I'm working on a gas mixing system where I have to inject between 1 to 100 Nm3/hr of air into a natural gas stream in order to stabilize the Wobbe Index of the gas. The injection rate of air is dependent on the flow rate and calorific value of natural gas. The flow rates of the two gas streams are measured by flow meters with temperature and pressure compensation and the flow rate of air is controlled by a 1" V30 control valve.
Due to the high turndown requirement, the control valve has to operate with a low differential pressure to increase controllability at low flow, but the problem is, at high flow, the droop from the regulator will cause the differential across the flow control valve to diminish. Using a pilot operated regulator such as the Type 99 might help, but the 2" might be oversized to give good performance and lockup response at low flows.
One solution I'm considering is to use the Fisher 630 with a remote droop compensator. The setup I envision is to supply an additional pneumatic loading pressure to the diaphragm via the 1/4" NPT vent port and the loading pressure controlled by an I/P converter. The regulator would be set to provide a relatively low discharge pressure say 1.15 barg to provide good controllability at low flow. As flow increases, the additional loading pressure also progressively increases to compensate the droop and also increases the discharge pressure at the high flow range to increase capacity of the flow control valve.
My question is, if the Fisher 630 is equipped with a steel body, will the spring case be able to withstand 1 barg of pneumatic pressure?
Has anybody tried this setup that can share some experience?
Eric,
Thanks for your inquiry. We do have a solution for this and it involves an electropneumatic controller (ER5000) that utilizes small solenoids to pressure load a regulator. The following video (www.youtube.com/watch) will explain in more detail how this controller works. This controller can be used on a number of other regulators.
For additional information:
http://www2.emersonprocess.com/en-us/brands/tescom/electropneumaticcontrollers-motors/er5000/pages/er5000.aspx
In reply to Michael Calaway:
Michael,
Thank you for offering this solution, which appears to be a very good control strategy for many pressure control applications. However, in my current system, I already have a pressure transmitter that communicates with the PLC via Profibus PA, hence if the ER5000 requires a pressure feedback signal in 4-20mA, I would have to install another 4-20mA pressure transmitter or use the PLC to repeat this pressure signal to the ER5000.
Nevertheless, if I use the ER5000, would it be possible to use it with the Fisher 630 with the primary pressure setpoint set by the loading spring and a secondary compensation signal supplied by the ER5000? Do you know if the Fisher 630’s spring casing is able to handle this additional loading pressure?
Best regards, Eric
In reply to Eric Tam:
Yes, the Type 630 spring case can be pressure loaded in the way that you are requesting. With a spring setpoint of 1.15 bar you would be using the Type 630 with a low-pressure actuator, which is limited to an outlet casing pressure of 4.6 bar. Your combination of spring setting + pressure load should not exceed 4.6 bar. You will need to request the special option of a "pressure load seal washer" from your sales office when purchasing. This washer will be installed around the adjusting screw, and will be clamped down by the lock nut. See the picture below. FYI: A Type 627 is a similar regulator option for you that can also be ordered with a seal washer as well.
I hope this is helpful information to help solve your selection needs.
Thanks,
Jason
{seal washer, lock nut, and adjusting screw}
You're right, you will need to add another transducer that has a 4-20 mAmp output to provide a feedback signal to the ER5000, however, depending on the level accuracy you need in your system, you should be able to get an inexpensive one. The payback comes from not needing to create a PID control algorithm in your PLC as you would with the I/P you spoke of earlier in your post. The PID control is internal to the ER5000, all you have to do is tell it what pressure to go to and it will automatically compensate for droop or any other disturbances in your system.
Best regards,
Linda
In reply to Linda Tharaldson:
Hi Linda,
In my case, since I already have a PLC performing other PID functions such as flow controls, it shouldn’t take more efforts to implement another control loop for the pressure than using the ER5000.
Hardware wise, I only need an additional 4-20mA output and an I/P converter. If I use the ER5000, I’ll need an additional 4-20mA pressure transmitter for the feedback or an additional 4-20mA output from the PLC to repeat the pressure signal to the ER5000.
For consideration of future applications, does the ER5000 work out of the box for closed-loop pressure control, or does it still require tuning of the PID in the field? Is the ER5000 available in Class 1 Div 1 certified version? This would be a requirement for most of my applications if it’s to be mounted directly on top of the air actuated regulator.
Cheers, Eric
In reply to Jason Journey:
Jason,
Thank you for the information, this is exactly what I need.
For further clarification, in the example that you provided, if the spring is set to 1.15 bar and the spring case is further loaded with 3.45 bar to give 4.6 bar discharge pressure, then at maximum flow with 20% droop, I’ll have to load the spring case to 3.68 bar to maintain 4.6 bar discharge pressure. Can the spring case be loaded to such pressure? From the product bulletin, the spring case material does look a lot thinner than the body material.
I just took a look at the Fisher 627 and it appears to have a much larger (1/2”) vent port that is intended to be used by the internal relief function. Would this spring case with such large orifice suitable for pressure loading?
Hi Eric,
The ER5000 often does work right out of box - it all depends on the accuracy and stability you need. A good number of our customers do just that. However, a lot of our customers are looking for very tight control - +/- 0.1% of full scale or maybe their application is very specialized. In that case, they can use the ERTune program that's included with every unit. ERTune provides an easy means of tuning the loop to your application's specifications.
The Explosion Proof ER5000 is still in development - due to be released 2nd quarter 2015. In the meantime, we still offer the Class 1, Div 1 version of the ER3000 which is the predecessor to the ER5000. The ER5000 was designed to be a drop-in replacement for the ER3000 making them easily interchangeable.
Regards,
The 627 has a 3/4" NPT vent that could use a 3/4"x1/4" reducing bushing to easily connect a loading line.
The 630 outlet pressure maximum is 4.6 bar. Please reduce your initial pressure load so that you never exceed the maximum. (spring setting + pressure load = 4.6 bar maximum).