I am a electrician and work in a Hospital . I have studied instrumentation but have a small amount of experience in the field and need some help in choosing a pressure transmitter for a steam boiler . The current situation is a gas steam boiler that operates at 800 Kpa, the engineer has asked for the PID controller a BTC9090 with 4-20ma input and output, to modulate the pressure over the range of 850 Kpa to 750 Kpa or better. The current pressure sensor used for the input to the controller is a GEMS 2000BGB1601A10A which has a range of 0-16 Bar and 4-20 ma with a accuracy of approx. 1% . I think this pressure sensor is not suitable (your coments welcome).
The range of control desired is 100 Kpa so the pressure sensor will have a output of 1 ma over the 100 Kpa range . The pressure sensor's accuracy of 1% over 16 Bar would give a resolution approx. 6.25 over a range of 100 Kpa .
I thing this is not good enough for good PID control .(your coments welcome).
Is there a pressure gauge that can withstand a pressure range of approx. 0-10 bar but have a 4-20 ma output when the pressure is in the range of approx. 700Kpa to 900kpa. This would give the controller input greater resolution and hopefully better control.
The other pressure transducer that may work is one with an adjustable span , but I am not sure they exist .
I an not going to be the one to make the modifications to the steam boiler because of safety regulations etc. but I am looking for info to make sure the contractors doe it right .
Any advise on the above would be appreciated.
Your best bet is to spec a honeywell pressure transducer, it is completely scalable and will usually provide +/-0.25% accuracy, if you need specifics on the model number drop me a line.
Send me your e-mail address, and I will send you specs of Yokogawa's EJA series digital pressure transmitter.
The range of your existing Gems pressure sensor of 0-16 Bar (15 kpa) looks good for normal pressure of 700Kpa to 900 Kpa (8 to 10 Bar). The 1% accuracy may be good enough for control. The sensors Repeatability specification may be more important for good control. Most process pressure transmitters (4-20 mA output) are accurate to +/- 0.25% of calibrated span or less.
Does your existing pressure control perform adequately? If it does not, does it hold a steady pressure in Manual mode with steady state conditions (steady steam and fuel flow)? The pressure controller needs to be "tuned" correctly and control valves need to perform smoothly, etc for good control.
You can search for a list of manufacturers of differential pressure transmitters at: http://www.isadirectory.org/online/prod1.cfm
The ECI Waste Heat Steam Boiler is heated by the exhaust of a 1 megawatt generator. The GEMS pressure transducer and BTC9090 controller were included with the as built specs about 5 years ago. The pressure transducer measures the steam pressure and the controller adjusts a damper to control the heat to the boiler.
Some time in the past some one bypassed the pressure transducer and controller with a pressure switch, when the steam pressure falls to 700Kpa, the pressure switch closes. the damper fully opens when the steam pressure reaches 800Kpsa, pressure switch opens and damper fully closes. Who and why bypassed the controller and pressure transducer is a mystery. The Heat supply and steam pressure usage should be smooth. I began looking at why the controller was bypassed at first I looked at the pressure transducer (downloaded speces from web) and thought the range we would like to control 850-750Kpa was too narrow for the pressure transducer's range 0-16Bar or 6.7% of the pressure transducer's range. The pressure transducer's accuracy is 0.25%-1.5%fs. I think the repeatability is part of the manufacturer's accuracy specs and do not know its value.
I averaged the accuracy to 1%fs, I thought a error of 1% over a range of 6.7% of the pressure transducer's range, would not give enough accuracy for good control.
I am not sure about this because of my limited experience so I thought it would be best to look for a transducer where the output is between 700Kpa and 800Kpa.
I have not tested the BTC9090 controller because I am still looking for a users instruction guide. I have limited experience in PID controllers but am looking forward to getting to steam boiler working correctly.
Thank you to all who have replied, I will be looking at all the info you have supplied.
You should also contact two companies Pressure Systems Inc. and Druck. If you tell them what you need they should be able to get you the right sensor for your application.
With a calibrated span of 1500 kPa for the pressure transducer 1% accuracy of full scale (typical specification) any pressure value will be +/- 15 kPa. If measured pressure is 800 kPa then accuracy at that value is 15/800 * 100% = 1.9%. What may be the most significant accuracy issue is how accurate do you calibrate the pressure transducer. Refer to PID loop tuning fundamentals at: http://www.manufacturing.net/ctl/index.asp?layout=article&articleid=CA307745
I could not find any manuals for the Brainchild BTC9090 controller either.
The presence of the pressure switch and bypass indicates a likely problem in the past with the controller. The controller is mainly for temperature control but will also do process control.
We (SMAR) make a pressure transmitter with 0.075% accuracy. It also has a built-in PID controller.
As another people described the importance of PID calibration, but if you think the problem is the transmiter you can buy a transmiter with an error less than 0,02% on request from SIEMENS.