There are other valves in the unit that are either open or closed and to stroke these, the DCS will use a (D.O.)digital output. Which means either on or off. Just like a light switch in your house. The DCS will send out the 120 volts to a solenoid which will open and allow air to open the valve all the way or turn off the solenoid and the valve will close. These valves will normally have limit switches on the indicator which will tell the DCS if the valve stroked all the way open or close. These are also switches that have to be set. You will have a ZSC for close and a ZSO for open. When the switch makes it closes and lets voltage come in and go back to the DCS where it receives it and knows that the valve is in the proper position. That would be a (D.I.) digital input to the DCS.
To trouble shoot a meter or a valve you will have to know how to use a meter. To measure AC or DC voltage you will have the leads in the meter on the two spots to the furthest right and then choose the correct symbol for AC or DC depending on what you’re measuring.
Pressure is the most basic and simple of the transmitters and you should be able to recognize a pressure transmitter immediately in the field. It will have a single piece of tubing going to it. It will read the pressure and tell the DCS what it sees. Some pressures are capillary and mounted right to a vessel. To calibrate these you will have to use a premade flange and gasket to assemble to the capillary and then pressure up the whole chamber.
This a Rosemount pressure transmitter with a manifold used to isolate and pressure up the transmitter.
The next simplest transmitter I would say is the temperature. There are two main types of inputs to a temperature transmitter. Those are thermocouples and (RTD) resistance temperature device. There are quite a few different thermocouples out there being used but the main ones you see in industry are the J and the K. The way a thermocouple works is to twist together or make a junction from two dissimilar metals and by doing so these two metals will create an (EMF) electromotive force. It means it will produce millivolts that are equivalent to the temperature. Type J is made of iron & constantan and the colors for them are black and red. Type K is made of chromel & alumel and the color for this is yellow and red. On a thermocouple the RED is negative. You will measure the thermocouple with a piece of test equipment and you just have to make sure you are chose to the right type thermocouple. You will hook up the wires to read the thermocouple and to calibrate or check a transmitter you will drive the signal. Your test equipment will read out in degrees but you are driving a millivolt signal that is equivalent to the temperature.
RTD is normally three wires and almost always is a 100 ohm platinum. Two of the three legs are common to each other and will be the same color. The third leg is a different color and to measure the RTD connect your meter to that wire and one of the others. If the RTD is in an ice bath at freezing temperature of 32 degrees F / 0 degrees C then you will read 100 ohms. If you were to raise the temperature to 215 degrees F / 203 degrees C you would read 139.66 ohms. There are a couple of different curves that are used for doing RTD and if you find yourself driving a temp and it is not reading correctly on the DCS then you might need to look at which curve the DCS is using. With the advancement of today’s DCS. Plants are now running the thermocouple wire or RTD directly to the DCS and the DCS reads the signal and turns this into a temperature. They eliminate the transmitter altogether. This is called a low level analog input. When using the test equipment you will either source or read the temperature. When you source the temperature this means you are driving the signal to the DCS for it to read.
Levels can be done many different ways. They can be done with a (D.P.) differential pressure transmitter or a float displacer or radar. A radar transmitter is mounted to the top of a vessel and it shoots a beam straight down in the tank or vessel and it will hit the liquid or whatever is in the tank and bounce back and the transmitter will say the level by how long it takes the signal to bounce back. The higher the level the less time it takes for the signal to come back. The transmitter turns this into a 4 to 20 ma output and sends that signal to the DCS.
A float is exactly what it sounds like. A long metal tube designed to be buoyant in the liquid it was chosen to measure is made and they put metal beads in it to make it weigh perfect for the liquid so that it rises and falls with the level. This float is then inserted into a chamber and will sink with no level and put out 4 ma and as the level rises it will rise and when full it will put out 20 ma to the DCS indicating 100%. The trick with levels is to remember that everything has a different specific gravity. Water is the standard and is a 1. In order to make a tube for something that is lighter than water it would have less weight in it than something that is heavier than water. There are times that you will need to measure an interface level where a tank has two different substances with two different SG with the lighter being on top and the heavy on bottom. The DCS wants to know where it is they separate in the vessel. You will fill the vessel with the light and that is the zero and then fill the vessel with the heavy and that is the 100%. Then when in operation the DCS will control the level at 50%.You will have a sight glass that you can see in and see where the two levels separate.
Alcohol, ethyl (ethanol) 25 0.787
Oil, Castor 25 0.959
Crude oil, Texas 60oF 0.876
Fluorine refrigerant R-22 25 1.197
Gasoline, Vehicle 60oF 0.739
Glycerin 25 1.263
Hexane 25 0.657
Mercury 25 13.633
Methane -164 0.466
Water, pure 39.2oF (4oC) 1
A DP transmitter can be used to measure a level. A DP transmitter works by measuring the pressure it senses on the low side and on the high side of the transmitter and then subtracts them. The difference between the low side and the high side of the transmitter is the DP. In this example we have a tank that is open on top. The liquid is water and is 3 feet high so it is exerting 36” H2O pressure to the transmitter on the high side. The low side is vented to atmosphere so it is reading zero. The difference between them is 36” H2O. At 100% the tank will have 4 feet of water which is 48” H2O. What ma output is the transmitter sending out? The answer is 16 ma = 75%
If we look at the same level but on a closed tank and the tank has a head pressure of 50 PSI the reading will be the same but you have to account for the head pressure. The high side will have the 50 PSI head pressure along with the pressure exerted from the level. The low side will just have the 50 PSI head pressure. The 50 will cancel itself out and you will have a differential of 36” H20.
DP measurement on vessels will sometimes be done with a capillary transmitter and once the transmitter is mounted and the capillary put in place on the vessel you will have to compensate for the fluid in the capillary. The distance between the two paddles is your span for the transmitter but it will be reading a negative number with zero level so you compensate for this. Once mounted the transmitter is seeing a DP of -23”H2O. The distance between the paddles is 20”. So to calibrate the transmitter correctly you will insert a range of -23” H2O to -3” H20.
There are many ways to measure flow and the most common one is to use a DP transmitter across an orifice plate. An orifice plate is designed to fit in between two pieces of pipe and create a DP across it. This is done by drilling a precise measured hole into the middle of the orifice plate. As liquid flows through the pipe it will come to the orifice plate and be forced to squeeze up and go through the hole. Tubing will come down from either side of the orifice plate to a transmitter. The transmitter will read the DP across the plate and send out the appropriate ma signal to the DCS. Everything in this system has been calculated out from the size of the pipe to the hole in the orifice and thus telling us what to calibrate the DP transmitter to. The flow is not linear though and for a correct reading the signal has to be square rooted. If the transmitter is calibrated 0 to 100” H2O so the (LRV) lower range value is 0 and the (URV) upper range value is 100”. You block in the low and high sides to the transmitter and vent both sides to atmosphere you will have 0” H2O DP and the output to the DCS will be 4 ma. Flows on the DCS are read different than a straight up pressure. A pressure is calibrated 0 to 1000 PSI and if you pump up the transmitter to 700 PSI in the field, the DCS will read 700 PSI. A flow can be in many different units such as GPM, SCFM, BPD and so on. For our example the DCS is 0 to 1000 GPM.
% ma range SQRT % SQRT range SQRT ma
100 20 100 100.00 1000 20.00
75 16 75 86.60 866 17.86
50 12 50 70.71 707 15.31
25 8 25 50.00 500 12.00
0 4 0 0.00 0 4.00
Some plants will have the square root done in the transmitter in the field and some will have the square root done in the DCS. If the square root is not done at either one then the flow will read way too low through the span. If it is accidentally done at both the transmitter and the DCS then the flow will read way too high throughout the span.
Other flow meters are not square rooted, just the ones going to a DP transmitter. It isn’t always an orifice plate that creates the DP. It can be a wedge, venture, or other ways but they all do the same thing and create a DP.
Some of the flow meters that do not use a DP transmitter are the Coriolis meter, votex shedder, mag meter and many more. These transmitters put out a linear signal so 12 ma is 50% flow and the DCS reads it as such.
An important note about these meters also is that a large amount of them are powered up with 120 volts AC and they put out their own 4 to 20 ma. Most all of the transmitters in the field will be powered up with 24 volts DC from the DCS. Then they turn that into 4 to 20 ma for the DCS to read. When doing new construction you will run into issues where the wires are landed wrong at the DCS and the DCs will be trying to give it 24 volts and the transmitter is sending in 24 volts.
Learning to read loop sheets and P & ID’s is essential to being a tech. The loop sheet will show you where all of the wiring is coming from for a particular loop. The wires will be a positive and a negative. At most plants the wires are black and white with the black being the positive and white the negative. They will leave the DCS card and go to a marshaling cabinet in the rack room normally taking up an entire wall. From there it will leave on a set of wires in a bundle of wires going to a junction box maybe a second junction box and then to the transmitter in the field. Some of the new systems have a box in the field that has wires going out to the transmitters and from that box to the DCS is fiber thus eliminating a bunch of wire. Most all older plants do not have this. The loop sheet will walk you from start to end of however the transmitter is talking to the DCS.
Piping and Instrument Design will show you where everything is located in the field and learning to read these will help you to find the transmitters in the field. Use this to find a starting point such as a pump or vessel and then find the right piping and follow it out to find your transmitter.
In the field we have talked about transmitters but there is also a large amount of switches that are set to trip at a specific temp, flow, level, or pressure. They are digital inputs to the DCS either on or off. These are normally used to trip and let the DCS know about critical situations.