How to calibrate DPT for boiler drum level control - calibration method at field without manometer as well as lab calibration method with menometer. How to decide range of DPT?
The drum level transmitter generally uses hydrostatic head method of measurement. The span is regulated by the length of the static leg in inches of water.
Calibrate with a manometer or similar source. Verify by raising and lowering water level (within a safe range while boiler is in operation) and record the transmitter output.
Well, if you use differential pressure transmitter like Siemens sitrans P DS III (for example9 you can use blind calibration option. Just reliable measure distance between "+" and "-" impulse pipe. Note that "+" pipe inserted through condensation pot on the steam side of a drum. When you know that, for instance, distance is 500 mm, than just calibrate your transmitter 4 mA=500mm and 20mA = 0.
Then, in software (PLC or DCS) you make correction regarding working pressure and temperature and perform offset. Usually, drum level is between some negative and positive value. With offset you declare zero and have level from -250mm to 250 mmm (just for example).
Now, in regular service maintenance works, you can check transmitter by connecting it to the water column and test its linearity.
Steam drum level calibration is not something you should attempt unless you really know what you are doing. Don't attempt it unless you have a boiler or process engineer verify your calculations. If you already have a set of figures that don't seem to make sense don't just assume that someone made a mistake, there might be something going on that is not obvious.
There are several different ways of connecting the DP cell to the boiler. If you post a sketch someone will be able to give you some better pointers. However, I think you are asking how to simulate the level once you have the correct figures. If you don't have a manometer you can easily simulate the pressures with a couple of lengths of clear plastic tubing filled with water, Use larger diameter e.g. 1/2" to avoid errors due to bubbles. The LP tube will be fixed at a height to simulate condensate pot and you will raise and lower the HP tube to simulate the change in level (remembering that this side is effected by drum density).
I have attached a link that shows some of the errors possible.
Google "Steam drum level" and you will find lots more.
Hope this helps.
For detailed usefull information about this topic please see the link : http://www2.emersonprocess.com/siteadmincenter/PM%20Rosemount%20Documents/00840-0100-4360.pdf
Steam drum calibrations are not as scary as some people make them out to be. Just stick to the basics and latest technology and you will see why I, for one, can feel confident enough to make this brave statement.
Let's go back to basics of level measurments making use of DPT's
In the old days we only had DPT's that could only measure in the positive meaning that if you have to do a wetleg on a application you had to swop the HP and LP legs around and do the measurment in the reverse. This have caused many technicians nightmares to do since it can become very confusing to do an application like this and you had to keep your wits together when you work on an applicatipon like this. It is not that it was all that difficult to do just confusing since everything have to work in the reserve and upside down. a Steam drum is a high pressure, high temperature application so a wetleg is the only option, for those who didn't know that.
All this is very basic so far and we all know this but what I find stange is that a lot of people seems to forget that we now have SMART transmitters that can measure in the positive as well as in the negative so why do people still want to stick to the old ways?
There is no neeed for this swopping around of the HP and LP legs anymore and to make the steam drum calibration even more safe and easy to do, make use of capiliries and chemical seals attached to the SMART transmitter.
I know a lot of technicians start to feel unconfortable when it coms to doing a calibration on a capliliry DPT but once you understand the basics of it and how it differs from an normal tubing DPT level installation, it becomes very simple and straight forward. Definitly much easier to do than trying to do a old wetleg application calculation and setup.
I cannot remember when the last time was that I have used a old wetleg installation. The new SMART transmitters with capiliries and chemical seals are solvinmg all the high pressure, tubing blockages and wetleg level applications problems for us.
If you need help on how to do a wetleg with a SMART transmitter and capiliries let me know and I will do a generic write-up that can be used and adapted to suit most wet-leg applications, even a steam drum application.
Ok I will make it as generic as possible however keep in mind this is to try and show you how it is done and is not for a specific application. Once you understand how to do it you can apply the method to your own applications, even to a boiler steam drum application.
Before we get to capillaries, you need to have a thorough understanding of how to do a normal dry-leg pressurized vessel level setup with a DPT otherwise you will not understand DPT's with capillaries on pressurized vessels applications.
To do a level setup on a closed pressurized vessel with a DPT making use of instrumentation tubing you will use a three or five way manifold. The reason for the manifold is to be able to open your transmitter to atmosphere before the calibration and do an atmospheric zero calibration. With a HART this is very easy to do by making use of the "zero trim" feature.
Once this is done you will calculate the height of your zero and span with the formulas (sg x h) = LRV and ((sgx(h+H)) = URV. "h" being the distance from your tubing where it comes out of your manifold to the zero mark on your vessel and "H" being the distance from the zero mark to your 100% mark on the vessel.
Now the initial atmospheric zero calibration is done but only if you work on a pressurized vessel of about 10Bar or less.
Once you start working on vessels with about 10Bar all the way to vessels with 150Bar pressure, a zero shift starts to take place in all transmitters and you have to do a process zero on the transmitter as well. To do this, isolate the transmitter from the process and connect a T-Piece from the HP side tubing, to both sides of the transmitter, keeping the equalizing valve close. Open the HP tap-off main isolation valve on the vessel in order to put the full process pressure on both sides of the transmitter's diaphragm simultaneously – again keep the equalization valve close on the manifold. The transmitter should stay on zero. If it is not on zero, a zero trim can to be done again yes, but experience have taught us that this is not the best way and you should use the error value as your new process zero value and just add your calibration calculated values to this value.
For example say the reading on your transmitter is -100mmH2O instead of 0mmH2O during your process zero check, without doing a zero trim again recalculate your calibration like this, -100 + (sg x h) = LRV and -100 + ((sg x (h+H)). Obviously if you have a calibrated span that is very big, this error is insignificant but in most cases you only have a calibrated span of 500mm or 1000mmH2O so this 100mm error will drastically affect your calibration once the transmitter is put back online. Also the error will be in relation to the size of your transmitter. If you use a to big transmitter for the application the error will be big as well so try and get a transmitter as close to your max span value to keep this zero shift error as small as possible.
Also be aware that the higher the process pressure the higher this error or zero shift will become. I know you think that this error is so small and the effort to do a process zero is so big that you will not bother to do it, but trust me once you get called by production again and again in the middle of the night for the same transmitter that keeps on giving incorrect readings directly after you calibrated it, you will start thinking about doing these process zero calibrations.
Also another point is if you do it like this and someone else comes to the transmitter and only do a atmospheric zero check he will still find it to be on zero and will not try and do a zero trim again since this could become a problem if different technicians work on the same transmitter and do not do their calibrations the same way. The other technician might wonder why you have put in the wrong values for your LRV and URV and modify them again the original values with only an atmospheric zero calibration. This could become very frustrating if every time you come to the transmitter, someone have gone and done a zero trim again and your process zero calculated values are gone again.
Ok that is all I have for you on DPT's making use of instrumentation tubing on pressurized vessel level applications. Please note the above is a normal dry leg installation on a pressurized vessel and not a wet leg installation and forms the basis of all pressurized vessel DPT level applications so it is important to have a thorough understanding of how it is done before moving on to the more complex level applications.
As a final note, when do you do a wet-leg and when do you do a dry leg installation? Look at the temperature of the product inside the vessel and compare it with the ambient temperature. If the product inside is colder than the ambient temperature condensation will form on the outside of the vessel, if the product is hotter than the ambient temperature condensation will form on the inside of the vessel and therefore also on the inside your impulse lines to your transmitter so therefore a wet leg installation must be done.
Since this write-up is getting to be too long I will do another post where we can look at how to work with DPT's with capillaries and chemical seals.
Ok the next step in the DPT learning process towards DPT's with capillaries, is to have a thorough understanding of wet leg applications making use of instrumentation tubing and a SMART DPT.
In the old days with pneumatic and electronic 4 to 20mA transmitters you had to swop the impulse lines around and do the calibrations in the reverse and also turn your display unit in the control room upside down. Very bad news and a nightmare to do for most technicians and from there the fear to work on steam drum applications.
Today with SMART transmitters this is not needed anymore since a SMART transmitter can measure the same amount in the negative as in the positive. If you look on the little plate or spec sheet of your SMART transmitter you will see that the range of the transmitter is given as something like -1200mmH2O to +1200mmH2O meaning that you can set zero and 4mA to -1200mmH2O and span and 20mA to +1200mmH2O giving you a full 2400mmH2O calibration with a transmitter like this.
Now if you install your transmitter on your vessel connect the HP side to the bottom and the LP side to the top as you would for a normal dry leg application. Install your T-Pieces or catch pots and fill the lines up with your buffer liquid. Now go back to your transmitter and see what it reads. It will read the same amount in the negative as the difference between the distance between your top and bottom tap off points. Knowing this you can actually calculate before going to your transmitter for the first time how much it should read by taking the distance between your top and bottom tap-off points and multiply that with the density of the liquid in the vessel.
Ok that is not all that important, just nice to know.
Once the installation is done, the lines filled up properly to the point where they will start to flow back into the vessel go back to your transmitter and take the reading as it is displayed and write it down.
Say the value is -1000mmH2O. This is what we call the ATM value.
Now to calculate the calibration values is pretty much the same as for a dry leg.
The formule is LRV = ATM + (sg x h) – again "h" being the distance from your transmitter tubing to the ZERO mark on your vessel.
The formule for the URV = ATM + ((sg x (h +H)) – again "H" being the distance from the zero mark to the 100% mark on your vessel.
If you have a high pressure, high temperature application disconnects the impulse lines from the vessel and pump the transmitter up to the same pressure as the process with the pressure line from the pump connected to both sides of the transmitter and the equalizing valve close. If the transmitter does not stay on zero use that error value and just add or subtract it from your ATM value during your calibrations. This will compensate for the zero shifts that will take place on high pressure applications. It is basically a manual process zero that you can do.
If it is safe enough to do you can also use the HP side again and connect a T-piece and take the process pressure to both sides of the transmitter but on a high pressure high temperature applications that is a bit too dangerous to do for my liking so the manual process zero method is safer to use although not as accurate as a actual process zero.
Anyway that is then a very easy and simple way to do a wet leg application on any type of vessel or application. Again if the temperature of the liquid inside the vessel is higher than the ambient temperature a wet-leg must be done since condensation will take place inside the vessel and inside tour impulse lines even if there is only half a degree difference.
The above will give a very good basis to work from and forms the basis for DPT's with capillary applications. I will show you how to work with them in my next post.
When you first get to work on DPT's with capillaries and chemical seals, it can be very confusing since they look completely different from your normal DPT's with manifolds and instrumentation tubing.
Like a technician asked me some time a go, "now how are we suppose to calculate the level correctly with all that capillaries all rolled up and all over the place?"
Obviously he was still thinking in terms of instrumentation tubing and was trying to relate the capillaries to what he knows about tubing level applications. Don't make the same mistake.
In order to understand how capillaries work you only need to understand a very simple concept and that is that the pressure of a vertical column is always measured on the vertical regardless of what the column looks like. So this means that you can work with capillaries in the same way you have worked with a wet-leg application and the only difference is the way you do a process zero on a DPT with capillaries and chemical seals.
To better explain let's create a scenario where you have to install a capillary DPT on high pressure and medium temperature vertical domed vessel. This is just an example and the chemical seal can be anywhere on the vessel the calibration procedure stays the same.
To start an installation like this first get the DPT form the stores and connect it up in the w/shop to a 24VDC supply and put both chemical seal next to the DPT on the work bench. In a situation like this the DPT must read zero and if it is not, do a zero trim with the HART on the transmitter, disconnect and go and install it on your vessel. Connect the HP seal to the bottom of the vessel and connect the LP seal to the top. Make sure the flanges are tight but keep your main isolation valve on the vessel closed, go back the the DPT and take down the reading. As with a wet leg with tubing the reading on the DPT will be the difference between the HP and LP leg and again can be calculated even before you get to the DPT.
Since capillaries have a tendency to drift stand and watch the ATM value for a while to make sure you have a good average if it is a bit unstable. Make a decision on the exact value you want to use and write that down as you're most stable and a good average ATM value.
Measure the distance from the middle of your HP seal to the zero mark on your vessel. (Distance=h)
Measure the distance from your zero mark on the vessel to the 100% mark.(Distance=H)
Now to calculate the calibration values is pretty much the same as for a dry and wet leg with tubing
The formulas are LRV = ATM + (sg x h) and URV = ATM + ((sg x (h +H))
If you have to do a process zero (higher than 10Bar pressure) you need to install flushing rings between the flanges where your chemical seals are connected and run a piece of tubing from the top to the bottom flushing ring with a needle valve in the middle and at the bottom of the bottom flushing ring to drain the liquid at the bottom. To do the process zero you need to isolate the HP side main isolation valve on the vessel and put the full gas pressure from the top of the vessel on both the top and bottom chemical seals simultaneously by making use of this tubing and needle valve. Once you have the new process zero value, which will be different from your atmospheric ATM zero value, use that and recalculate your calibration. Close the needle valve on the connection tubing and put your transmitter back on line. Use the bottom drain valve on the flushing ring to do a final drain to stabilize the system.
How to calibrate DPT for boiler drum level control? How to decide range of DPT?
We mount a level transmitter on boiler steam drum. The distance between tapping is 610MM. the upper tapping connected with Low and Lower Tapping with High side of transmitter. We Put range in transmitter LRV = -610 and URV = 0.
In Simple water transmitter show correct valve, but when pressure develop in boiler drum then it shows wrong value suggest me what I do.
Your calibration in the shop corresponds to 610 mmWg at ambient temperature where the density of the water = 1000 kg/m3. The density of the water in the steam drum is lower than 1000kg/m3 because of the operating temperature in the drum. The differential pressure required has to be calculated for an equivalent of 610 mm at the operating temperature of the drum. You are actually measuring the interface between water and steam in the drum. The density of the steam and the pressure head exerted by the steam have to be taken into account as it is not negligible.
Suppose LRV 800mmwc set to 4ma & 0mmwc to 20ma. The same transmitter calibrated to 0mmwc to 4ma & -800mmwc to 20ma. Is both calibrations give same result.
My experience both are same, but I have a serious doubt. my calendar -800 to 0 give any abnormality in some critical conditions.
What we haven't established is if this is a new installation or an existing one.
Sam gave us some excellent input with examples also pointing out some of the history and traps to watch out for like high pressure zero shift. This all goes to re-enforce my original statement that Steam drum level is not for the inexperienced.
For something as complicated as a boiler there should be a manual with drawings and calculations and I assume a calibration sheet for the transmitter with comprehensive notes on how the calculation was done.
When you have the calculations then it should be a simple matter of simulating the pressures.
Sam, could you please explain the zero procedure for over 10 bar, what is the reason for that requirement?